Protein Synthesis Flashcards
When and who provided evidence that DNA carries genetic information?
1994
Avery
What happened in 1966
Nirenberg, Ochoa, and Khorona elucidate the genetic code
Give the rules of the genetic code
1) Three bases encode one amino acid = codons
2) The code is non-overlapping
3) The code is degenerate ue some amino acids are specified by more than one codon
How many codons are there which code for amino acids?
61 codons for 21 amino acids
The others are stop codons
How many possible reading frames are there for one peice of DNA?
3
What is the start codon and which amino acid does it code for?
AUG = methionine
What are the three stop codons?
UAA
UAG
UGA
What do the tRNAs bind to?
One end base pairs with the codon = the anticodon loop
The other end (3’) end carries the amino acid
What gives tRNA its structure?
Intermolecular base pairing
Although tRNAs have a similar structure what varies?
The nucleotides vary, even with in the double stranded regions
Describe the structure of the tRNA molecules
Have a D loop, Anticodon loop and T loop
How are tRNAs modified?
Psuedouridine
Dihydrouridine
There are over 50 modifictions of tRNAs. What are their roles?
They affect the accuracy with which the tRNA is attached to the correct amino acid
They can facilitate the recognition of the appropriate mRNA codon by the tRNA molecule
True or false - there is a 1:1 ratio of tRNA per codon
FALSE
What allows the same tRNA anticodon to bind to more than one codon?
Wobble base pairing at position 3
Give one way that a wobble is made by modification
Deamination of A creates an inosine which can pair to U, C or A
How many tRNAs do bacteria have?
31
Coupling of the amino acid to tRNA is achieved by what?
Aminoacyl-tRNA synthetases
What are the steps of coupling amino acid to tRNA?
1) The amino acid is firstly activated by the linkage of AMP to the carboxyl group = adenylated amino acid
2) AMP linked carboxyl group is transferred to the hydroxyl group on the 3’ tRNA forming an ester linkage
This forms aminoacyl-tRNA
Aminoacyl tRNA is also known as what and why?
Charged tRNA
Because the energy of ATP hydrolysis is still contained in the ester linkage
Translation of mRNA to amino acids requires how many adapters?
2
What are the two adapters used?
1) Synthetase that pairs the correct amino acid to the correct RNA
2) tRNA that pairs the correct codon to the correct amino acid within the ribosome
Which terminus are new proteins added on to?
The C terminus
What are the two subunits of ribosomes?
The large subunit
The small subunit
What is the role of the large subunit?
It catalyses polymerisation
What is the role of the small subunit?
Facilitates tRNA/ mRNA interaction
Why is the formation of each new peptide bond energetically favourable?
The growing C terminus has been activated by the covalent attachment of a tRNA molecule
What are the sites of the large ribosomal sub unit?
E
P
A
Where do charged tRNAs enter?
At the A site - they leave uncharged at the E site
How many tRNAs are in the ribosome at any one time?
2
Give the steps that occurs in translation at the ribosome
1) Charged tRNAs enter at the A site
2) Peptidyl transferase catalyses amino acid addition and conformational changes move the tRNAs the the E and P sites
3) Conformational changes move the small subunit three nucleotides a long
4) tRNA leaves the E site
What are elongation factors?
They help translation and improve accuracy
Once an anticodon is bound, how does EF-1 causes what two delays before peptidyl transferase can act?
1) First GT must be hydrolysed to GDP
2) Then it has to dissociate from the tRNA
What are elongation factors called in eukaryotes and bacteria?
Eukaryotes = EF-1 and EF -2 Bacteria = EF-Tu and EF-G
Why are the time lags important?
It gives time for incorrectly bound tRNAs to fall off
Some of the correct tRNAs may also fall off but at a slower rate
The hydrolysis of GTP occurs more rapidly of what has happened?
If the codon and anticodon are incorrectly matched
If synthesis occurs in the absence of EF-1 what happens?
There are more errors in the proteins sequence
What is a ribozyme?
An RNA that catalyses a reaction
Methionone tRNA assembles what?
The ribosome
Only which tRNA with EiF-2 can bind to which subunit alone?
Met tRNA
mRNA has a cap and a tail which are bound by what?
eIF-4G and eIF-4E
This forms a loop
The loop formation is a checkpoint for what?
Broken mRNA
How far apart are ribosomes spaced apart on a polysome?
80 nucleotides apart
Stop codons are recognised by what?
Release factors
What are release factors?
They look like charged tRNAs - molecular mimicry and enter the A site
This causes dissociation of the ribosome
When does protein folding begin?
Immediately after leaving the ribosome
Many proteins initially fold in to what?
The molten globule which is roughly the correct confirmation
Misfolded proteins can lead to aggregation? How?
They have exposed hydrophobic regions that can lead to aggregation
What are the two major classes of molecular chaperones?
hsp60
hsp70
Why are they called heat shock proteins?
Their expression is elevated when the temperature is raised above the normal level
Why do chaperones function during normal folding as well as when a cell has been heated?
Because at high temperatures the proteins denature
How does the hsp70 class work?
Directly on proteins as they exit the ribosome binding to exposed hydrophobic amino acids
What does the hsp60 family do?
Put misfolded proteins into isolation
How does the HSP60 family work?
1) The hydrophobic entrance binds to the protein partially unfolding it
2) The GroES cap then seals the protein inside for about 15 seconds to allow refolding
What is poluubiuitination?
Marks translation failures for destruction in the proteosome
How many newly synthesised proteins are immediately recycled?
As much as 1/3 synthesised proteins
Protein aggregated cause what and why?
Diseases and cell death
They are large and protease resistant, and can sometimes cause a chain reaction to misfold more proteins
Give examples of diseases associated with protein aggregates
CJD
Huntingtons
Alzheimers
