4.1: Chromosomes, genes, alleles and mutations

Question 1

State that eukaryotic chromosomes are made of DNA and protein.

Answer 1

Eukaryotic chromosomes are made of DNA and proteins.

Question 2

Define gene, allele and genome:

Answer 2

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.

Question 3

Define gene mutation

Answer 3

A change to the base sequence of a gene.

Question 4

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

Answer 4

Cause of Sickle Cell Anaemia:
- A base substitution mutation is the change of a single base in a sequence of DNA, resulting in a change to a single mRNA codon during transcription
- In the case of sickle cell anaemia, the 6th codon for the beta chain of haemoglobin is changed from GAG to GTG (on the non-coding strand)
- This causes a change in the mRNA codon (GAG to GUG), resulting in a single amino acid change of glutamic acid to valine (Glu to Val)
• DNA: GAG to GTG (non-coding strand) • mRNA: GAG to GUG
• Amino Acid: Glu to Val
- The amino acid change alters the structure of haemoglobin, causing it to form fibrous, insoluble strands
- This causes the red blood cell to adopt a sickle shape

Consequences:

• The insoluble haemoglobin cannot effectively carry oxygen, causing individual to feel constantly tired
• The sickle cells may accumulate in the capillaries and form clots, blocking blood supply to vital organs and causing a myriad of health problems
• Also causes anaemia (low RBC count), as the sickle cells are destroyed more rapidly than normal red blood cells
• Sickle cell anaemia occurs in individuals who have two copies of the codominant ‘sickle cell’ allele (i.e. homozygotes)
• Heterozygous individuals have increased resistance to malaria due to the presence of a single ‘sickle cell’ allele (heterozygous advantage)