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may contain more than one copy of their genome. Genomic DNA from nearly all

prokaryotic and eukaryotic organisms is also complexed with protein and termed

chromosomal DNA. Each gene is located at a particular position along the chromo-

some, termed thelocus, whilst the particular form of the gene is termed theallele.In

mammalian DNA each gene is present in twoallelic formswhich may be identical

(homozygous) or which may vary (heterozygous). It is thought that there are approxi-

mately 20 000 genes present in the human genome, although not all will be expressed

in a given cell at the same time. However various processing events such as alternative

splicing or RNA editing can increase the number of actual proteins found in the cell in

relation to the number of genes to nearly 1 million. The occurrence of different alleles

at the same site in the genome is termedpolymorphism. In general the more complex

an organism the larger its genome, although this is not always the case since many

higher organisms have non-coding sequences some of which are repeated numerous

times and termedrepetitive DNA. In mammalian DNA repetitive sequences may be

divided into low copy number and high copy number DNA. The latter is composed of

repeat sequences that are dispersed throughout the genome and those that are clus-

tered together. The repeat cluster DNA may be defined into so-calledclassical satellite

DNA,minisatelliteandmicrosatellite DNA, the latter being mainly composed of

dinucleotide repeats (Table 5.2). These sequences are termed polymorphic, collectively

termed polymorphisms, and vary between individuals; they also form the basis of

genetic fingerprinting.

5.3.2 Single nucleotide polymorphisms (SNPs)

A further important source of polymorphic diversity known to be present in genomes

is termedsingle nucleotide polymorphismsor SNPs (pronouncedsnips). SNPs are

substitutions of one base at a precise location within the genome. Those that occur in

coding regions are termedcSNPs. Estimates indicate that an SNP occurs every once in

Table 5.2Repetitive satellite sequences found in DNA, and their characteristics

Types of repetitive DNA

Repeat unit

size (bp) Characteristics/motifs

Satellite DNA 5  200 Large repeat unit range (Mb) usually found at centromeres

Minisatellite DNA

Telomere sequence 6 Found at the ends of chromosomes. Repeat unit may span

up to 20 kb G-rich sequence

Hypervariable sequence 10 60 Repeat unit may span up to 20 kb

Microsatellite DNA 1  4 Mononucleotide repeat of adenine dinucleotide repeats
common (CA). Usually known as VNTR (variable number
tandem repeat)

Notes:bp, base-pairs; kb, kilobase-pairs.

146 Molecular biology, bioinformatics and basic techniques