Gene manipulation and recombinant DNA technology - Pharmaceutical Biotechnology: Concepts and Aapplications - page 43

Figure 3.6 (a) DNA structure. The two complementary polynucleotide strands in DNA are antiparallel to each
other in orientation (one runs 5' → 3', the other 3 → 5'; see Figure 3.5). The two strands are held together by
hydrogen bonds between opposite complementary bases, as well as hydrophobic interactions between stacked
bases, as described in the text. (b) The double polynucleotide chain adopts a double helical structure
interactions between the planar, largely hydrophobic bases effectively stacked above each other
along the length of each strand.
3.2.1 Genome and gene organization
The genome refers to the entire hereditary information present in an organism. As discussed
earlier, this is usually encoded by double-stranded DNA (the genome of most plant viruses and
some animal and bacterial viruses is RNA based). DNA-based genomes are largely or exclusively
organized into chromosomes, each chromosome being a single DNA molecule housing multiple
genes, as well as non-coding sequences.
Bacteria normally harbour a single, circular chromosome that tends to be tethered to the bacte-
rial plasma membrane and tends to have few if any closely associated proteins. Many bacteria also
contain extra-chromosomal DNA in the form of plasmids, as will be discussed later. Eukaryotes
(plants, animals and yeasts) posses multiple linear chromosomes contained within a cell nucleus,
and these chromosomes are normally closely associated with proteins termed histones (the pro-
tein-DNA complex is termed chromatin). Eukaryotes also invariably possess DNA sequences
within mitochondria and in chloroplasts in plants. The (usually circular) DNA molecules are much