protein synthesis & dna mutations

adapters in protein synthesis are …

tRNAs

tRNA structure

around 80 nucleotides long, single stranded, base pairs form within the chain, clover leaf structure - folds to make L shaped molecule, anticodon and amino acid binding site

what is the tRNA amino acid binding site

3' hydroxyl group at the end of RNA chain

how does tRNA become attached to the correct amino acid

through specific attachment, carried out by amino-acyl tRNA synthetases - activating enzymes

tRNA and amino acid attachment process

ATP hydrolysed and amino acid joined to AMP, correct tRNA binds and amino acid transferred from AMP to tRNA

wobble base pairing

bonding in RNA which doesn't follow watson-crick rules

ribosome structure

3 tRNA binding sites, large and small subunit, made of rRNA and proteins, exit tunnel for growing polypeptide

3 tRNA binding sites

A - aminoacyl tRNA binding site. P - peptidyl-tRNA binding sites. E - exit site

processes in ribosome during protein synthesis

binds mRNA and aminoacyl tRNAs, catalyses stepwise formation of peptide bonds, moves in 5' to 3' direction, amino acids added from N to C terminus

3 stages of protein synthesis

initiation, elongation, termination

initiation steps

small ribosomal subunit binds mRNA and recognises start codon - initiator tRNA binds to binds to start codon - large subunit binds - initiator tRNA fits into P site

elongation steps

aminoacyl tRNA base pairs with codon in A site, peptide bond formed, catalysed by peptidyl transferase, tRNA in P site ejected and ribosome moves along mRNA, growing chain in P site

how are peptide bonds formed

between amino group and COOH groups, by peptidyl transferase - proteins grow from N terminal to carboxy C terminal

termination steps

stop codon in A site - release factor enters A site instead of aminoacyl tRNA - water added to end, polypeptide released from P site - ribosome dissociates

steps for proteins destined for secretion - synthesis to secretion

synthesis starts on free ribosome - SRP (signal recognition particle) binds to signal peptide and stops synthesis - SRP binds to receptor protein in ER membrane - polypeptide keeps growing, translocates across membrane - signal cleaving enzyme cuts off signal peptide

mutations def

permanent change in cell dna

chromosomal mutation def

change in gene position or number

point mutation def

change in nucleotide sequence

somatic tissue mutations

not passed onto offspring, passed onto all cells arising from original, cause of ~85% of cancers

germline tissue mutations

passed onto offspring, causes inherited genetic diseases

spontaneous causes of mutations

through dna replication errors - base tautomerism, inherent instabilities in dna

induced causes of mutation

chemical - base analogues, modifying agents, intercalating agents, physical - ionising radiation, UV radiation

base analogues

chemicals that resemble DNA bases but pair incorrectly when incorporated

modifying agents can ..

remove amino group from A/C, add hydrocarbon groups to nucleotide bases - alkylating agents

effects of intercalating agents

insert between bases and distort helix, interfere with replication, cause frameshift mutations