When is chromosome number first reduced by one half

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. Chromosome choreography: The meiotic ballet. Science , — Petes, T. Meiotic recombination hot spots and cold spots.

Zickler, D. Meiotic chromosomes: Integrating structure and function. Annual Review of Genetics 33 , — Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer. Karyotyping for Chromosomal Abnormalities. Prenatal Screen Detects Fetal Abnormalities. Synteny: Inferring Ancestral Genomes. Telomeres of Human Chromosomes. Chromosomal Abnormalities: Aneuploidies. Chromosome Abnormalities and Cancer Cytogenetics.

Copy Number Variation and Human Disease. 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. In the case of males, each cell becomes a sperm.

In the case of females, one cell becomes an egg and the other three become polar bodies which are not used. Your parents each have at least one pair of alleles versions of a gene for every trait and many pairs of alleles for each polygenic trait. You ended up with half of mom's paired genes and half of dad's paired genes. But each non-identical-twin child of these parents ends up with a different combination.

Imagine, for example, that eye color was controlled by a single gene, and that mom could have B, the allele for brown eyes or b, the allele for blue eyes, and dad could also have B or b. This leads to four possibilities: You could get B from mom and B from dad, or B from mom and b from dad, or b from mom and B from dad, or b from mom and b from dad.

Each sperm and egg will end up with either B or b from mom and either B or b from dad. Mitosis is a fundamental process for life. During mitosis, a cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells. Because this process is so critical, the steps of mitosis are carefully controlled by certain genes.

When mitosis is not regulated correctly, health problems such as cancer can result. The other type of cell division, meiosis, ensures that humans have the same number of chromosomes in each generation. It is a two-step process that reduces the chromosome number by half—from 46 to 23—to form sperm and egg cells.