Translation (Protein Synthesis)
Translation • Processing of translating the language of RNA into the language of protein. • Components • mRNA transcript (which we’re translating – the message or genetic code) • Ribosome (machinery that makes the protein – our writing instrument) • tRNA (amino acid carrier – our translator) • Amino acids (what we connect together to make the protein – the words we use to form our translation)
Information translation • Ribosomes synthesize proteins by reading the nucleotide sequence of mRNAs and polymerizing amino acids in an N→C direction. • However, there is no obvious chemical affinity between the purine and pyrimidine bases and the 20 different amino acids. • Francis Crick reasoned that adapter molecules must bridge this information gap. These adapter molecules must interact specifically with both nucleic acids (mRNAs) and amino acids
The Genetic Code • A triplet codon specifies a single amino acid • There are 20 amino acids but 64 codons • The code is degenerate (3rd base wobble) • Code practically identical in all life forms • Three stop codons (don’t specify amino acids)
The Genetic Code Is a Triplet Code • A group of three bases codes for one amino acid. • The code is not overlapping. • The base sequence is read from a fixed starting point without punctuation. • The code is degenerate, meaning that, with the exception of Met and Trp, each amino acid can be coded by any of several triplets. • Codons representing the same amino acid tend to be similar in sequence. • The genetic code is unambiguous. Each of the 61 “sense” codons encodes only one amino acid. • The genetic code is “universal.” Codon assignments are virtually the same throughout all organisms
Some Codons Are biased • Because more than one codon exists for most amino acids, the possibility for variation in codon usage arises. • Even in organisms of average base composition, codon usage may be biased. • For example the Leu codons in of human genes, • CUG was used in excess of 48,000 times • CUC more than 23,000 times • UUA just 6000 times. • Preferred codons are represented by the most abundant isoacceptor tRNAs. Furthermore, mRNAs for proteins that are synthesized in abundance tend to employ preferred codons. • Rare tRNAs correspond to rarely used codons, and messages containing such codons might experience delays in translation
Translation Three types of RNA participate in translation mRNA: contains the genetic code to specify the amino acid sequence of the protein tRNA: carries the amino acids and reads the code Anticodon pairs with codon rRNA: part of the machinery (ribosome) that makes the protein product
tRNA Sets of tRNAs bind individual amino acids tRNAs have specific 2D and 3D shape tRNAs have 3-base anticodon Base pairs to codons in mRNA Decodes message
tRNA Sets of tRNAs bind individual amino acids 30-40 different tRNAs in bacterial cells Up to 150 in mammalian cells Isoacceptor tRNAs different anticodons – same amino acid
Charging the tRNA Enzymes add amino acid to tRNA Aminoacyl-tRNA synthetase (AARS) requires energy of ATP AMP added to amino acid Amino acid attached to tRNA at acceptor arm Connected at alpha-COO- group Alpha-NH2 group free