Genetics Exam 3

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RNA polymerase from E.coli

A. The complete enzyme (holoenzyme) is a complex composed of six polypeptide subunits.
B. The core enzyme lacks the sigma subunit
C. In vitro, the core enzyme synthesize RNA, but does not initiate transcription at the correct place on the DNA template
D. Alternative sigma are used by the cell to transcribe specific set of genes under adverse conditions such as thermal stress (heat-shock)
*E. Select this answer if choices A-D are all true.

Transcription Basics

*Initiation by RNA pol does not require a primer.
*A DNA sequence is read by RNA polymerase, which produces a complementary, antiparallel RNA strand and includes uracil (U) instead of thymine (T).
* Only the template strand is synthesized and since DNA is read from 3' to 5', the complementary RNA is created in the 5' to 3' direction.
*Step 1: DNA unwinds/"unzips" as the Hydrogen Bonds Break. Step 2: The free nucleotides of the RNA, pair with complementary DNA bases.
Step 3: RNA sugar-phosphate backbone forms. (Aided by RNA Pol.)
Step 4: Hydrogen bonds of the untwisted RNA+DNA "ladder" break, then the RNA leaves the nucleus through the small nucleur pores.
Step 5:This then goes to the cytoplasm to continue on to protein processing.

What is common to both DNA replication and transcription?

A phosphodiester bond is formed between the 3'-OH and 5'-phosphate of adjacent bases.

In the phenomenon of Hybrid Dysgenensis in Drosophila, a high level of mutagenesis is observed in the F2 progeny, but not in the F1 progeny, because:

The transponsase needed for mobilization of the p-element is only active in germline cells.

INTRONS

Suppose you have used the technique of R-looping to hybridize eukaryotic mRNA isolated from the cytoplasm of human cells with the corresponding genomic DNA Strand used as template for transcription. You then observed looping strands of nucleic acid when viewed by electron microscopy. What do these structures represent?

Major classes of RNA in Eukaryotic cells and their locations in the cell.

1. mRNA - carries information about a protein sequence to the ribosomes, the protein synthesis factories in the cell. (Eukaryotes) Once processed from pre-mRNA to mature mRNA, is then exported from the nucleus to the cytoplasm, where it is bound to ribosomes and translated into its corresponding protein form with the help of tRNA. (Prokaryotes) Since there is no nucleus or cytoplasm compartments, mRNA can bind to ribosomes while it is being transcribed from DNA.

2. tRNA - is a small RNA chain of about 80 nucleotides that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. It has sites for amino acid attachment and an anticodon region for codon recognition that binds to a specific sequence on the messenger RNA chain through hydrogen bonding.

3. rRNA - is the catalytic component of the ribosomes. It makes up 80% of the 10 mg/ml RNA found in a typical eukaryotic cytoplasm. Three of the rRNA molecules are synthesized in the nucleolus, and one is synthesized elsewhere.

4. snRNAs - found in the nucleus associated with numerous proteins to form small nuclear ribonucleoproteins (snRNPs).

5. snoRNAs - involved in the processing of rRNA in the nucleolus.

Which of the following major classes of RNA in Eukaryotic cells is not properly paired with its location in the cell?

a) mRNA is found in the ribosomes, nucleus, and cytoplasm.
b) tRNA is found in the ribosomes, nucleus, and cytoplasm.
c) rRNA is found in the ribosomes, nucleolus, and cytoplasm.
d) snRNAs are found in the nucleus.
e) snoRNAs are found in the nucleolus.

RNA polymerases

Eukaryotes have several types of RNAP, characterized by the type of RNA they synthesize:

1. RNA poly I synthesizes a pre-rRNA 45S, which matures into 28S, 18S and 5.8S rRNAs which will form the major RNA sections of the ribosome.
2. RNA pol II synthesizes precursors of mRNAs and most snRNA and microRNAs. This is the most studied type, and due to the high level of control required over transcription a range of transcription factors are required for its binding to promoters.
3. RNA pol III synthesizes tRNAs, rRNA 5S and other small RNAs found in the nucleus and cytosol.
4. RNA pol IV synthesizes siRNA in plants.
5. RNA pol V synthesizes RNAs involved in siRNA-directed heterochromatin formation in plants.

Which of the following is false concerning the telomeres of eukaryotic chromosomes?

a) The telomeres are short (usually 6 bp) repeating DNA sequence
b) Inhibitors of telomerase activity are being investigated as possible anti-cancer therapeutics
c) Telomerase is active in dividing normal cells and cancer cells, whereas the activity is extremely low in quiescent (Go) cells
d) Telomerase are the structure at the termini of eukaryotic chromosome
*e) Select this answer choices A-D are all true statements

The introns within pre-mRNA transcripts made in the nucleus of eukaryotic cells.....

Vary considerably in size and number among different species.

DNA polymerase I from E. coli was the first DNA polymerase to be purified. This enzyme has a unique enzymatic activity that has not been found with any other DNA polymerase in E.coli. That unique activity is ________.

5' to 3' exonuclease

Eukaryotic mRNAs are capped at the 5' end. Which of the following is FALSE concerning this 'cap' structure?

a) A modified guanosine nucleotide forms an unusual phosphodiester bond that has 5' to 5' triphosphate linkage.
b) A methyl (-ch3) group is added to 7 nitrogen of the guanine of the ribonucleotide added at the 5' end of the mRNA
c) The cap structure is required for translation.
d) Capped mRNAs frequently also have a methyl group added to the 2' -OH on the ribose sugar of the first 1-3 ribonucleotides attached to the cap Nucleotide.
*e) Select this answer if A-D is true.

Meselson and Stahl did a classic experiment using DNA labeled nitrogens with different densities for old and new DNA. This experiment proved.....

That DNA replication is Semi-Conservative:

-Two new DNA duplexes would each contain one DNA strand from the original DNA and one "new" strand of DNA.

Supercoiling of DNA results from over winding the DNA to create ___(a)___ super coiling or under winding to create ___(b)___ super coiling.

(a) positive
(b) negative

The polytene chromosome in the salivary glands of the fruit fly (Drosophilla).

Facts:
-Result from repeated rounds of DNA replication without cell division (Endoreduplication)
-Leads to about 500 copies of DNA for each chromosome
-Lie in alignment to produce very detailed bands when stained and viewed with the light microscope
-Chromosomal Puff: Unusual because the two homologous chromosomes are aligned (synapsed) during interphase

Answer = more than one above

Which of the following do not require a spliceosome to splice RNA because they are self splicing (no proteins needed):

a) Group I Introns
b) Group II Introns
c) Nuclear pre-mRNA Introns
d) tRNA Introns
*e) A and B

Which of the following enzymes creates the phosphodiester bond between adjacent Okazaki fragments?

a) Helicase: Unwinds a portion of the DNA Double Helix
b) RNA Primase: Attaches RNA primers to the replicating strands.
c) DNA Polymerase delta: Binds to the 5' to 3' strand in order to bring nucleotides and create the daughter leading strand.
d) DNA Polymerase epsilon: Binds to the 3' - 5' strand in order to create discontinuous segments starting from different RNA primers.
e) Exonuclease (DNA Polymerase I): Finds and removes the RNA Primers
*f) DNA Ligase: Adds phosphate in the remaining gaps of the phosphate - sugar backbone
g) Nucleases: Remove wrong nucleotides from the daughter strand.

Transposons....

-"Jumping Genes" are sequences of DNA that can move or transpose themselves to new positions within the genome of a single cell. The mechanism of transposition can be either "copy and paste" or "cut and paste". Transposition can create phenotypically significant mutations by making insertions in the coding region of a gene and alter the cell's genome size by making multiple copies of the same sequence resulting in unequal cross-overs, which is one of the main reasons for chromosome duplication.
-Genetic recombination between different transposons can lead to duplications, deletions and inversions of chromosome structure
-Answer = more than one are correct

Which of the following concerning core enzyme and the holoenzyme of prokaryotic cells are True:

-Answer = all are False

Facts: (for E.coli)
-Core Enzyme: NO Sigma factor; 5 subunits: ββ'ωα2
-Holoenzyme: Core Enzyme plus Sigma factor: ββ'ωα2σ

In e.coli a mutation that leads to an increased transcription rate of the affected gene and results in an increased homology to -10 box (tata box) consensus sequence is called.....

UP Mutation: refers to any mutation in the promoter region of a gene which can trigger transcription initiation.

Ribozymes

Enzymes that are composed exclusively of RNA; catalytic RNA; rRNA.

Which of the following is not required for replication and stable inheritance in eukaryotic chromosomes?

a) Topoisomerases
b)Telomeres
*c)Transposons
d) Primers
e) Polymerases

A genetic mutation that inactivated 5' to 3' exonuclease activity of DNA polymerase I would probably affect the ability of the enzyme:

To remove RNA primers from Okazaki fragments. DNA pol I removes the RNA primer (created by Primase) from the lagging strand and fills in the necessary nucleotides of the Okazaki fragments in 5' to 3' direction, proofreading for mistakes as it goes.

Enhancer sequences....

-Can enhance transcription even when located several thousand base pairs from a promoter.

-Enhancer DNA sequences bind transcription factors called enhancer-binding proteins which increase the rate of transcription. Enhancer sequences may be kilo bases away from the gene they influence. An enhancer complex may interact with promoter complexes by bringing the sites into direct contact.

Lariat shape intermediate that form during removal of an intron bond with an "A" has an extra phosphodiester bond ______ carbon:

Answer: -2 carbon.....?

Novel 5' to 2' phosphodiester bond between the G at the 5' end of the intron and the A in the branch point.

The DNA replication enzyme that most closely resembles RNA polymerase is?

DNA primase - an RNAp enzyme involved in the replication of DNA. Primase catalyzes the synthesis of a short RNA segment (called a primer) complementary to a ssDNA template. Primase is of key importance in DNA replication because no known DNA polymerases can initiate the synthesis of a DNA strand without an initial RNA or DNA primer (for temporary DNA elongation).

C- value refers to

The total mass of DNA in the haploid genome of a specific species. In polyploids the C-value may represent two genomes contained within the same nucleus. They are reported in picograms.

Which of the following is true concerning pre-mRNAs?

a) They are larger than the corresponding MRNA
b) These RNAs are the precursor molecules for the rRNA
c) These RNAs represent only the 55 rRNA and tRNA precursors
d) They are only found within the nucleus and not the cytoplasm of the cell
*e) More than one off the choices are correct

Replication of DNA cannot be continuous on both strand of DNA because:

The two strand of DNA in a duplexed molecule have an anti-parallel orientation.

The TATA binding protein TBP binds to the TATA box sequence in eukaryotic promoters. What is its function in transcriptional initiation?

-It bends and partly unwinds DNA as a promoter.
-TBP is involved in DNA melting (double strand separation) by bending the DNA by 80° (the AT-rich sequence to which it binds facilitates easy melting). The TBP is an unusual protein in that it binds the minor groove using a β sheet.

An intron is:

-Found in the pre-mRNA, but not in the mature mRNA.
-An intron is a DNA region within a gene that is not translated into protein. These non-coding sections are transcribed to precursor mRNA (pre-mRNA) and some other RNAs (such as long non-coding RNAs), and subsequently removed by a process called splicing during the processing to mature RNA. After intron splicing (ie. removal), the mRNA consists only of exon derived sequences, which are translated into a protein.

DNA Replication

-DNA replication is BIDIRECTIONAL.
-Helicase unwinds the double helix.
-SSBs hold the strands apart once separated.
-Enzymes called topoisomerases produce breaks in the DNA and then rejoin them in order to relieve the stress in the helical molecule during replication.

Which of the following is false concerning DNA replication in bacterial cell compared to Eukaryotic cell?

Answer = More than one of the choices.

Unlike prokaryotic DNA replication, eukaryotic DNA replication __________.

a) is completed by DNA polymerase
*b) cannot be completed by DNA polymerase
c) is semi-conservative
d) has a single origin
e) is error free

DNA POLYMERASES (replication)

-Prokaryotes - 3 types of DNA Pols:
1. DNA Pol III: 5' to 3' DNA synthesis; 3' to 5' exonuclease .
2. DNA Pol I: 5' to 3' DNA synthesis and exonuclease; 3' to 5' exonuclease; Allows for removal of RNA primer (while simultaneously putting down DNA in its place w/ the 5' to 3' DNA synthesis).

-Eukaryotes - atleast 15 types of DNA Pols but mainly:
1. Pol α (also called RNA primase): acts as a primase (removes the RNA primer), and then elongates that primer with DNA nucleotides.
2. Pol β: Implicated in repairing DNA, in base excision repair and gap-filling synthesis.
3. Pol γ: Replicates and repairs mitochondrial DNA and has proofreading 3' to 5' exonuclease activity.
4. Pol δ: Highly processive and has proofreading 3' to 5' exonuclease activity. Thought to be the main polymerase involved in lagging strand synthesis, though there is still debate about its role.
5. Pol ε: Also highly processive and has proofreading 3' to 5' exonuclease activity. Highly related to pol δ, and thought to be the main polymerase involved in leading strand synthesis, though there is again still debate about its role.

RNA POLYMERASES (transcription)

-Prokaryotes - only one type of RNA Pol:
5' to 3' synthesis and no exonuclease activity (Why? - Proofreading at mRNA level is not NEARLY as important in comparison to DNA level---but that's another topic).

-Eukaryotes - 3 main types of RNA Pols:
All have 5' to 3' synthesis and are large multimeric enzyme complexes containing more than 12 polypeptide subunits
1. RNA Pol I - transcribes rRNA
2. RNA Pol II - pre-mRNA, snoRNAs, snRNA, (transcribes genes that encode proteins)
3. RNA Pol III - tRNA, transcription of 5s-rRNA (promotors are downstream)

Eukaryotes vs Prokaryotes

*CHROMOSOMES:
-Eukaryotes - 46 linear chromosomes (humans),
-Prokaryotes - 1 circular chromosome (haploid)

*REPLICATION ORIGIN:
-Eukaryotes - Several ORiCs, therefore several replication bubbles
-Prokaryotes - One ORiC, and since they have a single circular chromosome, the replication bubble formed is going to make the entire thing look like the Greek letter Theta (why it is called Theta Replication).

*TRANSCRIPTION:
-Euk - Monocistronic (one transcript, one polypeptiden nucleus). In nucleus. Post transcriptional modifications: splicing, 5' methyl/guanine cap, poly A tail (termination).
-Prok - Polycistrionic. In cytoplasm. No post transcriptional modifications. Transcription and Translation occur at the same time. Rho dependent termination (hairpin loop so pol falls off DNA)

Answer: The gene probably does not contain an intron

In an R- Looping experiment, single stranded DNA from a gene of interest was hybridized to mRNA that was isolated from the cytoplasm of human cells. A single smooth uninterrupted DNA/RNA hybrid was observed using the electron microscope. This indicates that:

The R looping experiment was repeated except the source of RNA was pre mRNA for a different gene that had been isolated from the nucleus of the cell:

The number of intron and exon in the gene cannot be determined from this experiment

Okazaki fragments are

Synthesized only on the lagging strand of DNA

Topoisomerase

A class of enzymes that introduce either single or double stranded breaks in a DNA molecule changing relative to the position of the strand.
EX: Gyrase is a type II topoisomerase that introduces negative supercoils into, or relaxes positive supercoils of DNA by looping the template so as to form a crossing, then cutting one of the double helices and passing the other through it before releasing the break, changing the linking number by two in each enzymatic step.

The most notable features distinguishing termination of RNA transcripts requiring the rho factor from those that are rho-independent is the presence of:

a) An inverted repeat followed a string of uridine (u) residues in RNAs that terminate w/o the rho factor
b) An inverted repeat followed a string of Uridine (U) residues in RNAs that require the rho factor
c) A series of Deoxyadenosine (A) nucleotides in the DNA strand used as a template by RNA polymerase where termination is RHO independent
d) Poly (U) polymerase that enzymatically adds the poly (u) tail to rho independent transcripts.
*e) More than one is correct.

Intron are characterized by which of the following invariant consensus sequence:

5' GU- intron- AG 3'

Occasionally DNA polymerase III will add an incorrect nucleotide to the end of a growing DNA chain. The presence of a mismatched nucleotide will activate the polymerase's _______, which is one type of proofreading:

3' to 5' exonuclease

DNA polymerases are considered processive because:

Some polymerase molecules catalyzes multiple cycle of nucleotide additions to a polynucleotide chain. Once DNA is synthesized, DNA polymerase III can add many nucleotides without releasing from the template strand. Also, the leading strand is synthesized completely by the same DNA polymerase III enzyme complex.

The shine-dalgarno consensus sequence functions as:

The ribosome binding site on the mRNA molecules in bacterial cells.

What prevents super-coiling ahead of the replication fork?

Topoisomerases (Gyrase and SSBs)

What forms a phosphodiester bond between adjacent okazaki fragments to create a continuous DNA strand?

DNA Ligase

What synthesizes the leading strand of the DNA?

DNA Polymerase III

What synthesizes a 10-12 base segment of RNA?

DNA Primase

What fills in small regions of DNA where the RNA primers were located?

DNA Polymerase I

RNA proofreading activity:

Has not been found for any known RNA polymerase.
Why does RNA polymerase not proofread?
RNA polymerase reads only the "template strand" of DNA. There is therefore no way for it to "know" whether or not the template strand is really complementary to the non-template strand. The lack of RNA proofreading is not detramental to the cell since RNA doesn't last very long.

A lack of DNA proofreading would be detrimental to the cell, though. Without it, the DNA would become progressively damaged, and no longer be coding for the mRNA which would be translated to the proteins that the cell needs to continue living.

In eukaryotic cells, nearly all of the repetitive DNA is found in the _______

Heterochromatin

Isoforms are different polypeptides that are produced from the same pre-mRNA in eukaryotic cells. These polypeptides are usually the result of:

Alternative splicing of the exons of the pre-mRNA.

Some organisms that seem to be less complex than humans have much large genomes than humans. This phenomenon is known as:

C-value paradox. Reasoning is thought to be because of transposons.
The mechanism of transposition can be either "copy and paste" or "cut and paste". Transposition can create phenotypically significant mutations by making insertions in the coding region of a gene and alter the cell's genome size by making multiple copies of the same sequence resulting in unequal cross-overs, which is one of the main reasons for chromosome duplication.

What is the function of eukaryotic RNA Pol I?

Transcription of rRNA genes.

Consensus Sequence for the following Six nucleotide sequence:
AGGAGTT
AGCTATT
TGCAATA
TCCTAAT
TGCAATT

TAGCAATT

A circular DNA molecule that is 2,500 bp in length has 220 complete rotation of the helix. This molecule is:

Negatively super-coiled....There are 100bp/10 turns....2500bp/100bp = 25..... 25*10 = 250 turns needed..... A DNA molecule with 2500bp should have 250 turns, but there are only 220 here..... 250-220 = 30 turns underwound....This means that it is NEGATIVELY super-coiled.

Answer = Dispersive

You learn that a mars lander has retrieved a bacterial sample from the polar ice caps. You obtain a sample of this bacteria and perform the same kind of experiment Meselson and Stahl did to determine how the Mars bacteria replicates its DNA. Based on the following equilibrium centrifugation results, what type of the replication would you propose for this new bacteria?

If you radioactively labeled the phosphate between the last ribonucleotide of an exon and the guanosine at the 5' end of the adjoining intron- as shown by the boldface in exon-gp/gp Upintron- the radioactive phosphate would:

Be part of the lariat structure of the intron.

Most transposable elements are flanked by short direct repeats. How are these direct repeats formed?

(p.297) The presence of flanking direct repeats indicates that staggered cuts are made in the target DNA when a transposable element inserts itself. The staggered cuts leave short, single-stranded DNA on either side of the transposable element. Replication of the single-stranded DNA then creates the flanking direct repeats.

Which of the following is FALSE concerning DNA replication in bacterial cells when compared to eukaryotic cells?

Answer = more than one of the choices?

Which of the following is TRUE concerning RNA polymerase from E.coli?

(p.352) Only one type of RNA Pol. It is a large, multimeric enzyme (several polypeptide chains). Core enzyme has five subunits: (2α)ββ'ω. Holoenzyme has 6 subunits because there is a σ subunit which controls the binding of the RNA Pol to the promotor.

Which of the following is FALSE concerning the formation of aminoacyl-tRNA?

(p.407)
a) The 3' tRNA base is always Adenine.
b) Synthetases are the key to specificity so a tRNA links up with its appropriate amino acid during Translation.
c) A cell has 20 different amino acid synthetases.
d) Recognition of tRNAs by the synthetase depends on the different sizes, charges, and R groups of amino acids.
e) The attachment of a tRNA to its appropriate amino acid is called tRNA charging, which requires ATP.
*f) All of the above are TRUE.

Which of the following mutations would probably prevent transcription of a bacterial operon?

(p.436-437) Mutations in lactose metabolism at the promoter site. These mutations interfere with the binding of RNA polymerase to the promoter. Because this binding is essential for the transcription of the structural genes, E.coli strains with lacP- mutations don't produce lac enzymes either in the presence or in the absence of lactose.

In eukaryotic cells, nearly all of the unique DNA that encodes proteins is found in:

a) Heterochromatin
b) Homochromatin
*c) Euchromatin
d) Prochromatin
e) None of the above is correct because eukaryotic cells do not have repetetive DNA.

A. AACCGGAACCGG--------------GGCCAAGGCCAA
*B. TTTTTAAAAGGGG------------CCCCTTTTAAAAA
C. AAACCCGGG------------------TTTGGGCCC
D. None of the above
E. More than one of the above

Jr. Scientist thinks she has isolated a new transposable element (transposon).
It has all of the molecular features usually found with most transposons.
Which of the following sequences could most likely represent the termini of
this new transposon based on your understanding of transposon structure?

The polytene chromosomes in the salivary glands of the fruit fly, Drosophila
melanogaster......

a) Are much larger than the chromosomes found in the cells of other tissues in the fly.
b) Produce high resolution banding patterns that can easily be seen with a light microscope.
c) Are unique because their size makes them highly resistant to breakage by X-rays.
d) Are extremely fragile and, therefore spontaneously undergo chromosome deletions.
*e) More than one of the choices A-D are true.

DNA polymerases are considered processive because.....

A. they bind to a substrate, carry out one catalytic event, disengage and reinitiate synthesis on another DNA molecule.
B. they create a pool of nucleotides that covalently bind to their corresponding complimentary bases.
*C. the same polymerase molecule catalyzes multiple cycles of nucleotide additions to a polynucleotide chain.
D. they bidirectionally synthesize DNA as long continuous chains.
E. none of these

You have isolated what appears to be alien DNA. While studying its replication, you performed the exact experiment Meselson and Stahl did.
After three generations, the DNA is subjected to a CsCl gradient, and only one band appears. What type of replication does this DNA undergo?

A. Semiconservative
B. Conservative
*C. Dispersive
D. Replication of the alien DNA may be either semiconservative or conservative.
The experimental results cannot distinguish between the two possibilities.
E. The experimental results do not support any of the three possible mechanisms of replication.

Which controls supercoiling ahead the replication fork?

A. DNA ligase
B. DNA primase
*C. Topoisomerase
D. DNA polymerase I
E. DNA polymerase III

Which synthesizes a 10-12 base segment of RNA?

A. DNA ligase
*B. DNA primase
C. Topoisomerase
D. DNA polymerase I
E. DNA polymerase III

Which fills in small regions of DNA where the RNA primers were located?

A. DNA ligase
B. DNA primase
C. Topoisomerase
*D. DNA polymerase I
E. DNA polymerase III

Which forms a phosphodiester bond between adjacent Okazaki fragments to
create a continuous DNA strand?

*A. DNA ligase
B. DNA primase
C. Topoisomerase
D. DNA polymerase I
E. DNA polymerase III

Which synthesizes the leading strand of the DNA.

A. DNA ligase
B. DNA primase
C. Topoisomerase
D. DNA polymerase I
*E. DNA polymerase III

Which of the following is FALSE concerning the core enzyme and the holoenzyme of prokaryotic RNA polymerase?

A. The core enzyme completely lacks RNA pol activity.
B. Both the core enzyme and the holoenzyme accurately transcribe DNA in vitro.
C. The core enzyme has the sigma factor associated with it.
D. There is no physical difference between the core enzyme and the holoenzyme. They are just different names for the same protein complex
*E. Select this answer if all of the above are all false.

In E. coli, a mutation that leads to a decreased transcription rate of the affected gene and results in a decreased homology to the -10 box (TATA Box) consensus sequence is called a(n).....

A. Up mutation.
*B. Down mutation.
C. Constitutive mutation.
D. Consensus mutation.
E. Phenotypic mutation.

RNA proofreading activity

A. Is identical to proofreading activity for DNA except it involves RNA polymerase instead of DNA polymerase.
B. Has only been found in some types of eukaryotic cells, primarily fungi.
C. Has been recently discovered in E. coli.
D. Is critical for proper translation of mRNA.
*E. Has not been found for any known RNA polymerases

Introns are characterized by which of the following invariant consensus sequences?

*A. 5' GU-intron-AG
B. 5' CG-intron-CG
C. 5' GU-intron-UA
D. 5' GU-intron-UG
E. 5' UU-intron-GG

Which of the following is TRUE concerning pre-mRNAs?

A. They are larger than the corresponding mature mRNA.
B. These RNAs are the precursor molecules for the ribosomal RNA (rRNA).
C. These RNAs represent only the 5S rRNA and tRNA precursors.
D. They are only found within the nucleus and not the cytoplasm of the cell.
*E. More than one of the above.

Which of the following would most likely contain genes with the greatest number of introns?

A. E. coli cells
B. Yeast cells
*C. Human cells
D. Mitochondria
E. Bacteriophage lambda

Suppose you found a naturally occurring mRNA composed of alternating C and U residues. Which of the following peptides could be formed? The
codons for leucine are CUX and the codons for serine are UCX where X is any ribonucleotide.

*A. A polypeptide containing alternating leucine and serine residues.
B. A polypeptide containing either leucine or serine residues.
C. A polypeptide containing only leucine residues
D. A polypeptide containing only serine residues
E. Cannot be determined from the information provided.

Which of the following is correct for the one letter amino acid abbreviations for the following polypeptide: alanine, proline, glutamine, glycine, leucine, phenylalanine, arginine, tyrosine, tryptophan, asparagine, lysine

A. APYQHNKLFRR
B. APYGTRLKFNT
C. APGQWNKLFRY
D. APGNTRKLYRY
*E. APQGLFRYWNK

Which of the following is false regarding tRNA structure and function?

A. The enzyme that joins the amino acid to the proper tRNA is called aminoacyltRNA synthetase.
B. The anticodon of the tRNA is a three nucleotide sequence that base pairs with the corresponding codon of the mRNA.
C. Each tRNA has the sequence 5' CCA 3' at the 3' to which the amino acid is joined.
D. During translation, the tRNA with the attached amino acid enters the A site on the ribosome.
*E. Select this answer if choices A-D are all true.

Attenuation does not occur in eucaryotes because

*A. The nuclear membrane that does not allow coupled transcription and translation.
B. Eucaryotes only synthesize monocistronic mRNA.
C. Attenuation is unique to amino acid synthesis, and eucaryotic cells do not synthesize some amino acids.
D. Eucaryotic genes do not have well defined transcriptional termination signals.
E. Of the various forms of mRNA processing required prior to translation.

The lac operon is an example of a negative inducible genetic system because.....

A. It is prokaryotic instead of eukaryotic.
B. It requires an inducer (e.g. lactose) that binds directly to the promoter.
C. It is derepressed in the presence of the inducer.
D. It is repressed in the presence of the inducer.
*E. The binding of the lac repressor to the operator inhibits transcription by RNA polymerase.

Which of the following statements is FALSE?

*A. For a gene under negative repressible control, a small molecule is required to prevent the gene's repressor from binding to DNA.
B. An inducible gene is transcribed when a specific substance is present.
C. A repressible gene is controlled by a regulatory protein that inhibits transcription.
D. For a gene under positive repressible control, the normal state is transcription of a gene, stimulated by a transcriptional activator.
E. When a structural gene is under negative inducible control, a mutation that eliminates the repressor protein will cause the structural gene to be constitutively expressed.

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