Lecture Chapter 17 Flashcards
Where does transcripition take place?
nucleus
RNA is synthesized from the _ direction
it’s template is in the ____ direction
5’–3’
3’—5’
What enzyme creates RNA
RNA polymerase
What needs to be done so RNA can become mRNA?
tRNA and rRNA are also made in nucleus, so RNA processing must be done so it can become mRNA
How is pre mRNA matured?
poly A tail and 5’ cap
How does mRNA leave the nucleus?
pores
What is tRNA’s purpose?
Brings correct amino acid to each codon
What are degenerate codes?
different codes create the same amino acid
If you are given the anticodon, what must you do to determine the amino acid?
switch it so it is the mRNA
The anticodon is in the ____ direction
3’–5’
RNA polymerase— makes mRNA
2
RNA polymerase— makes rRNA
1
RNA polymerase— makes tRNA
3
What is the transcription complex?
RNA polymerase 2, transcription factors, and the promotor
Why are transcription factors needed
they can bind directly to the DNA
How does tRNA become active?
Enzymatically driven ( tyrosyl-tRNA synthetase)
Uses ATP
A bound ribosome creates proteins that are
in the ER lumen
Translation can begin before transcription is completed in prokaryotes, but not eukaryotes. Why is this?
prokaryotes don’t have a nuclear envelope, but eukaryotes need to transport the mRNA out of the nucleus before it is translated
What is reverse transcription?
A virus will use reverse transcriptase using viral RNA as a template to become DNA.
Th promotor sequences are in which direction?
5’—3’
How does the transcription initiation complex form?
- a eukaryotic promotor (TATA box)
- Several transcription factors bind to promotor to allow RNA polymerase to bind to DNA, either directly or through protein protein factors
- Transcription initiation complex forms as RNA polymerase II binds to
RNA polymerase 1 is found __ and responsible for ___
within the nucleoulus and has to do with ribosomal formation
RNA polymerase 3 generates
tRNA
Describe termination of transcription in eukaryotes
RNA polymerase II transcribes the polyadenylation signal sequence
The RNA transcript is released 10-35 nucleotides past this sequence
at the 5’ end during RNA processing, there is
a modified guanine that has been methylated
Contributes to stability and avoids degradation
At the 3’ end during RNA processing, there is
50-2590 adenine nucleotides added (poly A tail)
What are exons? Where are they located?
Regions that are expressed and spliced together to form mRNA
Spliceosomes consist of
proteins and small RNAs that recognize introns
The amino acid is attached to the ___ end of tRNA and corresponds with
3’
the anticodon attached to the bottom of the tRNA
When a tRNA is charged, it means
an amino acid is bound to it
Describe the purpose of the A P E sites in ribosomes
E= Exit (tRNA leaving)
P= Peptidyl tRNA binding site ( where the polypeptide is forming)
A=aminoacyl tRNA binding site (where the next amino acid to be added is chilling)
The aminoacyl synthetase is responsible for
catalyzing the covalent bonding between an amino acid and a tRNA using ATP
Stop codons are recognized by ____ that promotes ____.
a release factor that promotes hydrolysis
How does the ribosome know where to trasncribe the RNA?
all protein translation starts in cytosol
if a protein contains a signal peptide that is bound to an SRP, it causes ribosome to stop translating and transport it to ER in order to start translating again
What is a silent mutation?
A mutation that results in no change in expression because the amino acid can still be coded for
What is a missense substition?
a mutation that leads to a different amino acid
What is a nonsense mutation?
When an amino acid codon is translated into a stop codon. These are ideally destroyed
What is an insertion mutation?
Can lead to a frameshift , capable of causing immediate nonsense
What is a deletion? compare it to a 3 nucleotide pair deletion?
can also cause a frameshift
no frameshift but one amino acid missing, shape follows function so may not be detrimental
Describe the polypeptide product you would expect from a
poly-G mRNA that is 30 nucleotides long
A polypeptide made up of 10 Gly (glycine) amino acids
What is a promoter? Is it located at the upstream or down-
stream end of a transcription unit?
A promoter is the region of DNA to which RNA polymerase binds to begin
transcription. It is at the upstream end of the gene (transcription unit).
What enables RNA polymerase to start transcribing a gene
at the right place on the DNA in a bacterial cell? In a eukary-
otic cell?
In a bacterial cell, part of the RNA polymerase recognizes the gene’s pro-
moter and binds to it. In a eukaryotic cell, transcription factors must bind to
the promoter first, then the RNA polymerase binds to them. In both cases,
sequences in the promoter determine the precise binding of RNA polymerase
so the enzyme is in the right location and orientation.
Suppose X-rays caused a sequence change in the
TATA box of a particular gene’s promoter. How would that
affect transcription of the gene?
The transcription
factor that recognizes the TATA sequence would be unable to bind, so RNA
polymerase could not bind and transcription of that gene most likely would
not occur.
Given that there are about 20,000 human protein-coding genes,
how can human cells make 75,000–100,000 different proteins?
Due to alternative splicing of exons, each gene can result in multiple
different mRNAs and can thus direct synthesis of multiple different pro-
teins
Compare RNA splicing to how you would watch a pre-
recorded television show. What would introns correspond
to in this analogy?
In watching a pre-recorded show, you watch segments of the show
itself (exons) and fast-forward through the commercials, which are thus like
introns. However, unlike introns, commercials remain in the recording, while
the introns are cut out of the RNA transcript during RNA processing
What would be the effect of treating cells with an
agent that removed the 5′ cap from mRNAs?
Once
the mRNA has exited the nucleus, the cap prevents it from being degraded by
hydrolytic enzymes and facilitates its attachment to ribosomes. If the cap were
removed from all mRNAs, the cell would no longer be able to synthesize any pro-
teins and would probably die.
What two processes ensure that the correct amino acid is
added to a growing polypeptide chain?
First, each aminoacyl-tRNA synthetase specifically recognizes a single
amino acid and attaches it only to an appropriate tRNA. Second, a tRNA
charged with its specific amino acid binds only to an mRNA codon for
that amino acid.
Describe how a polypeptide to be secreted reaches the
endomembrane system.
A signal peptide on the leading end (amino end, or
N-terminus) of the polypeptide being synthesized is recognized by a signal-
recognition particle that brings the ribosome to the ER membrane. There,
the ribosome attaches and continues to synthesize
the polypeptide, depositing it in the ER lumen
In eukaryotic cells, mRNAs have been found
to have a circular arrangement in which proteins hold
the poly-A tail near the 5′ cap. How might this increase
translation efficiency?
When one
ribosome terminates translation and dissociates,
the two subunits would be very close to the cap.
This could facilitate their rebinding and initiating
synthesis of a new polypeptide, thus increasing the
efficiency of translation
What happens when one nucleotide pair is lost from the
middle of the coding sequence of a gene
In the mRNA, the reading frame downstream from the deletion is shifted,
leading to a long string of incorrect amino acids in the polypeptide, and in most
cases, a stop codon will occur, leading to premature termination. The polypep-
tide will most likely be nonfunctional
Individuals heterozygous for
the sickle-cell allele are generally healthy but show phenotypic effects of the allele under some circumstances. Explain in terms of gene expression.
Heterozygous individuals, said to
have sickle-cell trait, have a copy each of the wild-type allele and the sickle-cell
allele. Both alleles will be expressed, so these individuals will have both normal
and sickle-cell hemoglobin molecules. Apparently, having a mix of the two
forms of b-globin has no effect under most conditions, but during prolonged
periods of low blood oxygen (such as at higher altitudes), these individuals can
show some signs of sickle-cell disease.
In eukaryotic cells, transcription cannot begin until
(A) the two DNA strands have completely separated and
exposed the promoter.
(B) several transcription factors have bound to the promoter.
(C) the 5′ caps are removed from the mRNA.
(D) the DNA introns are removed from the template.
B
Which of the following is true of a codon?
(A) It never codes for the same amino acid as another codon.
(B) It can code for more than one amino acid.
(C) It can be either in DNA or in RNA
(D) It is the basic unit of protein structure
C
The anticodon of a particular tRNA molecule is
(A) complementary to the corresponding mRNA codon.
(B) complementary to the corresponding triplet in rRNA.
(C) the part of tRNA that bonds to a specific amino acid.
(D) catalytic, making the tRNA a ribozyme
A
Which of the following is true of RNA processing?
(A) Exons are cut out before mRNA leaves the nucleus.
(B) Nucleotides are added at both ends of the RNA.
(C) Ribozymes may function in the addition of a 5′ cap.
(D) RNA splicing adds a poly-A tail to the mRNA
B
Which of the following mutations would be most likely to have
a harmful effect on an organism?
(A) a deletion of three nucleotides near the middle of a gene
(B) a single nucleotide deletion in the middle of an intron
(C) a single nucleotide deletion near the end of the coding sequence
(D) a single nucleotide insertion downstream of, and close to,
the start of the coding sequence
D
