Describe the structure of DNA and the process of gene expression.
Genes are made up of DNA, a structure of which was proposed in 1953 by Watson and Crick based on X-ray crystallography studies conducted by themselves and others. The structure of DNA has to be simple enough that an identical replica is formed during each cell division, as well as having enough variability in the structure to produce the large variety of genes found in the chromosomes.
DNA is made of two chains of nucleotides wrapped around each other forming a double helix. Each nucleotide is made of a nitrogenous base (either a pyrimidine or a purine), a deoxyribose sugar and a phosphate molecule. The bases that make up DNA are adenine, guanine, cytosine and thymine and they always pair in the complimentary combinations adenine-thymine and guanine-cytosine. The backbone of the DNA molecule is formed as the phosphate attached to the hydroxyl group at the 5’ position on the sugar is attached to the hydroxyl group on the 3’ carbon of the adjacent sugar. These bonds are known as phosphodiester bonds. The two chains are held together by hydrogen bonds between the nitrogenous bases which point into the centre of the helix. The polarity of the DNA depends on the orientation of the sugar-phosphate backbone. The chain that ends at the 5’ carbon is known as the 5’ strand and the same for the 3’ strand. These run opposite to each other and are said to be antiparallel.
In a chromosome the DNA coils around proteins called histones forming nucleosomes. These nucleosomes then coil into chromatin fibres that form loops on a scaffold of non-histone acidic proteins which are then wound in a tight coil that makes up the chromosome as it would be seen under a light microscope.
DNA must be replicated during cell division. The process involves the action of enzyme DNA helicase which separates the two strands of the double helix. When separated, each of the strands can serve as a template to recreate the other strand. The process is semi-conservative as in each daughter cell only one strand has to be newly synthesised. The strands are prevented from re-annealing by single strand specific binding proteins. Replication through the action of DNA polymerases takes place at points known as origins of replication forming Y-shaped structures known as replication forks. As the polymerases can only act in a 5’ to 3’ direction, the leading strand is synthesised as a continuous process but the lagging strand is synthesised in pieces known as Okazaki fragments which are then joined together as a continuous fragment by the enzyme DNA ligase.
The distribution of genes throughout chromosomes is very varied. For example, chromosomes 19 and 22 are relatively gene rich whereas 4 and 18 are gene poor. The genes are also found more often in different areas of the chromosomes; there is a higher gene density in sub-telomeric areas. Genes are made up of coding sequences, known as exons are intervened by non-coding sequences called introns. Genes can vary in size of exons and introns, although larger genes tend to have more and larger exons.
Transcription is the process by which information stored in the genetic code is from the DNA of a gene to messenger RNA (mRNA). Every base in the mRNA molecule is complimentary to a corresponding base in the gene’s DNA but with uracil replacing the thymine. The transcribed RNA molecule is a copy of the sense strand of the DNA helix. This is the strand of DNA that is complementary to the template strand, also sometimes known as the antisense strand. The mRNA is transcribed by RNA polymerase which reads the template strand in the 3’ to 5’ direction and synthesises the mRNA in the 5’ to 3’ direction. Transcription is initiated by the polymerase recognising and binding to a promoter region of the gene. The mRNA undergoes splicing, capping, and