Molecular Biology MCQs
1. In order to clone eukaryotic DNA into prokaryotic cells:
A) DNA with both exons and introns must be used
B) DNA without introns must be added
C) RNA with both exons and introns must be used
D) exons must be removed from eukaryotic DNA
Correct Answer: B) DNA without introns must be added
Explanation: Prokaryotic cells (bacteria) do not have the molecular machinery (spliceosomes) to remove introns from eukaryotic gene transcripts. Therefore, to express a eukaryotic protein in bacteria, the DNA used must be a “pre-spliced” version containing only exons.
2. In the process of cloning eukaryotic DNA into prokaryotic cells, the role of reverse transcriptase is to make:
A) double-stranded eukaryotic cDNA from mature mRNA2
B) mature mRNA from precursor mRNA3
C) bacterial DNA from eukaryotic DNA4
D) double-stranded DNA with introns added back5
Correct Answer: A) double-stranded eukaryotic cDNA from mature mRNA
Explanation: Reverse transcriptase is an enzyme that synthesizes DNA from an RNA template. By using mature (already spliced) mRNA from a eukaryote, researchers can create complementary DNA (cDNA) that lacks introns and can be successfully expressed in a bacterial host.
3. cDNA is:
A) DNA with both introns and exons that can be cloned into prokaryotes6
B) DNA with only introns that can be cloned into prokaryotes7
C) eukaryotic DNA with only exons that can be cloned into prokaryotes8
Correct Answer: C) eukaryotic DNA with only exons that can be 9cloned into prokaryotes
Explanation: cDNA stands for complementary DNA. Because it is synthesized from mature mRNA, all the non-coding introns have been removed, leaving only the protein-coding exons.
4. The segment of the DNA molecule where messenger RNA synthesis begins is called the…
A) promoter region.
B) sigma factor.
C) transcription terminator.
D) polymerase.
Correct Answer: A) promoter region.
Explanation: The promoter is a specific DNA sequence located upstream of a gene. It serves as the binding site for RNA polymerase and signals the exact point where transcription (RNA synthesis) should begin.
5. The complimentary messenger RNA strand that would be synthesized from the DNA base sequence of CTGAC would be…
A) GACTG
B) UGACU
C) AGTUG
D) GACUG
Correct Answer: D) GACUG
Explanation: In RNA synthesis, Cytosine (C) pairs with Guanine (G), Guanine (G) with Cytosine (C), Thymine (T) with Adenine (A), and Adenine (A) with Uracil (U). Therefore, DNA CTGAC matches to RNA GACUG.
6. The name of the structure that causes the synthesis of RNA to cease is called the…
A) promoter region
B) sigma factor
C) transcription terminator
D) polymerase
Correct Answer: C) transcription terminator
Explanation: Transcription ends when RNA polymerase reaches a specific sequence called the terminator. This sequence causes the newly formed RNA strand and the enzyme to detach from the DNA template.
7. Restriction enzymes:
A) cut donor DNA evenly so smooth edges result10
B) cut donor DNA but do not affect plasmids11
C) make staggered cuts at specific sequences in DNA in both donor DNA and plasmid12
D) are used to incorporate plasmids into bacterial host cells13
Correct Answer: C) make staggered cuts at specific sequences in DNA 14in both donor DNA and plasmid
Explanation: Most restriction enzymes recognize specific palindromic sequences and make staggered cuts, leaving “sticky ends.” These single-stranded overhangs allow the donor DNA to easily base-pair with a similarly cut plasmid.
8. Which of the following are limitations of DNA polymerase?
A) it can only add bases to the exposed 5′ end of a pre-existing strand
B) it can only replicate the leading strand15
C) it can only replicate the lagging strand16
D) none of the above (Note: Correction—the typical limitation is the 3′ end)17
Correct Answer: D) none of the above18
Expla19nation: The actual primary limitation of DNA polymerase is that it can only add nucleotides to the 3′ end of a pre-existing strand (it works in a $5′ \rightarrow 3’$ direction). None of the specific options provided accurately describe this fundamental rule.
9. The enzyme ________ unzips and unwinds the DNA molecule.
A) DNA polymerase
B) helicase
C) primase
D) DNA ligase
Correct Answer: B) helicase
Explanation: Helicase is responsible for breaking the hydrogen bonds between the nitrogenous bases, effectively “unzipping” the double helix to create a replication fork.
10. Which of the following statements about DNA replication is TRUE?
A) the leading strand is replicated continuously, while the lagging strand is replicated discontinuously202122
B) the leading strand is replicated discontinuously, while the lagging strand is replicated continuously232425
C) both the leading and lagging strands are replicated continuously262728
D29) both the leading and lagging strands are replicated discontinuously3031
Correct Answer: A) the leading strand is replicated continuously, while the lagging strand is replicated discont32inuously33
Explanation: Because DNA polymerase can only work in one direction, the leading strand follows the helicase smoothly. The lagging strand, however, must be made in short segments called Okazaki fragments.
11. Which of the following events do NOT occur in prophase of mitosis?
A) DNA condenses to form chromosomes
B) nuclear membrane breaks down
C) nucleolus breaks down
D) chromosomes are replicated
Correct Answer: D) chromosomes are replicated
Explanation: DNA/Chromosomes are replicated during the S phase of Interphase, which occurs well before mitosis begins. Prophase is focused on organizing and preparing the already replicated DNA for division.
12. The mitotic spindle fibers attach to chromosomes via special structures termed:
A) centrioles.
B) asters.
C) kinetochores.
D) centrosomes.
Correct Answer: C) kinetochores.
Explanation: Kinetochores are protein complexes located at the centromere of each sister chromatid. They act as the “anchor points” for the spindle microtubules during mitosis.
13. Which of the following statements about microtubules during anaphase is TRUE?
A) those attached to chromosomes elongate, while those that are unattached shorten
B) those attached to chromosomes shorten, while those that are unattached elongate34
C) both attached and unattached microtubules shorten35
D) both attached and unattached microtubules elongate36
Correct Answer: B) those attached to chromosomes shorten, while those that are unattached elongate37
Explanation: During anaphase, kinetochore microtubules shorten to pull chromosom38es toward the poles, while non-kinetochore microtubules elongate to push the poles further apart, lengthening the cell.
14. Phagocytes are attracted by all of the following except:
A) phospholipids released by injured cells
B) components of the complement system
C) chemical products of microorganisms
D) lysosomes
Correct Answer: D) lysosomes
Explanation: Phagocytes are attracted via chemotaxis to inflammatory signals (like complement or bacterial products). Lysosomes are internal organelles within the phagocyte and do not act as an external chemoattractant.
15. In phagocytosis the digested contents are eliminated by:
A) endocytosis
B) exocytosis
C) opsonization
D) C3b
Correct Answer: B) exocytosis
Explanation: After the phagolysosome has digested the pathogen, the waste material is moved to the cell membrane and released into the extracellular environment via exocytosis.
16. A lysosome:
A) is found only inside the bacterium being ingested
B) is an organelle that makes a bacterium motile
C) contains lysozyme and proteases which digest bacteria
D) contains enzymes specific to the receptors on the bacterium being eliminated39
Correct Answer: C) contains lysozyme and proteases which digest bacteria40
Explanation: Lysosomes are the “digestive system” of the cell. They contain acidic hydrolytic enzymes (like41 proteases and lysozymes) that break down organic molecules and pathogens.
17. Lysosomes are membrane-bound vesicles that arise from the:
A) nucleus.
B) mitochondria.
C) endoplasmic reticulum.
D) Golgi apparatus.
Correct Answer: D) Golgi apparatus.
Explanation: Proteins meant for lysosomes are synthesized in the ER, but the final processing, sorting, and budding of the lysosome vesicle occurs at the Golgi apparatus.
18. The lysosome contains ________ enzymes.
A) photosynthetic
B) anabolic
C) hydrolytic
D) melancholic
Correct Answer: C) hydrolytic
Explanation: Hydrolytic enzymes use water to break chemical bonds. This allows the lysosome to decompose proteins, nucleic acids, lipids, and complex sugars.
19. Which of the following cells will be expected to contain the most active lysosomes?
A) brain cell
B) skin cell
C) kidney cell
D) phagocytosing white blood cell
Correct Answer: D) phagocytosing white blood cell
Explanation: Cells specialized for defense, such as macrophages or neutrophils (white blood cells), must constantly digest engulfed pathogens, requiring a very high number of active lysosomes.
20. The number of chromosomes in daughter cells produced by meiosis is ________ the number of chromosomes in daughter cells produced by mitosis.
A) one-fourth
B) half
C) equal to
D) twice
Correct Answer: B) half
Explanation: Mitosis produces diploid cells ($2n$), whereas meiosis produces haploid cells ($n$). Therefore, meiotic daughter cells have half the chromosomes of mitotic ones.
21. Which of the following is unique to meiosis?
A) nuclear membrane breaks down
B) DNA organizes into chromosomes
C) crossing-over
D) chromosomes align along the central plate of the cell
Correct Answer: C) crossing-over
Explanation: Crossing-over (genetic recombination) occurs during Prophase I of meiosis. This exchange of genetic material between homologous chromosomes is a key source of genetic diversity and does not occur in mitosis.
22. The purpose of PCR is to:
A) make more copies of DNA primers to increase protein synthesis
B) make many copies of an organism’s DNA sequence so a small number of organisms will become large enough to be identified
C) make more RNA so large units of protein can be synthesized
D) recycle DNA using thermocyclers
Correct Answer: B) make many copies of an organism’s DNA sequence so a small number of organisms will become large enough to be identified42
Explanation: The Polymerase Chain Reaction (PCR) is a method for “amplifying” DNA. It allows scientists to take a tiny sample of DNA and create millions of identical copies for 43analysis.
23. For DNA amplification to occur, which of the following are needed?
A) loose ribonucleotides
B) RNA primers
C) thermostable DNA polymerase
D) b and c
Correct Answer: C) thermostable DNA polymerase
Explanation: PCR requires a DNA template, DNA primers (not RNA), dNTPs (DNA nucleotides, not ribonucleotides), and a thermostable DNA polymerase (like Taq) that can survive the high temperatures of the denaturation step.
24. Taq polymerase starts copying at:
A) the end of free single-stranded RNA
B) any open point
C) RNA primers attached to the end of the desired gene
D) DNA primers attached to the end of the desired gene
Correct Answer: D) DNA primers attached to the end of the desired gene
Explanation: Like all DNA polymerases, Taq requires a double-stranded starting point. In PCR, synthetic DNA primers are designed to bind (anneal) to the specific ends of the target sequence to provide this starting point.
25. Which of the following statements about DNA replication is TRUE?
(Note: Question 25 was a duplicate of 11/13 in your source, provided here for completeness.)
Correct Answer: A) the leading strand is replicated continuously, while the lagging strand is replicated discontinuously
This occurs because the two strands of the DNA double helix are antiparallel, but the enzyme DNA polymerase can only add nucleotides in one specific direction ($5′ \rightarrow 3’$).