Protein Synthesis

• what are the two main stages in protein synthesis?

  • transcription
  • translation
• what is transcription?

  copying the section of DNA that codes for the protein required into mRNA, in a complementary way
• what is translation?

  the turning of code carried by mRNA into a protein
• what base does RNA not make?

  Thymine (so makes Uracil instead)
• how is mRNA transcribed?

  • the antisense strand ( 3' to 5') acts as a template strand during transcription - means complementary RNA strand formed carries the same base sequence as the sense strand
  • free RNA nucleotides will pair with the complementary base pairs exposed on the antisense strand as the DNA unzips
  • RNA polymerase forms phosphodiester bonds between RNA nucleotides
  • transcription stops at the ends of a gene and the completed strand of RNA is messenger (m) RNA
• how does a gene encode for a protein?

  a gene contains DNA that encodes how to make specific proteins, as transcription occurs the mRNA is made and can be translated to build the protein molecule (polypeptide) encoded by the origonal gene
• why is mRNA less stable than DNA?

  mRNA contains a hydroxide group meaning it is more suseptible to hydrolysis
• why is the instability of mRNA necessary feature?

  • it is only required for the synthesis of proteins in the short-term and it's perpetual presence would represent an energy cost to the cell
  • controls gene expression and influences DNA replication
  • so it can be broken down and read
• what are the stages involved in transcription?

  • DNA strand is unzipped by DNA helicase
  • antisense strand is replicated (complementary strand made)
  • RNA polymerase joins RNA polynucleotides to form mRNA chain - free RNA nucleotide base pair with complementary bases exposed on antisense strand (carries same base sequences as the sense (coding) strand
  • phosphodiester bonds form between polynucleotides
  • mRNA chain leaves through nuclear pore and binds to cytoskeleton where it is transported to a ribosome
• where does post-transcriptional modification to mRNA occur?

  Golgi-apparatus
• what are exons?

  sections of a gene that code for proteins
• what are introns?

  sections of a gene that are non-coding, and might have a regulatory role
• what is splicing?

  the removal of introns
• what is polyadenylation?

  where a poly-A tail is added to a free 3'-end
• what is the function of polyadenylation?

  • many mRNA molecules are broken down by 3'-5' exonucleases
  • having a long string of 'A' nucleotides at the ends will protect the coding part of mRNA from degradation
  • the longer the poly-A tail, the longer the mRNA will last (and so more protein will be synthesised)
• what is the function of the 5' guanine cap?

  • protects from degradation by 5'-3' exonucleases
  • acts as a recognition site so ribosomes can bind
  • transporting mRNA into cytoplasm
  • export DNA
  • encourages translation
• what is the start codon?

  AUG
• what are the steps in translation?

  • the anticodon on the tRNA binds to a complementary codon on the mRNA strand - 2 max at a time
  • the tRNA molecules carry an amino acid corresponding to a codon
  • when the complementary binding happens, the amino acids are brought together in the correct sequence to form the primary structure of the protein coded for by the mRNA
  • the ribosome moves along the mRNA, releasing the first tRNA and the second tRNA becomes the first
  • as primary structure is formed, they fold into secondary and tertiary structures
• why are introns removed in splicing?

  • role of a protein is dependent on the structure
  • 3D shape is dependent on primary structure
  • base codons on mRNA code for amino acids
  • introns would code for unnecessary amino acids/stop signals
  • codons would cause frameshift
• what is an advantage of being able to edit mRNA?

  different proteins can be produced from one gene
• why are introns present in genes?

  • could've originally been functional genes
  • mutations
  • changed base sequence
  • no longer code for a useful amino acid