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The chromosomes gather into nuclei, and the cells divide.

Each of the four cells has a nucleus with a haploid number of chromosomes.

9.14 Meiosis In meiosis,two sets of chromosomes are divided among four nuclei, each of which then has half as many chromosomes as the original cell.These four haploid cells are the result of two successive nuclear divisions.The photomicrographs show meiosis in the male reproductive organ of a lily. As in Figure 9.8, the diagrams show corresponding phases in an animal.

Like mitosis, meiosis I is preceded by an interphase with an S phase during which each chromosome is replicated. As a result, each chromosome consists of two sister chromatids, held together by cohesin proteins.

Meiosis I begins with a long prophase I (the first three frames of Figure 9.14), during which the chromosomes change markedly. The homologous chromosomes pair by adhering along their lengths, a process called synapsis. This process lasts from prophase I to the end of metaphase I.

By the time chromosomes can be clearly seen under light microscope, the two homologs are already tightly joined. This joining begins at the centromeres and is mediated by a recognition of homologous DNA sequences on homologous chromosomes. In addition, a special group of proteins may form a scaffold called the synaptonemal complex, which runs lengthwise along the homologous chromosomes and appears to join them together.

The four chromatids of each pair of homologous chromosomes form what is called a tetrad, or bivalent. In other words, a tetrad consists of four chromatids, two each from two homologous chromosomes. For example, there are 46 chromosomes in a human diploid cell at the beginning of meiosis, so there are 23 homologous pairs of chromosomes, each with two chromatids (that is, 23 tetrads), for a total of 92 chromatids during prophase I.

Throughout prophase I and metaphase I, the chromatin continues to coil and compact, so that the chromosomes appear ever thicker. At a certain point, the homologous chromosomes seem to repel each other, especially near the centromeres, but they are held together by physical attachments mediated by cohesins. These cohesins are different from the ones holding the two sister chromatids together. Regions having these attachments take on an X-shaped appearance and are called chiasmata (from the Greek chiasma, "cross"; Figure 9.15).

A chiasma reflects an exchange of genetic material between nonsister chromatids on homologous chromosomes—

9.15 Chiasmata: Evidence of Exchange between Chromatids

Chiasmata are visible near the middle of this scanning electron micrograph of some chromatids from a desert locust, and near the ends of others.Three chiasmata are indicated with arrows.

Sister chromatids

During prophase I, homologous chromosomes, each with a pair of sister chroma-^¿^^tids, line up to form a tetrad.

Homologous chromosomes

A crossover forms between adjacent chromatids of different homologs.

Crossover

The homologs break at the point of cross-over and rejoin, resulting in recombinant chromatids.

Recombinant |J chromatids

9.16 Crossing Over Forms Genetically Diverse Chromosomes

The exchange of genetic material by crossing over may result in new combinations of genetic information on the recombinant chromosomes.

Desert Locust Chromosomes

9.15 Chiasmata: Evidence of Exchange between Chromatids

Chiasmata are visible near the middle of this scanning electron micrograph of some chromatids from a desert locust, and near the ends of others.Three chiasmata are indicated with arrows.

Recombinant |J chromatids

9.16 Crossing Over Forms Genetically Diverse Chromosomes

The exchange of genetic material by crossing over may result in new combinations of genetic information on the recombinant chromosomes.

what geneticists call crossing over (Figure 9.16). The chromosomes begin exchanging material shortly after synapsis begins, but chiasmata do not become visible until later, when the homologs are repelling each other. Crossing over increases genetic variation among the products of meiosis by reshuffling genetic information among the homologous pairs. We will have a great deal to say about crossing over and its genetic consequences in the coming chapters.

There seems to be plenty of time for the complicated events of prophase I to occur. Whereas mitotic prophase is usually measured in minutes, and all of mitosis seldom takes more than an hour or two, meiosis can take much longer. In human males, the cells in the testis that undergo meiosis take about a week for prophase I and about a month for the entire meiotic cycle. In the cells that will become eggs, prophase I begins long before a woman's birth, during her early fetal development, and ends as much as decades later, during the monthly ovarian cycle.

Prophase I is followed by prometaphase I (not pictured in Figure 9.14), during which the nuclear envelope and the nucleoli disaggregate. A spindle forms, and microtubules become attached to the kinetochores of the chromosomes. In meiosis I, the kinetochores of both chromatids in each chromosome become attached to the same half-spindle. Thus the entire chromosome, consisting of two chromatids, will migrate to one pole. Which member of a homologous chromosome pair becomes attached to each half-spindle, and thus which member will go to which pole, is random. By metaphase I, all the chromosomes have moved to the equatorial plate. Up to this point, homologous pairs are held together by chiasmata.

The homologous chromosomes separate in anaphase I, when the individual chromosomes, each still consisting of two chromatids, are pulled to the poles, with one homolog of a pair going to one pole and the other homolog going to the opposite pole. (Note that this process differs from the separation of chromatids during mitotic anaphase.) Each of the two daughter nuclei from this division thus contains only one set of chromosomes, not the two sets that were present in the original diploid nucleus. However, because they consist of two chromatids rather than just one, each of these chromosomes has twice the mass that a chromosome at the end of a mitotic division has.

In some organisms, there is a telophase I, with the reappearance of the nuclear envelopes. When there is a telophase I, it is followed by an interphase, called interkinesis, similar to the mitotic interphase. During interkinesis the chromatin is partially uncoiled; however, there is no replication of the genetic material, because each chromosome already consists of two chromatids. Furthermore, the sister chromatids in in-terkinesis are generally not genetically identical, because crossing over in prophase I has reshuffled genetic material between the maternal and paternal chromosomes. In other organisms, the chromosomes move directly into the second meiotic division.

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Responses

  • danielle duncan
    How many chromosomes are there in prophase.1 of a locust?
    8 years ago
  • cecilia
    What are Recombinant chromatids?
    8 years ago

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