Mutations of a DNA sequence could alter the expression of a gene. Describe and explain the process of gene expression and how a mutation could lead to disease.

DNA is a universal code for all living beings known to man. There are four nucleotide bases in DNA and RNA, the combinations of these sequences are what determine the order and length of a polypeptide. Every three nucleotides code for an amino acid, they are called codons. Gene expression starts with transcription, this is where a gene (coding for a protein) is copied into an mRNA single strand. This is done thanks to the complementarity of purines and pyrimidines. The RNA nucleotide Uracil is complementary to Adenine and Guanine to Cytosine. Transcriptions (like most reactions in the body) are catalysed by enzymes, in this case it is RNA polymerase. The mRNA strand then travels outside the nucleus and to a ribosome for the next step to commence. The second step of gene expression is called translation. This is where a ribosome slides along an mRNA while the tRNA molecules assemble amino acids. tRNAs have anticodons that are complementary to the codons on the mRNA strand, therefore there are many different tRNA molecules, each with a different amino acid attached. When two adjacent tRNA molecules are bound to mRNA, their amino acids form a peptide bond. This keeps happening until the full protein is formed. This is gene expression. Mutations are errors in the DNA sequence which result in an error in the protein coded for in return. When the incorrect nucleotide sequence is translated, the protein will in many cases contain an incorrect primary structure which also affects how the protein is folded and therefore its function. For example, if the gene coding for haemoglobin was mutated, it may no longer be able to carry oxygen in the body, possibly resulting in death.