E. coli grown for many generations in 15N medium is moved to 14N medium. After two rounds of replication, density centrifugation of its DNA shows:

One-quarter heavy, rest light DNA

Where do the restriction endonucleases used in recombinant DNA technology cut DNA?

At specific palindromic sequences

At random points along the strand

Wherever adenine pairs with thymine

Only at the ends of the molecule

A man who eats no fruit or vegetables has bleeding gums and poor wound healing. Which post-translational step in collagen synthesis is impaired?

Hydroxylation of proline and lysine

Gamma-carboxylation of glutamate

Molecular biology and its techniques

56 Kerala PSC Lab Technician Gr II practice questions on Molecular biology and its techniques, part of Biochemistry. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
E. coli grown for many generations in 15N medium is moved to 14N medium. After two rounds of replication, density centrifugation of its DNA shows:
1. AAll hybrid DNA
2. BHalf heavy and half light DNA
3. CHalf hybrid and half light DNA
4. DOne-quarter heavy, rest light DNA
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Answer: C · Half hybrid and half light DNA
Replication is semi-conservative: each new molecule keeps one parent strand and gains one new strand. After the first round in 14N, every molecule has one heavy and one light strand, so all DNA is hybrid. In the second round, each hybrid molecule gives one hybrid and one fully light molecule, so the DNA is half hybrid and half light. This is the result Meselson and Stahl obtained, which ruled out conservative replication.
- A: All hybrid DNA is the result after only one round.
- B: Half heavy and half light would mean conservative replication after one round.
- D: One-quarter heavy is what conservative replication would give after two rounds.
Question 2 · Easy
Where do the restriction endonucleases used in recombinant DNA technology cut DNA?
1. AAt random points along the strand
2. BWherever adenine pairs with thymine
3. COnly at the ends of the molecule
4. DAt specific palindromic sequences
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Answer: D · At specific palindromic sequences
Type II restriction enzymes recognize short sequences, usually 4 to 8 base pairs, that read the same on both strands, and cut within them. EcoRI, for example, cuts GAATTC and leaves sticky ends. Cutting the vector and the insert with the same enzyme gives matching ends that can be joined. In bacteria these enzymes destroy invading phage DNA, while the host's own DNA is protected by methylation.
- A: Random cutting would make cloning impossible to plan.
- B: Each enzyme needs its own specific recognition sequence, not any A-T pair.
- C: Endonucleases cut inside the molecule; exonucleases work from the ends.
Question 3 · Medium
Leucine is coded by six different codons. What is this property of the genetic code called?
1. AAmbiguity
2. BUniversality
3. CDegeneracy
4. DNon-overlapping
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Answer: C · Degeneracy
There are 64 codons, and 61 of them code for only 20 amino acids, so most amino acids have more than one codon. This is degeneracy (redundancy), and the codons for one amino acid usually differ only in the third base. The code is not ambiguous, because each codon specifies only one amino acid. Degeneracy explains silent mutations, which change a codon without changing the amino acid.
- A: Ambiguity would mean one codon codes several amino acids, which does not happen.
- B: Universality means the same code is used by almost all organisms.
- D: Non-overlapping means each base belongs to only one codon.
Question 4 · Medium
A man who eats no fruit or vegetables has bleeding gums and poor wound healing. Which post-translational step in collagen synthesis is impaired?
1. APhosphorylation of serine
2. BGlycosylation of asparagine
3. CHydroxylation of proline and lysine
4. DGamma-carboxylation of glutamate
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Answer: C · Hydroxylation of proline and lysine
Prolyl and lysyl hydroxylases need vitamin C to keep their iron in the ferrous state. Hydroxyproline stabilizes the collagen triple helix, and hydroxylysine allows cross-linking. Without vitamin C, collagen is weak, which causes the bleeding gums, poor wound healing and fragile vessels of scurvy. Gamma-carboxylation of glutamate is the vitamin K-dependent step in clotting factors II, VII, IX and X.
- A: Serine phosphorylation regulates enzymes; it is not the step lost in scurvy.
- B: N-linked glycosylation of asparagine does not need vitamin C.
- D: Gamma-carboxylation needs vitamin K and affects clotting factors.
Question 5 · Easy
Which blotting technique is used to detect a specific RNA?
1. ASouthwestern blotting
2. BNorthern blotting
3. CSouthern blotting
4. DWestern blotting
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Answer: B · Northern blotting
Southern blotting, named after Edwin Southern, detects DNA, and Northern blotting was named by analogy for RNA. In a Northern blot, RNA is separated by size on a gel, transferred to a membrane and detected with a labelled complementary probe. It shows whether a gene is expressed in a tissue and the size of its mRNA. Western blotting detects proteins with labelled antibodies.
- A: Southwestern blotting detects DNA-binding proteins.
- C: Southern blotting detects DNA fragments.
- D: Western blotting detects proteins with antibodies.
Question 6 · Easy
All of the following are stop (termination) codons EXCEPT:
1. AUAA
2. BUGA
3. CUAG
4. DAUG
Answer and explanation in LearnFree account
Question 7 · Hard
A PCR starts with 100 copies of the target DNA. Assuming 100% efficiency, about how many copies are present after 20 cycles?
1. A2.0 × 10^3
2. B1.05 × 10^8
3. C5.24 × 10^7
4. D1.05 × 10^6
Answer and explanation in LearnFree account
Question 8 · Medium
In fluorescence in situ hybridization (FISH), how does the labelled probe attach to its target?
1. AComplementary base pairing
2. BAntigen-antibody binding
3. CRestriction enzyme cutting
4. DCovalent bonding to histones
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Question 9 · Medium
A child with xeroderma pigmentosum develops many skin cancers on sun-exposed areas. Which DNA repair pathway is defective?
1. ANucleotide excision repair
2. BDouble-strand break repair
3. CMismatch repair
4. DBase excision repair
Answer and explanation in LearnFree account
Question 10 · Easy
Which enzyme synthesizes messenger RNA in eukaryotic cells?
1. ADNA polymerase alpha
2. BRNA polymerase II
3. CRNA polymerase I
4. DRNA polymerase III
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Question 11 · Medium
In gene cloning, why does a plasmid vector carry an antibiotic resistance gene?
1. ATo help the plasmid enter the bacteria
2. BTo select bacteria that took up the plasmid
3. CTo make the insert easy to cut out
4. DTo make the plasmid copy itself faster
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Question 12 · Medium
How does a cDNA library differ from a genomic library?
1. AIs the same in every tissue of the body
2. BHas only expressed sequences, without introns
3. CHas introns, promoters and spacer DNA
4. DIs made directly from chromosomal DNA
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Question 13 · Easy
During DNA replication, which enzyme unwinds the double helix by breaking hydrogen bonds between base pairs?
1. ADNA ligase
2. BPrimase
3. CDNA gyrase
4. DHelicase
Answer and explanation in LearnFree account
Question 14 · Easy
Which enzyme finally joins the Okazaki fragments on the lagging strand?
1. ATopoisomerase I
2. BDNA ligase
3. CRNA primase
4. DDNA polymerase I
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Question 15 · Hard
In a conventional PCR run, the no-template control shows a band of the same size as the positive control. The most likely cause is:
1. AContamination with amplified product
2. BInhibitors in the patient samples
3. CToo few amplification cycles
4. DFormation of primer-dimers
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Question 16 · Easy
During translation, which RNA carries the anticodon that pairs with a codon on mRNA?
1. ASmall nuclear RNA
2. BMessenger RNA
3. CRibosomal RNA
4. DTransfer RNA
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Question 17 · Medium
Once a protein has been tagged with a chain of ubiquitin molecules, it is:
1. ADegraded in the proteasome
2. BAnchored in the cell membrane
3. CSecreted out of the cell
4. DImported into mitochondria
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Question 18 · Medium
In agarose gel electrophoresis, DNA fragments separate according to their length. Why?
1. ABase composition sets each fragment's speed
2. BShorter fragments carry more charge per base
3. CLonger fragments carry more charge per base
4. DCharge per length is equal, so the gel sieves by size
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Question 19 · Easy
Restriction-cut DNA is run on agarose, transferred to a nylon membrane and probed with labelled DNA. This method is called:
1. AWestern blotting
2. BNorthern blotting
3. CDot blotting
4. DSouthern blotting
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Question 20 · Easy
Adjacent nucleotides in one strand of DNA are joined by:
1. APeptide bonds
2. B3',5'-Phosphodiester bonds
3. CN-glycosidic bonds
4. DHydrogen bonds
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Question 21 · Easy
Reverse transcriptase, used in RT-PCR and to make cDNA, synthesizes:
1. ARNA from a DNA template
2. BDNA from an RNA template
3. CProtein from an mRNA template
4. DRNA from an RNA template
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Question 22 · Medium
Several members of one family had colon cancer before 50 years of age, without multiple polyps. Their tumors show loss of MLH1 and microsatellite instability. Which DNA repair system is defective?
1. AHomologous recombination repair
2. BBase excision repair
3. CNucleotide excision repair
4. DMismatch repair
Answer and explanation in LearnFree account
Question 23 · Medium
In eukaryotes, the introns of the primary transcript (hnRNA) are removed by:
1. ARibosomes
2. BRestriction endonucleases
3. CRNA polymerase II
4. DSpliceosomes
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Question 24 · Easy
DNA polymerase adds each new nucleotide to which end of the growing strand?
1. ABoth ends at the same time
2. BThe free 3'-hydroxyl end
3. CThe free 5'-phosphate end
4. DEither end, as needed
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Question 25 · Hard
Beta-globin PCR gives a 380 bp product. DdeI cuts the normal allele into 260 and 120 bp; the sickle mutation abolishes this site. After digestion, a known HbAA control shows only 260 and 120 bp bands. A child's sample shows 380, 260 and 120 bp bands. What does this show?
1. AHeterozygous for the sickle mutation
2. BHomozygous for the sickle mutation
3. CDigestion incomplete; repeat test
4. DNo sickle mutation present
Answer and explanation in LearnFree account

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Molecular biology and its techniques

E. coli grown for many generations in 15N medium is moved to 14N medium. After two rounds of replication, density centrifugation of its DNA shows:

Where do the restriction endonucleases used in recombinant DNA technology cut DNA?

Leucine is coded by six different codons. What is this property of the genetic code called?

A man who eats no fruit or vegetables has bleeding gums and poor wound healing. Which post-translational step in collagen synthesis is impaired?

Which blotting technique is used to detect a specific RNA?

All of the following are stop (termination) codons EXCEPT:

A PCR starts with 100 copies of the target DNA. Assuming 100% efficiency, about how many copies are present after 20 cycles?

In fluorescence in situ hybridization (FISH), how does the labelled probe attach to its target?

A child with xeroderma pigmentosum develops many skin cancers on sun-exposed areas. Which DNA repair pathway is defective?

Which enzyme synthesizes messenger RNA in eukaryotic cells?

In gene cloning, why does a plasmid vector carry an antibiotic resistance gene?

How does a cDNA library differ from a genomic library?

During DNA replication, which enzyme unwinds the double helix by breaking hydrogen bonds between base pairs?

Which enzyme finally joins the Okazaki fragments on the lagging strand?

In a conventional PCR run, the no-template control shows a band of the same size as the positive control. The most likely cause is:

During translation, which RNA carries the anticodon that pairs with a codon on mRNA?

Once a protein has been tagged with a chain of ubiquitin molecules, it is:

In agarose gel electrophoresis, DNA fragments separate according to their length. Why?

Restriction-cut DNA is run on agarose, transferred to a nylon membrane and probed with labelled DNA. This method is called:

Adjacent nucleotides in one strand of DNA are joined by:

Reverse transcriptase, used in RT-PCR and to make cDNA, synthesizes:

Several members of one family had colon cancer before 50 years of age, without multiple polyps. Their tumors show loss of MLH1 and microsatellite instability. Which DNA repair system is defective?

In eukaryotes, the introns of the primary transcript (hnRNA) are removed by:

DNA polymerase adds each new nucleotide to which end of the growing strand?

Beta-globin PCR gives a 380 bp product. DdeI cuts the normal allele into 260 and 120 bp; the sickle mutation abolishes this site. After digestion, a known HbAA control shows only 260 and 120 bp bands. A child's sample shows 380, 260 and 120 bp bands. What does this show?