AS/A-level Biology - Biological Molecules

• Nucleic acids come in two forms: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
• The monomer of all nucleic acids is called a nucleotide.
• Each single nucleotide is made up of 3 covalently bonded units:
  (1) one phosphate group
  (2) one pentose sugar (dexoyribose in DNA and ribose in RNA)
  (3) one organic nitrogenous base (5 possible bases: adenine, thymine, guanine, cystosine and uracil)

• A condensation reaction between the phosphate group of one nucleotide and the sugar of another nucleotide joins the two together.
• Repeating this bond gives a long chain of nucleotides.
• As nucleotides are bonded together to form chains, the 'backbone' of the molecule consists of a repeating sugar-phosphate chain.

Do you remember the differences between DNA and RNA in AS/A-level Biology?

• The five organic nitrogenous bases are grouped into purines and pyramidines. 
• Pyramidines are smaller than purines.
• Purines: Adenine (A) and Guanine (G)
• Pyramidines: Thymine (T), Uracil (U) and Cytosine (C)

• Purines bond with pyramidines as they are complementary bases, joined together by  2 or 3 hydrogen bonds.
• Guanine (G) and Cytosine (C) have 3 hydrogen bonds.
• Adenine (A), Thymine (T) and Uracil (U) have 2 hydrogen bonds.

• DNA is found in the nucleus.
• The molecule is twisted into a double helix in which each of the strands are antiparallel to each other, meaning the strands always run in opposite directions to each other.
• It has two sugar phosphate backbones attached to one another by complementary bases.
• These bases pair in the centre of the molecule by means of hydrogen bonds.
• The chains are always the same distance apart because the bases pair up in a specific way.
• Where a pyramidine appears on one side, a purine appears on the other.

1. The double helix unwinds.

2. Hydrogen bonds between the bases are broken apart to 'unzip' the DNA to form two single strands, exposing the bases.

3. Hydrogen bonds form between free DNA nucleotides and exposed bases through complementary base pairing.

4. Covalent bonds are formed between the phosphate of one nucleotide and the sugar of the next to seal the backbone using the enzyme DNA polymerase.

5. Each new DNA molecule consists of one conserved strand plus one newly built strand. This process of DNA replication is described as semi-conservative replication.

References:
1. http://www.a-levelnotes.co.uk/biology-ocr-as-notes-foundations-in-biology-nucleotides-and-nucleic-acids.html

That's the end of the topic!

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