translation

- translation from nt language to aa language
- codons -- the nucleotide triplets that represent an aa
- codons are nonoverlapping
- groups of three that code for amino acids (nonoverlapping - gene is only used in 1 amino acid - does not slide down one at a time) 3 per 1 amino acid

64
- 61 code for amino acid, however, only 20 different amino acids are commonly found in proteins (some just never used)
- 3 code for stop codons (tells to stop and release polypeptides)
- most aa have 2 or more codons
3 “stop” codons
- built in safeguards
-- each aa’s codons are related
-- similar aa have similar codons (acidic, polar uncharged, hydrophobic, basic)
- Genetic code relates codon sequences in mRNA to amino acids in proteins

the set of ruled by which the nucleotide sequence of a gene, through an intermediary mRNA molecule, is translated into the amino acid sequence of a protein as the “genetic code”

decoding codons - tRNA recognizes codon and brings it to the aa
- basically acts as a bridge for mRNA and aa
- composed of 2 sites:
1. anticodon (reads all messages present on mRNA) 5' to 3'
2. Codon (has aa site where it could recognize aa and attach to it)
- 3D structure w hydrogen bonds (between nt) and amino acid attachment site

- 73-93 nucleotides
- posttranscriptionally-modified unusal bases (e.g., pseudouridine, methylguanosine
- aa added to 3’ adenosine
- middle loop of 7 nts contains anticodon
- tRNA molecules are molecular adaptors, linking amino acids to codons. The conventional “cloverlead” structure shows the complementary base-pairing that creates the double helical regions of the molecule. The anticodon loop contains three nucleotides that base pair with Phe codon in mRNA.

Both could occur.
- The nucleotide sequence of an mRNA is translated into the amino acid sequence of a protein via genetic code
each codons mRNA specific an amino acid
- genetic code is redundant

- 61 codons encode for aa; but there are fewer than 61 tRNAs. Why?
- mRNA weak interaction = all different 3rd nt = 1 tRNA -- since it codes for the SAME aa
- steric (structural) requirement between tRNA anticodon and mRNA codon is strict for first two positions, but flexible at third
- thus a single tRNA can recognize more than one codon

1. tRNA to aa inside cytoplasm
2. needs aminoacyl-tRNA synthetase to become activated - this enzyme is specific to each amino acid; 20 different ones; enzyme covalently-links aa to correct tRNA. Single enzyme can recognize all tRNAs for that aa.
3. This enzyme has 2 sites for the attachement of ATP (energy for peptide formation)
4. AMP complex formed -- specific tRNA molecule attaches to specific aa and AMP is released (ATP provides energy)
- proofreading mechanism
- aa has no role in targeting

AUG

- need mRNA code AUG start codon -- small ribsomal subunit attaches to mRNA
- interaction with tRNA and mRNA need help from 10 initiation factors
- 43S complex binds to mRNA, when finds AUG, eIF2-GTP hydrolysis releases eIFs allowing large (60S) ribosome subunit to bind
- Kozak sequence

additional nts containing AUG; help ribosome distinguish start from internal AUG; base pairs with rRNA