protein synthesis

• what does it mean that the genetic code is universal

  the same base triplets code for the same amino acids in all living organisms
• what does it mean that the genetic code is non-overlapping

  the DNA bases in a sequence can only be used once
• what does it mean that the genetic code is degenerate

  many amino acids can be coded for by more than one codon
• what are the two stages of protein synthesis

  transcription and translation
• why is DNA transcribed into RNA

  a DNA molecule is too large to leave the nucleus
• describe the stages of transcription

  1. RNA helicase breaks the hydrogen bonds and separates the DNA strands

  2. free nucleotides will pair up with the bases on the template strand

  3. RNA polymerase catalyses the formation of phosphodiester bonds between the RNA nucleotides

  4. when the mRNA reaches a stop codon it detaches from the DNA template and leaves the nucleus through the nuclear pore and attaches to the ribosome. The DNA double helix reforms
• what is the sense strand

  it runs from 5' to 3' and codes for the protein
• what is the antisense strand

  it runs from 3' to 5' and is a complementary copy of the sense strand and so acts as a template strand for the mRNA
• describe the stages of translation

  1. the mRNA binds to the small subunit of the ribosome and tRNA molecules carry specific amino acids to the ribosome

  2. a tRNA molecule with an anticodon that's complementary to the start codon on the mRNA attaches itself to the mRNA by complementary base pairing

  3. rRNA in the ribosome catalyses the formation of a peptide bond between the amino acids attached to the tRNAs, the first tRNA moves away leaving its amino acid behind

  4. this process continues producing a polypeptide chain until there's a stop codon on the mRNA molecule

  5. the polypeptide chain moves away from the ribosome and translation is complete
• what do mRNA, rRNA and tRNA stand for

  messenger RNA, ribosomal RNA and transfer RNA
• What are ribosomes composed of?

  two subunits: one large and one small, they are composed of protein and rRNA
• what is tRNA composed of

  a strand of RNA folded so that the anticodon is at one end of the molecule