DNA structure and the genome
Each person’s genome contains a large amount of DNA that is a potential target for DNA profiling. The selection of the particular region of polymorphic DNA to analyse can change with the individual case and also the technology that is available. In this chapter a brief description of the primary structure of the DNA molecule is provided along with an overview of the different categories of DNA that make up the human genome. The criteria that the forensic geneticist uses to select which loci to analyse are also discussed.
DNA has often been described as the ‘blueprint of life’, containing all the information that an organism requires in order to function and reproduce. The DNA molecule that carries out such a fundamental biological role is relatively simple. The basic building block of the DNA molecule is the nucleotide triphosphate (Figure 2.1a). This comprises a triphosphate group, a deoxyribose sugar (Figure 2.1b) and one of four bases (Figure 2.1c). The information within the DNA ‘blueprint’ is coded by the sequence of the four different nitrogenous bases, adenine, guanine, thymine and cytosine, on the sugar- phosphate backbone (Figure 2.2a). DNA normally exists as a double stranded molecule which adopts a helical arrange- ment – first described by Watson and Crick in 1953 . Each base is attracted to its complementary base: adenine always pairs with thymine and cytosine always pairs with guanine (Figure 2.2b).
Organization of DNA into chromosomes
Within each nucleated human cell there are two complete copies of the genome. The genome is ‘the haploid genetic complement of a living organism’ and in humans con- tains approximately 3200000000 base pairs (bp) of information, which is organized into 23 chromosomes. Humans contain two sets of chromosomes – one version of each chromosome inherited from each parent giving a total of 46 chromosomes (Figure 2.3). Each chromosome contains one continuous strand of DNA, the largest – chromosome
DNA STRUCTURE AND THE GENOME
Figure 2.1 The DNA molecule is built up of deoxynucleotide 5′-triphosphates (2.1a).The sugar (2.1b) contains five carbon atoms (labelled C1 to C5); one of four different types of nitrogenous base (2.1c) is attached to the 1 prime (1′) carbon, a hydroxyl group to the 3′ carbon and the phosphate group to the 5′ carbon
Figure 2.2 In the DNA molecule the nucleotides are joined together by phosphodiester bonds to form a single stranded molecule (2.2a). The DNA molecule in the cell is double stranded (2.2b) with two complementary single stranded molecules held together by hydrogen bonds. Adenine and thymine form two hydrogen bonds while guanine and cytosine form three bonds