Lecture 16: Genteic Code and Translation Flashcards
What causes autosomal dominant ICA
A nonsense, a frameshift, and 5 missense mutations
One gene one enzyme hypothesis
Genes function by encoding enzymes, and each gene encodes a separate enzyme
Auxotroph
An organism with a mutation that prevents synthesis of a nutritional component (i.e. an amino acid) necessary for growth
How did one gene one enzyme hypothesis get proven
Mutation could be identified by the spores ability to grow in minimal medium supplemented y the substance that the mutant could not synthesize
-when they place arginine in minimal media, auxotrophic organism was able to grow
How did they identify the amino acid specified by a homopolymer
They placed a homopolymer (UUU) to a test tube with 1 radioactive AA and 19 unlabeled AAs
-The test tube with radioactive phenylalanine lit up so they knew UUU encoded phenyalanine
How did they determine AAs specified by heteropolymers
They added mRNA with a heteropolymer (GUU) and also added tRNAs. They washed out the tRNAs that did not bind and the ones left were bound to valine- showing that GUU corresponds to valine
Stop codons include
UAA, UAG, UGA
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 position
3rd codon pair is less important
A single anticodon can pair with more than one codon
Nonoverlapping
A single nucleotide may not be included in more than one codon
Amino acid attaches to the __’ end of tRNA
3’ end
carboxyl group of AA attaches to hydroxyl of 3’ end of tRNA
The base on the 3’ end of the tRNA is always
Adenine
T or F: ATP is used to attach tRNA to AA
True
Initiation of translation
Initiation factor IF-2, initiator tRNA with N-formylmethionine attached to form fmet-tRNA
GTP used for initiation**
Shine-Dalgarno sequence
In prokaryotes- this sequence in mRNA is required for attachment of small unit of ribosome
Kozak sequence
This sequence in mRNA signals the beginning of the coding sequence in eukaryotes
(AUG)
IF3 and initiation start
IF-3 binds small subunit so large cannot bind
- small unit can then attach to mRNA
- tRNA charged with fmet forms a complex with IF-2 and GTP, and binds initiation codon while IF-1 joins small subunit
- All IFs dissociate, GTP hydrolyzed to GDP, large subunit joins
What role does poly A tail play in initiation
Proteins attach to poly A tail and interact with cap-binding proteins, which enhance the binding of the ribosome to the 5’ end of mRNA
Elongation steps
fmet-tRNA occupies P site EF-Tu, GTP, charged tRNA enter A site GTP cleaved, EF-Tu-GDP complex released Peptide bond forms between AAs in A and p site Ribosome moves down mRNA to next codon
What is required for the ribosome to move down the mRNA to the next codon (translocation)
EF-G and GTP
Release factors
Proteins that are triggered by presence of stop codon to bind ribosome and trigger release of polypeptide
GTP is required
T or F: An mRNA molecule may be transcribed simultaneously by several ribosomes
True
