DNA (Leaving Cert Biology): Revision Notes

DNA

Structure of DNA

• Chromosomes are highly coiled structures made up of 40% DNA and 60% protein.
• The proteins involved are called histones. They help organise the DNA.
• DNA stands for deoxyribonucleic acid.
• It was discovered by Watson and Crick in 1953 (and Rosalind Franklin).
• Each human cell contains about 3 metres of DNA.
• DNA is heavily folded to fit into the nucleus.
• DNA is double-stranded and forms a double helix.
• The strands are held together by bases.
• DNA is made up of nucleotides, which join to form long chains called polynucleotides.

Nucleotides

A nucleotide is the building block of DNA.

It is made up of three parts:

1. A phosphate group
2. A sugar (called deoxyribose)
3. One of four nitrogen-containing bases

The four nitrogen-containing bases in DNA are:

• Adenine (A)
• Guanine (G)
• Cytosine (C)
• Thymine (T)

Base Pairing Rules:

• Adenine (A) always pairs with Thymine (T).
• Cytosine (C) always pairs with Guanine (G).
• These pairs are called complementary bases.

Hydrogen Bonds:

• A and T are joined by 2 hydrogen bonds.
• C and G are joined by 3 hydrogen bonds.

Purines and Pyrimidines:

• Adenine (A) and Guanine (G) are classified as purines.
• Cytosine (C) and Thymine (T) are classified as pyrimidines.