Chromosomes, genes and DNA (CCEA)

Chromosomes are long strands of DNA (deoxyribonucleic acid).

They are subdivided into genes.

In most cells, chromosomes are found in the nucleus in functional pairs.

Humans have 46 chromosomes arranged in 23 pairs.

Chromosomes are not in functional pairs in gametes (which have only half the number) or in bacterial cells (which have no nucleus).

The entire genetic material of an organism is known as the genome.

A gene is a short length of DNA found on a chromosome that codes for a particular characteristic or protein.

Alleles are different forms of the same gene. For example, eye colour is the gene but blue, green, brown etc are alleles.

DNA has a double helix structure, made up of two chains of nucleotides.

Each nucleotide contains a phosphate group, a sugar (deoxyribose), and a base.

The phosphate and sugar molecules link together to form the backbone, while the interlinking bases hold the two strands together.

There are four bases, which pair as follows:

• Adenine (A) with thymine (T)

• Cytosine ( C ) with guanine (G)

This is known as the complementary base-pair rule.

Each person (except for identical twins) has unique DNA, meaning their sequence of bases is different from others.

DNA carries the genetic code to make proteins.

Three bases (a base triplet) code for one amino acid.

Amino acids are then joined in this order to make the protein.

This is known as the base triplet hypothesis.