2.7 + 7.3 Translation

Question 1

Define codon

Answer 1

CODON: a triplet of bases of mRNA which codes for one amino acid in a polypeptide chain, the order of codons determines the order of amino acids in polypeptide chain

64 possible codons = 4³ = 61 for am a + START + 2 STOP

Question 2

Define genetic code

Answer 2

GENETIC CODE: set of rules which are encoded within mRNA and converted into amino acids and polypeptides by living cells

Question 3

START, STOP codons

Answer 3

Coding starts with START codon (Met) and terminates at STOP codon (no am a)

START: AUG (Met)

Question 4

Define translation

Answer 4

TRANSLATION: process of protein synthesis in which genetic information encoded in mRNA is translated into a sequence of am a in a polypeptide chain

Question 5

Translation sequence

Answer 5

1. INITIATION: In cytoplasm smaller ribosome subunit binds to 5’ end of mRNA - moves along until START codon reached - activated tRNA with Met and complementary anticodon binds to START codon - larger ribosome subunits aligns at P site - forms a complex
2. ELONGATION: another tRNA binds to A site - tRNA at P site deacylated (no am a) - moves into E site - released - am a covalently attached by peptide bond to am a in A site - polypeptide held by tRNA in A site ribosome moves in triplets - forms polypeptide chain

Translocation: ribosome moves 5’ to 3’ end mRNA in triplets

3. TERMINATION: elongation is completed in repeating cycles until STOP codon is read (no tRNA) - disassembly of ribosome releases polypeptide

Question 6

Termination diagram

Answer 6

Question 7

tRNA activation

Answer 7

tRNA activation - am a attached

In cytoplasm tRNA activating enzyme binds ATP to specific am a - complex formed - specific tRNA bound to am a by an energetic bond the enzyme (E used to form peptide bonds) - activated tRNA released

Question 8

Explain polysomes

Answer 8

POLYSOME (polyribosome): a group of two or more ribosomes translating an mRNA into polypeptides simultaneously

Question 9

Translation in eukaryotes vs prokaryotes

Answer 9

EUARYOTES: DNA and mRNA are separated by nucleus, mRNA must be transported out of nucleus - post-transcriptional modifications needed

PROKARYOTES: no nucleus - mRNA can be started to be translated while it is still transcribed, possible because both happen 5’ to 3’ end

Question 10

Proteins synthesised by free vs bound ER ribosomes

Answer 10

FREE ribosomes: proteins for cell use - intracellular

BOUND ER ribosomes: proteins for secretion out of cell - COTRANSLATIONAL IMPORT

Protein destination is determined by initial signal sequence - binds initial signal recognition particle (SRP) - halts translation - SRP-ribosome complex docs at receptor on ER membrane - rER forms - translation re-initiated - polypeptide continues to grow into rER lumen - SRP recycled

Protein from rER transported into Golgi/lysosome