Question
Which stage of DNA profiling involves the polymerase chain reaction (PCR)?
A. During extraction of the DNA sample
B. During replication of the DNA that has been cut with restriction enzymes
C. During electrophoresis to separate the DNA fragments on a gel
D. During incubation with labelled probes
▶️Answer/Explanation
Answer: B. During replication of the DNA that has been cut with restriction enzymes
Explanation:
- DNA profiling is a technique used to identify individuals by analyzing specific regions of their DNA.
- The polymerase chain reaction (PCR) is a laboratory method that creates many copies of a particular DNA segment. In DNA profiling, PCR is used to amplify the DNA fragments after they have been cut by restriction enzymes, making it easier to analyze these fragments in later steps.
Answer Evaluation:
A. Incorrect – Extraction is the process of isolating DNA from cells; PCR is not involved here.
B. Correct – PCR amplifies the DNA fragments after restriction enzymes cut the DNA, enabling further analysis.
C. Incorrect – Electrophoresis separates DNA fragments by size; it does not amplify DNA.
D. Incorrect – Incubation with labelled probes is for detecting specific sequences, not for amplifying DNA.
Question
What is the role of DNA polymerase during DNA replication?
A. It adds nucleotides to the growing strand.
B. It adds nucleotides to the template strand.
C. It builds messenger RNA.
D. It carries out translation.
▶️Answer/Explanation
Answer: A. It adds nucleotides to the growing strand.
Explanation:
- DNA polymerase is an enzyme that plays a crucial role during DNA replication by adding new nucleotides one by one to the end of a growing DNA strand. This enzyme reads the template strand and matches complementary nucleotides to create a new, accurate copy of the DNA.
- It cannot add nucleotides to the template strand itself; it only builds the new complementary strand.
- Building messenger RNA is the job of RNA polymerase, not DNA polymerase.
- Translation is the process of making proteins from mRNA and does not involve DNA polymerase.
Answer Evaluation:
A. Correct – DNA polymerase adds nucleotides to the new strand during replication.
B. Incorrect – DNA polymerase does not add nucleotides to the template strand; it uses it as a guide.
C. Incorrect – RNA polymerase builds messenger RNA, not DNA polymerase.
D. Incorrect – Translation is carried out by ribosomes, not DNA polymerase.
Question
Which sequence of bases and amino acids could be produced by transcription and translation of the DNA molecule shown?
Answer/Explanation
Answer: D
Explanation:
- Transcription is the process where a messenger RNA (mRNA) strand is made using the DNA template strand.
- Translation is when this mRNA is used to make a chain of amino acids, forming a protein.
- The template strand of DNA runs 3′ to 5′, and the mRNA is built in the 5′ to 3′ direction by matching complementary RNA bases.
- Using the DNA template strand:
3′ ATGAAATGCTTTTCGCGGG 5′
The complementary mRNA will be:
5′ UAC UUU ACG AAA AGC CCC 3′
This mRNA codes for the following amino acids using the codon chart:
UAC = Tyr, UUU = Phe, ACG = Thr, AAA = Lys, AGC = Ser, CCC = Pro
Answer Evaluation:
A. Incorrect – The amino acid sequence shown does not match the correct translation of the DNA template strand.
B. Incorrect – The mRNA sequence and amino acid chain do not align with the given DNA template strand.
C. Incorrect – Although the sequence starts with Met (AUG), the full base and amino acid sequence do not match the DNA template.
D. Correct – The base sequence is transcribed correctly from the DNA template strand, and the amino acid chain matches the codon table.
Question
What is replicated by a semi-conservative process?
A. Messenger RNA (mRNA) only
B. Messenger RNA (mRNA) and transfer RNA (tRNA) only
C. Messenger RNA (mRNA), transfer RNA (tRNA) and DNA only
D. DNA only
Answer/Explanation
Answer: D. DNA only
Explanation:
- The semi-conservative replication process refers specifically to how DNA is copied. During replication, each new DNA molecule contains one original (parental) strand and one newly synthesized strand, conserving half of the original molecule.
- Messenger RNA (mRNA) and transfer RNA (tRNA) are produced by transcription, not replication. They are synthesized from DNA templates but are not replicated semi-conservatively.
- Only DNA undergoes semi-conservative replication to ensure genetic information is accurately passed on during cell division.
Answer Evaluation:
A. Incorrect – mRNA is made by transcription, not semi-conservative replication.
B. Incorrect – Both mRNA and tRNA are synthesized by transcription, not replicated semi-conservatively.
C. Incorrect – mRNA and tRNA are not replicated by semi-conservative replication, only DNA is.
D. Correct – Semi-conservative replication applies only to DNA.
Question
What enables bacteria to produce human growth hormone?
A. DNA replication is semi-conservative.
B. The polymerase chain reaction can be used.
C. They need the hormone for growth.
D. The genetic code is universal.
Answer/Explanation
Answer: D. The genetic code is universal.
Explanation:
- The genetic code is the set of rules by which DNA sequences are translated into proteins. It is universal, meaning that almost all organisms—including bacteria and humans—use the same code to produce proteins.
- This universality allows bacteria to read and produce human proteins, such as human growth hormone (HGH), when given the human gene inserted into their DNA.
- DNA replication being semi-conservative and the use of PCR are important biological processes but do not explain why bacteria can produce human proteins.
- Bacteria do not need human growth hormone for their own growth.
Answer Evaluation:
A. Incorrect – Semi-conservative DNA replication does not enable bacteria to produce human proteins.
B. Incorrect – PCR is a lab technique for copying DNA but does not explain protein production in bacteria.
C. Incorrect – Bacteria do not require human growth hormone for growth.
D. Correct – The universal genetic code allows bacteria to produce human proteins from inserted human genes.
Question
What is a reason that Taq polymerase is a suitable enzyme for use in the polymerase chain reaction (PCR)?
A. It can work at a wide range of pH.
B. It works at higher temperatures than most enzymes.
C. It can separate two strands of DNA.
D. It allows DNA to be replicated without the use of primers.
▶️Answer/Explanation
Answer: B. It works at higher temperatures than most enzymes.
Explanation:
- Taq polymerase is an enzyme derived from the bacterium Thermus aquaticus, which lives in hot springs. Because of this, it is stable and functional at high temperatures, such as those used in PCR during the DNA denaturation step (around 95°C).
- Most enzymes would denature (lose function) at such high temperatures, but Taq polymerase remains active, making it ideal for PCR, where repeated heating and cooling cycles are necessary.
- Taq polymerase does not separate DNA strands; that is done by heat in PCR.
- Primers are essential for DNA replication in PCR; Taq polymerase cannot replicate DNA without primers.
- Its ability to function at a wide pH range is not the primary reason it is used in PCR.
Answer Evaluation:
A. Incorrect – Although Taq polymerase is stable, its pH range is not the main reason for its use in PCR.
B. Correct – Taq polymerase can withstand the high temperatures used in PCR, making it suitable for the process.
C. Incorrect – DNA strand separation in PCR occurs by heat, not by Taq polymerase.
D. Incorrect – DNA replication in PCR requires primers; Taq polymerase cannot replicate without them.
Question
Which are two proteins that assist in the unwinding and separation of DNA strands during replication?
A. Helicase and DNA polymerase III
B. DNA gyrase and DNA polymerase I
C. Helicase and DNA primase
D. Single-strand binding protein and DNA gyrase
▶️Answer/Explanation
Answer: D. Single-strand binding protein and DNA gyrase
Explanation:
- During DNA replication, the double helix must be unwound and the strands separated. Two key proteins help in this process:
- Helicase breaks the hydrogen bonds between base pairs to unwind the DNA helix, and DNA gyrase (a type of topoisomerase) reduces the tension caused by unwinding, preventing the DNA from becoming supercoiled.
- Single-strand binding proteins (SSBs) stabilize the separated strands and keep them apart so they don’t re-anneal.
- DNA polymerase enzymes synthesize new strands but do not assist with unwinding.
- DNA primase adds RNA primers but does not help separate or unwind DNA.
Answer Evaluation:
A. Incorrect – Helicase is correct, but DNA polymerase III synthesizes new DNA, not involved in unwinding.
B. Incorrect – DNA gyrase is involved in unwinding tension, but DNA polymerase I removes primers and fills gaps, not unwinding.
C. Incorrect – Helicase is correct, but DNA primase synthesizes RNA primers, not involved in strand separation.
D. Correct – DNA gyrase reduces torsional strain ahead of the replication fork, and SSBs keep the strands apart after they are separated.
Question
What is a function of the enzyme helicase?
A. It coils DNA up into a double helical shape.
B. It links DNA nucleotides in a new DNA strand.
C. It breaks hydrogen bonds between the DNA strands.
D. It forms temporary hydrogen bonds to produce messenger RNA.
▶️Answer/Explanation
Answer: C. It breaks hydrogen bonds between the DNA strands.
Explanation:
- Helicase is an essential enzyme in DNA replication. Its main function is to break the hydrogen bonds between the base pairs of the two DNA strands, allowing the double helix to unwind and separate into two single strands.
- This separation is necessary so that each strand can serve as a template for building a new complementary strand.
- Helicase does not coil DNA, link nucleotides, or produce RNA. Those are the roles of other enzymes like DNA polymerase and RNA polymerase.
Answer Evaluation:
A. Incorrect – DNA naturally coils into a helix; helicase unwinds it, not coils it.
B. Incorrect – DNA polymerase links nucleotides in the new strand, not helicase.
C. Correct – Helicase breaks hydrogen bonds between DNA strands during replication.
D. Incorrect – RNA polymerase forms RNA from DNA templates; helicase is not involved in RNA production.
Question
What is a function of DNA polymerase I?
A. Adds nucleotides in a 5’ to 3’ direction to elongate the chain
B. Uncoils the DNA double helix and splits it into two template strands
C. Removes RNA primer and replaces it with DNA
D. Produces sugar-phosphate bonds to link Okazaki fragments
▶️Answer/Explanation
Answer: C. Removes RNA primer and replaces it with DNA
Explanation:
- DNA polymerase I is an enzyme involved in DNA replication, especially on the lagging strand. Its main function is to remove the RNA primers laid down by primase and replace them with DNA nucleotides.
- It works after DNA polymerase III has added most of the nucleotides, and before DNA ligase connects fragments.
- DNA polymerase III is responsible for the main elongation of the DNA strand in the 5’ to 3’ direction.
- Unwinding and separating strands is the job of helicase, and the linking of Okazaki fragments is done by DNA ligase.
Answer Evaluation:
A. Incorrect – DNA polymerase III adds nucleotides for elongation, not DNA polymerase I.
B. Incorrect – Helicase unwinds and splits DNA strands, not DNA polymerase I.
C. Correct – DNA polymerase I removes RNA primers and replaces them with DNA nucleotides.
D. Incorrect – DNA ligase links Okazaki fragments by forming sugar-phosphate bonds, not DNA polymerase I.
Question
What is PCR used for?
A. Separate fragments of DNA by size
B. Amplify small amounts of DNA
C. Compare DNA samples
D. Genetically modify organisms’ DNA
▶️Answer/Explanation
Answer: B. Amplify small amounts of DNA
Explanation:
- The polymerase chain reaction (PCR) is a laboratory technique used to amplify, or make many copies of, a specific segment of DNA. This is especially useful when only a tiny amount of DNA is available, such as in forensic analysis or ancient DNA samples.
- PCR uses repeated cycles of heating and cooling along with primers, DNA polymerase (Taq polymerase), and free nucleotides to create millions of copies of the target DNA sequence.
- It does not separate DNA fragments (that is electrophoresis), compare samples (done after PCR), or alter DNA (as in genetic engineering).
Answer Evaluation:
A. Incorrect – DNA fragment separation by size is done by gel electrophoresis, not PCR.
B. Correct – PCR amplifies small amounts of DNA by making many copies.
C. Incorrect – Comparing DNA samples is done after PCR, often using gel electrophoresis or DNA profiling.
D. Incorrect – Genetically modifying DNA involves recombinant DNA technology, not PCR directly.