Lecture 3/6 Flashcards
can you have an odd number of chromosomes in a diploid cell? why or why not?
you need an even number of chromosomes in mitosis / as a diploid organism bc you wouldn’t be able to split into equal haploid cells if there’s an uneven number of chromosomes
what do aminoacyl-tRNA synthetases do? how?
they attach amino acids to tRNA at the 3’ end by recognizing a specific amino acid and the structural features of its corresponding tRNA
how many aminoacyl-tRNA synthetases are there?
20, one for each amino acid
what is a tRNA once it has an amino acid attached?
charged
what makes up a ribosome in proks?
2 subunits (50S and 30S) proteins and rRNAs
what makes up a ribosome in proks?
2 subunits (60S and 40S) proteins and more rRNAs
what are the two things critical for translation?
charged tRNA and ribosomes
what did the mutated tryptophan E. coli experiments tell us in terms of what determines a codon?
codons contain more than one ribonucleotide and each ribonucleotide is part of only one codon (no overlap)
is the genetic code overlapping? how do we know?
no, it’s not overlapping bc each ribonucleotide is part of only one codon
how many amino acids can one codon encode?
one
what is a mutagen?
things that mutate DNA
what is a revertant?
a second mutation that will restore WT function
what is intragenic suppression
a second mutation that will restore WT function
what is the difference between a revertant and intragenic suppression
they are the same
for proflavin-induced mutations, what was the phenotype following an insertion or deletion of one ribonucleotide?
mutant
what is a reading frame?
the way a sequence of codons is divided into consecutive, non-overlapping codons
codons must be read in a ________ ________
reading frame
what kind of mutation must happen in order for the second mutation to cause a revertant/intragenic suppressor? and what are three examples?
one that restores the WT reading frame
1. -+
2. — or ——
3. +++ or ++++++
how long is a codon?
3 ribonucleotides
how were the non-repetitive codons figured out?
put all aas, all tRNAs, all aminoacyl-tRNA synthetases, and ATP into a test tube. you have all of the charged tRNAs, but one is radioactively labeled. add ribosomes and synthetic triplets, they would trap the tRNA that hybridizes to the synthetic triplet codon and filter the codon to see if it had radioactivity at the top or if it went through.
if radioactivity went through the filter, what does that tell us?
that the ribosome had not hybridized to the codon, as the ribosome is too big to fit through the filter
what is wobble?
some tRNAs recognize more than one codon for the amino acid they carry
does a tRNA anticodon or mRNA codon have wobble?
the tRNA has the wobble
is the wobble on the 3’ or 5’ end of the tRNA’s anticodon?
5’ end
does the wobble position on the tRNA correspond to the 3’ or 5’ end of the mRNA?
3’
which nucleotides in the third position can bind to any of the 4 codons?
U and A
What can G bind with in the Wobble position?
C or U
What can C bind with in the wobble position?
just G
Why is genetic code degenerate/redundant?
because an amino acid can be encoded by multiple codons
What codons start and stop translation?
AUG is the initiator codon and there are stop codons
are there any halts/pauses in translation?
no, translation continues until a stop codon is reached
in what polarity direction does translation move on the mRNA?
5’ to 3’
in what polarity direction are proteins added?
from N-terminus to C-terminus
why is the genetic code considered nearly universal?
the codons for different amino acids are conserved through evolution from proks to euks
what are the three different types of mutations in the genetic code?
missense, frameshift, and nonsense
what is a frameshift mutation
if you take out or add a nucleotide, it shifts the frame of the codon
what is a missense mutation
a single base pair substitution in DNA leads to a different amino acid being incorporated into a protein
what is a nonsense mutation
premature stop codon
if there are a bunch of transcripts in the cytoplasm, how do we know where to start translation in proks?
a small ribosomal subunit scans for the Shine-Delgarno sequence and then start codon (AUG). it then recruits fMET-tRNA
at what end of the transcript is the shine delgarno sequence (AGGAGG)?
the 5’ end
when the small ribosomal subunit in proks binds to the transcript, what can now happen?
the fMet tRNA can bind
is the shine delgarno sequence only in proks?
yes
What is the anticodon for AUG?
5’ CAU
once the tRNA is bound to the sequence in proks, what can be recruited?
the large ribosomal subunit
what protein does the large ribosomal subunit have and what does it do?
it has peptidyl transferase which makes peptide bonds
what is the difference between an initiator in proks and euks?
the Met is modified in proks, it’s an fMet
what are the first three steps to starting translation in proks?
- find shine delgarno sequence and the following AUG using small ribosomal subunit
- recruit fMet tRNA to bind to codon
- recruit large subunit
when do the pockets in the ribosome appear?
when the two subunits come together
what are the 3 pockets in the ribosome?
E site (exit)
P site (peptidyl)
A site (aminoacyl)
what is used in tandem with the ribosomal complex?
elongation factors
once the charged tRNA with the initiator codon is in the P position, where can another tRNA come in?
the A site
why is only the large subunit capable of making the peptide bonds?
bc it has peptidyl transferase
what happens when the peptide bond is made between two amino acids?
the first amino acid is no longer attached to the tRNA and the two amino acids move over so the first amino acid is in the E site and the second is in the P site
once there are amino acids in the E and P sites, what happens?
a new amino acid can go into the A site and the tRNA that’s no longer attached to the first amino acid gets kicked out. then a new peptide bond is formed between the amino acids in the P and A sites
which terminus is the first amino acid?
the N-terminus
What do N and C terminals refer to ?
amino terminal and carboxy terminal
what chemical process occurs to make a peptide bond?
condensation (it makes water)
which terminal gets added to?
the C terminal
what chemical process can break a peptide bond?
hydrolysis
can multiple ribosomes translate a transcript simultaneously or do they work individually?
they can work simultaneously, otherwise it would take too long to make protein
when does translation stop?
at the stop codon
what enters the A site once the stop codon is in the P site?
release factors; no charged tRNA enters the A site
what amino acid does the stop codon encode?
it doesn’t encode any amino acids
what do release factors do?
they release the polypeptide in the ribosomal complex and tRNA is released
