Protein Synthesis

• Protein synthesis occurs in 2 stages

  • Transcription - DNA transcribed (makes a copy (template) of DNA called mRNA in nucleus) so mRNA molecule produced
  • Translation - mRNA translated (mRNA read by ribosomes to produce polypeptide chain in cytoplasm) so amino acid sequence produced
  • mRNA = single stranded molecule made up of many RNA nucleotides joined together
  • Role = to carry information encoded in DNA from nucleus to site of translation on ribosomes
• mRNA VS tRNA

  mRNA = messenger RNA

  • Made in nucleus during transcription
  • 3 adjacent bases - codon
  • Carries genetic code from DNA in nucleus to cytoplasm where it's used to make a protein during translation

  tRNA = transfer RNA

  • Found in cytoplasm
  • Has an amino acid binding site at one end and a sequence of 3 bases at other end - anticodon
  • Carries amino acids that are used to make proteins to ribosomes during translation
• • DNA molecules are found in nucleus of cells but ribosomes (responsible for protein synthesis) found in cytoplasm
  • DNA too large to move out of nucleus so a section is copied into mRNA - transcription
  • mRNA leaves nucleus and joins with ribosome in cytoplasm, where it can be used to synthesise a protein - translation
• Transcription

  1. In nucleus, enzyme RNA polymerase attaches to DNA double helix at beginning of a gene (start codon)

  2. This and DNA helicase causes hydrogen bonds

  (between complementary base pairings) to break

  separating strands so part of DNA molecule unwinds
• Transcription

  
  

  3. This exposes gene to be transcribed. Sequence on template strand used as template to make RNA copy - complimentary copy of code from gene made - mRNA molecule produced by free mRNA nucleotides in nucleoplasm

  
  

  4. RNA polymerase lines up free activated RNA nucleotides along template strand. They pair up using hydrogen bonds to complementary bases on exposed strand of unzipped DNA molecule
• Transcription

  
  

  5. Once RNA mono-nucleotides paired up with specific bases on DNA strand, they're joined together by RNA polymerase, forming mRNA molecule

  • Sugar phosphate groups of RNA nucleotides joined together by phosphodiester bonds, through condensation reaction catalysed by RNA polymerase to form sugar-phosphate backbone of mRNA molecule

  6. Once gene has been transcribed and mRNA molecule complete, RNA polymerase passes along DNA, separating strands and assembling mRNA strand
• Transcription

  
  

  7. Hydrogen bonds between DNA and mRNA strands break and double stranded DNA molecule reforms

  
  

  8. When RNA polymerase reaches stop codon, it stops making mRNA and detaches from DNA by breaking hydrogen bonds

  
  

  9. Completed mRNA molecule leaves nucleus through pore in nuclear envelope and into cytoplasm where it attaches to a ribosome
• Anti-sense and sense strands

  • In transcription, free RNA nucleotides pair up with exposed bases on DNA molecule
  • They only pair with bases on one strand of DNA molecule - called Anti-sense/template strands + used to produce mRNA molecule. Other strand = sense/coding strand
  • RNA polymerase moves along template strand in 3' to 5' direction so mRNA molecule will grow in 5' to 3' direction
  • mRNA formed by complementary pairings with DNA template strand so mRNA molecule contains exact same sequence of nucleotides as DNA coding strand (except mRNA will have uracil instead of thymine)
• Translation

  1. After transcription, mRNA moves into cytoplasm and attaches to ribosome. Cytoplasm contains free tRNA molecules

  • tRNA molecules have a triplet of unpaired bases at one end - anticodon, which are complementary to codons on mRNA molecule. They have a binding site at other end where one specific amino acid can attach

  2. tRNA molecules carry amino acids to ribosome to the mRNA molecule. They bind with their specific amino acids

  • Smaller subunit of ribosome binds mRNA, large subunit binds 2 tRNA
• Translation

  
  

  3. A tRNA molecule attaches itself to mRNA by complementary base pairing

  • Anticodon on each tRNA molecule pairs with complementary triplet on mRNA molecule - Start codon

  4. The second tRNA molecule attaches itself to next codon on mRNA in same way

  • 2 tRNA molecules fit onto ribosome at any one time, bringing amino acids they are each carrying to be side to side
• Translation

  
  

  5. Ribosome holds mRNA, tRNA ans amino acid in place while a peptide bond is formed by a condensation reaction between amino acids

  
  

  6. The first tRNA molecule moves away, leaving it's amino acid behind. The ribosome moves along to the next codon. A third tRNA molecule binds to codon on mRNA and it's amino acid binds to first 2, and second tRNA molecule moves away
• Translation

  
  

  7. Process continues until a stop codon on mRNA molecule is reached - acts as a signal for translation to stop and amino acid chain coded for mRNA molecule is complete

  
  

  8. Chain of linked amino acids forms final polypeptide which then moves away from ribosome - complete