3.1 – Chromosomes, Genes, Alleles and Mutations
Chromosomes are composed of two daughter chromatids which are joined at the centromere. Chromosomes are mainly comprised of DNA and histone proteins
3.1.2 – Define gene, allele, and genome
Allele – An allele is a specific form of a gene, differing for other alleles by one or a few bases
only. They occupy the same gene locus as the other alleles on the gene Genome – The whole of the genetic information of an organism
Genome – The whole of the genetic information of an organism 4.1.3 – Define gene mutation
3.1.3 – Define gene mutation
A gene mutation is a change in the base sequence of an allele This may produce a different amino acid sequence in the protein translated, which may not
This may produce a different amino acid sequence in the protein translated, which may not be beneficial. A substance that causes mutation is called a mutagen, including radiation and chemicals.
Deletion is when one of the bases is removed, changing the whole gene. Insertion involves the addition of a base, which also changes the whole gene. Substitution is when a base is changed, altering only one amino acid. However, this will still affect the shape of the protein.
3.1.4 – Explain the consequence of a base substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anaemia
Sickle cell anaemia is a genetic disease. It has a frequency of about 1 in 655 African Americans. The condition is inherited, and cannot be contracted by infectious routes. The affected gene is found on chromosome 11. The sequence that codes for the sixth amino acid normally has the base sequence GAG, which codes for glutamic acid. This amino acid carries a negative charge. However, the substitution produces a different sequence, GUG, which codes for the neutral amino acid valine. The result is that the beta chain changes shape.
Haemoglobin is made up of four proteins, two of which can affected by the mutation. The usual shape of the red blood cells is a biconcave disc. However, when there is mutation, the cells become shaped like a sickle. As a result, the red blood cells cannot carry oxygen, causing anaemia. Furthermore, the irregular shape of the cells means that they do not move through the bloodstream properly, causing blockages in places such as the kidney tubules. This may damage the kidney and possibly lead to death.
In areas where malaria is common, those with the sickle cell anaemia trait are resistant to the infection. This is because normal blood cells are affected by the disease. As a result, those who do not have the mutation are more likely to die from malaria. In these regions, sickle cell anaemia has become more common since it gives carriers an advantage.