When is diploid number of chromosomes restored

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Meiosis and mitosis Start this free course now. Free course Meiosis and mitosis. Figure 3 How the number of chromosomes changes at gamete production meiosis and at fertilisation in a hypothetical organism with only two pairs of homologous chromosomes.

For simplicity, each chromosome is shown as a single strand, rather than as a pair of chromatids Figures 1 and 2. Meiosis has both similarities to and differences from mitosis, which is a cell division process in which a parent cell produces two identical daughter cells.

Meiosis begins following one round of DNA replication in cells in the male or female sex organs. The process is split into meiosis I and meiosis II, and both meiotic divisions have multiple phases. Meiosis I is a type of cell division unique to germ cells, while meiosis II is similar to mitosis.

Meiosis I, the first meiotic division, begins with prophase I. During prophase I, the complex of DNA and protein known as chromatin condenses to form chromosomes. The pairs of replicated chromosomes are known as sister chromatids, and they remain joined at a central point called the centromere.

A large structure called the meiotic spindle also forms from long proteins called microtubules on each side, or pole, of the cell. Between prophase I and metaphase I, the pairs of homologous chromosome form tetrads.

Within the tetrad, any pair of chromatid arms can overlap and fuse in a process called crossing-over or recombination. Recombination is a process that breaks, recombines and rejoins sections of DNA to produce new combinations of genes. In metaphase I, the homologous pairs of chromosomes align on either side of the equatorial plate. Then, in anaphase I, the spindle fibers contract and pull the homologous pairs, each with two chromatids, away from each other and toward each pole of the cell.

Diploid describes a cell that contain two copies of each chromosome. Nearly all the cells in the human body carry two homologous, or similar, copies of each chromosome.

The only exception is cells in the germ line, which go on to produce gametes, or egg and sperm cells. Germ line cells are haploid, which means they contain a single set of chromosomes. In diploid cells, one set of chromosomes is inherited from the individual's mother, while the second is inherited from the father.