Genetics is rife with potentially confusing scientific terminology associated with chromosomes and chromosome number. What is ploidy, and which cells are haploid? Which are diploid? What are homologous chromosomes? What are duplicated chromosomes? The intent of this article is to clarify some of this lingo.
* Diplod Chromosome Number in Somatic Cells *
Most of the cells in our bodies are somatic, or non sex cells, and have a diplod (2n) chromosome number. Diploid means 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.
* Ploidy & Mitosis *
Mitosis is cell division that results in the duplication of somatic cells; the 'daughter cells' genetic copies of the 'parent cell'. This cell multiplication allows for replacement of old cells, tissue repair, growth and development.
In order to divide and produce identical 'daughter cells', a 'parent cell' must first duplicate (replicate) its genetic material (DNA). So, prior to cell division, somatic diplod cells have duplicated chromosomes. As soon as the cell divides, each new cell has a complete diplod number of 46, unduplicated chromosomes.
* Haploid Chromosome Number in Gametes *
Most, but not all, of the cells in our body and diploid. Sex cells, also called gametes (sperm or eggs), have half the number of chromosomes as do somatic cells. These gametes are referred to as being haploid (1n).
* Ploidy & Meiosis *
In sexually reproducing organisms, gametes are produced by another method of cell division called meiosis. 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 have 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.
* Human Life Cycle: Haploid to Diploid *
The merging of haploid 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.
* Sources *
Campbell, N. A. & Reece J. B. (2005) Biology, seventh edition. Pearson Education Inc.
Campbell, N. A., Reece J. B. & Simon, E. (2004) Essential Biology with Physiology. Pearson Education Inc.