BSC 219

Translation

10/22/12

15.1 Many Genes Encode Proteins

The One Gene One Enzyme Hypothesis:

Genes function by encoding enzymes, and each gene encodes a separate enzyme.

More specific: one gene one polypeptide hypothesis

Hypothesis that each gene encoded a single, unique polypeptide

Codon: a triplet RNA code that corresponds to an amino acid in a protein

Degenerate code: Amino acid may be specified by more than one codon.

Synonymous codons: codons that specify the same amino acid

Isoaccepting tRNAs: different tRNAs that accept the same amino acid but have different anticodons

Wobble hypothesis-the third position of a codon has some variability.

Sense codons: encoding amino acid

Initiation codon: AUG

Termination codon: UAA, UAG, UGA

15.1 Many Genes Encode Proteins

The One Gene One Enzyme Hypothesis:

Genes function by encoding enzymes, and each gene encodes a separate enzyme.

More specific: one gene one polypeptide hypothesis

15.2 The Genetic Code Determines How the Nucleotide Sequence Specifies the Amino Acid Sequence of a Protein

Reading frame: three ways in which the sequence can be read in groups of three. Each different way of reading encodes a different amino acid sequence.

Nonoverlapping: A single nucleotide may not be included in more than one codon.

The universality of the code: near universal, with some exceptions

Aminoacyl-tRNA syntheses and tRNA charging

The specificity between an amino acid and its tRNA is determined by each individual aminoacyl-tRNA synthesis. There are exactly 20 different aminoacylt-tRNA syntheses in a cell.

Initiation factors IF-3, initiator tRNA with N-formylmethionine attached to form fmet-tRNA

Energy molecule: GTP

The Shine–Dalgarno consensus sequence in bacterial cells is recognized by the small unit of ribosome.

AG rich sequence approximately 6-8 nucleotides upstream of initiation AUG

The Kozak sequence in eukaryotic cells facilitates the identification of the start codon.

ACCAUGG

Exit site E-where depleted tRNA leaves ribosome

Peptidyl site P –growing polypeptide attached to tRNA

Aminoacyl site A-tRNA carrying next appropriate amino acid

Elongation factors: Tu, Ts, and G

Termination codons: UAA, UAG, and UGA

Release factors-proteins that bind each of the termination codons and cause ribosome dissociation.

The three-dimensional structure of the ribosome

Polyribosomes:

An mRNA with several ribosomes attached

Messenger RNA surveillance:

Detect and deal with errors in mRNA

Nonsense – mediated mRNA decay: eliminating mRNA containing premature termination codons

The posttranslational modifications of proteins

Glycosylation in ER and Golgi

Cleavage by specific enzymes

Specific folding by chaperones

Ribosomes that stall at the end of an mRNA molecule without hitting a termination codon can be released by a special mechanism.

Transfer-messenger RNA binds to A site of ribosome and acts as template.

Alanine is initially added, then 10 amino acids encoded by tmRNA

These serve as tag for rapid degradation of protein

Termination codon is contained on tmRNA and termination takes place