Those germ cells that enter meiosis become oocytes, the source of future eggs. Consequently, females are born with a finite number of oocytes arrested in the first meiotic prophase. Within the ovary, these oocytes grow within follicle structures containing large numbers of support cells. The oocytes will reenter meiosis only when they are ovulated in response to hormones. Human females, for example, are born with hundreds of thousands of oocytes that remain arrested in the first meiotic prophase for decades.
Over time, the quality of the oocytes may deteriorate; indeed, researchers know that many oocytes die and disappear from ovaries in a process known as atresia. Two divisions, meiosis I and meiosis II , are required to produce gametes Figure 3.
Meiosis I is a unique cell division that occurs only in germ cells; meiosis II is similar to a mitotic division. Before germ cells enter meiosis, they are generally diploid , meaning that they have two homologous copies of each chromosome.
Then, just before a germ cell enters meiosis, it duplicates its DNA so that the cell contains four DNA copies distributed between two pairs of homologous chromosomes. Compared to mitosis, which can take place in a matter of minutes, meiosis is a slow process, largely because of the time that the cell spends in prophase I. During prophase I, the pairs of homologous chromosomes come together to form a tetrad or bivalent , which contains four chromatids.
Recombination can occur between any two chromatids within this tetrad structure. The recombination process is discussed in greater detail later in this article.
Crossovers between homologous chromatids can be visualized in structures known as chiasmata, which appear late in prophase I Figure 4. Chiasmata are essential for accurate meioses. At the end of prometaphase I, meiotic cells enter metaphase I. Here, in sharp contrast to mitosis, pairs of homologous chromosomes line up opposite each other on the metaphase plate , with the kinetochores on sister chromatids facing the same pole.
Pairs of sex chromosomes also align on the metaphase plate. In human males, the Y chromosome pairs and crosses over with the X chromosome. These crossovers are possible because the X and Y chromosomes have small regions of similarity near their tips. Crossover between these homologous regions ensures that the sex chromosomes will segregate properly when the cell divides. Next, during anaphase I , the pairs of homologous chromosomes separate to different daughter cells.
Before the pairs can separate, however, the crossovers between chromosomes must be resolved and meiosis-specific cohesins must be released from the arms of the sister chromatids. Failure to separate the pairs of chromosomes to different daughter cells is referred to as nondisjunction , and it is a major source of aneuploidy.
Overall, aneuploidy appears to be a relatively frequent event in humans. Meiosis II resembles a mitotic division, except that the chromosome number has been reduced by half. Thus, the products of meiosis II are four haploid cells that contain a single copy of each chromosome. In mammals, the number of viable gametes obtained from meiosis differs between males and females. In males, four haploid spermatids of similar size are produced from each spermatogonium.
In females, however, the cytoplasmic divisions that occur during meiosis are very asymmetric. Fully grown oocytes within the ovary are already much larger than sperm, and the future egg retains most of this volume as it passes through meiosis. As a consequence, only one functional oocyte is obtained from each female meiosis Figure 2. The other three haploid cells are pinched off from the oocyte as polar bodies that contain very little cytoplasm. Prophase I is the longest and arguably most important segment of meiosis, because recombination occurs during this interval.
For many years, cytologists have divided prophase I into multiple segments, based upon the appearance of the meiotic chromosomes. Thus, these scientists have described a leptotene from the Greek for "thin threads" phase, which is followed sequentially by the zygotene from the Greek for "paired threads" , pachytene from the Greek for "thick threads" , and diplotene from the Greek for "two threads" phases.
In recent years, cytology and genetics have come together so that researchers now understand some of the molecular events responsible for the stunning rearrangements of chromatin observed during these phases.
Recall that prophase I begins with the alignment of homologous chromosome pairs. Historically, alignment has been a difficult problem to approach experimentally, but new techniques for visualizing individual chromosomes with fluorescent probes are providing insights into the process. Recent experiments suggest that chromosomes from some species have specific sequences that act as pairing centers for alignment.
In some cases, alignment appears to begin as early as interphase, when homologous chromosomes occupy the same territory within the interphase nucleus Figure 5.
The formation of DSBs is catalyzed by highly conserved proteins with topoisomerase activity that resemble the Spo11 protein from yeast. Genetic studies have shown that Spo11 activity is essential for meiosis in yeast, because spo11 mutants fail to sporulate. As the invading strand is extended, a remarkable structure called synaptonemal complex SC develops around the paired homologues and holds them in close register, or synapsis. The stability of the SC increases as the invading strand first extends into the homologue and then is recaptured by the broken chromatid, forming double Holliday junctions.
Investigators have been able to observe the process of SC formation with electron microscopy in meiocytes from the Allium plant Figure 6. Bridges approximately nanometers long begin to form between the paired homologues following the DSB. Only a fraction of these bridges will mature into SC; moreover, not all Holliday junctions will mature into crossover sites.
Gerton, J. Homologous chromosome interactions in meiosis: Diversity amidst conservation. Nature Reviews Genetics 6 , — doi Hassold, T. To err meiotically is human: The genesis of human aneuploidy. Nature Reviews Genetics 2 , — doi Lopez-Maury, L. Tuning gene expression to changing environments: From rapid responses to evolutionary adaptation. Nature Reviews Genetics 9 , — doi Marston, A. Meiosis: Cell-cycle controls shuffle and deal. Nature Reviews Molecular Cell Biology 5 , — doi Page, S.
But a child who's going to get half of their inheritance from their mother and father, they can't get 46 from both of them--that would be So there has to be a way to take those paired chromosomes and break them apart in order to make gametes; sperm and eggs. That's what meiosis is all about. It's taking that complete set and breaking it down to a so-called haploid set and also providing the opportunity for recombination to occur to sort of scramble the copies and produce new kinds of outcomes, which is critical for the diversity of a species.
Meiosis is one of those things that's hard to really get your mind around. There's meiosis 1, there's meiosis Remember, these "parent" cells already have half the number of chromosomes of the original parent cell thanks to meiosis I.
This page appears in the following eBook. Aa Aa Aa. What happens during meiosis I? Prophase I. Figure 1: Recombination is the exchange of genetic material between homologous chromosomes. At the end of prophase I, the nuclear membrane finally begins to break down.
Outside the nucleus, the spindle grows out from centrosomes on each side of the cell. As in mitosis, the microtubules of the spindle are responsible for moving and arranging the chromosomes during division. Metaphase I. Figure 2: Near the end of metaphase I, the homologous chromosomes align on the metaphase plate.
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?
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