BIOCH Y1 S1: Protein Synthesis

Question 1

3 types of RNA

Answer 1

• mRNA: codes for proteins
• rRNA: contributes to structure and function of ribosomes
• tRNA: brings the correct and specific amino acid to a ribosome to allow the polypeptide chain to grow

Question 2

transcription

Answer 2

• initiation: RNA polymerase attaches to promoter region and DNA unwinds
• elongation: DNA TEMPLATE strand copied into pre-mRNA from 5’-3’ using RNA polymerase and complementary base pairing (the coding strand is the complementary DNA sequence)
• termination: stop codon is encountered > binds a protein release factor
• everything dissociates, DNA rewinds
• pre-mRNA undergoes RNA processing to form mature mRNA
  -mRNA leaves the nucleus and goes to a ribosome for translation

Question 3

RNA processing

Answer 3

• splicing: introns removed by spliceosomes and exons spliced back together (alternative splicing): multiple proteins from same gene
• 5’ methyl cap and 3’ poly-A-tail added > affect the half life of the mRNA outside the nucleus and prevents degradation by exonucleases

Question 4

types of RNA polymerase in prokaryotes

Answer 4

• only one type of RNA polymerase that makes mRNA, rRNA and tRNA

Question 5

types of RNA polymerase in eukaryotes

Answer 5

• eukaryotes have RNA polymerase I, II and III
• I makes rRNA
• II makes mRNA
• III makes tRNA

Question 6

is RNA proofread like DNA?

Answer 6

no because it is supposed to be transient (degraded after use) whereas DNA is storing genetic code for several cell generations so needs to be accurate and consistent

Question 7

2 characteristics of the genetic code

Answer 7

• near universal: points to a common ancestor for all life
• redundant: several diff codons code for the same AA
  but NOT ambiguous: a codon will always code for an amino acid

Question 8

difference between transcription and translation in prokaryotes and eukaryotes

Answer 8

• spatial and temporal separation in eukaryotes (transcription in nucleus then translation in ribosome) whereas both occur simultaneously in cytoplasm (coupled) in prokaryotes > more room for error

Question 9

subunit structure of the ribosome

Answer 9

• 40s subunit (S for small): initiation of translation
• 60s subunit (L for large): joins later on in translation for tRNA binding and polymerisation
• 2 subunits are separate when not translating
• A, P, E sites

Question 10

translation

Answer 10

• activation: aninoacyl-tRNA synthetases link a specific AA to the 3’ end of the tRNA, forming charged tRNAs
• initiation: start codon found
• elongation: A, P, E sites
• termination: stop codon found but no S&C tRNA so everything dissociates

Question 11

3 sites on a ribosome where tRNA traverses in order

Answer 11

• A (aminoacyl) - charged tRNA anticodon binds to mRNA codon
• P (peptidyl) - tRNA adds AA to growing polypeptide chain
• E (exit) - where the tRNA (w/o its AA) goes before leaving the ribosome

Question 12

does the redundancy of the DNA code affect protein synthesis?

Answer 12

• no b/c of the Wobble hypothesis
• bases @ positions 1 and 2 are generally consistent between codons
• 3rd base forms non-std. base pairing w/ the 3rd base in the anticodon (wobble)

Question 13

post-translational modification

Answer 13

• folding: folds into 3D shape, assisted by chaperone proteins
• addressing: address label on protein tells it which organelle to go to (or remain in cytoplasm if no label)
• sequestration (part of addressing): protein is sequestered in a compartment and has no effect. When conditions change, protein migrates to correct compartment and completes function
• chemical modification: glycosylation, proteolysis, ubiquitination

Question 14

glycosylation

Answer 14

• sugar group added
• role in protein trafficking
• prevents degradation via proteases

Question 15

proteolysis

Answer 15

• unneeded parts of a protein are cleaved out to turn it into its active form
• e.g. preproinsulin > proinsulin > insulin

Question 16

ubiquitination

Answer 16

• ubiquitin group added to protein
• other ubiquitin molecules attach to primary one
• proteasome binds to ubiquitin
• proteases digest the protein

Question 17

how to make transcription faster or slower

Answer 17

• general transcription factors: proteins that bind to promoter and initiate transcription
• activators: bind to enhancer region and speed up transcription
• repressors: bind to silencer region and slow down transcription

Question 18

methylation

Answer 18

• add methyl group (CH3) to histone tails > 5’ position of cytosine in the CpG sequence
• +ve charge of lysine maintained
• DNA wraps tighter around histones (heterochromatin)
• less accessible for transcription

Question 19

acetylation

Answer 19

• add acetyl group (C2H) to histone tails
• +ve charge of lysine neutralised
• DNA (-ve charged) no longer as strongly attracted to lysine b/c less diff in charge
• DNA wraps looser around histones (euchromatin)
• more accessible for transcription

Question 20

explain how miRNA inhibits translation

Answer 20

• miRNA precursor transcribed from DNA and cleaved by Dicer enzyme to form miRNA (double-stranded)
• miRNA loaded onto RNA-induced silencing complex (RISC) and converted into single-stranded RNA
• RISC scans mRNA and binds to S&C target sequence, preventing translation