8.5 - Protein synthesis: translation

Question 1

is tRNA specific to amino acids

Answer 1

• yes
• each type of tRNA has a specific anticodon that attaches to a specific amino
  –> therefore each amino acid has one or more tRNA molecule, with its own anticodon of bases

Question 2

what happens once mRNA has passed out of the nuclear pore

Answer 2

it determines the synthesis of a polypeptide

Question 3

describe the steps to create a polypeptide

Answer 3

• a ribosome attaches to the starting codon at one end of he mRNA molecule (mRNA binds to small subunit)
• tRNA molecule with complementary anticodon sequence moves to ribosome and pairs up with the codon on the mRNA (this tRNA carries specific amino acid)
• another tRNA with a complementary anticodon sequence pairs with the next codon along (this tRNA carries another specific amino acid)
• the ribosome moves along the mRNA, bringing together the two tRNA molecules at any one time (each paring up with the corresponding two codons on the mRNA)
• the 2 specific amino acids on the tRNAs are joined by a peptide bond using and enzyme and ATP (for required energy)
• the ribosome move onto the 3rd codon in the sequence of the mRNA, thereby linking the 2 specific amino acids on the 2nd and 3rd tRNA molecules
• as this happens, the 1st tRNA is released from its amino acid and is free to collect another amino acid from he amino acid pool in the cell
• the process continues this way, with up to 15 amino acids being added each second, until a polypeptide chain is built up
• up to 50 ribosomes can follow this initial ribosome, therefore assembling their own polypeptide chains simultaneously
• the synthesis of a polypeptide continues until a ribosome reaches a stop codon. then the ribosome, mRNA and the last tRNA molecule all separate and the polypeptide chain is complete

Question 4

what determines which proteins the cell manufactures

Answer 4

• the DNA sequence of triplets that make up a gene determines the sequence of codons on the mRNA
• the sequence of codons on mRNA then determine the order in which the tRNA molecules line up
• they, in turn, determine the sequence of amino acids in the polypeptide.
• in this way genes precisely determine which proteins a cell manufactures
• as many of these proteins are enzymes = genes effectively control the activities of cells

Question 5

what’s a functional protein

Answer 5

• a single polypeptide chain (or sometimes a few) is a functional protein

Question 6

what usually happens next to a polypeptide depending upon the protein being made

Answer 6

• the polypeptide is coiled/folded = producing its secondary structure
• the secondary structure is folded, producing the tertiary structure
• different polypeptide chains, along with any non-protein groups, are linked to form the quaternary structure