Why is meiosis important heredity

During meiosis in humans, 1 diploid cell with 46 chromosomes or 23 pairs undergoes 2 cycles of cell division but only 1 round of DNA replication. The result is 4 haploid daughter cells known as gametes or egg and sperm cells each with 23 chromosomes — 1 from each pair in the diploid cell. At conception, an egg cell and a sperm cell combine to form a zygote 46 chromosomes or 23 pairs.

This is the 1st cell of a new individual. The halving of the number of chromosomes in gametes ensures that zygotes have the same number of chromosomes from one generation to the next. This is critical for stable sexual reproduction through successive generations. Replication of DNA in preparation for meiosis. After replication, each chromosome becomes a structure comprising 2 identical chromatids.

The chromosomes condense into visible X shaped structures that can be easily seen under a microscope, and homologous chromosomes pair up. Recombination occurs as homologous chromosomes exchange DNA. At the end of this phase, the nuclear membrane dissolves.

The pairs of chromosomes separate and move to opposing poles. Either one of each pair can go to either pole. Nuclear membranes reform. Cell divides and 2 daughter cells are formed, each with 23 chromosomes.

There are 4 new haploid daughter cells. Meiosis and fertilization create genetic variation by making new combinations of gene variants alleles. In some cases, these new combinations may make an organism more or less fit able to survive and reproduce , thus providing the raw material for natural selection. Genetic variation is important in allowing a population to adapt via natural selection and thus survive in the long term.

Improve this page Learn More. Skip to main content. Module 8: Cell Division. Search for:. Genetic Variation in Meiosis Learning Outcomes Understand how meiosis contributes to genetic diversity.

Mitosis creates two identical daughter cells that each contain the same number of chromosomes as their parent cell. In contrast, meiosis gives rise to four unique daughter cells, each of which has half the number of chromosomes as the parent cell.

Because meiosis creates cells that are destined to become gametes or reproductive cells , this reduction in chromosome number is critical — without it, the union of two gametes during fertilization would result in offspring with twice the normal number of chromosomes! Apart from this reduction in chromosome number, meiosis differs from mitosis in yet another way. Specifically, meiosis creates new combinations of genetic material in each of the four daughter cells. These new combinations result from the exchange of DNA between paired chromosomes.

Such exchange means that the gametes produced through meiosis exhibit an amazing range of genetic variation.

Finally, unlike mitosis, meiosis involves two rounds of nuclear division, not just one. Despite this fact, many of the other events of meiosis are similar to those that occur in mitosis. For example, prior to undergoing meiosis, a cell goes through an interphase period in which it grows, replicates its chromosomes, and checks all of its systems to ensure that it is ready to divide.

Like mitosis, meiosis also has distinct stages called prophase, metaphase, anaphase, and telophase. A key difference, however, is that during meiosis, each of these phases occurs twice — once during the first round of division, called meiosis I, and again during the second round of division, called meiosis II. As previously mentioned, the first round of nuclear division that occurs during the formation of gametes is called meiosis I. It is also known as the reduction division because it results in cells that have half the number of chromosomes as the parent cell.

During prophase I , the chromosomes condense and become visible inside the nucleus. Because each chromosome was duplicated during the S phase that occurred just before prophase I, each now consists of two sister chromatids joined at the centromere. This arrangement means that each chromosome has the shape of an X. Once this chromosomal condensation has occurred, the members of each chromosome pair called homologous chromosomes , because they are similar in size and contain similar genes , align next to each other.

At this point, the two chromosomes in each pair become tightly associated with each other along their lengths in a process called synapsis. Then, while the homologous chromosomes are tightly paired, the members of each pair trade adjacent bits of DNA in a process called crossing over , also known as recombination Figure 1. This trading of genetic material creates unique chromosomes that contain new combinations of alleles. During meiosis II , the two cells once again cycle through four phases of division.

Meiosis II is sometimes referred to as an equational division because it does not reduce chromosome number in the daughter cells — rather, the daughter cells that result from meiosis II have the same number of chromosomes as the "parent" cells that enter meiosis II. 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.