Molecular Biology

Step 1: Understanding the Concept:
Agarose gel electrophoresis is a technique used to separate macromolecules like DNA based on their size.
DNA has a uniformly negative charge, so when an electric field is applied, it migrates from the negative electrode (at the top, by the wells) towards the positive electrode (at the bottom).
The agarose gel acts as a sieve. Smaller DNA fragments move faster and farther through the gel than larger fragments.
Step 2: Detailed Explanation:


The starting points are the "Wells" at the top.

The distance traveled is inversely proportional to the size of the DNA fragment.

Observing the positions on the gel:

Fragment A has traveled the farthest distance, making it the smallest.

Fragment D has traveled the next farthest distance.

Fragment C has traveled less distance than D.

Fragment B has traveled the shortest distance, making it the largest.


The question asks for the order of increasing number of base pairs, which means from smallest to largest.

Step 3: Final Answer:
The correct order from smallest to largest is A, D, C, B.

Step 1: Understanding the Concept:
Telomerase is a ribonucleoprotein enzyme responsible for maintaining the length of telomeres, which are the protective caps at the ends of eukaryotic chromosomes.
It prevents the progressive shortening of chromosomes that occurs during standard DNA replication.
The term "reverse transcriptase" refers to an enzyme that synthesizes DNA from an RNA template.
Step 2: Detailed Explanation:
Let's break down the function of telomerase:


Telomerase contains its own built-in RNA molecule that serves as a template.

It uses this RNA template to synthesize DNA, adding repetitive nucleotide sequences to the 3' end of the DNA strand.

The process of synthesizing DNA using an RNA template is called RNA-dependent DNA synthesis.

Let's analyze the other options:


DNA-dependent RNA synthesis is transcription, carried out by RNA polymerase.

RNA-dependent RNA synthesis is carried out by RNA replicase, found in some RNA viruses.

DNA-dependent DNA synthesis is standard DNA replication, carried out by DNA polymerase.

Step 3: Final Answer:
Since telomerase is a reverse transcriptase that uses an RNA template to synthesize DNA, its activity is RNA-dependent DNA synthesis.

Step 1: Understanding the Concept:
Blotting techniques transfer macromolecules (DNA, RNA, proteins) from an electrophoresis gel to a solid membrane for detection.
Step 2: Detailed Explanation:
The specific names of the blotting techniques correspond to the type of molecule being detected:


Southern blotting: Used to detect specific DNA sequences. The process described in the question—separating DNA on a gel and transferring it to a membrane for hybridization—is the definition of a Southern blot.

Northern blotting: Used to detect specific RNA sequences.

Western blotting: Used to detect specific proteins using antibodies.

Eastern blotting: A less common technique for detecting post-translational modifications.

Step 3: Final Answer:
The technique for transferring separated DNA from a gel to a membrane is called Southern blotting.

Step 1: Understanding the Concept:
Chromosomes are classified into four types based on the position of the centromere, which divides the chromosome into two arms (a short 'p' arm and a long 'q' arm).
Step 2: Detailed Explanation:
Let's match each chromosome type in LIST-I with its corresponding centromere position in LIST-II:


A. Metacentric chromosome: The centromere is located exactly in the center, creating two arms of equal length. This matches with III. The centromere is located in the center of chromosome.

B. Submetacentric chromosome: The centromere is located slightly off-center, resulting in one arm being slightly shorter than the other. This matches with I. The centromere is located on one side of the central point of the chromosome.

C. Acrocentric chromosome: The centromere is located very close to one end, resulting in one very short arm (often with satellites) and one very long arm. This matches with IV. The centromere is located close to one end of the chromosome.

D. Telocentric chromosome: The centromere is located at the very end (telomere) of the chromosome, so there is only one arm. This matches with II. The centromere appears to be located at one end of the chromosome.

Step 3: Final Answer:
The correct matching is: A-III, B-I, C-IV, D-II. This corresponds to option (C).

Step 1: Understanding the Concept:
Recombinant DNA technology involves creating a new DNA molecule by combining genetic material from different sources. The question asks for the logical sequence of steps in a standard gene cloning experiment.
Step 2: Detailed Explanation:
Let's arrange the steps logically:


Step B: First, you must prepare the DNA fragments. This involves cutting both the target DNA (gene of interest) and the cloning vector (e.g., a plasmid) with the same restriction enzyme to generate compatible ends.

Step A: Next, the prepared target DNA fragment is joined with the cleaved vector DNA. The enzyme DNA ligase is used to form phosphodiester bonds, creating the recombinant DNA molecule.

Step D: The newly created recombinant DNA must be introduced into a host organism, usually a bacterium like E. coli. This process is called transformation.

Step C: Transformation is an inefficient process, so you need to select the few cells that successfully took up the recombinant DNA (transformants) from the vast majority that did not. This is done using selectable markers like antibiotic resistance or color indicators (e.g., blue-white screening).

Step 3: Final Answer:
The correct sequence of occurrence is B A D C.

Step 1: Understanding the Concept:
The process of translation is initiated at the start codon AUG, which codes for the amino acid methionine (or a modified form, fMet, in bacteria). The question asks about the fate of this initial methionine in the final protein product.
Step 2: Detailed Explanation:
Let's evaluate each statement:


(A) All proteins have methionine as the first amino acids. This is incorrect. While protein synthesis always begins with methionine, it is very often removed from the N-terminus of the polypeptide chain after translation.

(B) Methionine never added at the beginning of protein synthesis. This is incorrect. The start codon AUG explicitly directs the incorporation of methionine as the first amino acid.

(C) An enzyme can cleave the methionine amino acid from the polypeptide after synthesis. This is correct. This process is a common post-translational modification called N-terminal methionine excision (NME). It is carried out by enzymes called methionine aminopeptidases (MAPs). Whether the methionine is cleaved or not depends on the nature of the second amino acid in the chain.

(D) Methionine fall off automatically from the polypeptide... This is incorrect. The removal of the N-terminal methionine is an active, enzymatic process and does not happen automatically.

Step 3: Final Answer:
The correct statement is that the initial methionine can be enzymatically removed from the polypeptide after it has been synthesized.

Step 1: Understanding the Concept:
The question asks to identify the bacterial source of colibactin, a genotoxin (a toxin that damages DNA) produced by certain gut bacteria and linked to colorectal cancer.
Step 2: Detailed Explanation:


Colibactin is a secondary metabolite produced by a specific genomic island called the polyketide synthase (pks) island.

This pks island is found in certain strains of Escherichia coli, particularly those belonging to the B2 phylogroup, as well as some strains of Klebsiella pneumoniae and other Enterobacteriaceae.

These pks E. coli strains are part of the normal gut microbiota in some individuals.

The colibactin they produce can act as a DNA alkylating agent, causing double-strand breaks in host intestinal epithelial cells. This DNA damage can lead to chromosomal instability and promote the development of colorectal cancer.

Helicobacter pylori is associated with stomach cancer, Bacteroides fragilis can produce a different toxin (BFT) linked to colon cancer, and Bifidobacterium are generally considered probiotic.

Step 3: Final Answer:
Colibactin is synthesized by certain strains of E. coli.

Concept: Blotting techniques are widely used in molecular biology to detect specific biomolecules such as DNA, RNA, or proteins. Each blotting method is designed for a particular type of molecule.
Step 1:Understanding blotting techniques.
Different blotting methods detect different biomolecules:

• Southern blotting → DNA
• Northern blotting → RNA
• Western blotting → Proteins

Step 2:Principle of Southern blotting.
In Southern blotting, DNA fragments are first separated by gel electrophoresis. These fragments are then transferred onto a membrane and hybridized with a labeled DNA probe that is complementary to the target sequence.
Step 3:Detection of specific DNA sequences.
If the probe binds to the complementary DNA sequence on the membrane, the presence of the specific DNA fragment can be detected using radioactive or fluorescent signals. Therefore, the technique used to detect a specific DNA sequence is Southern blotting.

Concept: Gel electrophoresis is a technique used to separate DNA fragments based on their size by applying an electric field across a gel matrix. DNA molecules carry an overall negative charge due to the presence of phosphate groups in their backbone.
Step 1:Charge on DNA molecules.
The backbone of DNA contains phosphate groups, each carrying a negative charge. As a result, DNA fragments behave as negatively charged molecules in an electric field.
Step 2:Movement in an electric field.
When an electric current is applied, negatively charged molecules migrate toward the positively charged electrode (anode).
Step 3:Separation in the gel matrix.
As DNA fragments move through the agarose gel, smaller fragments migrate faster and travel farther than larger fragments, allowing separation based on size. Therefore, DNA fragments migrate toward the positive electrode (anode) during gel electrophoresis.

Concept: Polymerase Chain Reaction (PCR) is a molecular biology technique used to amplify a specific segment of DNA. It involves repeated cycles of temperature changes that allow DNA strands to separate, primers to bind, and new DNA strands to be synthesized. Each PCR cycle consists of three main steps.
Step 1:Denaturation.
The reaction mixture is heated to about (94^\circ C - 98^\circ C). At this high temperature, the double-stranded DNA separates into two single strands as the hydrogen bonds between complementary bases break.
Step 2:Annealing.
The temperature is lowered to approximately (50^\circ C - 65^\circ C). At this stage, short DNA primers bind (anneal) to their complementary sequences on the single-stranded DNA templates.
Step 3:Extension (Elongation).
The temperature is raised to around (72^\circ C), which is the optimal temperature for the enzyme {Taq polymerase}. The enzyme adds nucleotides to the primer and synthesizes a new DNA strand complementary to the template. Thus, the correct order of PCR steps is: [ \text{Denaturation} \rightarrow \text{Annealing} \rightarrow \text{Extension} ]