Students often become confused during discussions of genetics, when it comes to the terminology associated with chromosomes and chromosome number. What are homologous chromosomes? What are duplicated chromosomes? What are sister chromatids? How do all of these terms differ and how do they relate to each other? Here is an explanation of the difference between duplicated (replicated) DNA and homologous pairs of chromosomes.
* Copying DNA Before Cell Division *
During the cell cycle, somatic cells (the cells of your body that are not eggs or sperm) grow and divide. During this process, called mitosis, a single cell, called the “parent cell”, splits into two identical “daughter cells”. Before a cell divides, it must make a copy of all of its DNA (nucleic acid), so that each daughter cell has a complete copy of the genetic information. Each individual DNA molecule is the material of one chromosome, and the process of duplicating or copying the DNA is called replication.
* Chromatin Condensing *
Most of the time, the DNA molecules in each of your cells do not look like chromosomes, but instead exist in very long strands called chromatin. When the DNA undergoes the process of replication, it is in these long chromatin strands. But when the cell prepares to divide, it must pack its DNA for the move. So prior to cell division, the chromatin condenses, and your cells have 46 duplicated chromosomes. The duplicated chromosomes each look kind of like the letter “X” or the letter “H”.
* Homologous Chromosomes *
Most of the cells in our bodies are somatic, 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 (homologous) set of 23 via your father’s sperm. At fertilization, these two gametes joined together, and formed a fertilized egg; the first of your cells. These chromosomes that made up the fertilized egg are now the genetic material inside nearly every cell of your body. Each cell contains 23 pairs of homologous chromosomes.
* Homologous, Duplicated Chromosomes *
Homologues are seperate chromosomes, and each has the same type of information on it. Prior to cell division, all these chromosomes are duplicated. The duplicates, called sister chromatids, result from each DNA molecule being copied. Somatic cells of diploid (2n) organisms always have pairs of homologous chromosomes. So homologous chromosomes are only in duplicated form prior to cell division.
* 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.