Sex Cells

Mitosis Is How Cells Make More Cells

Mitosis and meiosis are two types of cell division. Mitosis is cell division that results in the duplication of cells; the daughter cells genetic copies of the parent cell. This cell multiplication allows for replacement of old cells, tissue repair, growth and development.

Meiosis is Sex Cell (Gamete) Formation

In sexually reproducing organisms, some cells are able to divide by another method called meiosis. This type of cell division results in the production of gametes (eggs or sperm).
Meiosis is much more complex than mitosis. Whereas mitosis involves the duplication and subsequent division of chromosomes, meiosis involves two divisions of genetic material. Gametes are haploid (1n) with half the number of chromosomes than the progenitor cell that they arose from. These haploid sex cells arise in specialized reproductive tissue called the gonads. Ovaries (female gonads) and testes (male gonads) are the sites of meiosis.

Ploidy – Diploid & Haploid

Most of the cells in our bodies are somatic, or non sex cells and have a diplod (2n) chromosome number, meaning that chromosomes come in pairs called homologues. Every somatic cell in your body has 46 chromosomes. You received a set of 23 from your mother’s egg and a matching set of 23 via your father’s sperm, and now these chromosomes are the genetic material inside nearly every cell of your body.
Sex cells (sperm or eggs) have half the number of chromosomes as do body (somatic) cells. The merging of sperm and egg at fertilization brings the chromosome count back to 2n diploid number necessary for a zygote to have complete genetic information; 2 sets of genetic instructions in 23 pairs of chromosomes.
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Meiosis I & II

As is the case in mitosis, in meiosis the cell duplicates its chromosome number prior to beginning cellular division. Then nuclear division, the sorting out of the genetic material, begins, and unfolds over the course of 2 cellular divisions that result in 4 gametes.
Meiosis I
During meiosis I, homologuous pairs of chromosomes buddy up at the cell’s equator and then each homologue moves to opposite ends of the cell. What started with one cell, now consists of two cells that each have a 23 duplicated chromosomes.
There are two events that occur during meiosis I that contribute to the astounding genetic variation the sexual reproduction produces:
  • Independent assortment
  • Crossing over
Independent Assortment
Consider the fact that, in human cells there are 23 pairs of chromosomes. This means that during meiosis I there are many different possible arrangements of these pairs.
How homologous chromosomes line up during meiosis I determines, in the end, the combination of chromosomes that end up ends up in each gamete. This is one of the reasons why sexual reproduction produces such astounding genetic variation.
When homologous chromosomes are lined up next to each other during meiosis I they also can swap pieces. This is called crossing over and results in more ‘shuffling of the genetic deck.’
Meiosis II
Meiosis II unfolds much like mitosis does, but without the chromosomes replicating before nuclear division. Each duplicated chromosome lines up at the cells equator and the sister chromatids (as the duplicates are called) separate and move to opposite poles. Meiosis II ultimately results in 4 cells that each have 23 (1n) chromosomes.

Human Fertilization

Independent assortment and crossing over of meiosis are only part of the reason sexual reproduction allows for so much genetic variation between individuals. Human males can produce millions of sperm and human females millions of eggs (although only a fraction mature and are ovulated). Put these one-of-a-kind gametes together and the product is a genetically unique individual. Fertilization is the final genetic shuffle.

More Information

To learn more about cell biology, see the excellent websites **Science Prof Online** and Cells Alive, or look to additional Suite101 biology articles, including Sexual and Asexual Reproduction, What Is Mitotic Cellular Division, Nucleotides & Nucleic Acids, Prokaryotic & Eukaryotic Cells and Nucleic Acids & Replication.

Read more at Suite101: Meiosis & Sexual Reproduction: How Sex Cell Formation and Fertilization Shuffle Our Genes