Lecture 6 Flashcards
Why are tRNA needed
mRNA cannot act as a physical template for amino acids therefore adapters are required to link mRNA and amino acids. These adapters are tRNA
tRNA structure
-Approx. 80 nucleotides in length
-Single stranded but base pairs from within the chain G-C, A-U
-Clover leaf structure further folds to make L-shaped molecule
-Anticodon is at one end - base pairs with codon
-Amino acid attachment site is the 3’ hydroxyl group at the end of the RNA chain
Attaching an amino acid to is tRNA
Carried out by acyl-tRNA synthetases
One for each of the 20 amino acids
There are binding sites for:
-specfic tRNA
-corresponding amino acid
-ATP
2-step process
1. ATP hydrolysed and amino acid joined to AMP
2. Correct tRNA binds and amino acid transferred from AMP to the tRNA
The ribosome
Composed of ribosomal RNA and proteins
Two subunits - large and small
Binds mRNA and amino acyl-tRNAa
Three tRNA binding sites:
-A (aminoacyl-tRNA binding site)
-P (peptidyl-tRNA binding site)
-E (exit site)
Catalyses stepwise formation of peptide bonds (amino acids added from N terminus to C terminus)
Moves in 5’-3’ direction along mRNA
By recgonising the correct start codon, ribosomes ensure correct reading frame is used
Basic three stages of protein synthesis
Intitation - small subunit binds mRNA and intiator amino acyl-tRNA then large subunit binds
Elongation - peptide bonds are formed as the ribosome moves along the mRNA
Termination - one of the three stop codons enters A-site and the completed protein is released
Initiation steps
- Small ribosomal subunit binds mRNA near its 5’ end
- Initiator tRNA binds to AUG start codon
- Large subunit binds so that the intiator tRNA fits into the P-site on the large subunit
-requires energy from GTP hydrolysis and proteins call intitation factors
The elongation cycle
- Incoming aminoacyl tRNA base pairs with codon in the A-site which requires hydrolysis of GTP
- Peptide bond formed between amino group of the new amino acid and th COOH group of the amino acid in the P-site which is catalysed by peptidyl transferase
- Growing polypeptide chain now in the A-site
- Translocation - tRNA in the P-site is ejected and the ribosome moves along the mRNA by precisely one codon which requires hydrolysis of GTP
- Growing chain now in the P-site and the A-site is free to accept the next incoming aminoacyl tRNA
Termination
- Stop codon in A-site
- There are no tRNAs for stop codon
- Release factor enters A-site instead of amino acyl tRNA
- Water added to end of the polypeptide chain
- Completed polypeptide released from tRNA in P-site
- Ribosome dissociates, 2xGTP hydrolysed
Mutations in somatic tissues
-Not passed onto offspring
-Passed on to all the cells descended from the original mutant
-~85% cancers caused by somatic mutations
Mutations in germ line tissues
-Passed onto offspring
-Cause of inherited genetic diseases
-Raw material from which natural selection provides evolutionary chainge
Causes of mutations
Spontaneous (through DNA replication)
Induced (mutagens)
-Chemical (base analogues, modifying agents, intercalating agents)
-Physical (ionising radiation/ultraviolet)
Three classes of chemical agents that act on DNA and can result in mutations
- Chemicals that resemble DNA bases but pair incorrectly when incorporated in DNA (base analogues)
2a. Chemicals that remove the amino group from adenine or cytosine eg nitrous acid/nitrite
2b. Chemicals that add hydrocarbon groups to nucleotide bases
