DNA analysis has many applications – in medical and pharmaceutical research – in forensic science – in genetic engineering – for determining close family relationship – and in ancestry research or genealogy. This last application forms the subject of my blog.
First of all, what is DNA? The letters stand for Deoxyribonucleic acid, a molecule encoding the genetic instructions used in the development and functioning of all known living organisms. Its structure was first described by Nobel Prize winners Crick and Watson in 1953. The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.
Figure 1. The double helix of the DNA molecule.
Written out the base pairs in DNA make a sequence, e.g. A T A T C G C G T A A T G C. More than 99.9% of those bases are the same in all people. The order, or sequence, of the letters determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.
Fig. 2. A single-nucleotide polymorphism (SNP) in a DNA sequence.
Within cells, DNA is organized into long structures called chromosomes. Most DNA occurs in the cell nucleus, but a small amount can also be found in the mitochondria. These are specialised structures within cells, supplying chemical energy as well as with other functions. This DNA is abbreviated as mtDNA.
The Y chromosome is a special chromosome, passed on from fathers to their sons, while mothers pass on mtDNA to both their sons and daughters. But mtDNA dies with men and so it survives only in the female line. This means that a man’s lineage can be followed along both paternal and maternal lines, while in a woman only her maternal or mtDNA line can be followed.
Every human carries two copies of the genetic code, one inherited from the mother and one from the father, some 6 billion letters in all. Apart from identical twins, no two individuals have the same genetic code. With the exception of the egg and sperm cells, all the cells of our bodies have 23 pairs of chromosomes, 46 in all. One chromosome of the pair is inherited from the father and one from the mother. However, in males the 23rd pair consists of a so-called Y-chromosome and an X-chromosome, whereas females have two X-chromosomes. The Y chromosome contains a gene which triggers embryonic development as a male and carries information about the male’s paternal lineage.
In sexual reproduction in mammals the DNA in the sperm and egg joins up so that homologous sequences are aligned with each other. This is followed by exchange of genetic information to form a new recombined chromosome which is passed on to the offspring. Cell division then takes place and the chromosomes are duplicated in the process of DNA replication, providing each cell its own complete set of chromosomes. The double-stranded structure of DNA provides a simple mechanism for DNA replication. In this process the two strands are separated and then each strand’s complementary DNA sequence is recreated by an enzyme.
Figure 3. DNA replication
Note that DNA only survives in nature for around 500,000 years. This means that recovery of DNA from most fossils is impossible.