Molecular Biology MCQ Quiz - Objective Question with Answer for Molecular Biology - Download Free PDF

The correct answer is option 3.

Key Points

• Human insulin was the first hormone to be produced using recombinant DNA technology. Hence, option 3 is correct.
• It was developed by Genentech and Eli Lilly in the early 1980s, marking a major breakthrough in biotechnology and medicine.
• This synthetic insulin is called "Humulin", and it is produced by inserting the human insulin gene into E. coli bacteria, which then synthesize insulin.

The correct answer is Statement I is incorrect but Statement II is correct

Concept:

• The differentiation between recombinant and non-recombinant colonies using blue-white screening is a widely used method in molecular biology.
• It is based on the insertional inactivation of the lacZ gene, which encodes the enzyme β-galactosidase.
• In this method, a chromogenic substrate such as X-gal is used. The β-galactosidase enzyme cleaves X-gal, producing a blue-colored product.
• Recombinant colonies are identified by the insertion of foreign DNA into the multiple cloning site of the plasmid, which disrupts the lacZ gene and prevents the production of β-galactosidase, resulting in white colonies.
• Non-recombinant colonies retain an intact lacZ gene and produce β-galactosidase, leading to blue-colored colonies.

Explanation:

Statement I: "The blue-colored colonies have DNA insert in the plasmid and they are identified as recombinant colonies."

• This statement is incorrect because the blue-colored colonies represent non-recombinant colonies.
• These colonies have an intact lacZ gene, which produces β-galactosidase, resulting in the cleavage of X-gal and the formation of blue color.
• No DNA insert is present in these colonies, and thus they are not recombinant.

Statement II: "The colonies without blue color have DNA insert in the plasmid and are identified as recombinant colonies."

• This statement is correct because the absence of blue color (white colonies) indicates the disruption of the lacZ gene by the insertion of foreign DNA.
• The lack of β-galactosidase activity results in no cleavage of X-gal, and thus no blue color is produced.
• These colonies are recombinant as they contain the inserted DNA fragment in the plasmid.

The correct answer is ρ (Rho)

Explanation:

• Transcription is the process by which a DNA sequence is copied into RNA. It is carried out by RNA polymerase and includes three main stages: initiation, elongation, and termination.
  • Initiation: In this stage, RNA polymerase binds to the promoter region of the DNA, forming a transcription initiation complex. In prokaryotes, this process often involves the transient association of a sigma factor (σ) with the RNA polymerase core enzyme, which helps in recognizing and binding to the promoter sequence.
  • Elongation: Once the initiation complex is formed, RNA polymerase begins moving along the DNA template, synthesizing RNA in the 5' to 3' direction. During elongation, RNA polymerase does not require the transient association of initiation or termination factors. It simply catalyzes the addition of ribonucleotides to the growing RNA chain based on the complementary base pairing with the DNA template.
  • Termination: In the termination stage, RNA polymerase recognizes specific sequences in the DNA template that signal the end of the gene or transcription unit. In prokaryotes, termination often involves the transient association of a termination factor (ρ) with the RNA polymerase complex, leading to the release of the newly synthesized RNA molecule and dissociation of RNA polymerase from the DNA template.

Other Options:

• α (Alpha): Alpha subunits are part of the core RNA polymerase enzyme complex and are involved in the assembly and stability of RNA polymerase. They play a role in initiating transcription but are not involved in termination.
• σ (Sigma): Sigma factors are essential for the initiation of transcription. They help RNA polymerase recognize and bind to specific promoter sequences but dissociate from RNA polymerase after initiation. Sigma factors are not involved in transcription termination.
• γ (Gamma): Gamma is not a factor associated with transcription in prokaryotes.

The correct answer is C and D only

Explanation:

• Gene cloning is a method used to create identical copies of a specific gene or DNA segment. It involves isolating a desired gene, inserting it into a vector, and introducing it into a host organism to amplify and express the gene.
• Enzymes play a crucial role in the various steps of gene cloning. However, not all enzymes are essential for the process.
• The most commonly used enzymes in gene cloning are restriction enzymes, DNA ligase, and DNA polymerase.
  • Restriction enzymes: These are essential enzymes for gene cloning. They recognize specific DNA sequences and cut the DNA at or near these sites, producing fragments that can be inserted into vectors.
  • DNA ligase: This enzyme is critical for gene cloning. It joins the DNA fragments (e.g., the insert DNA and the vector) by forming phosphodiester bonds, making a stable recombinant DNA molecule.
  • DNA polymerase: DNA polymerase is sometimes used in gene cloning for amplifying DNA through techniques like PCR (Polymerase Chain Reaction) or for filling in DNA overhangs after restriction digestion.
• DNA mutase: This enzyme is not essential for gene cloning. DNA mutase is involved in introducing mutations into DNA, which is not a requirement for cloning
• DNA recombinase: While recombinases facilitate site-specific recombination and have applications in advanced genetic engineering (e.g., CRISPR or recombineering), they are not essential for basic gene cloning processes.

The correct answer is George Gamow

Explanation:

• The genetic code refers to the set of rules by which information encoded in DNA or RNA is translated into proteins, the functional molecules in cells.
• Proteins are composed of amino acids, and the sequence of amino acids is determined by the sequence of nucleotides in the genetic material.
• The concept of the genetic code being made up of three nucleotides, known as codons, was first proposed by George Gamow, a physicist.

George Gamow:

• George Gamow proposed the idea of the triplet code in 1954. He theorized that a combination of three nucleotides could encode one amino acid.
• He argued that there are only 4 bases and if they have to code for 20 amino acids, the code should constitute a combination of bases.
• He suggested that in order to code for all 20 amino acids, the code should be made up of three nucleotides. This was a very bold proposition, because a permutation combination of 4³ (4 × 4 × 4) would generate 64 codons; generating many more codons than required.

Other Options:

• Francis Crick: James Watson and Francis Crick are credited with discovering the double helix structure of DNA in 1953.
• Jacques Monod: Jacques Monod was a molecular biologist known for his work on gene regulation, particularly the lac operon in bacteria.
• Franklin Stahl: Franklin Stahl is known for his work on DNA replication, specifically the Meselson-Stahl experiment that demonstrated the semi-conservative mechanism of DNA replication.

Explanation:

Similarities between DNA & RNA are:

• Both DNA and RNA are genetic material.
• DNA and RNA are both large biological polymers of nucleotides.
• Both DNA and RNA consist of sugar, nitrogenous bases, and a phosphate backbone.
• On both DNA & RNA Guanine & Cytosine are complementary to each other i.e. they form pair
• Complementary base pairs are connected by hydrogen bonding. Two hydrogen bonds form between adenine and either thymine or uracil, while three hydrogen bonds form between cytosine and guanine.

Additional Information 

Difference between DNA and RNA-

Concept:

• DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule.
• It is an enzymatically governed process, many enzymes participate in DNA replication.

Explanation:

• The principal enzyme for DNA replication is DNA-dependent DNA polymerase
• DNA replication process uses DNA polymerase III to catalyzes the joining of deoxyribonucleoside 5′-triphosphates (dNTPs) at a growing chain of DNA.
• DNA is read by DNA polymerase in the 3′ to 5′ direction, meaning the new strand is synthesized in the 5' to 3' direction.
• In the DNA replication process, one new strand (leading strand) is made as a continuous piece. The other (lagging strand) is made in small pieces.
• At the leading strand, DNA polymerase III synthesizes the new strand in the 5′ to 3′ direction.
• The lagging strand is elongated in an antiparallel direction, by the addition of short RNA primers (Okazaki fragment) which are filled with other joining fragments.
• The Okazaki fragments formed at the lagging strand are joined together by the enzyme DNA ligase, for this reason, DNA ligase is also called molecular glue
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            DNA Replication Fork
Additional Information

Other Important Enzyme in DNA Replication

Concept:

• Central dogma represents the flow of genetic information from DNA to RNA to protein.
• It refers to a process in which the information in DNA is converted into a functional product.

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Explanation:

• The formation of DNA from DNA is replication.
• The formation of mRNA from DNA is called transcription.
• The formation of protein from mRNA is called translation.
• Transduction is the transfer of genetic material from one bacterium to another with the help of a virus or a bacteriophage.
• Therefore, the correct answer is option 4.​

Concept:

• Adenosine Deaminase (ADA) is an enzyme required for the proper functioning of the immune system.
• ADA deficiency is caused due to deletion of the gene for the enzyme adenosine deaminase.

Explanation:

• ADA deficiency can be treated through gene therapy. 
• In gene therapy, the lymphocytes from the patient are grown outside the body and a functional ADA cDNA is introduced (using a retroviral vector) into the lymphocytes.
• The genetically engineered lymphocytes are then introduced back into the patient.
• These genetically engineered lymphocytes are immortal hence, the patient has to be infused with such lymphocytes regularly.

So, the correct answer is option 1.

Additional Information:

• Introducing genes for ADA into the cells at early embryonic development can lead to a permanent cure for ADA deficiency.

The correct answer is Keratin.

Key Points

• Keratin Composition: Keratin is a family of fibrous structural proteins, also known as scleroproteins. It is the key structural component that makes up hair, nails, and the outer layer of human skin.
• Durable and Protective: Keratin is an incredibly strong protein, which is why it's used by the body to form structures that need to withstand stress or protect the body. It forms both the internal (cortical) structure and the external (cuticle) layer of hair.
• Other Proteins: The other proteins listed - Dystrophin, Myosin, and Tubulin - have different roles in the body. Dystrophin helps protect muscle fibers from injury, Myosin is involved in muscle contraction, and Tubulin is a key protein in the structural network (cytoskeleton) inside cells. While they are important proteins, they aren't involved in the formation of hair.

Correct answer: 3)

Concept:

• Gel electrophoresis is a method of separation of a mixture of DNA samples based on their molecular size and charge.
• Generally, agarose gel is used to carry out gel electrophoresis.

Explanation:

• Gel electrophoresis is carried out in a series of steps- agarose gel is prepared, a sample of DNA is prepared, then the mixture of DNA sample is loaded into the wells in the gel, and then an electric field is applied.
• On applying electric field DNA fragments start to move towards the positive terminal as they are negatively charged.
• This separates the sample based on size and charge.
• Ethidium bromide (EtBr) is a fluorescent dye and an intercalation agent used in gel electrophoresis.
• EtBr inserts itself in between the stacked bases. When visualized under the UV lamp, it will appear as bright orange bands.

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So, the correct answer is option 3.

Concept-

• Gene expression is the mechanism at the molecular level by which a gene can express itself in the phenotype of an organism.
• The mechanism that stimulates the expression of certain genes and inhibits that of others is called the regulation of gene expression.
• Francis Jacob and Monod proposed a mechanism called the operon model for the regulation of gene action in E.coli.
• An operon is a part of genetic material that acts as a single regulated unit having one or more structural genes, a promoter gene, a regulator gene.
• Operons are of two types- inducible and repressible.
• The best-known operon is lac operon.

Explanation-

• In the absence of lactose, the repressor protein binds to the operator region of the operon.
• It prevents RNA polymerase from transcribing the operon.

Thereby option B is the correct answer.

Additional Information

• In the presence of an inducer, such as lactose or allolactose, the repressor is inactivated by interaction with the inducer.
• This allows RNA polymerase access to the promoter and transcription proceeds.
• Essentially, regulation of lac operon can also be visualized as regulation of enzyme synthesis by its substrate. 

Concept:

• A DNA molecule is made up of several nucleotides. Each nucleotide consists of a nitrogenous base, a phosphate group, and a 5-carbon sugar.
• There are 4 types of nitrogenous bases - Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
• The pairing between the nitrogenous bases is as follows: Adenine pairs with Thymine by double hydrogen bonds and Guanine pairs with Cytosine by triple hydrogen bonds.
• A DNA molecule is subjected to mutations that result in mispairing between these nitrogen bases.
• A chemical mutagen is a substance that can induce such mutations in a DNA molecule.
• It changes the genetic material of an organism.
• Ethyl methane sulfonate and N-methyl-N-nitrosourea are the most common chemical mutagens that induce point mutations in DNA.

Explanation:

• In the given question, the original sequence of DNA is: CAG GAC.
• It is given that a chemical mutagen alters the pairing properties of adenine (A) in the original sequence. 
• The mutation is such that adenine (A) pairs with cytosine (C) instead of thymine (T).
• This indicates a point mutation, where adenine is replaced by guanine that can pair with cytosine.
• The altered sequence after the mutation will be: CGG GGC.
• Transcription is the process by which a sequence of nucleotides is copied from a DNA template to an RNA molecule.
• On transcription, the mRNA sequence transcribed from the altered DNA sequence will be: GCC CCG.

So from the above-given information, the correct answer is option 3 (GCCCCG).

The correct answer is All of the above.

Key Points

Characteristics features of genetic code:-

• It is always universal: CORRECT
  • Being universal means each codon codes for a specific amino acid in all organisms.
  • ​The genetic code is a set of codes by which a linear sequence of nucleotides specifies the linear sequence of a polypeptide.
  • That is, they specify how the nucleotide sequence of an mRNA is translated into the amino acid sequence of a polypeptide.
• It is a triplet of nucleotides bases corresponding to 20 amino acids: CORRECT
  • ​The nucleotide sequence is read as triplets called codons, each of which codes for a specific amino acid. 
  • There 20 non-essential amino acids that are produced in the body, but there are only 4 nucleotide bases.
  • Thus, a combination of bases is required.
  • Doublet codes are not adequate to code for 20 amino acids because it gives only 4² = 16 codons.
  • Triplet code gives 4³ = 64 codons.
  • Therefore, it was pointed out that the triplet code is the minimum required.
• It is non-overlapping, non-ambiguous, and commaless: CORRECT
  • ​Non-overlapping - It is read in a continuous manner without overlapping any codon.
  • Non-ambiguous - One codon codes for only one specific amino acid.
  • Commaless - It is comma-free and there is no other punctuation to indicate the end of one codon and the beginning of the next.
• It has a start and a stop codon: CORRECT
  • The codon AUG codes for Methionine as well as acts as a START codon.
  • It initiates the process of polypeptide chain formation.
  • The 3 codons UAA, UAG, UGA do not code for any amino acid, but act as STOP codons.
  • When any one of them occurs it causes the release of the polypeptide chain from the ribosome. 

The correct answer is option 2.

Solution

Concept:

• Restriction enzymes act as molecular scissors.
• There are two kinds of restriction enzymes: endonucleases and exonucleases.
• Exonuclease helps in removing nucleotides from ends of DNA while endonuclease makes cuts at the specific sites in the DNA.

Explanation:

Let us understand the working of restriction endonuclease:

• Restriction endonuclease inspects the entire length of the DNA. 
• Upon finding a specific nucleotide sequence it will bind to it on the DNA and will slice the sequence from the DNA strand.
• It recognizes palindrome nucleotide sequences (the sequence of base pairs reads the same on the two strands of DNA when the reading orientation is kept the same).   
• They cut the DNA strand a little away from the centre of the palindrome nucleotide sequence but this occurs between the two same pairs on the opposite strands of DNA double helix.
• This leaves single-stranded portions at the ends. There are overhanging stretches called sticky ends on each strand.
• Sticky ends form hydrogen bonds with their complementary cut counterparts.

• This stickiness of the ends facilitates the action of the enzyme DNA ligase.

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Statement I: Restriction endonucleases recognise specific sequence to cut DNA known as palindromic nucleotide sequence; this is true. As restriction endonuclease recognizes palindrome sequence on the DNA.

Statement II: Restriction endonucleases cut the DNA strand a little away from the centre of the palindromic site; this is true. As restriction endonuclease cut the DNA strand a little away from the centre of the palindrome nucleotide sequence that occurs between the two same pairs on the DNA double helix.

So, the correct answer is option 2.