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Molecular Basis of Heredity: Part 2. Genomes

Author(s): Raye L. Alford, PhD

The Chromosome

Each chromosome of a living cell is a DNA double helix. The atoms that make up DNA are organized into two primary molecular components: bases and pentose sugars with attached phosphate groups. The bases are the key informational components of DNA, the letters of the DNA alphabet. The bases of DNA include adenine (A), cytosine (C), guanine (G), and thymine (T). Each base consists of a nitrogen-containing component called an amine. The side groups attached to the amines differ among the bases. The pentose sugars and phosphate groups serve as the links that connect the bases in a string, or strand, of DNA. When DNA strands form a double-stranded molecule, two strands of DNA are joined together through hydrogen bonds that form between the bases. In the double-stranded DNA molecule, an A base always pairs with a T base and a C base always pairs with a G base. Once joined, the bases are referred to as base pairs.

Embedded within the DNA sequence of each chromosome are the organism’s genes. The chromosome can be thought of as the DNA scaffolding within which the genes reside. An organism’s set of chromosomes is called its genome. Genes are interspersed unevenly along the lengths of most eukaryotic chromosomes. Across the human genome, for example, there are gene-rich regions and gene-poor regions.

The genomes of most eukaryotes are fairly large and often complex. In addition to genes themselves, most eukaryotic genomes also contain a variety of non-coding structural and regulatory elements and introns. Some of the genetic material also serves as a fossil record, a history book written in biological terms and handed down from generation to generation.

In addition to the DNA inside the cell nucleus, eukaryotic cells have separate genetic material in certain organelles such as mitochondria and, in plants, chloroplasts. In general, organelles, prokaryotes and viruses have greater biological constraints than nuclei on the tolerable sizes of their genomes because of the small genome size that can be incorporated into the organelle, bacterial cell, or viral capsid. As such, the genes of mitochondria, bacteria and viruses typically lack many of the complex non-coding elements commonly found in the nuclear genes of eukaryotes.