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# Chromosomes, Genes and DNA ## Learning Outcomes After studying this section, you should be able to: - Explain the structural relationship between chromosomes, genes, and DNA. - Describe the molecular structure of DNA. - Explain the terms 'autosome' and 'sex chromosome'. - Define the terms 'mutati...

# Chromosomes, Genes and DNA ## Learning Outcomes After studying this section, you should be able to: - Explain the structural relationship between chromosomes, genes, and DNA. - Describe the molecular structure of DNA. - Explain the terms 'autosome' and 'sex chromosome'. - Define the terms 'mutation', 'genome', 'haploid', 'diploid', and 'karyotype'. ## Chromosomes Nearly every body cell contains, within its nucleus, an identical copy of the entire complement of the individual's genetic material. Two important exceptions are red blood cells (which have no nucleus) and the gametes or sex cells (see later). In a resting cell, the chromatin (genetic material) is diffuse and hard to see under the microscope; but when the cell prepares to divide, it is collected into highly visible, compact, sausage-shaped structures called chromosomes. Each chromosome is one of a pair, one inherited from the mother and one from the father, so the human cell has 46 chromosomes that can be arranged as 23 pairs. A cell with 23 pairs of chromosomes is termed diploid. Gametes (spermatzoa and ova) with only half of the normal complement, i.e., 23 chromosomes instead of 46, are described as haploid. Chromosomes belonging to the same pair are called homologous chromosomes. The complete set of chromosomes from a cell is its karyotype (Fig. 17.1). Each pair of chromosomes is numbered, the largest pair being number 1. The first 22 pairs are collectively known as autosomes, and the chromosomes of each pair contain the same amount of genetic material. The chromosomes of pair 23 are called the sex chromosomes (Fig. 17.2) because they determine the individual’s gender. Unlike autosomes, these two chromosomes are not necessarily the same size. ### Figures - **Figure 17.1**: Chromosomal complement (karyotype) of a normal human male. There are 22 pairs of autosomes and 1 pair of sex chromosomes (XY, pair 23). The same karyotype, shown after coloured fluorescence staining. - **Figure 17.2**: Coloured scanning electron micrograph of replicated human sex chromosomes. Upper left, Y chromosome; centre, X chromosome. ### Table of Chromosomes **Figure 17.1** (A): - Chromosome pairs: - 1, 2, 3, 4, 5 - 6, 7, 8, 9, 10 - 11, 12, 13, 14, 15 - 16, 17, 18, 19, 20 - 21, 22, X, Y **Figure 17.2** (B): - Chromosome pairs: - 1, 2, 3, 4, 5 - 6, 7, 8, 9, 10 - 11, 12, 13, 14 - 15, 16, 17, 18, 19, 20, 21, 22 **Page Number**: 476