Lecture 20 Flashcards
(29 cards)
translation
biological polymerization of amino acids into polypeptide chains
translation requires
- amino acids
- messenger RNA (mRNA)
- ribosomes
- transfer RNA (tRNA)
tRNAS: transfers RNAS
- adapt genetic information present as specific triplet codons in mRNA to corresponding amino acid
- tRNA anticodons complement mRNAs
- tRNAs carry corresponding amino acids
ribosomes
- have an essential role in expression of genetic information
- consist of ribosomal proteins and ribosomal RNAs (rRNAs)
- consists of large and small subunits
- prokaryote ribosomes are 70S
- eukaryote ribosomes are 80S
ribosomal DNA (rDNA)
- encode for the rRNA genes
- moderately repetitive DNA fraction present in clusters at various chromosomal sites (remember copy number variant)
- each cluster contains tandem repeats separated by noncoding spacer DNA
tRNAs
- small in size and very stable
- 75-90 nucleotides
- transcribed fron DNA
- contain post-transcriptionally modified bases
tRNAs have a cloverleaf structure
anticodon
- tRNA has anticodon that complementarily base-pairs with codon in mRNA
- corresponding amino acid is covalently linked to CCA sequences at 3’ end of all tRNAs
aminoacylation: tRNA charging
- before translation can proceed, tRNA molecules must be chemically linked to respective amino acids
aminoacyl tRNA synthetase
enzyme that catalyzes aminoacylation
how many different synthetases for each amino acid
20 different synthetases (1:1 ratio)
- if there are higher or lower number of amino acids there will be an equal number of tRNAs
- highly specific, recognize only one amino acid
initiation requires
- small and large ribosomal subunits
- mRNA molecule
- GTP
- charged initiator tRNA
- Mg 2+
- initiation factors
Shine-Dalgarno sequence
(AGGAGG)
- precedes AUG start codon in bacteria
- base-pairs with region on 16S rRNA of 30S small subunit, facilitating initiation
initiation complex
small ribosomal subunit + initiation factors + mRNA at codon AUG
- combines with large ribosomal subunit
elongation
- both ribosomal subunits assembled with mRNA
- forms P site and A site
23S rRNA
- catalyzes peptide bond formation between amino acid on tRNA at A site and growing peptide chain bound to tRNA in P site
- uncharged tRNA movers to E (exit) site
- tRNA bound to peptide chain moves to P site from the A site
- sequences of elongation and translocation is repeated over and over
Peptidyl transferase
was initially believed to be catalytic enzyme for above reactions. It is in fact the catalytic activity of 23S rRNA
termination
- signaled by stop codons (UAG, UAA, UGA) in the A site)
- codons do not specify and any amino acid
GTP-dependent release factors
stimulates hydrolysis of polypeptide from peptidyl tRNA - released from translation complex
polysomes or (polyribosomes)
- mRNAs with several ribosomes translating at once
- as mRNA passes through ribosome, its free to associate with another small subunit
translation in eukaryotes
- ribosomes are larger and longer lived than bacteria
transcription occurs in nucleus
- 5’ end of mRNA capped with 7-methylguanosine residue at maturation, which is essential for translation
- poly-A tail added 3’ end of mRNA
translation occurs in the
cytoplasm
Eukaryotic mRNAs contain
purine (A or G), three bases upstream from AUG initiator codon, followed by G Kozak sequence (A/GNNAUGG
Kozak sequence
is considered to increase efficiency of translation by interacting with initiator tRNA
