D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 28
GENE EXPRESSION II: TRANSLATION
1.
Translation: (fig. 17 – 13& ppt. 1)
• codon: triplet mRNA base sequence coding for 1 amino acid in a protein
(fig. 17 – 4 & ppt. 2)
• genetic code: 64 triplet permutations of 4 bases (codons) code for 20 amino
acids; nearly universal (fig. 17 – 5 & ppt. 3)
- one codes for methionine &/or “start”; one codes for tryptophan
- three code for “stop”; three code for isoleucine
- five amino acids have 4 codons each (differing only by the third base in
sequence - “degeneracy”); = (total of 20 codons)
- 9 amino acids have 2 codons each (degeneracy); = (total of 18 codons)
- 3 amino acids have 6 codons each (degeneracy); = (total of 18 codons)
- deciphered by production of artificial mRNAs (poly-A, poly-C, poly-G,
poly-U, poly-UG, etc.), study of resultant proteins (Nirenberg)
• ribosomes: rRNAs & proteins: small & large subunits; feature P (peptidyl)
and A (amino acyl) sites on large subunit – sites for charged tRNAs (fig. 17 – 16 & ppt.
4)
• transfer RNA (fig. 17 – 14 & ppt. 5): self-complementary double-stranded
regions separating open loops (including anticodon loop); stem has specific amino
acid binding site
- anticodon = complement of codon for amino acid bound by stem
Bio 102
lec. 28 - p. 2
- binding of amino acid produces aminoacyl tRNA (catalyzed by aminoacyl
tRNA synthase) (fig. 17 - 15 & ppt. 6)
• mechanism of translation: initiation: mRNA, methionyl tRNA, and small
ribosomal subunit combined by initiation factors; large subunit joins these to form
initiation complex (fig. 17 – 17 & ppt. 7)
- A-site (amino acyl) & P-site (peptidyl) on ribosome bind amino acyl
tRNAs according to adjacent codon displayed by mRNA (codon-anticodon
complementary pairing)
- stems bring amino acids into favorable positioning for peptide bonding
(peptidyl transferase attaches P-site peptide occupant to A-site amino acid
occupant; ribosome moves along 3 bases (translocation), process repeats (fig. 17 – 18
& ppt. 8)
- termination codon signals release of new peptide chain & disassembly
of ribosome – mRNA – tRNA complex (fig. 17 – 19 & ppt. 9 + movie); summary of
paradigm (fig. 17 - 25 & ppt. 10)
• polyribosomes: mRNA with several ribosomes concurrently translating the
codons; found in prokaryotes (fig. 17 - 20 & ppts. 11 & 12)
2.
Endomembrane system proteins: (fig. 17 – 21 & ppt. 14)
• proteins for secretion synthesized on rough ER, passed into cisterna
• signal peptide: short sequence of amino acids on polypeptide destined for
endomembrane system facilitates ribosome attachment to ER & translocation of protein
into cisterna
Bio 102
3.
lec. 28 - p. 3
Point mutations:
• alterations of individual bases in nucleotide sequence of DNA; results range
from harmless or inconsequential changes to deleterious effects (figs. 17 - 22, 17 - 23 &
ppts. 15 & 16)