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22.2B: Prokaryotic Reproduction - Biology


Prokaryotes reproduce asexually by binary fission; they can also exchange genetic material by transformation, transduction, and conjugation.

Learning Objectives

  • Distinguish among the types of reproduction in prokaryotes

Key Points

  • Binary fission is a type of reproduction in which the chromosome is replicated and the resultant prokaryote is an exact copy of the parental prokaryate, thus leaving no opportunity for genetic diversity.
  • Transformation is a type of prokaryotic reproduction in which a prokaryote can take up DNA found within the environment that has originated from other prokaryotes.
  • Transduction is a type of prokaryotic reproduction in which a prokaryote is infected by a virus which injects short pieces of chromosomal DNA from one bacterium to another.
  • Conjugation is a type of prokaryotic reproduction in which DNA is transferred between prokaryotes by means of a pilus.

Key Terms

  • transformation: the alteration of a bacterial cell caused by the transfer of DNA from another, especially if pathogenic
  • transduction: horizontal gene transfer mechanism in prokaryotes where genes are transferred using a virus
  • binary fission: the process whereby a cell divides asexually to produce two daughter cells
  • conjugation: the temporary fusion of organisms, especially as part of sexual reproduction
  • pilus: a hairlike appendage found on the cell surface of many bacteria

Reproduction

Reproduction in prokaryotes is asexual and usually takes place by binary fission. The DNA of a prokaryote exists as as a single, circular chromosome. Prokaryotes do not undergo mitosis; rather the chromosome is replicated and the two resulting copies separate from one another, due to the growth of the cell. The prokaryote, now enlarged, is pinched inward at its equator and the two resulting cells, which are clones, separate. Binary fission does not provide an opportunity for genetic recombination or genetic diversity, but prokaryotes can share genes by three other mechanisms.

In transformation, the prokaryote takes in DNA found in its environment that is shed by other prokaryotes. If a nonpathogenic bacterium takes up DNA for a toxin gene from a pathogen and incorporates the new DNA into its own chromosome, it, too, may become pathogenic. In transduction, bacteriophages, the viruses that infect bacteria, sometimes also move short pieces of chromosomal DNA from one bacterium to another. Transduction results in a recombinant organism. Archaea are not affected by bacteriophages, but instead have their own viruses that translocate genetic material from one individual to another. In conjugation, DNA is transferred from one prokaryote to another by means of a pilus, which brings the organisms into contact with one another. The DNA transferred can be in the form of a plasmid or as a hybrid, containing both plasmid and chromosomal DNA.

Reproduction can be very rapid: a few minutes for some species. This short generation time, coupled with mechanisms of genetic recombination and high rates of mutation, result in the rapid evolution of prokaryotes, allowing them to respond to environmental changes (such as the introduction of an antibiotic) very rapidly.


Prokaryotic Cell Division

The cell division process used by prokaryotes (such as E. coli bacteria) and some unicellular eukaryotes is called binary fission. For unicellular organisms, cell division is the only method to produce new individuals. The outcome of this type of cell reproduction is a pair of daughter cells that are genetically identical to the original parent cell. In unicellular organisms, daughter cells are whole individual organisms. This is a less complicated and much quicker process than cell division in eukaryotes. Because of the speed of bacterial cell division, populations of bacteria can grow very rapidly.

Figure 1: An E. coli bacteria dividing into two identical daughter cells

To achieve the outcome of identical daughter cells, there are some essential steps. The genomic DNA must be replicated (using DNA replication) to produce two identical copies of the entire genome. Then, one copy must be moved into each of the daughter cells. The cytoplasmic contents must also be divided to give both new cells the machinery to sustain life. Since bacterial cells have a genome that consists of a single, circular DNA chromosome, the process of cell division is very simple.

/>Figure 2: Prokaryotic cell division occurs via a process called binary fission.


Binary Fission

Due to the relative simplicity of the prokaryotes, the cell division process is a less complicated and much more rapid process than cell division in eukaryotes. As a review of the general information on cell division we discussed at the beginning of this chapter, recall that the single, circular DNA chromosome of bacteria occupies a specific location, the nucleoid region, within the cell (). Although the DNA of the nucleoid is associated with proteins that aid in packaging the molecule into a compact size, there are no histone proteins and thus no nucleosomes in prokaryotes. The packing proteins of bacteria are, however, related to the cohesin and condensin proteins involved in the chromosome compaction of eukaryotes.

The bacterial chromosome is attached to the plasma membrane at about the midpoint of the cell. The starting point of replication, the origin , is close to the binding site of the chromosome to the plasma membrane (Figure). Replication of the DNA is bidirectional, moving away from the origin on both strands of the loop simultaneously. As the new double strands are formed, each origin point moves away from the cell wall attachment toward the opposite ends of the cell. As the cell elongates, the growing membrane aids in the transport of the chromosomes. After the chromosomes have cleared the midpoint of the elongated cell, cytoplasmic separation begins. The formation of a ring composed of repeating units of a protein called FtsZ (short for “filamenting temperature-sensitive mutant Z”) directs the partition between the nucleoids. Formation of the FtsZ ring triggers the accumulation of other proteins that work together to recruit new membrane and cell wall materials to the site. A septum is formed between the daughter nucleoids, extending gradually from the periphery toward the center of the cell. When the new cell walls are in place, the daughter cells separate.

These images show the steps of binary fission in prokaryotes. (credit: modification of work by “Mcstrother”/Wikimedia Commons)


13.1 Prokaryotes

Chapter 13.1 workbook pages – Note: The first page of chapter 13.2’s true / false assignment is included at the end of the chapter 13.1 packet. Complete it when you get to chapter 13.2.

Also, the 2nd vocabulary assignment gives one of the answers away on number 4. Enjoy the free answer, lol.

Vocabulary

  • antibiotic drug
    • drug that kills bacteria and cures bacterial infections and diseases.
    • ability to withstand antibiotic drugs that has evolved in some bacteria.
    • one of two prokaryote domains that includes organisms that live in extreme environments.
    • domain of prokaryotes, some of which cause human diseases.
    • colony of prokaryotes that is stuck to a surface such as a rock or a host’s tissue.
    • Gram-positive blue-green photosynthetic bacteria.
    • spores that form inside prokaryotic cells when they are under stress, enclosing the DNA and helping it survive conditions that may kill the cell.
    • any type of Archaea that lives in an extreme environment, such as a very salty, hot, or acidic environment.
    • long, thin protein extensions of the plasma membrane in most prokaryotic cells that help the cells move
    • method of increasing genetic variation in prokaryotes that involves cells “grabbing” stray pieces of DNA from their environment or exchanging DNA directly with other cells.
    • type of bacteria that stain red with Gram stain and have a thin cell wall with an outer membrane.
    • type of bacteria that stain purple with Gram stain and have a thick cell wall without an outer membrane.
    • small, circular piece of DNA in a prokaryotic cell.
    • organism such as an insect that spreads pathogens from host to host.

    Introduction

    No doubt you’ve had a sore throat before, and you’ve probably eaten cheese or yogurt. If so, then you’ve encountered the fascinating world of prokaryotes. Prokaryotes are single-celled organisms that lack a nucleus. They also lack other membrane-bound organelles. Prokaryotes are tiny and sometimes bothersome, but they are the most numerous organisms on Earth. Without them, the world would be a very different place.

    Here is a printable to help you remember the material in the video above:

    Classification of Prokaryotes

    Prokaryotes are currently placed in two domains. A domain is the highest taxon, just above the kingdom. The prokaryote domains are Bacteria and Archaea (see Figure below). The third domain is Eukarya. It includes all eukaryotes. Unlike prokaryotes, eukaryotes have a nucleus in their cells.

    Here is another printable you may want to take a look at to help you remember the video material:

    Characteristic Bacteria Archaea Eukarya
    Flagella Unique to Bacteria Unique to Archaea Unique to Eukarya
    Cell Membrane Unique to Bacteria Like Bacteria and Eukarya Unique to Eukarya
    Protein Synthesis Unique to Bacteria Like Eukarya Like Archaea
    Introns Absent in most Present Present
    Peptidoglycan (in cell wall) Present Absent in most Absent

    Domain Bacteria

    Bacteria are the most diverse and abundant group of organisms on Earth. They live in almost all environments. They are found in the ocean, the soil, and the intestines of animals. They are even found in rocks deep below Earth’s surface. Any surface that has not been sterilized is likely to be covered with bacteria. The total number of bacteria in the world is amazing. It’s estimated to be 5 × 10 30 , or five million trillion trillion. You have more bacteria in and on your body than you have body cells!

    Bacteria called cyanobacteria are very important. They are bluish green in color (see Figure below) because they contain chlorophyll. They make food through photosynthesis and release oxygen into the air.

    Thousands of species of bacteria have been discovered, and many more are thought to exist. The known species can be classified on the basis of various traits. One classification is based on differences in their cell walls and outer membranes. It groups bacteria into Gram-positive and Gram-negative bacteria, as described in Figure below.

    Domain Archaea

    Scientists still know relatively little about Archaea. This is partly because they are hard to grow in the lab. Many live inside the bodies of animals, including humans. However, none are known for certain to cause disease.

    Archaea were first discovered in extreme environments. For example, some were found in hot springs. Others were found around deep sea vents. Such Archaea are called extremophiles, or “lovers of extremes.” Figure below describes three different types of Archaean extremophiles.

    Archaea are now known to live just about everywhere on Earth. They are particularly numerous in the ocean. Archaea in plankton may be one of the most abundant types of organisms on the planet. Archaea are also thought to play important roles in the carbon and nitrogen cycles. For these reasons, Archaea are now recognized as a major aspect of life on Earth.

    Mastering Science: Extremophile Hunt (Note: This video mentions billions of years.)

    BBC Extremophiles – weird animals (optional)

    Prokaryote Structure

    Most prokaryotic cells are much smaller than eukaryotic cells. Although they are tiny, prokaryotic cells can be distinguished by their shapes. The most common shapes are helices, spheres, and rods (see Figure below).

    Plasma Membrane and Cell Wall

    Like other cells, prokaryotic cells have a plasma membrane (see Figure below). It controls what enters and leaves the cell. It is also the site of many metabolic reactions. For example, cellular respiration and photosynthesis take place in the plasma membrane.

    Most prokaryotes also have a cell wall. It lies just outside the plasma membrane. It gives strength and rigidity to the cell. Bacteria and Archaea differ in the makeup of their cell wall. The cell wall of Bacteria contains peptidoglycan (composed of sugars and amino acids). The cell wall of most Archaea lacks peptidoglycan.

    Cytoplasm and Cell Structures

    Inside the plasma membrane of prokaryotic cells is the cytoplasm. It contains several structures, including ribosomes, a cytoskeleton, and genetic material. Ribosomes are sites where proteins are made. The cytoskeleton helps the cell keeps its shape. The genetic material is usually a single loop of DNA. There may also be small, circular pieces of DNA, called plasmids (see Figure below). The cytoplasm may contain microcompartments as well. These are tiny structures enclosed by proteins. They contain enzymes and are involved in metabolic processes.

    Extracellular Structures

    Many prokaryotes have an extra layer, called a capsule, outside the cell wall. The capsule protects the cell from chemicals and drying out. It also allows the cell to stick to surfaces and to other cells. Because of this, many prokaryotes can form biofilms, like the one shown in Figure below. A biofilm is a colony of prokaryotes that is stuck to a surface such as a rock or a host’s tissues. The sticky plaque that collects on your teeth between brushings is a biofilm. It consists of millions of bacteria.

    Most prokaryotes also have long, thin protein structures called flagella (singular, flagellum). They extend from the plasma membrane. Flagella help prokaryotes move. They spin around a fixed base, causing the cell to roll and tumble. As shown in Figure below, prokaryotes may have one or more flagella.

    Endospores

    Many organisms form spores for reproduction. Some prokaryotes form spores for survival. Called endospores, they form inside prokaryotic cells when they are under stress. The stress could be UV radiation, high temperatures, or harsh chemicals. Endospores enclose the DNA and help it survive under conditions that may kill the cell. Endospores are commonly found in soil and water. They may survive for long periods of time.

    Prokaryote Metabolism

    Like all living things, prokaryotes need energy and carbon. They meet these needs in a variety of ways. In fact, prokaryotes have just about every possible type of metabolism. They may get energy from light (photo) or chemical compounds (chemo). They may get carbon from carbon dioxide (autotroph) or other living things (heterotroph). Table below shows all the possible types of metabolism. Which types of prokaryotes are producers? Which types are consumers?

    Type of Energy Source of Carbon: carbon dioxide Source of Carbon: other organisms
    Light Photoautotroph Photoheterotroph
    Chemical Compounds Chemoautotroph Chemoheterotroph

    Most prokaryotes are chemoheterotrophs. They depend on other organisms for both energy and carbon. Many break down organic wastes and the remains of dead organisms. They play vital roles as decomposers and help recycle carbon and nitrogen. Photoautotrophs are important producers. They are especially important in aquatic ecosystems.

    Prokaryote Habitats

    Prokaryote habitats can be classified on the basis of oxygen or temperature. These factors are important to most organisms.

    • Aerobic prokaryotes need oxygen. They use it for cellular respiration. An example is the bacterium that causes the disease tuberculosis (TB). It infects human lungs.
    • Anaerobic prokaryotes do not need oxygen. They use fermentation or other methods of respiration that don’t require oxygen. In fact, some cannot tolerate oxygen. An example is a bacterium that infects wounds and kills tissues, causing a condition called gangrene.

    Temperature

    Like most organisms, prokaryotes live and grow best within certain temperature ranges. Prokaryotes can be classified by their temperature preferences, as shown in Table below. Which type of prokaryote would you expect to find inside the human body?

    Type of Prokaryote Preferred Temperature Where It Might Be Found
    Thermophile above 45°C (113°F) in compost
    Mesophile about 37°C (98°F) inside animals
    Psychrophile below 20°C (68°F) in the deep ocean

    Reproduction in Prokaryotes

    Prokaryote cells grow to a certain size. Then they divide through binary fission.

    Binary Fission

    Binary fission is a type of asexual reproduction. It occurs when a parent cell splits into two identical daughter cells. This can result in very rapid population growth. For example, under ideal conditions, bacterial populations can double every 20 minutes. Such rapid population growth is an adaptation to an unstable environment. Can you explain why?

    Genetic Transfer

    In asexual reproduction, all the offspring are exactly the same. This is the biggest drawback of this type of reproduction. Why? Lack of genetic variation increases the risk of extinction. Without variety, there may be no organisms that can survive a major change in the environment.

    Prokaryotes have a different way to increase genetic variation. It’s called genetic transfer. It can occur in two ways. One way is when cells “grab” stray pieces of DNA from their environment. The other way is when cells directly exchange DNA (usually plasmids) with other cells. Genetic transfer makes bacteria very useful in biotechnology. It can be used to create bacterial cells that carry new genes.

    Bacteria and Humans

    Bacteria and humans have many important relationships. Bacteria make our lives easier in a number of ways. In fact, we could not survive without them. On the other hand, bacteria can also make us sick.

    Benefits of Bacteria

    Bacteria provide vital ecosystem services. They are important decomposers. They are also needed for the carbon and nitrogen cycles. There are billions of bacteria inside the human intestines. They help digest food, make vitamins, and play other important roles. Humans also use bacteria in many other ways, including:

    • Creating products, such as ethanol and enzymes.
    • Making drugs, such as antibiotics and vaccines.
    • Making biogas, such as methane.
    • Cleaning up oil spills and toxic wastes.
    • Killing plant pests.
    • Transferring normal genes to human cells in gene therapy.
    • Fermenting foods (see Figure below).

    TED Ed: The beneficial bacteria that makes delicious foods

    Bacteria and Disease

    You have ten times as many bacteria as human cells in your body!

    TED Ed: You are your microbes

    Most of these bacteria are harmless. However, bacteria can also cause disease. Examples of bacterial diseases include tetanus, syphilis, and food poisoning. Bacteria may spread directly from one person to another. For example, they can spread through touching, coughing, or sneezing. They may also spread via food, water, or objects.

    TED Ed: How do germs spread?

    Another way bacteria and other pathogens can spread is by vectors. A vector is an organism that spreads pathogens from host to host. Insects are the most common vectors of human diseases. Figure below shows two examples.

    How dangerous is Lyme Disease?

    Humans have literally walked into some new bacterial diseases. When people come into contact with wild populations, they may become part of natural cycles of disease transmission. Consider Lyme disease. It’s caused by bacteria that normally infect small, wild mammals, such as mice. A tick bites a mouse and picks up the bacteria. The tick may then bite a human who invades the natural habitat. Through the bite, the bacteria are transmitted to the human host.

    Controlling Bacteria

    Bacteria in food or water usually can be killed by heating it to a high temperature (generally, at least 71°C, or 160°F). Bacteria on many surfaces can be killed with chlorine bleach or other disinfectants. Bacterial infections in people can be treated with antibiotic drugs. For example, if you ever had “strep” throat, you were probably treated with an antibiotic.

    Antibiotics have saved many lives. However, misuse and over-use of the drugs have led to antibiotic resistance in bacteria. Figure below shows how antibiotic resistance evolves. Some strains of bacteria are now resistant to most common antibiotics. These infections are very difficult to treat.

    TED Ed: What causes antibiotic resistance
    NOTE: There is a brief mention of evolution in this video.

    Lesson Summary

    • Prokaryotes include Bacteria and Archaea. An individual prokaryote consists of a single cell without a nucleus. Bacteria live in virtually all environments on Earth. Archaea live everywhere on Earth, including extreme environments.
    • Most prokaryotic cells are much smaller than eukaryotic cells. They have a cell wall outside their plasma membrane. Prokaryotic DNA consists of a single loop. Some prokaryotes also have small, circular pieces of DNA called plasmids.
    • Prokaryotes fulfill their carbon and energy needs in various ways. They may be photoautotrophs, chemoautotrophs, photoheterotrophs, or chemoheterotrophs.
    • Aerobic prokaryotes live in habitats with oxygen. Anaerobic prokaryotes live in habitats without oxygen. Prokaryotes may also be adapted to habitats that are hot, moderate, or cold in temperature.
    • Prokaryotic cells grow to a certain size. Then they divide by binary fission. This is a type of asexual reproduction. It produces genetically identical offspring. Genetic transfer increases genetic variation in prokaryotes.
    • Bacteria and humans have many important relationships. Bacteria provide humans with a number of services. They also cause human diseases.

    Lesson Review Questions

    Recall

    2. Distinguish between Gram-positive and Gram-negative bacteria, and give an example of each.

    4. What are extremophiles? Name three types.

    5. Identify the three most common shapes of prokaryotic cells.

    6. Describe a typical prokaryotic cell.

    7. What are the roles of flagella and endospores in prokaryotes?

    8. List several benefits of bacteria.

    Apply Concepts

    9. Assume that a certain prokaryote is shaped like a ball, lives deep under the water on the ocean floor, and consumes dead organisms. What traits could you use to classify it?

    10. Apply lesson concepts to explain why many prokaryotes are adapted for living at the normal internal temperature of the human body.

    Think Critically

    11. Compare and contrast Archaea and Bacteria.

    12. Why might genetic transfer be important for the survival of prokaryote species?

    Points to Consider

    In this lesson, you read that some bacteria cause human diseases. Many other human diseases are caused by viruses.


    Signalling Mechanism in Prokaryotes and Eukaryotes | Microbiology

    In this article we will discuss about the signalling mechanisms, both in eukaryotes and prokaryotes.

    1. Eukaryotic Cell-to-Cell Signaling:

    The integrative nature of biological systems could be understood after the pioneering work of Claude Bernard (1813-1878) of France. He gave the concept of the miliew interieur and suggested the system of ductless gland (i.e. endocrine glands) for integrating function and maintaining homeostasis.

    In 1902, Bayliss and Staling demonstrated a marked flow of pancreatic juice in dogs after injecting an acid extract of duodenum. Starling coined the term ‘hormone’ (Greek, I excite) for such intercellular messenger molecules. An American physiologist Water Cannon coined the term ‘homeostasis’ (a condition that may vary but is relatively constant).

    There are three types of signalling systems in multicellular organisms like mammals: neuronal, endocrine and cytokine signalling (Fig. 27.3). Neuronal signalling occurs over very long distance i.e. brain to toe. Synaptic junctions communicate rapidly. Many chemicals are involved in signalling at junctions and associated with inflammation.

    Endocrine signalling involves the release of a hormone from its gland and its transport to blood to a limited number of cells in the target tissue. It occurs at a long distance and limited by the rate of blood flow and diffusion from blood to tissues.

    Most of the intracellular signalling is that of the cytokines. Much of this signalling occurs through paracrine signalling (over short distance cell to nearby cell) or by auto-signalling (stimulation of the cell producing the cytokines).

    Certain bacterial endotoxins target neuronal signalling hence these are called neurotoxins as produced by Clostridium tetani and CI. botulinum. These have metalloproteinase activity and cleave specific intracellular proteins. Thus they prevent the neurotransmitters. A recent discovery also points to the interaction of a bacterial toxin with neuroendocrine signalling and synapsis of cytokines.

    (i) Endocrine Hormone Signalling:

    Endocrine hormones are mostly produced by specific glands (such as pituitary, hypothalamus, and parathyroid glands) and glandular tissues (such as pancreas and intestine).

    There are three major groups of hormones: peptide hormones (produced especially by the intestine which has neurotransmitter-like activity), steroid hormones (produced by adrenal cortex, gonads and skin), and thyrosine derivatives (e.g. thyroid hormones T3, T4, etc. and catecholamines, noradrenaline, adrenaline and dopamine having neurotransmitter activity).

    After secretion from glands, endocrine hormones are circulated as free hormone or bound to carrier proteins, for example a serum protein, albumin. It binds to several circulating hormones and exerts action (only in bound form) to specific cell receptors in target tissues.

    The peptide hormones bind to specific membrane receptors which result in specific intracellular signalling pathway. On the other hand the steroid and sterol hormones enter into cells and bind to cytoplasmic receptor proteins and then move to nucleus and act as a factor for transcription.

    Endocrine hormones control the energy metabolism through insulin and glycogen, adrenaline and noradrenaline involving production and breakdown of carbohydrate stored as glycogen in the liver and muscle.

    The lack of control of this system is visible in diabetes. Bacterial infection results in hormonal imbalance in body. Certain bacteria and viruses affect neural tissue. Mycobacterium leprae and Treponema pallidum have a tropism for nervous tissue.

    The tissue of the gastric and intestinal mucosae are highly regulated. They respond and produce several endocrine signals including gastro-intestinal hormones e.g. gastrin, secretin, cholecystokinin and guanylin. E. coli alters fluid imbalance in the intestine and causes diarrhoea. Now seven different strains of E. coli have been reported which induce different pathological symptoms.

    The enter toxigenic E. coli strains produce heat-labile toxin (LT) and heat-stable toxin (ST). ST is the first bacterial analogue of an endocrine hormone (guanyl) that activates guanyl cyclase and control fluid release from intestinal cells so that mucin layer could be kept wet (Fig. 27.4). There are other heat-stable guanyl-like toxin of other strains of E. coli and other bacteria.

    (ii) Cytokines:

    Cytokines are a large group of over 1000 proteins which are involved in cell- to-cell signalling and control the inflammatory response to bacterial infection. These are polypeptide hormones secreted by a cell that affects growth and metabolism of the same cell (autocrine signalling) or another cell (paracrine signalling).

    Their over production causes disease. These are found at the site of infection by the agents. These induce lipid mediators (prostaglandins, leukotrienes, lipo xins, platelet-activating factor and the mediators from mast cells (e.g. histamine and enzymes such as tryptase).

    (a) Nomenclature of cytokines:

    Cytokines are divided into six sub-families (Table 27.7) on the basis of several criteria such as historical types, sequence homology, localisation of chromo­somes and biochemical actions. In 1979, the term interleukin (inter: between, leukin: leukocytes) was coined to denote the proteinaceous factors which modulate the function of the other leukocytes.

    At present there are over 20 interleukins (IL-1 to IL-18). The endotoxin-injected mice expressed a tumour necrosis factor (TNF) grouped under cytotoxic cytokines. The TNF kill certain tumour cell lines via induction of apoptosis and are potent pro-inflammatory molecules. The TNF receptor family is itself membrane-bounded proteins (e.g. CD27, CD30 and CD40).

    Table 27.7 : Cytokines: nomenclature and sub-families.

    The interferon’s (IFNs) are such cytokines which were discovered first. These are involved in inhibiting the growth and spread of viruses. They are of three types: INF-a, INF-P, and INF- Y- Interferon’s also act against protozoa, rickettsia and mycobacteria.

    The colony-stimulating fac­tors (CSFs) control the growth and differentiation of neutrophils, monocytes and cell populations derived from monocytes in the bone marrow. The monocytes/macrophages are the phagocytic cells which engulf and kill bacteria. Hence, they are also called as antigen-presenting cells and stimu­late T and B lymphocytes.

    Growth factors include families of pro­teins such as fibroblast growth factor (FGF) family, platelet-derived growth factors (PDGF), and transforming growth factor-β (TGFβ). The FGF cytokines act on mesenchyme cells and epithelial cells also.

    The peptide chemotactic factors are called chemokines which is a large sub-group of cytokines. Chemokines have molecular mass of 8-10 kDalton, with 20-50% sequence homology at protein level and cysteine as conserved residues which form disulphide bonds within the molecules.

    On the basis of chromosomal location of genes and protein structure, chemokines are divided into two families: α-chemokine and β-chemokine families. A third family of chemokines discovered in 1994 currently has one member called lympholactin which is a strong attractant of T cells.

    (b) Receptors of cytokines:

    Cytokine receptors have high affinity for their ligand. The number of individual receptor present on target cell is low. On the basis of sequence homology and structural motifs cytokine receptors are grouped into a small number of families. At present there are nine receptors for CC chemokines (CCR), five receptor for CXC chemokines and CXCR1, one receptor for fractalkine.

    The cytokine receptors are shed from cell via proteolytic cleavage. Cell surface metalloproteinases (sheddases) help the release of cytokine receptors. The released receptors bind the soluble cytokines and inhibit their activity or stimulate the cytokine-receptor lacking cells.

    (c) Biological action of cytokines:

    Cytokines play a role in physiological development. They are found at all developmental stages in mammals. On the other hand, cytokine receptors present on cell membrane also play a physiological role. They act as portals for vital entry into cells. For example HIV enters through binding to cytokine receptors. Similarly, herpes simplex virus enters through binding the TNF receptor family.

    After binding receptors induce selective intracellular signalling resulting in switching on or switching off of particular genes and production of cyclooxygenase II, and nitric oxide (NO) is synthesised after induction of nitric oxide synthetase.

    Aspirin and ibuprofen are the non-steroid anti-inflammatory drugs which block cyclooxygenase activity. These drugs reduce pain and fever as the prostaglandins and prostacyclin lower threshold in pain nerve resulting in a relief of pain and fever.

    Various molecules are produced after binding cytokines to cytokine-receptor which produce pathology [prostaglandins, NO, tissue plasminogen activator (tPA) and plasminogen activator inhibitor and collaginases]. Tissue damage is directly induced by collagenase and tPA.

    Besides, cytokines also induce the synthesis of their own and other cytokines which result in a complex network of interactions. Cytokines can also modify the behaviour of cells in many ways. Various actions of cytokines on cells are shown in Fig. 27.5.

    2. Prokaryotic Cell-to-Cell Signalling: Quorum Sensing and Bacterial Pheromones:

    Until the 1980s, no attention was paid that bacteria could talk to one another. Thereafter, examples were put forth for cell-to-cell signalling in bacteria. Conjuga­tion is one of the methods of DNA transfer between two bacteria. To establish conju­gation, both the bacteria must establish cell-to-cell contact. Enterococcus faecalis is a Gram-positive mammalian pathogen.

    Its aggregation in controlled by the secretion of small peptide pheromones. Pheromones induce adhesion production consequently bacteria form cell clumps which facilitate conjugation. Several pheromones have been isolated which are hepta- or octa-peptides found in low concentration (5吆 -11 M).

    Endospores of Clostridium tetani are regarded as resting forms of bacteria and a part of virulence mechanism. In contrast some bacteria such as a myxobacterium under adverse environ­mental conditions undergo complex morphological changes. Polyangium vitellinum forms cyst-like structure consisting of an outer covering of polysaccharide to resist from dehydration.

    Myxococcus xanthus forms myxospores (fruiting body) and alternate with vegetative cells This programme is triggered by starvation which causes morphological changes within 4 hours. A dense mound-shaped structure is formed when a cell density of bacteria has reached to about 10 5 . After 20 hours of starvation the cells inside this mound differentiate into myxospores.

    Myxospores are heat- and starvation-resistant dormant cells. They germinate during favourable conditions and produce vegetative cells. Again myxospores are formed when conditions are unfavourable. This type of cell differentiation is controlled by extracellular signals. Cell-to-cell signalling mechanism is given in Table 27.8.

    Quorum Sensing:

    The term quorum refers to ‘a fixed number of members of any committee of the society whose presence is mandatory for proper transaction of business’. Quorum sensing in bacteria is a mechanism through which they take a census of their number. After reaching a quorum of cell number they can transact the business of switching on or switching off of specific genes.

    The current knowledge of quorum sensing began with the study of luminescence in Vibrio fischeri and V. harveyi. They are marine bacteria forming symbiotic relationship with monocentrid fish and with bobtail squids (e.g. Euprymna scolopes). The bobtail squid consists of very high concentration of V. fischeri. The light organ is supposed to be part of a counter illumination the details of which are not clear.

    The newly hatched squids develop symbiotic association with only certain strains of V.fischeri. Within hours after hatching, light organ is colonised by V.fischeri. The light organ positively selects only certain strains of V.fischeri and negatively selects the others to exclude colonisation of other bacteria present in sea water.

    It is not known how this selection is made. One of the possible mechanisms may be the expression of specific adhesin for V.fischeri by epithelium of light organ. The epithelium is exposed to trypsin which di­rectly triggers a specific morphogenetic response in the squid. This results in formation of the complex.

    (a) Mechanism of quorum sensing:

    It is the feedback control system. Bacteria continu­ously produce a small amount of signal called auto inducer. Most of the Gram-positive bacteria produce auto inducer which are acylhomoserine lactones (AHLs). Staphylococcus aureus and other bacteria produce peptide auto inducers. E. colt and S. typhimurium produce a quorum sensing mol­ecule of 1 kDalton. These extracellular inducers are diffused out.

    Besides, bacteria also recognise the pres­ence of auto inducer. The bacterial membrane protein does this function. It acts both as receptor of auto inducer and activator of gene transcription. V. fischeri produces luminescence. V. fischeri system is the best studied quorum sensing system.

    Luminescence is associated with lux operon system which consists of two main regulatory genes luxl and luxR (Fig. 27.6) and other genes (luxCDABEG) which synthesise chemicals to produce light. LuxI encodes a protein which catalyses the synthesis of a wide range of AHLα. Autoinducer of V. fischeri is N-(3-oxo-hexanoyl)-L- homoserine lactone.

    LuxR encodes a protein which acts both as a receptor for AHL and as a transducer of the signal that activates the other genes of lux operon. The luxCDABEG genes are expressed after binding AHL to the luxR protein (Fig. 27.6). The luxA and luxB genes synthesise the α- and β- subunits of bacterial luciferase. The other genes encode polypeptides which facilitate the synthesis of the substrate and produces light.

    (b) Quorum sensing as a virulence mechanism:

    In addition to V. fischeri, there is a large number of Gram-negative bacteria which produce AHLs to quorum sense. These are medically important bacteria, for example Pseudomonas aeruginosa, Proteus mirabilis, Serratia liquefaciens and Yersinia enterocolitica. In these bacteria LuxI/LuxA homologues are involved in quorum sensing system. Ps. aeruginosa utilises two quorum systems, the las and rhl.

    The las operon expresses LasR protein which is similar to LuxR and acts as transcriptional activator in the presence of PAI of Pseudomonas. The LasI (the Luxl homologue) produces AHL. The autoinducer of P. aeruginosa at a threshold concentration swich on a group of virulence gene including lasB, lasA apv and toxA.

    The rhl system is the second quorum sensing system which involves RhIR (the transcriptional activator protein) along with the autoinducer (N-butyryl-L-homoserine lactone) synthesised by RhIR. This quorum sensing system results in production of extra virulence factor e.g. elastase which cleaves and inhibits the interleukin-2 (the key host defence cytokines). The las system is dominant which is activated before the rhl system.

    Many Gram-positive bacteria use oligopeptide as signalling molecules. For example, two different peptides are secreted by Bacillus subtilis. These are necessary for competence (ability for DNA uptake) and sporulation.

    In Staphylococcus aureus, a locus agr controls the expression of many virulence factors, namely exotoxins, capsular polysaccharide type 8 and V8 protease. An octapeptide quorum sensing autoinducer is encoded by the agr lucus which induces the agr locus. The quorum sensing autoinducer interacts with host defence system and inhibits the albeit at high concentration (Fig. 27.7).


    Biology 171

    By the end of this section, you will be able to do the following:

    • Describe the process of binary fission in prokaryotes
    • Explain how FtsZ and tubulin proteins are examples of homology

    Prokaryotes, such as bacteria, produce daughter cells by binary fission. For unicellular organisms, cell division is the only method to produce new individuals. In both prokaryotic and eukaryotic cells, the outcome of cell reproduction is a pair of daughter cells that are genetically identical to the parent cell. In unicellular organisms, daughter cells are individuals.

    To achieve the outcome of cloned offspring, certain steps are essential. The genomic DNA must be replicated and then allocated into the daughter cells the cytoplasmic contents must also be divided to give both new cells the cellular machinery to sustain life. As we’ve seen with bacterial cells, the genome consists of a single, circular DNA chromosome therefore, the process of cell division is simplified. Karyokinesis is unnecessary because there is no true nucleus and thus no need to direct one copy of the multiple chromosomes into each daughter cell. This type of cell division is called binary (prokaryotic) fission .

    Binary Fission

    Due to the relative simplicity of the prokaryotes, the cell division process is a less complicated and much more rapid process than cell division in eukaryotes. As a review of the general information on cell division we discussed at the beginning of this chapter, recall that the single, circular DNA chromosome of bacteria occupies a specific location, the nucleoid region, within the cell (Review). Although the DNA of the nucleoid is associated with proteins that aid in packaging the molecule into a compact size, there are no histone proteins and thus no nucleosomes in prokaryotes. The packing proteins of bacteria are, however, related to the cohesin and condensin proteins involved in the chromosome compaction of eukaryotes.

    The bacterial chromosome is attached to the plasma membrane at about the midpoint of the cell. The starting point of replication, the origin , is close to the binding site of the chromosome to the plasma membrane ((Figure)). Replication of the DNA is bidirectional, moving away from the origin on both strands of the loop simultaneously. As the new double strands are formed, each origin point moves away from the cell wall attachment toward the opposite ends of the cell. As the cell elongates, the growing membrane aids in the transport of the chromosomes. After the chromosomes have cleared the midpoint of the elongated cell, cytoplasmic separation begins. The formation of a ring composed of repeating units of a protein called FtsZ (short for “filamenting temperature-sensitive mutant Z”) directs the partition between the nucleoids. Formation of the FtsZ ring triggers the accumulation of other proteins that work together to recruit new membrane and cell wall materials to the site. A septum is formed between the daughter nucleoids, extending gradually from the periphery toward the center of the cell. When the new cell walls are in place, the daughter cells separate.


    The precise timing and formation of the mitotic spindle is critical to the success of eukaryotic cell division. Prokaryotic cells, on the other hand, do not undergo karyokinesis and therefore have no need for a mitotic spindle. However, the FtsZ protein that plays such a vital role in prokaryotic cytokinesis is structurally and functionally very similar to tubulin, the building block of the microtubules which make up the mitotic spindle fibers that are necessary for eukaryotic nuclear division. FtsZ proteins can form filaments, rings, and other three-dimensional structures that resemble the way tubulin forms microtubules, centrioles, and various cytoskeletal components. In addition, both FtsZ and tubulin employ the same energy source, GTP (guanosine triphosphate), to rapidly assemble and disassemble complex structures.

    FtsZ and tubulin are considered to be homologous structures derived from common evolutionary origins. In this example, FtsZ is the ancestor protein to tubulin (an evolutionarily derived protein). While both proteins are found in extant organisms, tubulin function has evolved and diversified tremendously since evolving from its FtsZ prokaryotic origin. A survey of mitotic assembly components found in present-day unicellular eukaryotes reveals crucial intermediary steps to the complex membrane-enclosed genomes of multicellular eukaryotes ((Figure)).

    Cell Division Apparatus among Various Organisms
    Structure of genetic material Division of nuclear material Separation of daughter cells
    Prokaryotes There is no nucleus. The single, circular chromosome exists in a region of cytoplasm called the nucleoid. Occurs through binary fission. As the chromosome is replicated, the two copies move to opposite ends of the cell by an unknown mechanism. FtsZ proteins assemble into a ring that pinches the cell in two.
    Some protists Linear chromosomes exist in the nucleus. Chromosomes attach to the nuclear envelope, which remains intact. The mitotic spindle passes through the envelope and elongates the cell. No centrioles exist. Microfilaments form a cleavage furrow that pinches the cell in two.
    Other protists Linear chromosomes wrapped around histones exist in the nucleus. A mitotic spindle forms from the centrioles and passes through the nuclear membrane, which remains intact. Chromosomes attach to the mitotic spindle, which separates the chromosomes and elongates the cell. Microfilaments form a cleavage furrow that pinches the cell in two.
    Animal cells Linear chromosomes exist in the nucleus. A mitotic spindle forms from the centrosomes. The nuclear envelope dissolves. Chromosomes attach to the mitotic spindle, which separates the chromosomes and elongates the cell. Microfilaments form a cleavage furrow that pinches the cell in two.

    Section Summary

    In both prokaryotic and eukaryotic cell division, the genomic DNA is replicated and then each copy is allocated into a daughter cell. In addition, the cytoplasmic contents are divided evenly and distributed to the new cells. However, there are many differences between prokaryotic and eukaryotic cell division. Bacteria have a single, circular DNA chromosome but no nucleus. Therefore, mitosis (karyokinesis) is not necessary in bacterial cell division. Bacterial cytokinesis is directed by a ring composed of a protein called FtsZ. Ingrowth of membrane and cell wall material from the periphery of the cells results in the formation of a septum that eventually constructs the separate cell walls of the daughter cells.

    Free Response

    Name the common components of eukaryotic cell division and binary fission.

    The common components of eukaryotic cell division and binary fission are DNA duplication, segregation of duplicated chromosomes, and division of the cytoplasmic contents.

    Describe how the duplicated bacterial chromosomes are distributed into new daughter cells without the direction of the mitotic spindle.

    As the chromosome is being duplicated, each origin moves away from the starting point of replication. The chromosomes are attached to the cell membrane via proteins the growth of the membrane as the cell elongates aids in their movement.

    Glossary


    Preparation for Reproduction

    Before prokaryotes are able to reproduce, they have to meet certain guidelines that allow them to successfully complete the operation. For example, a prokaryote first has to reach a certain size in order to split into two different cells. This is important because without the necessary nutrients, the cell may not have the ability to reproduce if it is too malnourished.

    Another condition that influences prokaryote reproduction is the environment. For example, if the temperature is too hot or cold, this may impact the rate of reproduction of binary fission. If the conditions are ideal however (such as in a laboratory setting) prokaryotes have the ability to rapidly produce from millions to billions of new cells.


    PROKARYOTIC CELLS

    The word procariota is derived from the Greek “prokaryota”. They are all those that lack a cell nucleus, mitochondria or other organelles . This means that genetic material as well as DNA is known. It is dispersed in the cytoplasm.

    Usually most of the organisms constituted by this type of cell have only one of these and are called unicellular organisms,except for some cases such as myxobacteria , some of which have multicellular stages in their life cycle. Compartmentalization is also common in the prokaryotic world in the form of compartments bounded by proteins and other delimited by lipids. They are microorganisms that possess a single chromosome called nucleoid , its reproduction are asexual by binary fission other cases they create large colonies, as in cyanobacteria .

    Living things, which are made up of prokaryotic cells, are divided into two groups:

    In short we tell you the differences between cell eukaryote and prokaryote .


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    1 . Question

    Which one occurs in binary fission of prokaryotes?

    • DNA replication
    • Spindle fibre formation
    • Metaphase
    • Pairing of sister chromatids
    2 . Question

    In which of these ways does binary fission differ from mitosis?

    • DNA replicates and separates at the same time
    • DNA does not replicate
    • DNA replicates after cell division
    • The cell does not divide
    3 . Question

    By what process does cell division take place in prokaryotes?

    4 . Question

    Which of the following divide by binary fission?

    5 . Question

    During division by binary fission the two copies of the circular DNA are attached to which structure?

    6 . Question

    The two copies of circular DNA are separated by what process during binary fission in prokaryotes?

    • Cell growth
    • Microtubule depolymerisation
    • Kinesin movement along microfilaments
    • Brownian motion
    7 . Question

    Binary fission of a parent cell results in how many daughter cells?

    8 . Question

    In most cases, binary fission results in which of the following?

    • Two daughter cells with an exact copy of the DNA of the parent cell
    • Two daughter cells with half of the DNA content of the parent cell
    • Four daughter cells with half of the DNA content of the parent cell
    • Four daughter cells with an exact copy of the DNA of the parent cell
    9 . Question

    Why might the genetic content of a daughter cell differ from a parent cell after binary fission?

    10 . Question

    Which of the following is true of binary fission?

    • Plasmids are also replicated and distributed between each daughter cell
    • Only the circular DNA is replicated and moved into each daughter cell
    • The circular DNA is separated using spindle fibres
    • Plasmids use flagella to swim into the new daughter cell
    11 . Question

    Why might the number of plasmids vary between each daughter cell following prokaryote cell division?

    • Because the many plasmid floating in the cytoplasm are randomly segregated when the cell divides
    • Because plasmids stay in the parent cell only
    • Because there is only a single copy of the plasmid which stays in one of the daughter cells
    • Because plasmids only ever occur in odd numbers, so one daughter will always inherit more
    12 . Question

    Which of the following does not divide by binary fission?

    13 . Question

    Which of the following phases do not occur during binary fission? 1. Metaphase 2. Telophase 3. Interphase

    14 . Question

    Which of the following divide by binary fission?

    • Archaea and Bacteria
    • Archaea and Fungi
    • Bacteria and Fungi
    • Archaea, Bacteria and Fungi
    15 . Question

    Which of the following prokaryotic features are not involved in cell division?

    16 . Question

    During binary fission, what happens after the cell membrane pinches inward and divides the cytoplasm in two?

    • New cell wall forms between the two daughter cells
    • Separation of the two copies of circular DNA begins
    • Plasmids are moved along the cell membrane into each daughter
    • The cell membrane forms a tube that connects the daughter cells
    17 . Question

    Explain why one daughter cell may not be antibiotic resistant following cell division in bacteria

    • It may not have inherited any of the plasmids confering antibiotic resistance
    • The antibiotic may have become more potent
    • It may not have inherited a complete copy of the circular DNA
    • It's cell wall may have reformed
    18 . Question

    Which of these microbial pathogens divides by binary fission?

    • Mycobacterium tuberculosis (TB)
    • Human Immunodeficiency Virus (AIDS)
    • Plasmodium falciparum (Malaria)
    • Candida albicans (Thursh)
    19 . Question

    If a single bacterium takes 20 minutes to complete its cell cycle, how many bacteria will there be after 2 hours?

    20 . Question

    If a single bacterium takes 20 minutes to complete its cell cycle, how many bacteria will there be after 4 hours?

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    What are the differences between prokaryotic and eukaryotic cells?

    Learn about the differences between prokaryotic and eukaryotic cells.

    Prokaryotic cells and eukaryotic cells are the two types of cells that exist on Earth. There are several differences between the two, but the biggest distinction between them is that eukaryotic cells have a distinct nucleus containing the cell's genetic material, while prokaryotic cells don't have a nucleus and have free-floating genetic material instead.

    What are prokaryotic and eukaryotic cells?

    All living things can be divided into three basic domains: Bacteria, Archaea and Eukarya. The primarily single-celled organisms found in the Bacteria and Archaea domains are known as prokaryotes. These organisms are made of prokaryotic cells &mdash the smallest, simplest and most ancient cells.

    Organisms in the Eukarya domain are made of the more complex eukaryotic cells. These organisms, called eukaryotes, can be unicellular or multicellular and include animals, plants, fungi and protists. Many people are unclear on whether yeasts or fungi are prokaryotes or eukaryotes. Both are eukaryotes and share similar cell structure to all other eukaryotes.

    Eukaryotes developed at least 2.7 billion years ago, following 1 to 1.5 billion years of prokaryotic evolution, according to the National Institutes of Health (NIH). Scientists hypothesize that the nucleus and other eukaryotic features may have first formed after a prokaryotic organism swallowed up another, according to the University of Texas. According to this theory, the engulfed organism would have then contributed to the functioning of its host.

    What do prokaryotes and eukaryotes have in common?

    Although prokaryotic and eukaryotic cells have many differences, they share some common features, including the following:

      : Genetic coding that determines all the characteristics of living things.
  • Cell (or plasma) membrane: Outer layer that separates the cell from the surrounding environment and acts as a selective barrier for incoming and outgoing materials.
  • Cytoplasm: Jelly-like fluid within a cell that is composed primarily of water, salts and proteins.
  • Ribosomes: Organelles that make proteins.
  • How do prokaryotes and eukaryotes differ?

    Nucleus/DNA: Eukaryotic cells have a nucleus surrounded by a nuclear envelope that consists of two lipid membranes, according to Nature Education. The nucleus holds the eukaryotic cell's DNA. Prokaryotic cells do not have a nucleus rather, they have a membraneless nucleoid region (open part of the cell) that holds free-floating DNA, according to Washington University.

    The entire DNA in a cell can be found in individual pieces known as chromosomes. Eukaryotic cells have many chromosomes which undergo meiosis and mitosis during cell division, while most prokaryotic cells consist of just one circular chromosome. However, recent studies have shown that some prokaryotes have as many as four linear or circular chromosomes, according to Nature Education. For example, Vibrio cholerae, the bacterium that causes cholera, has two circular chromosomes.

    Organelles in Eukaryotic Cells: Eukaryotic cells have several other membrane-bound organelles not found in prokaryotic cells. These include the mitochondria (convert food energy into adenosine triphosphate, or ATP, to power biochemical reactions) rough and smooth endoplasmic reticulum (an interconnected network of membrane-enclosed tubules that transport synthesized proteins) golgi complex (sorts and packages proteins for secretion) and in the case of plant cells, chloroplasts (conduct photosynthesis). All of these organelles are located in the eukaryotic cell's cytoplasm.

    Although only eurkaryotes carry membrane-bound organelles, recent evidence suggests that both eukaryotes and prokaryotes can produce organelle-like structures that lack membranes, according to a 2020 report published in the journal Proceedings of the National Academy of Sciences (PNAS).

    For instance, in the bacterium Escherichia coli, molecules and proteins cluster together to form liquid "compartments" within the cytoplasm, according to the PNAS study. These compartments form similarly to how oil forms droplets when mixed with water, according to a statement from the University of Michigan. Such membraneless structures have been reported in many bacterial species, including Mycobacterium tuberculosis, which causes tuberculosis, and cyanobacteria, a type of photosynthetic bacteria that can also cause disease.

    Ribosomes: In eukaryotic cells, the ribosomes are bigger, more complex and bound by a membrane. They can be found in various places: Sometimes in the cytoplasm on the endoplasmic reticulum or attached to the nuclear membrane (covering on the nucleus).

    In prokaryotic cells, the ribosomes are scattered and floating freely throughout the cytoplasm. The ribosomes in prokaryotic cells also have smaller subunits. All ribosomes (in both eukaryotic and prokaryotic cells) are made of two subunits &mdash one larger and one smaller. In eukaryotes, these pieces are identified by scientists as the 60-S and 40-S subunits. In prokaryotes, the ribosomes are made of slightly smaller subunits, called 50-S and 30-S.

    The difference in types of subunits has allowed scientists to develop antibiotic drugs, such as streptomycin, that attack certain types of infectious bacteria, according to the British Society for Cell Biology. On the downside, some bacterial toxins and the polio virus use the ribosome differences to their advantage they're able to identify and attack eukaryotic cells' translation mechanism, or the process by which messenger RNA is translated into proteins.

    Reproduction: Most eukaryotes reproduce sexually (although some protists and single-celled fungi may reproduce through mitosis, which is functionally similar to asexual reproduction). Prokaryotes reproduce asexually, resulting in the offspring being an exact clone of the parent. Some prokaryotic cells also have pili, which are adhesive hair-like projections used to exchange genetic material during a type of sexual process called conjugation, according to Concepts of Biology. Conjugation can occur in bacteria, protozoans and some algae and fungi.

    Cell Walls: Most prokaryotic cells have a rigid cell wall that surrounds the plasma membrane and gives shape to the organism. In eukaryotes, vertebrates don't have a cell wall but plants do. The cell walls of prokaryotes differ chemically from the eukaryotic cell walls of plant cells, which are primarily made of cellulose. In bacteria, for example, the cell walls are composed of peptidoglycans (sugars and amino acids), according to Washington University.

    Additional resources:

    • Check out this animated video by the Amoeba Sisters that explains the difference between prokaryotic and eukaryotic cells.
    • Learn how prokaryotes evolved into eukaryotes.
    • Compare microscopic images of prokaryotic and eukaryotic cells.

    This article was updated on June 18, 2021 by Live Science staff writer Nicoletta Lanese.