protein synthesis

• RNA nucleotides


  ribose sugar

  uracil instead of thymine

  condensation reaction with phosphodiester bonds
• why is uracil used in RNA nucleotides?


  thymine quickly gets destroyed outside of nucleus

  however uracil has risks of mutating inside of cells so thymine is used in DNA
• RNA function 


  RNA takes information from DNA to ribosomes where it can synthesise proteins
• mRNA


  messenger RNA

  single strand of RNA nucleotides

  helix shaped - represented in straight line

  sends information from nucleus to ribosomes
• 1st stage of transcription

  RNA polymerase breaks apart DNA bases held together by hydrogen bonds

  RNA polymerase binds to bases and unzips the strands
• 2nd stage of transcription

  RNA polymerase adds free nucleotides to exposed complementary base pairs on one strand

  RNA polymerase joins two nucleotides together using condensation reactions which forms phosphodiester bonds
• 3rd stage of transcription

  RNA polymerase also fixes DNA helicase as it moves along

  once complete, RNA polymerase and RNA strand detach from DNA

  multiple rounds of transcription occur using both DNA strands
• template strand 


  the strand that is used to produce RNA
• what is the outcome of transcription?


  a strand called pre-mRNA which is a copy of one of the strands of DNA

  it is complementary to the other strand of DNA
• gene definition 


  a base sequence of DNA that codes for the amino acid sequence of a polypeptide
• pre mRNA vs mRNA

  pre mRNA contains exons and introns whereas mRNA only contains exons

  pre mRNA is longer than mRNA
• introns


  non-coding sequence in a gene
• exons


  coding sequence in a gene
• splicing


  proteins called spliceosome cluster around pre-mRNA, removes the introns and attach the remaining exons back together

  this results in a complete strand of mRNA
• transcription in prokaryotes


  prokaryotic DNA does not contain introns so there is no need for splicing
• codons


  set of 3 bases that code for one amino acid

  in DNA is called a triplet and codons in mRNA form from each complementary triplet in the original DNA
• why is the genetic code universal?

  the amino acid coded for is typically the same in all organisms
• why is the genetic code non-overlapping?

  each base is only read once
• why is the genetic code degenerate?

  amino acids can be coded for by several different triplets
• anticodons


  have 3 complementary bases to a codon on mRNA

  are usually on a molecule carrying an amino acid
• tRNA


  polypeptide chains formed through translation, folded to form a clover shape

  base pairs are held together by hydrogen bonds

  has amino acid binding site and an anticodon on the other side

  transfer RNA-transfers amino acids to polypeptide chains

  single strand of RNA
• 1st stage of translation


  ribosome attaches to mRNA strand

  molecule carrying amino acid binds to start codon
• 2nd stage of translation

  as there is still room in the ribosome, another molecule will join the first molecule

  ribosome joins amino acid together via condensation reaction- forms a peptide bond
• 3rd stage of translation

  ribosome moves to next codon, forcing first molecule to detach from start codon and the amino acid (amino acid is still attached to other amino acid with peptide bond)

  there is room for another amino acid carrying molecule to join on
• 4th stage of transpiration

  ribosome moves along the strand until chain is complete

  once finished, everything detaches from each other

  the new polypeptide chain can perform its function or can fold for a secondary structure
• start codons


  ribosome starts translation when 3 specific bases (AUG) is identified
• stop codons


  translation stops when ribosome reaches this
• rRNA 


  rRNA combines with other proteins to form ribosomes
• functional RNA


  tRNA and ribosomal RNA

  both produced by transcription- do not get translated

  is coded for by genes