Topic 7.3

Question 1

Initiation stage in translation

Answer 1

• the small ribosomal subunit binds to the 5’ end of the mRNA strand & slides along in a 5’-3’ direction until it reaches the start codon on the mRNA strand (AUG)
• the appropriate tRNA molecule bind to the codon on the mRNA strand via its anticodon(UAC) (according to complementary base pairing)
• the large ribosomal subunit aligns itself to the tRNA molecule at the P site and forms a complex with the small subunit

Question 2

What does initiation involve?

Answer 2

The 1st stage of translation involves the assembly of the three components that carry out the process (mRNA, tRNA, ribosome)

Question 3

What are the stages of transcription

Answer 3

• initiation
• elongation
• translocation
• termination

Question 4

Elongation

Answer 4

• the anticodon of incoming tRNA pairs w its complementary mRNA codon at the amino acid site(A site) of the ribosome
• the amino acid at the P site is covalently attached to the amino acid at the A site via a peptide bond (condensation reaction)
• the tRNA in the P site is now deacylated (no amino acid), while the tRNA in the A site carries the peptide chain

Question 5

Translocation

Answer 5

• The ribosome moves along the mRNA strand by one codon position (in a 5’→ 3’direction)
• The deacylated tRNA moves into the E site and is released, while the tRNA carrying the peptide chain moves to the P site
• Another tRNA molecules attaches to the next codon in the now unoccupied A site and the process is repeated

Question 6

Termination

Answer 6

• involves the disassembly of the components and the release of a polypeptide chain
• Elongation and translocation continue in a repeating cycle until the ribosome reaches a stop codon
• These codons do not recruit a tRNA molecule, but instead recruit a release factor that signals for translation to stop
• The polypeptide is released and the ribosome disassembles back into its two independent subunits

Question 7

Primary protein structure

Answer 7

• theorder / sequence of amino acids which comprise the polypeptide chain
• formed bycovalent peptide bondsbetween the amine and carboxyl groups ofadjacent amino acids
• controls all subsequent levels of protein organisation because it determines the nature of the interactions between R groups of different amino acids

Question 8

Secondary protein structure

Answer 8

• the way a polypeptide folds in arepeating arrangement to formα-helices andβ-pleated sheets
  
  • folding is a result ofhydrogen bondingbetween the amine & carboxyl groups ofnon-adjacentamino acids
• Sequences that do not form either an alpha helix or beta-pleated sheet will exist as a random coil
• Secondary structure provides the polypeptide chain with a level of mechanical stability (due to the presence of hydrogen bonds)

Question 9

Tertiary protein structure

Answer 9

• the way the polypeptide chain coils and turns to form a complex molecular shape (i.e. the3D shape)
• caused byinteractions between R groups; including H-bonds, disulfide bridges, ionic bonds and hydrophobic interactions
• Relative amino acid positions are important (e.g. non-polar amino acids usually avoid exposure to aqueous solutions)
• Tertiary structure may be important for the function of the protein (e.g. specificity of active site in enzymes)

Question 10

Quaternary protein structures

Answer 10

• Multiple polypeptidesor prosthetic groupsmay interact to form a single, larger, biologically active protein (quaternary structure)
  
  • prosthetic group → an inorganic compound involved in protein structure or function (e.g. the heme group in haemoglobin)
• A protein containing a prosthetic group is called a conjugated protein
• Quaternary structures may be held together by a variety of bonds (similar to tertiary structure)

Question 11

What is a polysome?

Answer 11

a group of two or more ribosomes translating an mRNA sequence simultaneously

Question 12

How are differences in polypeptide length affected by the polysome cluster?

Answer 12

Ribosomes located at the 3’-end of the polysome cluster will have longer polypeptide chains than those at the 5’-end

Question 13

What is protein destination determined by?

Answer 13

by the presence or absence of an initial signal sequence on a nascent polypeptide chain