4.1 Chromosomes, Genes, Alleles and Mutations


4.1.1  State that eukaryotic chromosomes are made of DNA and protein

  • Eukaryotic chromosomes consist of DNA wrapped around histone proteins
  • This forms the basic structure of the nucleosome, which is packed together to form chromatin (in a 'beads on a string' arrangement)
  • Chromatin will supercoil and condense during prophase to form chromosomes that can be visualised under a light microscope
  • Prokaryotic DNA is not wrapped around proteins and is thus considered to be 'naked'

Arrangement of DNA into chromosomes

4.1.2  Define gene, allele and genome

Gene:  A heritable factor that controls a specific characteristic, consisting of a length of DNA occupying a particular position on a chromosome (locus)

Allele:  One specific form of a gene, differing from other alleles by one or a few bases only and occupying the same locus as other alleles of the gene

Genome:  The whole of the genetic information of an organism

4.1.3  Define gene mutation

Gene mutation:  A change in the nucleotide sequence of a section of DNA coding for a particular feature

Types of Mutations

4.1.4  Explain the consequence of a base substitution mutation in relation to the process of transcription and translation using the example of sickle cell anaemia

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

                                                         Normal Red Blood Cell                                                                                     Sickle Cell

Consequences of Sickle Cell Anaemia

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)

Sickle Cell Anaemia