Biology Reference
In-Depth Information
2
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 structure
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 that adopts a helical
arrangement - first described by Watson and Crick in 1953 [1]. 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
contains approximately 3 200 000 000 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 [2]
(Figure 2.3).
