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IB Biology

GENETICS 1

4.1 Chromosomes, genes, alleles and mutations4.1.1 State that eukaryote chromosomes are made of DNA and proteins.

Eukaryote chromosomes are made of DNA and proteins.

4.1.2 Define gene, allele and genome.

Gene: a heritable factor that controls a specific characteristic.Allele: one specific form of a gene, differing from other alleles by one or a few bases only

and occupying the same gene locus as other alleles of the gene.Genome: the whole of the genetic information of an organism.

4.1.3 Define gene mutation.

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Gene mutation: a permanent change in the sequence of base pairs in the DNA that makesup a gene

DNA sequence is altered due to a base addition, deletion or substitution The specific sequence for amino acids is altered resulting in an altered protein This may have little or no effect on the organism, or it may have a major influence on

physical characteristics

E.g.) Base substitution mutation: a mutation that results from the substitution of a singlenitrogenous base for another, altering the protein

4.1.4 Explain the consequence of a base substitution mutation in relation to theprocesses of transcription and translation, using the example of sickle-cell anemia.

Sickle-cell anemia (Hbs) a chronic usually fatal anemia marked by sickle-shaped redblood cells (RBCs). The disease occurs in individuals who are homozygous for the mutanthemoglobin gene. Hbs Hbs

Hemoglobin (Hb) quaternary protein structure that carries oxygen in RBCs

Made up of four polypeptide chains In one of the chains a single base substitution results in a different amino acid being

produced The 6th amino acid GAG (glutamic acid) becomes GTG (valine) This altered amino acid results in an altered protein structure The valine is hydrophobic and this replaces the hydrophilic glutamic acid This causes the clumping together of Hbs cells into rigid structures causing the

sickling of the cells The sickle-cell RBCs can block blood vessels and cause damage to organs Someone who is heterozygous for the mutant gene (Hbs Hb) will not exhibit

symptoms

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Sickle-cell anemia is a genetic disease that affects red blood cells in the body. It is due to amutation on the Hb gene which codes for a polypeptide of 146 amino acids which is part ofhemoglobin (hemoglobin is an important protein component in red blood cells). In sickle-cellanemia the codon GAG found in the normal Hb gene is mutated to GTG. This is called abase substitution mutation as adenine (A) is replaced by thymine (T). This means that whenthe mutated gene is transcribed, a codon in the mRNA will be different. Instead of the

normal codon GAG, the mRNA will contain the codon GUG. This in turn will result in amistake during translation. In a healthy individual the codon GAG on the messenger RNAmatches with the anticodon CUC on the transfer RNA carrying the amino acid glutamic acid.However, if the mutated gene is present then GUG on the messenger RNA matches withthe anticodon CAC on the transfer RNA which carries the amino acid valine. So the basesubstitution mutation has caused glutamic acid to be replaced by valine on the sixth positionon the polypeptide. This results in hemoglobin S being present in red blood cells instead ofthe normal hemoglobin A. This has an effect on the phenotype as instead of normal donutshaped red blood cells being produced some of the red blood cells will be sickle shaped. Asa result these sickle shaped red blood cells cannot carry oxygen as efficiently as normal redblood cells would. However, there is an advantage to sickle cell anemia. The sickle cell redblood cells give resistance to malaria and so the allele Hbs on the Hb genewhich causes

sickle cell anemia is quite common in parts of the world where malaria is found as it

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provides an advantage over the disease.

Summary of important steps:

Normal Gene Mutated gene

Codon GAG GTG

Transcription GAG on mRNA GUG on mRNA

Translation Anticodon CUC andamino acid glutamic

acid on tRNA.

Anticodon CAC andamino acid valine on

tRNA.

Haemoglobin HbA

HbS

Phenotype Normal donut shapedred blood cells.

Sickle cell shaped redblood cells.

Effects Carry oxygen efficientlybut are affected by

malaria.

Do not carry oxygenefficiently but give

resistance to malaria.