Topic 4.1: Chromosomes, genes, alleles and mutations

4.1.1 State that eukaryote chromosomes are made of DNA and proteins

An eukaryotic cell's chromosomes are made up of DNA and proteins. They are structured around 8 histone balls to form a nucleosome. This is shown in the image below.

A collection of nucleosomes are called a chromatin. It is important to not that, DNA is a nucleic acid. It is different from protein even if part of the DNA is made up of nitrogenous bases. Hence it is incorrect to state that it is wrong to write that histones are made up of protein.


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 (loci)

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.

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

A base substitution mutation is the change of a single base in a sequence of DNA

This resulting in a change to a single mRNA codon during transcription

In sickle cell anemia, the DNA sequence for the beta chain of haemoglobin is changed from GAG to GTG

This changes the mRNA codon (GAG to GUG), resulting in a single amino acid change (Glu to Val)

The amino acid change alters the structure of the haemoglobin causing it to form insoluble strands

This causes the red blood cells to adopt a sickle shape

The insoluble haemoglobin can't effectively carry oxygen, causing individuals to feel constantly tired

The sickle cells may accumulate in the capillaries and form clots, blocking blood supply to organs

Also causes anemia (low RBC count), as sickle cells are destroyed more rapidly than normal cells

Sickle cell anemia occurs in individuals who have two copies of the codominant "sickle cell" allele

Heterozygous individuals have increased resistance to malaria