Genetics
Polypeptide Synthesis
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Get access- Transcription
Begins with the binding of transcription factors to DNA within a promoter sequence (such as the TATA box)
- One strand of DNA forms a template for the synthesis of messenger RNA
- This is catalysed by RNA polymerase which binds to a DNA promoter region
The process moves along the DNA strand as the RNA molecule is built in the 5’ to 3’ direction
- The process stops when it meets the termination signal on the DNA molecule
- This transcript is an exact replica of the DNA strand
- The introns are excised and the exons spliced together at spliceosomes
Some molecules undergo ‘alternative’ splicing and thus form other active mRNA
- This is another mechanism of regulating gene expression
- Splicing occurs in the nucleus
- The mRNA molecule then leaves the nucleus
- NB: mRNA is monocistronic meaning it only encodes for one protein
Its 5’ end is blocked by 7-methylguanine and a poly-adenine tail is added to the 3’ end which aids transport to the cytoplasm and provides stability
- Translation
Performed by specific transfer RNA (tRNA) which is specific for an amino acid and transports them to the mRNA template on the ribosomes
- Only 1% of cellular RNA is mRNA
Amino acids specific for the three nucleotide bases are attached by their C-terminal ends
- Each tRNA anticodon is linked with the matching codon on the mRNA
The process is controlled by aminoacyl-tRNA synthetases which couples amino acids to the matching tRNA molecule and peptidyl transferase which uncouples tRNA from amino acids and joins it to the chain
- Translation is always initiated at a codon specifying methionine
Initiation factor 2 (IF-2) catalyses the reaction between specific tRNA molecules and the mRNA
- The mRNA is read a codon at a time from the 5’ to 3’ direction
- The polypeptide chain is built from the N-terminal end to the C-terminal end
The stop codon initiates release factor to bind the A site (this is where incoming tRNA molecules bind), blocking further tRNA and releasing the polypeptide into the cytoplasm
- The polypeptide chain can be further modified after translation