W8.2_Protein Synthesis

Question 1

Briefly explain what the Central Dogma is.

Answer 1

• DNA (replication) -transcription-> RNA -translation-> Protein
• 20 amino acids -> different combinations -> different proteins

Question 2

What are the other names for template strand and mRNA steand? Briefly explain transcription and why it is needed.

Answer 2

• Template strand ≈ Non-coding strand
• mRNA strand ≈ Transcript strand
• Between non-coding and mRNA strands: complementary base pairing
• ∵ DNA has genetic code for protein synthesis
• ∵ DNA is double stranded, too large to leave nucleus
• ∵ RNA is single stranded, can leave the nucleus through nuclear pores
• ∴ There is a need to transcribe DNA -> mRNA

Question 3

Explain what translocation does. What are the purposes of different sites in ribosomes and tRNA?

Answer 3

• mRNA transported from nucleus to cytoplasm -> bind to ribosome (a lot in rough ER to reduce distance to grab mRNA) -> decoding of mRNA message into polypeptide chain
• Ribosomes separated into small and large subunits
• A site: aminoacyl receptor site
• P site: peptidyl site for bond formation
• E site: exit site
• tRNA: carry amino acids from cytoplasm to ribosome

Question 4

Describe in detail how mRNA is decoded into amino acid chains through tRNA.

Answer 4

• mRNA (codon) <-complementary base pairing-> tRNA (anti-codon) <-covalent bond to increase strength and lower disruption during environment changes-> amino acid
• Amino acids activated by addition of ATP to become amino acid-AMP complex -> bind to tRNA catalysed by specific tRNA synthetase

Question 5

Describe in detail on the three different steps of protein synthesis.

Answer 5

• Initiation
• Small subunit complexes with initiation factors
• Base pairing occurs between small subunit & upstream of start codon (Met) on mRNA
• Start codon of mRNA is positioned in P site and formyl-methionyl tRNA joins
• Large subunit joins, GTP hydrolysed into GDP, initiation factors leave ribosome
• Elongation
• Ribosome has 3 sites tRNA can bind, but only 2 can be occupied at one time
• Aided by additional/elongation factors
• Translocation (shifts in the direction of -APE->)
• Termination
• Synthesis continues until stop codon is reached
• Special release factor binds to A site
• Polypeptide in P site hydrolysed from its tRNA and leaves via exit tunnel, tRNA exits ribosome
• Catalysed by 2 GTP hydrolysis, ribosomal subunits dissociate, ready to be used again

Question 6

State the different drug targets that impacts the protein synthesis process. How can it relate to antibiotic resistance?

Answer 6

• Inhibit initiation, prevent large subunit binding, prevent tRNA bind to A site, inhibit peptide bond formation, block exit tunnel of ribosome
• Mutation of a single base in rRNA/single amino acid -> antibiotic resistance

Question 7

Explain the post-translational modification of polypeptide chains. What is its key role?

Answer 7

• Folding to proteins, enzymatic modification -> different versions of a protein (ex. lipidation, phosphorylation, glycosylation, acetylation)
• *The further the process, the higher the complexity
• Key role in activity/localisation/interaction of proteins, affect structure and function (ex. important in study of heart diseases, cancer, neurodegenerative diseases, diabetes)