Eukaryotic translation

Question 1

What are polysomes?

Answer 1

Ribosomes that work together as a collective

Question 2

What are ribosomes made up of?

Answer 2

2 subunits

bacteria: 30s & 50s = 70s
eukaryotes: 40s & 60s =80s

Question 3

In eukaryotes what is the small subunit made up of?

Answer 3

1 rRNA molecule and many proteins

Question 4

S = svedberg units

Answer 4

Heavier molecules/ribosomes sink more quickly and travel further so get a higher rating

Question 5

In centrifugation how do different bands form

Answer 5

Sucrose solution - based on how quickly the macromolecules move which depends on how big/heavy they are and due to their shape

Question 6

What is the active site of a ribosome?

Answer 6

Catalytic site (where the peptide bond in the new protein is formed) made up of rRNA

Question 7

What is a ribozyme?

Answer 7

ribosomes that are an RNA enzyme

Question 8

Transfer RNA

Answer 8

Bring amino acids to the ribosome to be added to the new protein
There is a tRNA for each codon as they must recognise the correct amino acid and codon

Question 9

Describe E.coli 16s Ribosomal RNA

Answer 9

Small subunit of the ribosome. Single RNA molecule made up of a 5’ domain, central domain, 3’ major domain and 3’ minor domain

Question 10

Anticodons

Answer 10

3 bases complementary to the codon of the mRNA

Question 11

Aminoacyl tRNA synthetase

Answer 11

Joins a specific amino acid to the tRNA
Specific amino acyl tNRAs are assembled for each different amino acid
It is accurate and corrects its own mistakes

Question 12

Describe the structure of tRNA

Answer 12

acceptor arm 
T psi C arm 
Variable loop
Anticodon arm 
D arm

Question 13

T psi C arm

Answer 13

Thymine pseudo uridine cytosine arm

Question 14

The variable loop is…

Answer 14

varied in length and may not be a complete loop

Question 15

What is the acceptor arm of the tRNA?

Answer 15

The point at which the amino acid will attach. The specificity is achieved through modification to bases

Question 16

What is the step by step process to form an aminoacyl- tRNA?

Answer 16

Amino acid + ATP => Amino acid-AMP + pyrophosphate

Amino acid-AMP + tRNA => aminoacyl-tRNA + AMP

Question 17

What is the name of the process of sticking an amino acid on a tRNA?

Answer 17

amino-acyl tRNA charging

Question 18

What does every tRNA have?

Answer 18

its own synthetase to load the correct amino acid

Question 19

What are the 3 steps for translation?

Answer 19

Initiation
Elongation
Termination

Question 20

What is the initiator tRNA?

Answer 20

A special tRNA for the first amino acid (methionine)
Eukaryotes use: met-tRNAi^met
Bacteria use: met-tRNA^fmet

Question 21

What is used to locate where the small subunit of the ribosome needs to bind?

Answer 21

Shine-Dalgarno sequence

3-10 subunits upstream of the first translated codon

Question 22

What are the steps to initiation?

Answer 22

mRNA with start/initiation codon
(+small subunit. IF-3)
small subinit and IF-3 locate the AUG & sit there enveloping the AUG initiation codon. IF-3 keeps the large subunit out the way.
(+Initiator tRNA, IF-2, GTP)
IF-2 ensures that the ribosome is lined up with the mRNA and the mRNA is lined up with the initiator tRNA.
(+IF-1)
large subunit binds, all initiation factors released. Assembly complete.
Small and large subunits form on active site

Question 23

How is initiation in eukaryotes different?

Answer 23

Small SUBUNIT, IFs, Met-tRNAi^met,
Locates the start codon through the kozak sequence
more IFs
Poly(A)tail assists small subunit binding

Question 24

What do tRNAs need to attach to?

Answer 24

binding sites on ribosomes

Question 25

E P A

Answer 25

E: exit site, tRNA ejected after delivery amino acid P: peptidyl-tRNA site, second binding site for tRNA in the ribosome A: aminoacyl-tRNA site, entry

Question 26

Where does the initiator tRNA go?

Answer 26

Straight into the P site

Question 27

What is elongation?

Answer 27

Adding to the chain beyond the first initial amino acid

Question 28

What are the steps to elongation?

Answer 28

The first tRNA is attached to the AUG initiation codon carrying formyl methionine (P site). The A site is empty aminoacyl-tRNAs are trying to fit onto the next codon. tRNA binds bringing elongation factor (EF) 1A bound to GTP. EF-1A-GTP. tRNA in A site. GTP must be hydrolysed to provide energy to form the peptide bond between adjacent amin oacids. EF-1A-GTP

Question 29

Function of EF-1B

Answer 29

re-generates EF-1A after EF-1A has yielded the energy in its attached GTP

Question 30

Explain how translocation occurs in elongation

Answer 30

Two amino acids are still attached to their transfer tRNAs and attached to eachother by a peptide bond. EF-2-GTP provides energy to move the second amino acidand tRNA to the next position. The first amino acid is in the exit position and the enrgy from the hydrolysis of the GTP is used to dissolve the bond between the amino acid and tRNA. De-acylated tRNA is released with EF2 & GDP.

Question 31

Where does synthesis of the peptide bond occur?

Answer 31

In the P-site. Peptidyl transferase activity that comes from the tRNA catalyses joiningof the carboxyl group of one amino acid with the amino group of another amino acid in the A site.

Question 32

What are the steps in phase 3- termination

Answer 32

The stop codon enters the A site and does not signal for an aminoacyl-tRNA. (+RF-1, RF-3 OR RF-2 & RF-3) RF-1 & RF-2 minic tRNA molecules and they can bind to the A site. Once bound the last amino acid in the chain tries to make a peptide bond with the releasing factor but is unable to. Chain released from tRNA. RF-3 gets rid of RF-1/RF-2. Complete polypeptides. RF leaves. The 2 ribodomse subunits dissociate. Ribosome recycling factor RRF separates the ribosome subunits and separates it from the mRNA

Question 33

Why is RF-1 & RF-3 used OR RF-2 & RF-3?

Answer 33

RF-1 recognises UAG & UAA | RF-2 recognises UGA & UAA

Question 34

Quality control mechanisms

Answer 34

Errors in mRNA could lead to faulty proteins. mRNA surveillance helps stop cells making useless or toxic versions of a protein. The 2 mechanisms used are: Non-sense mediated decay (NMD)- elimination of mRNA with premature stop codons (Eukaryotes) tmRNA (prokaryotes)

Question 35

tmRNA

Answer 35

Ribosome stalls when the end of an mRNA lacking a termination codon is reached. tmRNA, which is charged with Ala, binds to the A site of the ribosome. Alanine is added to the polypeptide chain. The ribosome reads the mRNA and adds amino acids continuing translation. The amino acids encoded by the tmRNA are added to the polypeptide chain. A stop codon is then reached.

Question 36

How does tmRNA work?

Answer 36

It allows 10 amino acids in a polypeptide chain to be added and labelled as defective. These amino acids are a tag. This produces a labelled aberrant protein that the cell can recognise and target for degradation. The system may also detect the aberrant mRNA and destroy this.