DNA (A2 Only)
Chemistry: AQA A Level, topic 6.7.3
DNA
DNA is essential to pass on genetic information in living organisms. It provides the necessary code in cell replication and protein synthesis.
Basic structure of DNA
- A strand of DNA is a polymer of nucleotides - in other words, a chain of nucleotides.
- Nucleotides are made up of a deoxyribose sugar molecule attached to a phosphate ion (PO43−) and a complementary base pair - as shown above.
- Nucleotides join together to produce a chain by a condensation reaction between a phosphate and a deoxyribose molecule.
Basic structure of DNA
- There are four possible bases:
- Adenine (A).
- Thymine (T).
- Guanine (G).
- Cytosine (C).
Double helix in DNA
- DNA has a double helix structure constructed from two antiparallel nucleotide chains.
- The double helix is held together by hydrogen bonding between complementary base pairs:
- A with T and vice versa.
- C with G and vice versa.
Base pairing
- Above shows how the two complementary base pairs couple together through hydrogen bonding.
- C-G form THREE hydrogen bonds.
- A-T form TWO hydrogen bonds.
- The double helix structures itself based around these hydrogen bonds.
- It twists and turns until the hydrogen bonds are in the correct placement (correct alignment and distance between the two bases).
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DNA replication
- DNA replication and protein synthesis rely on the breaking and reforming of hydrogen bonds between strands.
- In DNA replication, the strands separate and new strands are constructed anti-parallel to the two old strands.
- Each new helix contains one new (daughter) strand and one old (parent) strand.