1.3 Structure of a chromosome

It is important to know what a chromosome is and what the difference between a replicated and unreplicated chromosome. The diagram below illustrates the structure of chromosomes. It starts at DNA level and shows how DNA is supercoiled to form a chromosome.

DNA Replication

Replication takes place during interphase where two copies of the chromosome is made. Each copy is now called a chromatid which is joined by a centromere. This is illustrated by the diagram below:

Chromosome number

• Every species has a specific number of chromosomes in the nucleus.
• Somatic cells (body cells) have the diploid number (2n) (equal amount of chromosomes).
• There are two chromosomes of each kind, one from the mother and one from the father.
• Sex cells (gametes) contain only half the number of chromosomes (n).
• When a female (n) gamete and a male gamete (n) fuse the resultant zygote is diploid (2n).
• Human somatic cells have 46 chromosomes in the nuclei and gametes have 23 chromosomes.