4.1: Chromosomes, genes, alleles and mutations
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Identify the composition of chromosomes in eukaryotes.
Chromosomes in eukaryotes consist of DNA and protein.
Distinguish between a gene and an allele
A gene is a heritable factor that determines a specific characteristic, e.g. gene for eye colour. An allele is one form of a gene that can occupy the locus for that characteristic, e.g. blue eye colour or brown eye colour.
The genome is the complete genetic information of an organism.
Define gene mutation
A gene mutation is a permanent change in the genetic information in a gene. the mutation can involve one or more base pairs.
Distinguish between a spontaneous mutation and an induced mutation.
A spontaneous mutation occurs when the causative agent cannot be identified. In an induced mutation, the causative agent can be identified, e.g. exposure to radiation such as x-rays, gamma rays or certain chemical substances such as benzene.
What is a base substitution mutation?
A base substitution mutation is a point mutation where one base is substituted for another in the genetic code for a particular gene.
Identify the mutation involved in sickle-cell anaemia.
IN sickle-cell anaemia, the DNA code is changed from a T to an A on the β chain of haemoglobin on chromosome 11.
Explain how the mutation in sickle-cell anaemia affects transcription and translation.
the gene mutation GAG to GTG causes the amino acid glutamic acid to be replaced with the amino acid valine in transcription and translation. The expression of the slightly different polypeptide with the different amino acid changes the properties of the haemoglobin in red blood cells. The effect of the presence of the different polypeptide causes a distortion of the red blood cells to the sickle-cell shape and a loss of elasticity of the rad blood cells. Thus a single base substitution can dramatically affect body processes in an individual.
Describe how sickling affects the functioning of red blood cells.
Sickling decreases the elasticity and flexibility of the red blood cells, making them more rigid and unable to flow through narrow capillaries. The sickle cells clump together causing vessel occlusion. This deprives the body's cells of oxygen and cellular metabolism is affected. The spleen can be affected as it breaks down old red blood cells and blockages of blood supply can lead to necrosis (death) of spleen tissue.
Explain why sickle-cell trait is advantageous in some regions of the world.
Sickle-cell trait refers to the heterozygous condition where a person has one defective gene for sickle cell and one gene for a normal red blood cell. Due to incomplete dominance, heterozygous (carriers of the sickling gene) show some signs of sickling and produce some sickle cells, which does not normally cause problems for the individual under normal conditions. Problems arise for heterozygotes if they are in low oxygen conditions (high altitudes) or if they are dehydrated. Heterozygotes have increased resistance to malaria. Malaria is caused by a protozoan that invades red blood cells. In malarial areas of the world, heterozygots for sickle-cell anaemia have increases resistance to malaria. Thus the sickle-cell gene is present in higher frequencies in malarial areas that would be expected for a recessive gene that reduces life expectancy.
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