Chromosome Number and Morphology
In more complex organisms, such as plants and animals, each somatic cell (any cell exclusive of sex cells) contains one set of chromosomes inherited from the maternal (female) parent and a set of homologous chromosomes (or homologues) from the paternal (male) parent. Homologous chromosomes (other than sex chromosomes) are similar in size, physical structure, and gene composition. The number of chromosomes in this dual set is called the diploid (2n) number. The suffix "-ploid" refers to chromosome "sets." The prefix indicates the degree of ploidy. Sex cells, or gametes, which contain half the number of chromosome sets found in somatic cells, are referred to as haploid cells (n).A genome refers to the genetic information contained in the cells (i.e., on the chromosomes) of a particular species. The number of chromosomes in the nucleus of each somatic cell is the same for all members of a given species. For example, human somatic cells contain 46 chromosomes, tobacco cells have 48, cattle 60, the garden pea 14, the fruit fly 8, etc. The chromosomes in the nucleus make up the nuclear genome, while the chromosomes of the mitochondria and chloroplasts make up the mitochondrial genome and the chloroplast genome, respectively.
Chromosomes are composed of DNA associated with a variety of proteins. This complex of DNA and proteins can often be seen inside cells as chromatin. Members of one group of proteins, called histones, help to organize the long strands of DNA into a structure known as a nucleosome. The structure of chromosomes becomes most easily visible during certain phases of nuclear division when the chromosomes are highly coiled (e.g., metaphase). Each chromosome can generally be distinguished from all others by several criteria, including the relative lengths of the chromosomes, the position of a structure called the centromere that divides the chromosome into two arms of varying length, the presence and position of enlarged areas called "knobs" or chromomeres and the presence of tiny terminal extensions of chromatin material called "satellites". A chromosome with a median centromere (metacentric) will have arms of approximately equal size. A submetacentric or acrocentric chromosome has arms of distinctly unequal size; the size difference is more pronounced in an acrocentric chromosome. The shorter arm is called the p arm and the longer arm is called the q arm. If a chromosome has its centromere at or very near one end of the chromosome, it is called telocentric. Each chromosome of the genome (with the exception of sex chromosomes) is numbered consecutively according to length, beginning with the longest chromosome first.
Autosomes vs. Sex Chromosomes
In the males of some species, including humans, sex is associated with a morphologically dissimilar (heteromorphic) pair of chromosomes called sex chromosomes. Such a chromosome pair is usually labeled X and Y. Genetic factors on the Y chromosome in humans determine maleness. Females have two morphologically identical X chromosomes. The members of any other homologous pairs of chromosomes (homologues) are morphologically indistinguishable, but usually are visibly different from other pairs (nonhomologous chromosomes). All chromosomes exclusive of the sex chromosomes are called autosomes. Figure 1-2 shows the chromosomal complement of the fruit fly Drosophila melanogaster (2n = 8), with three pairs of autosomes (2, 3, 4) and one pair of sex chromosomes. Figure 7-8 shows the karyotype of a human male.
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