Genetic code

• Exons are coding sequences in pre-mRNA.
• What does the one gene-one polypeptide hypothesis state?

  One gene codes for one polypeptide
• Haemoglobin requires 4 different genes because it has 4 different polypeptide chains.
• Collagen has 3 identical alpha helices, so only one gene is needed.
• What is a gene?

  Section of DNA coding for a protein
• The genetic code is a triplet code, where 3 bases code for each amino acid.
• A codon is a triplet of bases in mRNA that codes for a particular amino acid or a punctuation signal.
• What are introns in pre-mRNA?

  Non-coding sequences
• Introns are cut out of pre-mRNA by enzymes called endonucleases
• What does it mean for the DNA code to be degenerate?

  More than one triplet encodes an amino acid
• Stop codons mark the end of translation in mRNA.
• The genetic code is universal because the same triplet codes for the same amino acid in all known organisms.
• Why is the genetic code described as non-overlapping?

  Each base occurs in only one triplet
• The genetic code determines the amino acid sequences in proteins.