protein synthesis

• mRNA:

  • linear
  • has no hydrogen bonds
  • has codons
  • different mRNA's have different lengths
  • has more nucleotides
• tRNA:

  • clover leaf shaped
  • has hydrogen bonds
  • has anticodons
  • all tRNA's are the same length
  • has less nucleotides
• protein synthesis: transcription to form mRNA

  1. start of a gene is marked by a specific sequence of DNA bases called the promoter
  2. RNA polymerase breaks the hydrogen bonds between base pairs to separate the two DNA strands
  3. only one strand acts as a template
  4. free RNA nucleotides bind to the exposed bases on the template strand to form complimentary base pairs
  5. uracil base pairs with adenine, replacing thymine
  6. RNA polymerase joins adjacent RNA nucleotides by phosphodiester bonds, this requires energy from ATP
  7. pre-mRNA is spliced to form mRNA
• protein synthesis: translation to form polypeptide

  1. mRNA attaches to ribosomes
  2. ribosomes move to find the start codon on mRNA
  3. tRNA anticodons bind to complimentary mRNA codons
  4. tRNA brings a specific amino acid
  5. amino acids are joined together by peptide bonds
  6. tRNA is released after amino acid is joined to polypeptide
  7. the ribosome moves along the mRNA to the next codon to form the polypeptide
• genetic code:

  main features of the genetic code are:

  1. universal
  2. degenerate
  3. non-overlapping
• universal - all living organisms share the same codons for the same amino acids
• degenerate - more than one codon codes for a single amino acid
• non-overlapping - each base is used in only one codon
• ribosomes are made from proteins and ribosomal RNA
• proteome - full range of different proteins made by a cell
• RNA polymerase:

  • breaks hydrogen bonds to separate the 2 DNA strands
  • joins adjacent nucleotides by forming phosphodiester bonds
• when RNA polymerase reaches the STOP codon it stops making mRNA
• mRNA moves out the nucleus via nuclear pores into the cytoplasm to attach to a ribosome for translation
• post-transcriptional modification: splicing

  1. pre-mRNA is an exact copy of the gene so contains exons and introns
  2. during splicing, the introns are cut out and the remaining exons join together
  3. the introns are broken down and the nucleotides are recycled
  4. mRNA is formed and is smaller than the original gene
• prokaryotic genes do not contain introns so DNA is transcribed straight into mRNA
• comparison of DNA replication & transcription: similarities

  • in both, hydrogen bonds are broken between base pairs to separate the DNA strands
  • in both, DNA acts as a template for complementary base pairing
  • both use polymerase enzymes
• comparison of DNA replication & transcription: differences

  • both strands act as as a template in DNA replication but only one strand acts as a template in transcription
  • DNA replication uses DNA polymerase but transcription uses RNA polymerase
  • thymine base in DNA replication is replaced by uracil base in transcription
  • all DNA is copied is DNA replication but only the gene is copied in transcription
• a polyribosome is a group of ribosomes all attached to one piece of mRNA
• tRNA charging:

  1. amino acyl enzyme has an amino acid site, ATP site and tRNA site
  2. a specific amino acid and a ATP molecule bind to the enzyme
  3. ATP hydrolyses to form AMP and 2 phosphates are released
  4. the specific tRNA with the complimentary anti-codon binds to the tRNA site
  5. through induced fit the amino acid is attached to the tRNA and the AMP is released
  6. the charged tRNA is released
• post-translational modification:

  • chain cutting (using enzymes)
  • adding sugars or lipids
• tRNA in translation:

  • anticodons bind to complimentary mRNA codons
  • brings a specific amino acid
  • to the ribosome
  • forms the correct sequence of amino acids
• eukaryotic genes contain intron and exons

  prokaryotic genes contain only exons