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Translation - Term 1 Human Body Notes
The Human Body (PY4010)
Kingston University
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The Human Body- Translation, the Ribosome and Antibiotic Specificity
After transcription, mRNA leaves the nucleus via nuclear pores. This involves the binding of several proteins one of which is a nuclear export receptor protein, this allows the mRNA strand to be
exported out to the cytoplasm in an active manner.
The cap at the 5’ end identifies the RNA as mRNA, and is where the ribosome first binds. The poly A tail controls stability or degradation of the mRNA.
The sequence between the cap and start of coding region contains sequences that control the rate of translation (5’ untranslated region or 5’UTR).
Sequence between end of coding region and poly A tail is the 3’UTR. It contains sequences that control the stability of the mRNA.
Start codon- always AUG. AUG also codes for methionine, meaning newly synthesised proteins always start with methionine. It is often later cleaved off to form the mature polypeptide.
Stop codon- always UAG, UAA or UGA
The coding region consists of what needs to be translated, and spans from the N-terminus to the C- terminus.
3 bases are called a codon. Some amino acids coded for by more than one triplet (degenerate). The genetic code is a triplet code, it’s unambiguous (triplet will always code for something) and degenerate.
The ribosome consists of large and small subunits, each of which contains around 50% proteins and 50% RNAs by mass.
Molecules known as tRNA are important. Characteristics include:
- Cloverleaf shape
- Attachment site for amino acid
- Anticodon (3 bases)
Protein synthesis performed in the ribosome. The mRNA is pulled through the ribosome, and the tRNAs are matched codon to anticodon and the nucleotide sequence is translated into a protein sequence.
Stages of protein synthesis:
- Charging of tRNAs with amino acids, where amino acyl tRNA transferase attaches amino acid residue to tRNA
- Initiation of polypeptide synthesis
- Elongation and formation of peptide bonds
- Termination and release of fully formed protein and mRNA
Ribosomes contain 3 active sites, E, P and A. Once start codon has been read, tRNA with methionine is loaded into the small ribosomal subunit and is bound at the P site. An amino-acyl-tRNA molecule binds to the vacant A site. The codon-anticodon base pairing ensures correct association of the correct tRNA, as a proof-reading method. A peptide bond is formed, and mRNA is pulled through the ribosome moving the empty tRNA into the E site. The next tRNA joins into the A position. Cycle is
repeated and a polypeptide is produced.
Elongation
Elongation of the process is speeded up by associate proteins called elongation factors, these hydrolyse GTP and help to increase the speed and accuracy of the procedure.
The end of the protein coding message is signalled by the STOP codon. These are not recognized by the tRNA. A release factor binds to the A site – stops translation. Polypeptide is released.
Each mRNA will have many ribosomes translating it. This is called a polysome.
Prokaryotes Eukaryotes -No cap structures -Bacterial ribosomes recognise an internal sequence in the 5’UTR, called the Shine Dalgarno sequence -Start codon AUG codes for a modified methionine amino acid, N-formyl methionine -Bacterial ribosomes have 50s and 30s subunit
-Have 60s and 40s subunit
Antibiotics that inhibit protein synthesis:
- Streptomycin targets 30s subunit
- Tetracyclines target 30s subunit
Translation - Term 1 Human Body Notes
Module: The Human Body (PY4010)
University: Kingston University
