MCG - Gene Products and Protein Synthesis Flashcards
What is the start codon (in amino acids)?
AUG (which codes for methionine - all proteins start with methionine).
What are the 3 stop codon (in amino acids)?
UAA
UAG
UGA
What is a reading frame?
The genetic code is read in triplets.
The position in which you begin to read the code will determine which triplets are produced.
Each possible grouping of triplets is called a ‘reading frame’.
As a recap, how does RNA differ from DNA?
RNA is single-stranded, contains U instead of T and ribose instead of deoxyribose.
As a recap, what are the three types of RNA involved in translation, and what are their functions?
- mRNA: Copied from DNA. Encodes proteins.
- rRNA: Structural and enzymatic component of the ribosome.
- tRNA: Delivers amino acids to the ribosome.
Describe the tRNA molecule.
Transfer RNA (tRNA) is the adaptor molecule that links an mRNA codon with a specific amino acid. It uses an anticodon to link them.
It has a clover leaf structure. It has 75-90 nucleotides, and extensive internal base pairing.
It contains unusual bases, and has a CCA-OH sequence at the 3’ end.
It also has an anticodon on the central loop.
How does a tRNA molecule work (by explaining the anticodon)?
The anticodon is a triplet of bases that are complementary to the codon.
These bases are unpaired and available for hydrogen bonding.
We have 61 amino acid codons but less than 61 tRNA molecules, therefore some must recognise more than 1 codon.
‘Wobble pairing’ allows this to happen.
What is ‘wobble pairing’?
Often first 2 letters in codon are the same, third base can vary.
The third position less critical and can follow non-Watson-
Crick base pairing between mRNA and tRNA.
This allows a single tRNA species to recognize more than one codon.
Note: only tryptophan and methionine are encoded by single codon.
What are charge tRNAs, and how are the created?
A ‘charged’ tRNA has an amino acid attached at the 3’ end via an ester linkage. It uses the hydrolysis of ATP to create this high energy bond.
Each tRNA can accept only the single amino acid that is appropriate for its anticodon sequence.
Specific aminoacyl-tRNA-synthetases ‘load’ tRNA molecules with amino acids.
amino acid + tRNA + ATP → aminoacyl-tRNA + PPi + AMP
Describe the different sites of a ribosome.
There are 3 sites: A, P and E sites.
A is where we get the amino acid.
P site if where the nascent polypeptide is formed.
E is where the tRNA has to leave the ribosome.
Where are polyribosomes in eukaryotes?
In eukaryotes, polyribosomes are free in the cytoplasm or can be bound to rough endoplasmic reticulum (RER).
What are the 3 steps of translation?
- initiation
- elongation
- termination
Describe the process of translation initiation.
The initiation factors (IFs) IF1 and IF3 bind the 30S subunit.
Together, this complex binds mRNA.
The fMet-tRNAfMet in complex with IF2-GTP enters the P site.
16S rRNA binds to the Shine-Dalgarno sequence in the mRNA to line up fMet-tRNAfMet with AUG start codon.
The Shine-Dalgarno sequence is in the 5’ UTR of the mRNA.
The large 50S subunit binds, and is accompanied by hydrolysis of GTP.
The GDP + Pi, and IFs (1,2,3) released.
Describe the process of translation elongation.
The next aminoacyl tRNA binds to elongation factor EF-Tu GTP, and enters A site in the ribosome.
If the anticodon of the incoming tRNA is complementary to the codon, then hydrolysis of GTP takes place, and EF-Tu GDP + Pi are released.
The translocation of the ribosome occurs with hydrolysis of the GTP bound to EF-G. The A site is now free again.
The discharged tRNA is released from the E site.
What is the peptidyl transferase process.
The protein is synthesized by ‘lifting’ the incomplete polypeptide, and placing the incoming (‘new’) amino acid underneath.
In chemical terms, the free –NH2 of the incoming amino acid attacks the carbonyl carbon of the previous amino acid to form the peptide bond.
