Genes can be switched on or off, this is known as expression. In our cells, we have all the genes possible but not all of them are switched on, because if they were, we would be in pretty big trouble. For example, the cells in our stomach can produce hydrochloric acid to create the acidic conditions needed but imagine if the gene which causes this was turned on in the cells in our eyes. We would be crying hydrochloric acid! For genes to be switched on they must be transcribed and used to be a functional product (like proteins). The process of gene expression requires several stages, so the first to focus on is transcription.
Transcription is the process by which the information in a strand of DNA is copied into a new molecule of messenger RNA. There are two types of transcription, prokaryotic and eukaryotic. Deoxyribonucleic acid (DNA) consists of nucleotides joined together by phosphodiester bonds, there are two strands of these nucleotide chains, which are held together by hydrogen bonds in a helix. The two strands are antiparallel, as DNA has polarity. There is a directionality of the strands, going from the 5’ end to the 3’ end. Nucleotides are the monomers which together make the polymer DNA (a nucleic acid.) Mer means thinks, so monomer is one thing, polymer means lots of things. Each nucleotide contains a pentose sugar, called deoxyribose, a phosphate group and a nitrogenous base. A nucleoside is deoxyribose attached to a phosphate group. It becomes a nucleotide when we add the nitrogenous base. There are four types of bases in DNA, adenine, thymine, guanine, and cytosine. Adenine and guanine are called purine bases, this means they consist of two carbon-nitrogen rings attached together. Thymine and cytosine are pyrimidines (one carbon-nitrogen ring), thus purines are bigger than pyrimidines. Purines can only bind to pyrimidines, so adenine binds to thymine, and cytosine binds to guanine. In the double helix, the hydrogen bonds form between the bases. Adenine and thymine are held together by 2 hydrogen bonds whilst guanine and cytosine are held by 3 bonds.
Transcription requires Ribonucleic acid (RNA). This nucleic acid is similar to DNA, but rather than there being two strands, there is one. Secondly, rather than the base thymine, we have uracil. Uracil forms bonds with thymine. The question raises as to why uracil is not found in DNA. Cytosine can undergo spontaneous deamination to produce uracil. If DNA replication occurred after deamination it would replace the cytosine to guanine pair with uracil to adenine. This would introduce a mutation (and we don’t want this to happen.) To prevent this from happening in DNA, any uracil is removed by the enzyme uracil-DNA glycosylase, generating an abasic site, which is removed and repaired by DNA polymerase. Uracil is energetically less expensive to produce than thymine, hence its use in RNA. As it is easily produced in DNA from cytosine, the fact that thymine is present makes the normal base detection and repair of mutations highly efficient.