BIOCELL GENETICS Lecture 5.1 genetics

• DNA strands


  • Antiparallel arrangement
• DNA polymerases


  • Add nucleotides only to the 3' end of a growing strand

  
• Leading strand

  Replicated towards the replication fork

  
• Lagging strand


  Replication away from the replication fork
• Leading strand synthesis


  Synthesised as one piece in the 'bubble'
• Lagging strand synthesis


  Made of 100-200 nucleotide pieces, Okazaki fragments, which are joined together by DNA ligase into a single strand

  
• DNA polymerases cannot initiate the synthesis of DNA

  
• Primer


  The start of a new DNA chain which is made of RNA, ~10bp (base pairs) long

  
• Primase


  An enzyme which joins RNA nucleotides into a primer, and can initiate the process from scratch

  
• The leading strand requires just 1 primer, but each Okazaki fragment requires a primer
• Another DNA polymerase replaces primer with DNA before ligase can join fragments

  
• Each DNA strand


  • Has a leading strand and lagging strand at opposite ends of the replication 'bubble'

  
• Helicases


  Enzyme which untwists and separates DNA helix

  
• Single-strand binding proteins


  Bind to separates strands and hold them apart

  
• Enzymes assisting in proofreading and repairing DNA


  • DNA polymerase
  • Mismatch repair enzyme
  • Nuclease
  • Nucleotide excision repair

  
• The 5' problem


  As polymerase can only add nucleotides to the 3' end, there is no way to complete the 5' end
• Prokaryotes


  Have circular DNA which avoids the 5' problem
• Eukaryotes

  Have telomeres to avoid the 5' problem
• Telomeres

  100-1000 repeated short sequences of DNA, such as TTAGGG in humans
• Telomerase


  Enzyme containing RNA which further lengthens the 3' end to allow completion of the 5' end
• Telomerase is generally only present in germ-cell lines, thus somatic cell DNA strands tend to get shorter with each division
• The faster telomeres shrink


  The shorter a bird's lifespan
• Telomerase


  Reverse transcriptase capable of restoring telomere ends, not expressed in human somatic cells
• Telomeres


  Shorten with age, the cell's machinery cannot replicate right to the end of the chromosome, every cell division results in telomere shortening
• Cellular senescence


  When telomeres become critically shortened the cell stops dividing
• Telomeropathies


  Disorders which cause premature telomere shortening due to defects in the telomere maintenance machinery
• Telomeres


  Shorten with age in various species including Seychelles warblers, chimpanzees, humans, badgers, sea lions
• Telomeres do not shorten with age in the longest lived bats
• DNA contains genes which code for RNA, which can result in the production of proteins, and those proteins 'do things' in cells
• One gene - one enzyme hypothesis

  Not all proteins are enzymes, many proteins are comprised of more than one polypeptide chain, each coded by a single gene
• Genetic code


  DNA 'language' of A, C, G, T, RNA 'language' of A, C, G, U, Protein 'language' of 20 amino acids
• Replication, Transcription, Translation


  Linking the genetic code to polypeptides

  
• mRNA


  Messenger RNA, carries the full building instructions from a gene, immediately translated in prokaryotes, further processed in eukaryotes
• Triplet code


  3 DNA bases code for a single amino acid, read in the 5' to 3' direction
• Codon


  The mRNA triple code for an amino acid
• Template strand

  The coding strand of DNA for a gene

  
• Reading frame


  Triplet grouping
• Codons to amino acids

  All polypeptides start with methionine, redundancy present but no ambiguity, read as triplets without overlap from 5' to 3'
• DNA's code is nearly universal, allows sections of DNA to be transferred from one organism to another and still produce 'meaningful' protein

  
• RNA polymerase


  Pries apart the DNA helix and hooks together the RNA nucleotides, can only add nucleotides at the 3' end