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Are gametes diploid or haploid?


Some sources say that gametes are haploid, some say that they are diploid.

I'm confused.


Actually there is some confusion here, and that's quite excusable, because it's extremely common reading that monoploid and haploid are synonyms and have the same meaning. However, they are different terms. According to Hartl and Ruvolo (2012):

The potential confusion arises because of diploid organisms, in which the monoploid chromosome set and the haploid chromosome set are the same.

As we, human beings, are diploid organisms, it's easy to see why haploid and monoploid ended up being considered as synonyms.

However, a more precise terminology would be:

  • Monoploid: the total number of chromosomes in a single complete set of chromosomes (this does not change whether we are talking about a somatic cell or a gamete).
  • Haploid: half of the total number of chromosomes in a somatic cell. The haploid chromosome set is the set of chromosomes present in a gamete, irrespective of the chromosome number of the species.

That being said, diploid and haploid are not antonyms nor mutually exclusive terms. A cell can be diploid and haploid at the same time. Let's exemplify this with organisms that perform gametic meiosis:

Human beings have diploid somatic cells, with 46 chromosomes. When a somatic human cell perform meiosis, it produces haploid cells which are monoploid. Human gametes are haploid and monoploid.

In wheat (Triticum aestivum), somatic cells are hexaploid, having 42 chromosomes (that is, 6 full sets of 7 chromosomes). When a wheat cell perform meiosis (producing micro and mega spores, and later on gametes), it produces haploid cells which are triploid. Wheat gametes are haploid and triploid.

The same whay, a tetraploid organism would produce, by meiosis, a cell which is haploid and diploid. Thus, depending on the number of chromosome sets in the somatic cell of a given species, you can say that a gamete is diploid (as stated in this other answer).

In a nutshell, a gamete that was produced by meiosis (there are life cycles where the gamete is not produced meiotically) is always haploid, regardless the number of chromosome sets it has (which will determine if it is monoploid, diploid, triploid, hexaploid etc… ).

Sources:

  • Hartl, D. and Ruvolo, M. (2012). Genetics. 1st ed. Burlingham, Mass.: Jones and Bartlett Learning.

  • Genetics-notes.wikispaces.com. (2017). genetics-notes - Ploidy. [online] Available at: https://genetics-notes.wikispaces.com/Ploidy [Accessed 1 May 2017].


It depends on the organism in question. (https://en.wikipedia.org/wiki/Polyploid#Examples)

Notice that the gametes carry half number of copies of the normal cells. As such a tetraploid organism will have diploid gametes.


Don't get confused by the number of chromosomes. Haploid refers to 1 set of chromosome, diploid refers to 2 set of chromosomes, triploid means 3 set of chromosomes. They don't represent the numbers of chromosome present on a set.

We human beings have 23 chromosomes on a single set. We are diploid organisms and thus all the cells of our body carries two set of chromosomes (thus 23*2=46). Our germ cells however are formed through meiosis cell division and thus they are haploid (23 chromosomes).

So, sperm cell carries 23 chromosome and egg carries 23 chromosome each. When they fuse, zygote is formed and as you can see, zygote carries 23+23=46 chromosomes. Zygote undergoes mitotic cell division and a complete human is formed. So, human zygote definitely is diploid.


Gametes must be haploid because they will be combining with another gamete. Sexual reproduction works to increase genetic diversity by having two haploid gametes combine to form a new organism that has a different combination of genes than either of its parents. The new organism has half the chromosomes from its mother and half from its father.

Source from Chromosomes and Meiosis Interactive

For example, in order for humans to reproduce, a sperm cell must fuse with an egg cell, producing a zygote that has a unique set of genetic information. If the gametes were diploid instead of haploid, the resulting organism would have too many chromosomes. By having two haploid gametes fuse together, it is ensured that the new organism will be genetically distinct and still have the correct number of chromosomes that it needs.


Gametes

Gametes are reproductive cells or sex cells that unite during sexual reproduction to form a new cell called a zygote. Male gametes are called sperm and female gametes are ova (eggs). Sperm are motile and have a long, tail-like projection called a flagellum. Ova are non-motile and relatively large in comparison to the male gamete.

In seed-bearing plants, pollen is a male sperm-producing gametophyte and female sex cells are contained within plant ovules. In animals, gametes are produced in male and female gonads, the site of hormone production. Read to learn more about how gametes divide and reproduce.


Diploid-Dominant Life Cycle

Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells , are produced within the gonads, such as the testes and ovaries. Germ cells are capable of mitosis to perpetuate the cell line and meiosis to produce gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state (Figure 1).

Figure 1. In animals, sexually reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring. The germ cells are generated early in the development of the zygote.


Meiosis II

In meiosis II, the connected sister chromatids remaining in the haploid cells from meiosis I will be split to form four haploid cells. In some species, cells enter a brief interphase, or interkinesis, that lacks an S phase, before entering meiosis II. Chromosomes are not duplicated during interkinesis. The two cells produced in meiosis I go through the events of meiosis II in synchrony. Overall, meiosis II resembles the mitotic division of a haploid cell.

In prophase II, if the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed. In prometaphase II, the nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles. In metaphase II, the sister chromatids are maximally condensed and aligned at the center of the cell. In anaphase II, the sister chromatids are pulled apart by the spindle fibers and move toward opposite poles.

Figure 7.2.3: In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to individual kinetochores of sister chromatids. In anaphase II, the sister chromatids are separated.

In telophase II, the chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four genetically unique haploid cells. At this point, the nuclei in the newly produced cells are both haploid and have only one copy of the single set of chromosomes. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombination of maternal and paternal segments of chromosomes&mdashwith their sets of genes&mdashthat occurs during crossover.


Life Cycles of Sexually Reproducing Organisms

Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends on the organism. The process of meiosis reduces the resulting gamete’s chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. There are three main categories of life cycles in multicellular organisms: diploid-dominant, in which the multicellular diploid stage is the most obvious life stage (and there is no multicellular haploid stage), as with most animals including humans haploid-dominant, in which the multicellular haploid stage is the most obvious life stage (and there is no multicellular diploid stage), as with all fungi and some algae and alternation of generations, in which the two stages, haploid and diploid, are apparent to one degree or another depending on the group, as with plants and some algae.

Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. The gametes are produced from diploid germ cells, a special cell line that only produces gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state (Figure 7.2 a).

Figure 7.2 (a) In animals, sexually reproducing adults form haploid gametes from diploid germ cells. (b) Fungi, such as black bread mold (Rhizopus nigricans), have haploid-dominant life cycles. (c) Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism. (credit c “fern”: modification of work by Cory Zanker credit c “gametophyte”: modification of work by “Vlmastra”/Wikimedia Commons)

If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?

Most fungi and algae employ a life-cycle strategy in which the multicellular “body” of the organism is haploid. During sexual reproduction, specialized haploid cells from two individuals join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores (Figure 7.2 b).

The third life-cycle type, employed by some algae and all plants, is called alternation of generations. These species have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes because they produce gametes. Meiosis is not involved in the production of gametes in this case, as the organism that produces gametes is already haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will develop into the gametophytes (Figure 7. 2 c).


Contents

Animals produce gametes directly through meiosis from diploid mother cells in organs called gonads (testis in males and ovaries in females). Males and females of a species that reproduce sexually have different forms of gametogenesis:

Stages Edit

However, before turning into gametogonia, the embryonic development of gametes is the same in males and females.

Common path Edit

Gametogonia are usually seen as the initial stage of gametogenesis. However, gametogonia are themselves successors of primordial germ cells (PGCs) from the dorsal endoderm of the yolk sac migrate along the hindgut to the gonadal ridge. They multiply by mitosis, and, once they have reached the gonadal ridge in the late embryonic stage, are referred to as gametogonia. Once the germ cells have developed into gametogonia, they are no longer the same between males and females.

Individual path Edit

From gametogonia, male and female gametes develop differently - males by spermatogenesis and females by oogenesis. However, by convention, the following pattern is common for both:

Cell type ploidy/chromosomes in humans DNA copy number/chromatids in human [Note 1] Process
gametogonium diploid (2N)/46 2C before replication, 4C after
46 before, 46 × 2 after
gametocytogenesis (mitosis)
primary gametocyte diploid (2N)/46 2C before replication, 4C after
46 before, 46 × 2 after
gametidogenesis (meiosis I)
secondary gametocyte haploid (N)/23 2C / 46 gametidogenesis (meiosis II)
gametid haploid (N)/23 C / 23
gamete haploid (N)/23 C / 23

In vitro gametogenesis Edit

In vitro gametogenesis (IVG) is the technique of developing in vitro generated gametes, i.e., "the generation of eggs and sperm from pluripotent stem cells in a culture dish." [1] This technique is currently feasible in mice and will likely have future success in humans and nonhuman primates. [1]

Fungi, algae, and primitive plants form specialized haploid structures called gametangia, where gametes are produced through mitosis. In some fungi, such as the Zygomycota, the gametangia are single cells, situated on the ends of hyphae, which act as gametes by fusing into a zygote. More typically, gametangia are multicellular structures that differentiate into male and female organs:

In angiosperms, the male gametes (always two) are produced inside the pollen tube (in 70% of the species) or inside the pollen grain (in 30% of the species) through the division of a generative cell into two sperm nuclei. Depending on the species, this can occur while the pollen forms in the anther (pollen tricellular) or after pollination and growth of the pollen tube (pollen bicellular in the anther and in the stigma). The female gamete is produced inside the embryo sac of the ovule.

Meiosis is a central feature of gametogenesis, but the adaptive function of meiosis is currently a matter of debate. A key event during meiosis is the pairing of homologous chromosomes and recombination (exchange of genetic information) between homologous chromosomes. This process promotes the production of increased genetic diversity among progeny and the recombinational repair of damage in the DNA to be passed on to progeny. To explain the adaptive function of meiosis (as well as of gametogenesis and the sexual cycle), some authors emphasize diversity, [2] and others emphasize DNA repair. [3]


Diploid-Dominant Life Cycle

Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells , are produced within the gonads, such as the testes and ovaries. Germ cells are capable of mitosis to perpetuate the cell line and meiosis to produce gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state ([Figure 1]).

Figure 1: In animals, sexually reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring. The germ cells are generated early in the development of the zygote.


Gametogenesis (Spermatogenesis and Oogenesis)

Gametogenesis, the production of sperm and eggs, includes the process of meiosis to produce haploid cells, and growth and maturation of these cells into oocytes and sperm. The production of sperm is called spermatogenesis and the production of eggs is called oogenesis.

Spermatogenesis

Figure 3. During spermatogenesis, four sperm result from each primary spermatocyte.

Spermatogenesis, illustrated in Figure 3, occurs in the wall of the seminiferous tubules, with stem cells at the periphery of the tube and the spermatozoa at the lumen of the tube. Immediately under the capsule of the tubule are diploid, undifferentiated cells. These stem cells, called spermatogonia (singular: spermatagonium), go through mitosis with one offspring going on to differentiate into a sperm cell and the other giving rise to the next generation of sperm.

Meiosis starts with a cell called a primary spermatocyte. At the end of the first meiotic division, a haploid cell is produced called a secondary spermatocyte. This cell is haploid and must go through another meiotic cell division. The cell produced at the end of meiosis is called a spermatid and when it reaches the lumen of the tubule and grows a flagellum, it is called a sperm cell. Four sperm result from each primary spermatocyte that goes through meiosis.

Stem cells are deposited during gestation and are present at birth through the beginning of adolescence, but in an inactive state. During adolescence, gonadotropic hormones from the anterior pituitary cause the activation of these cells and the production of viable sperm. This continues into old age.

Link to Learning

Oogenesis

Oogenesis, illustrated in Figure 4, occurs in the outermost layers of the ovaries. As with sperm production, oogenesis starts with a germ cell, called an oogonium (plural: oogonia), but this cell undergoes mitosis to increase in number, eventually resulting in up to about one to two million cells in the embryo.

Figure 4 The process of oogenesis occurs in the ovary’s outermost layer.

The cell starting meiosis is called a primary oocyte, as shown in Figure 4. This cell will start the meiosis I and be arrested in its progress very early on, in a stage called the first prophase stage. At the time of birth, all future oocytes are in the prophase stage no additional oocytes or precursors are produced after birth. At adolescence, anterior pituitary hormones cause the development of a number of follicles in an ovary. This results in the primary oocyte finishing meiosis I. The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell. This second cell is called a polar body and usually dies. A secondary meiotic arrest occurs, about halfway through the meiosis II in a stage called the metaphase II stage. At ovulation, this secondary oocyte will be released and travel toward the uterus through the oviduct. If the secondary oocyte is fertilized, the cell continues through meiosis II, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm. If the oocyte is not fertilized, however, it does not complete meiosis II.

Oocyte production begins before birth, is arrested during meiosis until puberty, and then individual cells continue through at each menstrual cycle. One oocyte is produced from each meiotic process, with the extra chromosomes and chromatids going into polar bodies that degenerate and are reabsorbed by the body.


Spermatogenesis

Males can produce sperm when they reach puberty at the age between 10-16 years old. Approximately 200 million sperms produce in a day. These sperms happen in the seminiferous tubules of the testes of male. In this case, the seminiferous tubules are separated by the blood-testis barrier from the systematic circulation.

The spermatogenesis is a process to produce sperms which occurs in the male gonads or testes. The human testes consist of many seminiferous tubules which are lined by the cells of germinal epithelium.

This germinal epithelium plays an important role to produce sperms through the process of spermatogenesis. The germinal cell also contains some somatic cells, known as sertoli cells which have a role in nourishing the developing spermatozoa or sperms.

Image Showing Spermatogenesis Process: Image credit-wikimedia commons

The spermatogenesis is a continuous process and it can be described in four different headings: