Gene Expression and Protein Synthesis Part 3 - (Week 4)

Question 1

What happens to genetic information during translation?

Answer 1

• it flows from mRNA to proteins

Question 2

What three properties of RNA that enable it to function?

Answer 2

• can form 3-dimensional structure, due to the ability to base-pair itself
• some bases in RNA contain functional groups that may participate in catalysis
• RNA may hydrogen-bond with other nucleic acid molecules

Question 3

Other properties of RNA:

Answer 3

• single helix

- can make H-bonds between complementary base pairs

Question 4

What are some properties of tRNA?

Answer 4

• the molecules are not identical
• each carry a specific amino acid on one end and an anticodon on the other
• the anti codon base-pairs with a complementary codon on mRNA
• due to H-bonds, tRNA twists + folds into a 3-dimensional molecule ( L - shaped)

Question 5

What is translation?

Answer 5

• complex biochemical + mechanical process
• 2 steps
• correct match between a tRNA & an amino acid by the enzyme aminoacyl-tRNA synthetase
• correct match between the tRNA anticodon & an mRNA codon

Question 6

What is the wobble position?

Answer 6

• this is the 3rd nucleotide in a codon

Question 7

What are the characteristics of the wobble position?

Answer 7

• binding of a codon in an mRNA, the cognate tRNA = much looser
• permitting several types of non Watson- Crick base pairing to occur at the 3rd position
• redundancy of genetic code, where several different codons code for the same amino acid is specified

Question 8

What is aminoacyl-tRNA synthetase?

Answer 8

• an enzyme that attaches the appropriate amino acid onto its corresponding tRNA.
• by catalysing the transesterification of a specific cognate amino acid
• most living cells possess of set of 20 of this enzyme

Question 9

What are ribosomes and what do they do?

Answer 9

• they are organelles which facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis
• has 2 ribosomal units (l&s) - made of proteins & ribosomal RNA (rRNA)

Question 10

What happens in the subunits of ribosomes?

Answer 10

• (small subunit) - info processing & where mRNA attaches

- (large subunit) - containing catalytic site forming bonds between amino acids

Question 11

Ribosomes are universally conserved. True or False?

Answer 11

True - exist in bacteria, the common core, lower eukaryotes and higher eukaryotes

Question 12

What exists in the schemtic model of a ribosome?

Answer 12

• P-site (peptidyl- tRNA binding site), holding the tRNA that carries that growing polypeptide chain
• E site (exit site), where discharged tRNA leaves
• the exit tunnel
• A site (aminoacyl-tRNA binding site) , holding the tRNA that carries the next amino acid
• large subunit
• small subunit

Question 13

What are the stages of building a polypeptide of translation?

Answer 13

(IET)

• Initiation
• Elongation
• Termination

Question 14

What happens in initiation stage? (translation)

Answer 14

• brings together mRNA, tRNA, the 1st amino acid + 2 ribosomal subunits
• small ribosomal subunit binds with mRNA & a special initiator tRNA
• small subunit moves along the mRNA until it reaches the start codon (AUG)

Question 15

What do we need in order for initiation to start?

Answer 15

• a ribosome (small &large pieces)
• mRNA with instruction in building
• initiator tRNA carrying the first amino acid in the protein, (methionine)

Question 16

How does the methionine get itself to the ribosome?

Answer 16

• by attaching itself to the tRNA that contains the right anitcodon (e.g. UAC for AUG)
• complementary base paring

Question 17

What is elongation?

Answer 17

• addition of amino acids by the formation of peptide bonds

- creating a polypeptide

Question 18

What happens in the elongation step?

Answer 18

• each addition involves proteins called elongation factors
• 3 steps
• codon recognition
• peptide bond formation
• translocation
• energy expenditure occurs in the 1st & 3rd steps

Question 19

What happens in the termination step?

Answer 19

• a stop codon in the mRNA reaches the A site of the ribosome
• A site accepts a protein - release factor
• release factor causes the addition of a water molecule instead of an amino acid
• reaction releases polypeptide, + the translation assembly comes apart

Question 20

What happens at the very end of translation? (before the process starts again)

Answer 20

• equipment used in termination = very reusable
• after the small & large subunits separate from mRNA & each other, each element takes part in another round of translation