Biology Meiosis

Chromosomes—those strands of DNA and protein inside the cell nucleus—are the carriers of genes.

The genes are located in specific positions on chromosomes.

Human Body cells have 46 chromosomes

Each parent contributes 23 chromosomes

Homologous chromosomes—one of two paired chromosomes, one from each parent

Same length

Same Centromere position

Carry the genes that control the same intertied traits

A cell that contains both sets of homologous chromosomes is diploid, meaning “two sets.”

The diploid number of chromosomes is sometimes represented by the symbol 2n.

For the fruit fly, the diploid number is 8, which can be written as 2n = 8, where N represents twice the number of chromosomes in a sperm or egg cell.

Some cells contain only a single set of chromosomes, and therefore a single set of genes.

Such cells are haploid, meaning “one set.”

The gametes of sexually reproducing organisms are haploid.

For fruit fly gametes, the haploid number is 4, which can be written as N = 4.

Meiosis is a process in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

Meiosis usually involves two distinct divisions, called meiosis II.

By the end of meiosis II, the diploid cell becomes four haploid cells.

Just prior to meiosis I, the cell undergoes a round of chromosome replication called interphase I.

Each replicated chromosome consists of two identical chromatids joined at the center.

The cells begin to divide, and the chromosomes pair up, forming a structure called a tetrad, which contains four chromatids.

As homologous chromosomes pair up and form tetrads, they undergo a process called crossing-over.

First, the chromatids of the homologous chromosomes cross over one another.

Then, the crossed sections of the chromatids are exchanged.

Crossing-over is important because it produces new combinations of alleles in the cell.

As prophase I ends, a spindle forms and attaches to each tetrad.

During metaphase I of meiosis, paired homologous chromosomes line up across the center of the cell.

During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell.

When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cell.

During telophase I, a nuclear membrane forms around each cluster of chromosomes.

Cytokinesis follows telophase I, forming two new cells.

Meiosis I results in two cells, called daughter cells, each of which has four chromatids, as it would after mitosis.

Because each pair of homologous was separated, neither daughter cell has the two complete sets of chromosomes that it would have in a diploid cell.

The two cells produced by meiosis I have sets of chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

The two cells produced by meiosis I now enter a second meiotic division.

Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.

As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible.

The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I.

During metaphase of meiosis II, chromosomes line up in the center of each cell.

As the cell enters anaphase, the paired chromatids separate.

These four daughter cells now contain the number (n)—just two chromosomes each.

The haploid cells produced by meiosis II are gametes.

In male animals, these gametes are called sperm. In some plants, pollen grains contain haploid sperm cells.

In female animals, generally only one of the cells produced by meiosis is involved in reproduction. The female gamete is called an egg in animals and an egg cell in some plants.

Fertilization—the fusion of male and female gametes—generates new combinations of alleles in a zygote.

The zygote undergoes cell division by mitosis and eventually forms a new organism.

How is meiosis different from mitosis?

In mitosis, when the two sets of genetic material separate, each daughter cell receives one complete set of chromosomes. In meiosis, homologues chromosomes line up and then move to separate daughter cells.

Mitosis does not normally change the chromosome number of the

Original cell. This is not the case for meiosis, which reduces the chromosome number by half.

Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells.

Mitosis is a form of asexual reproduction, whereas meiosis is an early step in sexual reproduction.

There are three other ways in which these two processes differ.