🧬DNA replication and the genetic code 🧬

• In DNA replication, the two strands of DNA double helix separate and each strand acts as a template to create a new double stranded DNA molecule

  Complementary base pairing rules ensure the two new strands are identical to the original
• Semi-conservative replication involves the following steps:

  • the double helix unwinds for replication so the hydrogen bonds holding complementary base pairs together are broken to separate the DNA molecule into two strands
  • free nucleotides in the nucleus pair with their complementary bases and hydrogen bonds form between them
  • the new nucleotides join to their adjacent nucleotides with phosphodiester bonds
  • This results in two new DNA molecules, each consisting of one old strand (parental strand) of DNA and one new strand (daughter strand)
• The enzymes involved in DNA replication are:

  • DNA helicase- it travels along the DNA backbone, catalysing reactions that break the hydrogen bonds between complementary base pairs as it reaches them (unzips DNA molecule)
  • DNA polymerase- it catalyses the formation of phosphodiester bonds between new nucleotides
• DNA polymerase can only bind to the 3' end so always travels in the 3' to 5' direction on the template strand

  The strand that is unzipped from 3' to 5' can be continuously replicated as the strands unzip. This is called the leading strand

  The other strand is unzipped from 5' to 3' so the DNA polymerase has to wait until a section of the strand has unzipped and the work back along the strand because it can only work from 3' to 5'. This is called the lagging strand and it undergoes discontinuous replication which results in the DNA being produced in sections called Okazaki fragments
• Replication errors that can lead to mutation:

  • sequence of bases is not always matched exactly
  • incorrect sequence may occur in the newly copied strand
  • random and spontaneous mutations
• The genetic code is how DNA codes for a sequence of amino acids
• A codon is a sequence of three bases that code for one amino acid
• A gene is a section of DNA that contains the complete sequence of codons to code for an entire protein
• The genetic code is universal. All organisms use this same code but the sequences of bases coding for each individual protein will be different.
• The genetic code is known as degenerate because only 20 different amino acids occur regularly in biological proteins. However, there are 4 different bases so 64 different codons possible so there are more codons than amino acids

  This means many amino acids can be coded for by more than one codon

  One of the 64 codons acts as a start codon signalling the beginning of a gene. This ensures the base sequence is read in frame and doesn't overlap with other genes
• If the start codon is in the middle of a gene it just codes for the amino acid methionine
• There are three stop codons that don't code for any amino acids they just signal the end of a gene