Meselson and Stahl
Meselson and Stahl were able to identify the semi-conservative replication of DNA by using labeled nitrogen isotopes. Which of the following reflects the result of their investigation that proves that DNA undergoes a semi-conservative replication process?
D
The Hershey and Chase experiment served as powerful independent confirmation that DNA was indeed the genetic material. They made their discovery using which type of organism?
S. pneumoniae
mice
strains of bacteria
bacteriophage
bacteriophage
In the Hershey-Chase experiment, why was radioactive phosphorus (32P^{32}P32P) used to label DNA?
A. Phosphorus is only found in the sugar-phosphate backbone of DNA.
B. Phosphorus is unique to DNA and absent from proteins.
C. Phosphorus is incorporated into amino acids during protein synthesis.
D. Phosphorus is the only element found in bacteriophage DNA.
Answer: A. Phosphorus is only found in the sugar-phosphate backbone of DNA.
During DNA replication, why does DNA polymerase require a primer to initiate synthesis?
A. It needs a free 3'-OH group to add nucleotides.
B. It cannot bind to single-stranded DNA directly.
C. It requires ATP from the primer for energy.
D. It can only function on RNA templates.
Answer: A. It needs a free 3'-OH group to add nucleotides.
In prokaryotes, which DNA polymerase is primarily responsible for replacing RNA primers with DNA?
A. DNA polymerase I
B. DNA polymerase II
C. DNA polymerase III
D. RNA polymerase
Answer: A. DNA polymerase I
If we assume that DNA replication is according to the conservative model. Starting with 14N14N (lighter) DNA, and after two generations in the 15N medium, bacterial cells will produce the following band/s in density-gradient centrifugation.
b
In the famous Avery, MacLeod, and McCarty experiment, the tube that was treated with _______ resulted in the destruction of the "transforming material."
lipase
RNase
DNase
protease
DNAse
What genetic mechanism explains the coat color patterns in calico cats?
A. Autosomal recessive inheritance
B. X-inactivation causing random silencing of coat color alleles
C. Polygenic inheritance influenced by environmental factors
D. Y-linked inheritance determining coat color
Answer: B. X-inactivation causing random silencing of coat color alleles
What feature of DNA polymerase ensures high fidelity during replication?
A. Its ability to synthesize both leading and lagging strands.
B. Its proofreading activity via 3'→5' exonuclease function.
C. Its rapid elongation rate during synthesis.
D. Its inability to synthesize de novo.
Answer: B. Its proofreading activity via 3'→5' exonuclease function.
What is the role of single-strand binding proteins (SSBs) during DNA replication?
A. To synthesize primers for DNA polymerase.
B. To unwind the DNA double helix.
C. To prevent reannealing of separated DNA strands.
D. To add nucleotides to the growing DNA strand.
Answer: C. To prevent reannealing of separated DNA strands.
Starting with 14N14N (lighter) DNA, and after two generations in the 15N medium, bacterial cells will produce the following band/s in density-gradient centrifugation:
C
When Frederick Griffith coined the term "transformation," this was in reference to the genetic re-programming of
strains of bacteria.
laboratory mice.
patients with pneumonia.
bacteriophages
strains of bacteria.
Why are male calico cats extremely rare?
A. Coat color genes are autosomal, and males lack the gene.
B. Males usually carry only one X chromosome and cannot be heterozygous for coat color.
C. Barr body formation prevents coat color variation in males.
D. Male calico cats result from a rare mutation in autosomal genes.
Answer: B. Males usually carry only one X chromosome and cannot be heterozygous for coat color.
Which enzyme prevents supercoiling ahead of the replication fork during DNA replication?
A. Helicase
B. DNA polymerase
C. Topoisomerase
D. Primase
Answer: C. Topoisomerase
Why does the synthesis of the lagging strand pose a challenge for DNA polymerase?
A. DNA polymerase cannot bind to the lagging strand directly.
B. The lagging strand is synthesized in the 5'→3' direction away from the replication fork.
C. DNA polymerase is unable to attach to RNA primers.
D. The lagging strand contains more A-T base pairs, requiring slower synthesis.
Answer: B. The lagging strand is synthesized in the 5'→3' direction away from the replication fork.
Meselson and Stahl transferred the bacteria from 15N medium to 14N medium. They allowed them to replicate for two generations. DNA extracted from these cells is centrifuged. The DNA obtained after the first round of replication is
A)Hybrid of one lighter and one heavier strand
a
Which experimental evidence directly led Watson and Crick to propose the double-helix model of DNA?
A. Chargaff's rules showing A=T and G≡C ratios
B. X-ray diffraction data showing a helical structure
C. Experiments showing DNA as the transforming principle
D. Density gradient experiments confirming semiconservative replication
Answer: B. X-ray diffraction data showing a helical structure
What would happen if X-inactivation did not occur in females?
A. All females would display uniform coat color patterns like males.
B. Females would express twice the amount of X-linked gene products compared to males.
C. Barr bodies would form in both males and females.
D. Females would have no active X chromosomes.
Answer: B. Females would express twice the amount of X-linked gene products compared to males.
Why are Okazaki fragments necessary during DNA replication?
A. The leading strand cannot synthesize DNA continuously.
B. DNA polymerase synthesizes DNA only in the 3'→5' direction.
C. The lagging strand is synthesized discontinuously due to antiparallel DNA strands.
D. DNA ligase needs fragmented DNA to function.
Answer: C. The lagging strand is synthesized discontinuously due to antiparallel DNA strands.
Telomerase is essential for maintaining chromosome length in eukaryotic cells because:
A. It synthesizes RNA primers for replication.
B. It adds repetitive sequences to the ends of chromosomes to prevent shortening.
C. It unwinds the DNA helix at telomeres.
D. It prevents mismatches during replication.
Answer: B. It adds repetitive sequences to the ends of chromosomes to prevent shortening.
Meselson and Stahl conducted an experiment to demonstrate that DNA replication is semiconservative. Imagine you perform a similar experiment, growing E. coli in a medium containing heavy nitrogen (15N^{15}N15N) for many generations. Then, you switch the E. coli to a medium with light nitrogen (14N^{14}N14N) and allow the bacteria to replicate their DNA.
You extract DNA after one generation and centrifuge it in a cesium chloride gradient. You observe a single band at an intermediate density.
What does this single band indicate about the nature of DNA replication?
Predict the band pattern if DNA was extracted after the second generation and centrifuged.
Suppose DNA replication was conservative instead of semiconservative. What band pattern would you observe after the first and second generations?
If you allowed the E. coli to continue replicating for three more generations, what would you observe in the banding pattern for semiconservative replication?
Which observation from Griffith’s transformation experiment suggested that DNA could transfer genetic information?
A. Smooth (S) strain bacteria killed mice, while rough (R) strain did not.
B. Heat-killed S strain could transform live R strain into virulent S strain.
C. DNAase treatment prevented transformation in R strain.
D. Proteinase treatment did not prevent transformation in R strain.
Answer: B. Heat-killed S strain could transform live R strain into virulent S strain.
A pedigree is presented where a rare genetic condition affects multiple generations. Analyze the following pedigree description and answer the questions below:
Description
Answer: C. X-linked dominant
Answer: C. 100%
Answer: C. The condition is never passed from father to son.
Answer: C. 50%
Answer: B. Affected fathers passing the condition only to their sons.
Answer: C. Affected males pass the condition to all daughters but no sons.
Telomerase plays a critical role in maintaining chromosome stability in eukaryotic cells.
Answer:
Telomerase is an enzyme that adds repetitive nucleotide sequences (e.g., TTAGGG in humans) to the ends of linear chromosomes (telomeres). During replication, the lagging strand cannot be fully synthesized at the 3' ends, leading to progressive shortening of chromosomes. Telomerase prevents this shortening by extending the template strand, allowing the synthesis of complete lagging strands.
Telomerase activity is higher in stem cells and cancer cells because these cells undergo frequent divisions. Without telomerase, stem cells would lose essential genetic material, leading to senescence or apoptosis. In cancer cells, telomerase activity is upregulated, enabling unlimited division and contributing to cellular immortality.
In the absence of telomerase, telomeres shorten with each cell division, eventually triggering DNA damage responses, cellular senescence, or apoptosis. Disorders associated with defective telomerase function include dyskeratosis congenita, which leads to premature aging, bone marrow failure, and other degenerative symptoms.
DNA replication involves the coordinated synthesis of leading and lagging strands at the replication fork.
Answer:
The leading strand is synthesized continuously in the 5'→3' direction toward the replication fork. The lagging strand is synthesized discontinuously in the 5'→3' direction away from the replication fork, forming short Okazaki fragments.
The lagging strand is more prone to replication errors because it requires frequent priming, synthesis, and ligation of Okazaki fragments, increasing the likelihood of mismatches or gaps. Cells mitigate these risks through proofreading by DNA polymerase (3'→5' exonuclease activity) and mismatch repair mechanisms, which identify and correct replication errors after synthesis.