DNA Replication and Genetic code

  • Cells divide to produce more cells needed for growth or repair of tissues.

  • the 2 daughter cells produced from cell division = genetically identical to parent cell with identical base sequence.

  • When a cell divides, 2 strands of double Helix DNA separate and each strand acts as a template for creation of a new double strand DNA molecule. Due to certain Pyrimidines and Purines having to match with each other (from complementary base pairing rules), this makes the 2 new strands identical - DNA REPLICATION

Semi-conservative replication

  • Essentially what is written above

  • For DNA to replicate, helix structure has to unwind to 2 separate strands therefore H bonds broken

  • Each strand finds complementary base and forms H bond between them + phosphodiester bonds from adjacent nucleotides bonding.

  • As a result 2 new molecules of DNA produced - each consists of the old strand and a new strand = semi-conservative (half the same) replication

Roles of Enzymes in replication

  • DNA replication controlled by enzymes (proteins acting as catalysts for biochemical reactions) - which can only carry out their function by recognising and attaching to specific molecules (lock + key)

  • Before replication, unwinding and separation of 2 strands of DNA double helix is carried out by enzyme ‘DNA helicase’. Travelling along DNA ‘backbone’, catalysing reactions that break H bonds between complementary base pairs - ‘unzipping’

  • 2nd enzyme ‘DNA polymerase’ catalyses the formation of phosphodiester bonds between these nucleotides that were previously unzipped.

Continuous and Discontinuous Replication

  • DNA polymerase can only add nucleotides in one direction (5’ → 3’), so the two strands are copied differently at the replication fork.

Continuous replication (leading strand)

  • New DNA strand is made in one long piece.

  • DNA polymerase follows helicase and keeps adding nucleotides as the strand unzips.

Discontinuous replication (lagging strand)

  • New DNA strand is made in short sections (Okazaki fragments).

  • Each fragment is started, then the fragments are joined together by DNA ligase to form one continuous strand.

  • (DNA ligase): joins the Okazaki fragments by sealing the sugar-phosphate backbone (connecting the 3’ on one end to the 5’ on the next fragment) into one long strand.

Replication Errors

  • sequences of bases sometimes incorrect which occur randomly which lead to a change of sequence bases known as a ‘Mutation’

Genetic Code

  • DNA must carry the ‘blueprint’ needed to synthesise the many different proteins need by organisms. Proteins are the foundation for different physical and biochemical characteristics of living things.

  • Proteins made up of a sequence of amino acids - folded into complex structures. therefore DNA must code for a sequence of amino acids - genetic code

Degenerate Code

  • 1 codon - 1 amino acid/// A codon is a triplet of bases that code for an amino acid

  • 64 codons (4^3) possible, different codons have different functions - 3 codons that signal the end of the sequence (no more adding on amino acids to create the protein), A codon to signal to start sequence.

  • Only 20 amino acids therefore amino acids can be coded by more than 1 codon therefore ‘degenerate’