In addition to the differences in gamete size and number, in mammals the timing of meiosis differs between males and females. In males, germ cells are pre-meiotic at birth and do not enter meiosis until the onset of puberty. Mitosis maintains a population of precursor cells, so that sperm production can continue throughout adulthood. In females, germ cells enter meiosis I during embryonic development.
These primary oocytes remain arrested "stuck" in meiosis I until puberty. After this time, one primary oocyte per month roughly for humans, depends upon cycle length for other mammals completes meiosis I, enters meiosis II and is ovulated. Actually, meiosis II is only completed if the oocyte is successfully fertilized! The timing of meiotic arrest can differ between different species. For example, in the nematode C.
In plants, the products of meiosis reproduce a few times using mitosis as they develop into functional male or female gametes. The purpose of gametes is to allow reproduction generation after generation.
By uniting two gametes with half the number of chromosomes, the full chromosome number is restored each generation. Remember that homologous recombination and assortment of chromosomes create a genetically diverse population of gametes. Like any biological process, errors can occur during meiosis. If homologous chromosomes or sister chromatids are not correctly distributed during meiosis known as nondisjunction , gametes can have too many or too few chromosomes 5.
Learning Objectives Describe and draw the key events and stages of meiosis that lead to haploid gametes. Recall that homologous chromosomes separate during meiosis I a reductional division and that sister chromatids separate during meiosis II an equational division. Compare mitosis and meiosis. Compare the processes of oogenesis and spermatogenesis in humans, including the chromosome complements of the gametes. Meiosis I In meiosis I replicated, homologous chromosomes pair up, or synapse, during the pachytene stage of prophase I, line up in the middle of the cell during metaphase I, and separate during anaphase I.
Chromosome condensation during meiosis As meiosis proceeds, chromatin becomes increasingly condensed. Copyright CC by Chelysheva et al. PLoS Genet 4 12 : e Homologous Recombination Within the synaptonemal complex during prophase 1, homologous recombination, or crossing over, occurs.
Meiosis II At the completion of meiosis I there are two haploid cells, each with one, replicated copy of each chromosome 1n. Outcomes of meiosis The outcome of meiosis is a cell or cells with half the number of chromosomes as the starting cell. Gamete maturation In animals and plants, the cells produced at the end of meiosis need to mature before they become functional gametes.
Spermatogenesis In most male animals, the four products of meiosis are called spermatids. These cells undergo meiosis II to produce spermatids that undergo differentiation to produce mature sperm and residual bodies with unneeded cellular material. Oogenesis In female animals, the gametes are oocytes. How can you make a really small cell? Timing of Spermatogenesis and Oogenesis In addition to the differences in gamete size and number, in mammals the timing of meiosis differs between males and females.
Each chromosome looks like an elongated X-shaped structure. In the pair of chromosomes at top, the chromosome at left is mostly green, but the lower region of the right chromatid is orange. The chromosome at right is mostly orange, but the lower region of the left chromatid is green. A second pair of chromosomes exhibiting the same pattern of coloration on their arms is shown below the topmost pair.
Mitotic spindles are located at each side of the cell. Each spindle apparatus is composed of several white lines, representing fibers, emanating from two oval-shaped structures, representing centrosomes. The fibers attach the centrosomes to the centromeres of each chromosome. Shorter fibers also emanate from the mitotic spindle but are not attached to chromosomes. At the start of metaphase I , microtubules emerge from the spindle and attach to the kinetochore near the centromere of each chromosome.
In particular, microtubules from one side of the spindle attach to one of the chromosomes in each homologous pair, while microtubules from the other side of the spindle attach to the other member of each pair. With the aid of these microtubules, the chromosome pairs then line up along the equator of the cell, termed the metaphase plate Figure 2. Anaphase I. Figure 3: During anaphase I, the homologous chromosomes are pulled toward opposite poles of the cell. The chromosome at right is moving toward the right-hand mitotic spindle.
The chromosome is mostly orange, but the lower region of the left chromatid is green. A second pair of chromosomes exhibiting the same pattern of coloration on their arms is shown below the topmost pair, mirroring the movements of the chromosomes above.
During anaphase I, the microtubules disassemble and contract; this, in turn, separates the homologous chromosomes such that the two chromosomes in each pair are pulled toward opposite ends of the cell Figure 3. This separation means that each of the daughter cells that results from meiosis I will have half the number of chromosomes of the original parent cell after interphase.
Also, the sister chromatids in each chromosome still remain connected. As a result, each chromosome maintains its X-shaped structure. Telophase I. Figure 4: Telophase I results in the production of two nonidentical daughter cells, each of which has half the number of chromosomes of the original parent cell. As the new chromosomes reach the spindle during telophase I , the cytoplasm organizes itself and divides in two.
There are now two cells, and each cell contains half the number of chromosomes as the parent cell. In addition, the two daughter cells are not genetically identical to each other because of the recombination that occurred during prophase I Figure 4. At this point, the first division of meiosis is complete. The cell now rests for a bit before beginning the second meiotic division.
During this period, called interkinesis , the nuclear membrane in each of the two cells reforms around the chromosomes. In some cells, the spindle also disintegrates and the chromosomes relax although most often, the spindle remains intact. It is important to note, however, that no chromosomal duplication occurs during this stage. What happens during meiosis II? Prophase II. As prophase II begins, the chromosomes once again condense into tight structures, and the nuclear membrane disintegrates.
In addition, if the spindle was disassembled during interkinesis, it reforms at this point in time. Metaphase II. Figure 5: During metaphase II, the chromosomes align along the cell's equatorial plate. The events of metaphase II are similar to those of mitotic metaphase — in both processes, the chromosomes line up along the cell's equatorial plate, also called the metaphase plate, in preparation for their eventual separation Figure 5. Anaphase II. Figure 6: Anaphase II involves separation of the sister chromatids.
During anaphase II , microtubules from each spindle attach to each sister chromatid at the kinetochore. The sister chromatids then separate, and the microtubules pull them to opposite poles of the cell. As in mitosis, each chromatid is now considered a separate chromosome Figure 6. Therefore, chromosome number remains constant n as the previous cell. At the end of the equational division, four haploid cells are produced. Reduction division is the first division of meiosis.
It is also known as meiosis I. As the name suggests, the chromosome number reduces in half. Therefore, the chromosome number decreases from diploid 2n to haploid n state during the reduction division. There is a long interphase before meiosis I.
Reduction division occurs via four subphases: prophase I, metaphase I, telophase I and anaphase I. During prophase I, homologous chromosomes recognize each other and form pairs. Then they form tetrads and exchange their genetic material between them. During prophase I, genetic recombination takes place.
Genetic recombination increases the genetic variability within a species. During anaphase I, homologous chromosomes migrate towards the opposite poles. Since homologous chromosomes migrate into each pole, the chromosome number becomes half.
Each daughter cell has only one copy of each chromosome. At the end of reduction division, two haploid daughter cells are produced. Reduction division is followed by equational division.
In equational division, genetic material is transmitted equally into daughter cells. In reduction division, genetic material is reduced in half and transmits to daughter cells.
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