DNA (A level Bio)
From WikiTextbook
The genetic material is DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Both molecules are macromolecules and polymers. The monomers which make up DNA are called nucleotides.
Nucleotides are made up of three parts:
- Nitrogen base
- Pentose (5 carbon atoms) sugar
- Phosphate group
The Nitrogen base is one of 4 (5 in RNA):
- Guanine
- Cytosine
- Thymine
- Adenine
Adenine and Guanine are pyrine bases (large, with 2 carbon rings). Cytosine and Thymine are pyrimidines (smaller, with 1 carbon ring).
The Pentose sugar in DNA is deoxyribose.
DNA is formed during interphase of the eukaryotic cell cycle. Nucleotides are linked together by covalent bonds between the sugars (deoxyribose) and phosphates. Two polynucleotides running in opposite directions are linked by hydrogen bonds between the bases forming base pairs. A pyrine base (A or G) ALWAYS pairs with a pyrimidine base (C or T). Cytosine always pairs with Guanine (3 hydrogen bonds) and Adenosine with Thymine (2 hydrogen bonds). This is called complementary base pairing. The two polynucleotides now twist around each other to form a left handed (sinistral) double helix.
