Although a chromosome is a complex three-dimensional structure, the genes on a chromosome are in linear order and can be represented by segments of a bar, as shown here. Genes are often given alphabetical designations in genetic diagrams.
Hand-tinted transmission electron micrograph (TEM) of circular bacterial DNA. This figure actually shows a small plasmid, rather than a full-size chromosome. The double-stranded DNA is yellow. An individual gene has been mapped by using an RNA copy of the gene. The RNA base pairs to one strand of the DNA forming a DNA/RNA hybrid (red). The other strand of the DNA forms a single-stranded loop, known as an "R-loop" (blue). Magnification: x28,600. Courtesy of: P. A. McTurk and David Parker, Science Photo Library.
Genes are not mere abstractions. They are segments of DNA molecules carrying encoded information.
Chromosomes Are Long, Thin Molecules That Carry Genes
Genes are aligned along very long, string-like molecules called chromosomes (Fig. 1.05). Organisms such as bacteria usually fit all their genes onto a single circular chromosome (Fig. 1.06); whereas, higher, eukaryotic organisms have several chromosomes that accommodate their much greater number of genes. Genes are often drawn on a bar representing a chromosome (or a section of one), as shown in Figure 1.05.
One entire chromosome strand consists of a molecule of deoxyribonucleic acid, called simply DNA (see Ch. 3). The genes of living cells are made of DNA, as are the bacteria Primitive single-celled organisms without a nucleus and with one copy of each gene chromosome Structure containing the genes of a cell and made of a single molecule of DNA
Different Organisms may Have Different Numbers of Chromosomes 7
FIGURE 1.07 Genes Match on Each of a Pair of Homologous Chromosomes
Higher organisms possess two copies of each gene arranged on pairs of homologous chromosomes. The genes of the paired chromosomes are matched along their length. Although corresponding genes match, there may be molecular variation between the two members of each pair of genes.
Different organisms differ greatly in the number of genes, the number of copies of each gene, and the arrangement of the genes on the DNA.
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