Genetic Code Flashcards
who used fungus Neurospora to work out the relation of genes to protein
Beadle and Tatum
protein are polymers consisting of amino acids are linked by
peptide bonds
the linear sequence of amino acid is
the primary structure
how is the secondary and tertiary structure developed
primary structure folds
two or more polypeptide chains associate to form
quartenary structure
level of structural organization of protein
1, primary
2. secondary
3. tertiary
4. quaternary
what determines how the nucleotide sequence specifies the amino acid sequence
genetic code
what are the examples of breaking the genetic code
- use of homopolymers
- use of random copolymers
- use of ribosome-bound tRNAs
what amino acid are specifies by codons composed of only one type of base?
UUU=phenylalanine
Nirenberg and Matthaei developed a method for identifying amino acid specifies by
a homopolymer
the use of poly (U) directs
incorporation of phenylalanine
the use of poly (A) directs
incorporation of lysine
use of poly (C) directs
incorporation of proline
use of poly (G) directs
did not yield interpretable results
effect of using random copolymers
synthesized RNAs containing 2 or 3 nucleotides which gives info about the base composition but not EXACt sequence
what does ribosome-bound tRNAs do
provide information about genetic code
codon GUU specifies
valine
out of 64 codons, how many encode amino acids
61 called sense codons
what is the initiation codon
AUG=methionine
what are stop codons
UAA
UAG
UGA
Why can an aminoacid be specified by more than one codon
because they are degenerate
how is codon in an mRNA decoded
it is decoded by base pairing with its anticodon in tRNA
what are isoaccepting tRNAs?
different tRNAs that accept the same amino acid but have different anticodons
where is the wobble position
third (3’) position of the codon
first (5’) position of anticodon
who proposed the wobble hypothesis
Francis Crick
THE first codon of an open reading frame is called
initiation codon or start codon
which sequence is used for initiation in a few bacterial mRNAs and encode for methionine
GUG or UUG
which sequence is used for initiation codon in eukaryotes
CUG
In bacteria, translation begina with
N-formylmethionine (fMet)
fMET is delivered to the ribosome by
a special tRNA (tRNAi Met)
initaiton codon establishes the correct or incorrect reading frame
correrect reading frame
eukaryotes begin translation with
methionine
tRNA Met is used for
elongation
genetic code is generally overlapping or non overlapping
non overlapping
alternate names for termination codons are
stop codons or nonsense codons
exceptions of each codon encodes the same amino acid is found in
Mitochondrial genes
how are amino acid are assembled into a protein through
translation
translation of an mRNA molecule takes place where
on a ribosome
protein is synthesized from which terminus
N to C
what is the first stage of translation
the attachment of a tRNA to its amino acid
—— attaches an amino acid to the 3’ end of a tRNA to form an aminoacyl-tRNA
Aminoacyl-tRNA synthetase
certain positions on tRNA are recognized by the appropriate aminoacyl-tRNA synthetase. TRUE/FALSE
TRUE
step of charging tRNA
- Amino acid reacts with atp TO PRODUCE aminocyl-AMP and 2 Pi
- Amino acid is transfered to the tRNA forming aminoacyl-tRNA. AMP is released
what is the second step of translation
initiation
which components assemble during intiation
- mRNA
- the large and small ribosomal subunits
- the initiator aminoacyl-tRNA
- Initiation factors
- GTP
Initiation occurs in 3 steps
- mRNA binds to the small ribosomal subunit
- The initiator aminoacyl-tRNA base pairs to the initiation codon
- The large ribosomal subunit joins the initiation complex
what sequence in bacterial mRNA is required for the attachment of the small lsubunit of the ribosome
Shine-Dalgarno sequence
Eukaryotic mRNAs do not have Shine-Dalgarno sequence, so what is the alternate
5’ cap play a role in initiation
describe initiation in eukaryotes
- the small ribosomal subunit, IFs, and Met-tRNAi Met form a complex that binds to the 5’cap
- initiation complex scan down the mRNA until reaching the initiation codon
- Kozak sequence helps identify the initiation codon in eukaryotic mRNAs
what affects initiation of mRNA in eukaryote
The proteins that attach to the poly A tail interact with cap-binding protein and enhance binding of ribosome to 5’end of the mRNA
what is the third stage of translation
elongation
what do elongation require
70s initiation complex, Aminoacyl-tRNAS, elongation factors & GTP
tRNAs occupy which sites on ribosome
3 sites
‘Exit (E) site
Peptidyl (P) site
Aminoacyl (A) site
3 steps in elongation stage of translation comprises
- fMET-tRNA fMet occupies the P site of the ribosome
- Ef-Tu, GDP and charged t-RNA form a complex that enters the A site
- Peptide bond form between the P and A sites, and the tRNA in the P site is released. Ribosome moves down the mRNA and A is ready to receive another t-RNA
function o fRelease Factors (RF) in termination
Binds to ribosome when stop codon is reached and terminate translation
an mRNA molecule may be translated simultaneously by several ribosomes. this is called
Polyribosomes
examples of Messenger RNA surveillance
Nonsense-mediated mRNA decay (NMD)
Stalled ribosomes and nonstop mRNAs
No go decay
what do messenger RNA surveillance do
they detect and correct errors in mRNA
what results in the rapid degradation of an mRNA containing a premature stop codon
Nonsense mediated mRNA decay (NMD)
what does bacteria use to remove stalled ribosomes
transfer-messenger RNA (tmRNA)
how does eukaryote degrade abnormal RNA from the 3’end
process called nonstop RNA decay
what removes stalled ribosomes resulting from secondary structures or chemical damage to the mRNA, premature stop codons
NGD no-go decay
tmRNA is charged with —- and binds to
Ala and binds to A site
which antibiotic blocks binding of aminoacyl-tRNA to A site of ribosome
Tetracycline
which antibiotic prevents the transition from initation complex to chain elongation and also causes miscoding
streptomycin
which antibiotic blocks tehpeptidyl transferase reaction on ribosomes
chloramphenicol
which antibiotic bloxks teh translocation reaction on ribosomes
cycloheximide
which antibiotic blocks initiation of transcription by binding to RNA Polymerase
Rifamycin
