3.7Chromosomes and the Genome
Fig. 3-6 shows a diagram of chromosomes. Chromosome is a single piece of DNA coiled with the aid of proteins to form the shape shown in the figure. Humans have 46 chromosomes, and they are named as 1 through 22 starting with the longest chromosome. They come in pairs called autosomes. The remaining two chromosomes are the sex chromosomes that decide human gender (XY in males and XX in females). Normally, chromosomes are found as individual units; however, during cytokinesis, they are doubled by replication and form the shape shown in Fig. 3-6A.
Fig. 3-6 Human Chromosomes
(A) shows chromosomes as observed during cytokinesis. During this process, each homologous chromosome duplicates and the resulting sister chromatids connect at the site of the centromere to form an X shape. (B) is a diagram showing the pattern after Giemsa staining. Chromosomes are often represented in diagrams using this shape because it is convenient to show the stripes colored by staining and the physical locations of the genes. A is reprinted from “Molecular Biology” (D. P. Clark), Academic Press, 2005. B is amended from “Essentials of Neural Science and Behavior” (E. R. Kandel et al.), Appleton & Lange, 1996.
Genes are found in every region of a chromosome (see Chapter 8, Fig. 8-4). The genome refers to all genes that determine the traits of a specific organism. For example, the human genome comprises approximately three billion base pairs and contains approximately 25,000 genes. Generally, the size of the genome and the complexity of the organism are proportionate; however, there are exceptions. Humans and mice are examples of two organisms with comparable genome sizes and number of genes, but they show a large difference in higher cognitive function (see Chapter 7, Fig. 7-2).