C. what types of cells are produced from meiosis

Because the genes contained in the duplicate chromosomes are transferred to each successive cellular generation, all mitotic progeny are genetically similar. However, there are exceptions. For example, there are genetic variations that arise in clonal species , such as bacteria , due to spontaneous mutations during mitotic division. Furthermore, chromosomes are sometimes replicated multiple times without any accompanying cell division. This occurs in the cells of Drosophila larvae salivary glands, for example, where there is a high metabolic demand.

The chromosomes there are called polytene chromosomes, and they are extremely large compared to chromosomes in other Drosophila cells.

These chromosomes replicate by undergoing the initial phases of mitosis without any cytokinesis Figure 2. Therefore, the same cell contains thick arrangements of duplicate chromosomes side by side, which look like strands of very thick rope. Scientists believe that these chromosomes are hyper-replicated to allow for the rapid and copious production of certain proteins that help larval growth and metamorphosis Gilbert, The greatest impact of Sutton's work has far more to do with providing evidence for Mendel's principle of independent assortment than anything else.

Specifically, Sutton saw that the position of each chromosome at the midline during metaphase was random, and that there was never a consistent maternal or paternal side of the cell division.

Therefore, each chromosome was independent of the other. Thus, when the parent cell separated into gametes, the set of chromosomes in each daughter cell could contain a mixture of the parental traits, but not necessarily the same mixture as in other daughter cells.

To illustrate this concept, consider the variety derived from just three hypothetical chromosome pairs, as shown in the following example Hirsch, Each pair consists of two homologues: one maternal and one paternal.

Here, capital letters represent the maternal chromosome, and lowercase letters represent the paternal chromosome:. When these chromosome pairs are reshuffled through independent assortment , they can produce eight possible combinations in the resulting gametes:. A mathematical calculation based on the number of chromosomes in an organism will also provide the number of possible combinations of chromosomes for each gamete.

In particular, Sutton pointed out that the independence of each chromosome during meiosis means that there are 2 n possible combinations of chromosomes in gametes, with "n" being the number of chromosomes per gamete.

Thus, in the previous example of three chromosome pairs, the calculation is 2 3 , which equals 8. Furthermore, when you consider all the possible pairings of male and female gametes, the variation in zygotes is 2 n 2 , which results in some fairly large numbers.

But what about chromosome reassortment in humans? Humans have 23 pairs of chromosomes. That means that one person could produce 2 23 different gametes. In addition, when you calculate the possible combinations that emerge from the pairing of an egg and a sperm, the result is 2 23 2 possible combinations. However, some of these combinations produce the same genotype for example, several gametes can produce a heterozygous individual.

Of course, there are more than 23 segregating units Hirsch, While calculations of the random assortment of chromosomes and the mixture of different gametes are impressive, random assortment is not the only source of variation that comes from meiosis.

In fact, these calculations are ideal numbers based on chromosomes that actually stay intact throughout the meiotic process. In reality, crossing-over between chromatids during prophase I of meiosis mixes up pieces of chromosomes between homologue pairs, a phenomenon called recombination. Because recombination occurs every time gametes are formed, we can expect that it will always add to the possible genotypes predicted from the 2 n calculation. In addition, the variety of gametes becomes even more unpredictable and complex when we consider the contribution of gene linkage.

Some genes will always cosegregate into gametes if they are tightly linked, and they will therefore show a very low recombination rate. While linkage is a force that tends to reduce independent assortment of certain traits, recombination increases this assortment.

In fact, recombination leads to an overall increase in the number of units that assort independently, and this increases variation. While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction , genes get mixed up. Sexual reproduction actually expands the variety created by meiosis, because it combines the different varieties of parental genotypes. Thus, because of independent assortment, recombination, and sexual reproduction, there are trillions of possible genotypes in the human species.

During cell division, chromosomes sometimes disappear. This occurs when there is some aberration in the centromere , and spindle fibers cannot attach to the chromosome to segregate it to distal poles of the cell. Consequently, the lost chromosome never properly groups with others into a new nuclear envelope , and it is left in the cytoplasm , where it will not be transcribed. Also, chromosomes don't always separate equally into daughter cells.

The four stages of meiosis I are as follows, according to " Molecular Biology of the Cell. Prophase I : At this stage, chromosomes become compact, dense structures and are easily visible under the microscope. The homologous chromosomes pair together. The two sets of sister chromatids resemble two X's lined up next to each other.

Each set exchanges bits of DNA with the other and recombines, thus creating genetic variation. This process is known as crossing over, or recombination. Even though in humans the male sex chromosomes X and Y are not exact homologs, they can still pair together and exchange DNA. Crossing over occurs within only a small region of the two chromosomes.

Metaphase I : The meiotic spindle, a network of protein filaments, emerges from two structures called the centrioles, positioned at either end of the cell. The meiotic spindle latches onto the fused sister chromatids. By the end of metaphase I, all the fused sister chromatids are tethered at their centromeres and line up in the middle of the cell.

The homologs still look like two X's sitting close together. Anaphase I : The spindle fibers start to contract, pulling the fused sister chromatids with them.

Each X-shaped complex moves away from the other, toward opposite ends of the cell. Telophase I : The fused sister chromatids reach either end of the cell, and the cell body splits into two. Meiosis I results in two daughter cells, each of which contains a set of fused sister chromatids.

The genetic makeup of each daughter cell is distinct because of the DNA exchange between homologs during the crossing-over process. In other words, by the end of the process, the chromosome number is unchanged between the cells that enter meiosis II and the resulting daughter cells.

Prophase II : The nuclear membrane disintegrates, and meiotic spindles begin to form once again. Metaphase II : The meiotic spindles latch onto the centromere of the sister chromatids, and they all line up at the center of the cell.

Chromosomes are bundles of tightly coiled DNA located within the nucleus of almost every cell in our body. Humans have 23 pairs of chromosomes. Mitosis is a process where a single cell divides into two identical daughter cells cell division. DNA or deoxyribonucleic acid is a long molecule that contains our unique genetic code. Like a recipe book it holds the instructions for making all the proteins in our bodies. Cells divide and reproduce in two ways, mitosis and meiosis. Mitosis results in two identical daughter cells, whereas meiosis results in four sex cells.

Below we highlight the keys differences and similarities between the two types of cell division. If you have any other comments or suggestions, please let us know at comment yourgenome. Can you spare minutes to tell us what you think of this website? Open survey. In: Facts In the Cell. During meiosis one cell divides twice to form four daughter cells. These four daughter cells only have half the number of chromosomes of the parent cell — they are haploid.

Meiosis produces our sex cells or gametes eggs in females and sperm in males. These are divided between the first time the cell divides meiosis I and the second time it divides meiosis II : Meiosis I 1.

Interphase: The DNA in the cell is copied resulting in two identical full sets of chromosomes. Outside of the nucleus are two centrosomes, each containing a pair of centrioles, these structures are critical for the process of cell division. Their formation depends on the RecA recombination protein homologues that are expressed in this species. In the next phase of homologue interaction, these nascent interactions are converted to stable strand-invasion events.

This nucleates the formation of the synaptonemal complex SC. Homologous chromosome interactions in meiosis: diversity amidst conservation. Nature Reviews Genetics 6, All rights reserved. References and Recommended Reading Gerton, J. Science , — Petes, T. Article History Close. Share Cancel. Revoke Cancel.

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