DNA and protein synthesis

• Describe structural features of mRNA

  -Single-stranded

  -Linear in shape

  -Each 3 bases is a codon

  -4 bases = adenine, uracil, guanine, cytosine
• Describe structural features of tRNA

  -Single-stranded, but some complementary base pairing with H bonds when it folds

  -'clover leaf' shape

  -Contains an anti codon complementary to an mRNA codon

  -4 bases = adenine, uracil, guanine, cytosine

  -Contains an amino acid bindsite, so can transport amino acids when ATP hydrolysis is occuring
• Describe transcription

  -DNA helicase breaks H bonds between complementary bases

  -One strand (non-coding) is used as the template

  -Free RNA nucleotides are attracted and form H bonds between complementary bases

  -RNA polymerase joins RNA nucleotides with phosphodiester bonds until a stop codon is reached

  -In eukaryotes, introns in pre-mRNA are removed and exons are spliced, forming mature mRNA. In prokaryotes, mature mRNA is formed instantly as no introns are present
• Describe translation

  -mRNA leaves the nucleus and binds to the small subunit on a ribosome

  -A tRNA molecule, with an anticodon complementary to the first mRNA codon binds to the large subunit of the ribosome

  -A ribosome can hold 2 tRNAs at once, so a 2nd tRNA with a complementary anticodon to the second mRNA attaches

  -Amino acids can join by peptide bonds. The 1st tRNA detaches, leaving behind its amino acid

  -tRNA attaches to another amino acid in the cytoplasm

  -RIbosome keeps moving down the mRNA. This continues until the stop codon is reached and the ribosome releases the mRNA