Translation and Genetic Code

genetic code

genetic code

specifies the relationship between the sequence of bases in mRNA and the sequence of amino acids in polypeptide

codon

sequence of 3 nucleotide bases that specify a particular amino acid

in what direction are codon sequences read?

5’ to 3’ direction

how many codon possibilities are there?

64 (4³)

bacterial mRNA binding site

5’ binding site; start codon is AUG; 100 amino acids in length

stop codons

UAA, UAG, UGA

characteristics of genetic code

• each codon specifies one amino acid

• code is degenerate

• 18 of amino acids can be specified by more than one codon

• code non-overlapping (each ribonucleotide within mRNA is part of only one codon)

why are codons a sequence of 3 bases

triplet nature of the codon allows only 4 nucleotides to code for 20 essential amino acids

anticodon

attached to tRNA; allows binding of tRNA to mRNA codon

which amino acids are encoded by a single codon?

methionine and tryptophan

degeneracy

redundancy of a genetic code

wobble hypothesis

multiple codons specify one amino acid, only the third nucleotide in the codon differs

what is the initiator codon in eukaryotes?

Methionine (AUG)

what is the initiator codon in bacteria?

formylmethionine

termination codons

• signal the end of mRNA

• DO NOT code for an amino acid

• bound by release factors

aminoacyl-tRNA synthetase

catalyzes attachment of amino acids to tRNA by charging the tRNA

ribosome structure

large and small subunits that join only when mRNA attached to molecule

three binding sites for tRNA

A site, P site, E site

A site (aminoacyl site)

holds tRNA that carries the next amino acid to be added

P site (peptidyl site)

holds tRNA that carries growing aminoacyl site

E site (exit site)

exit site where discharged tRNAs leave ribosomes