In Griffith's experiment, why did mice injected with heat‑killed S‑type bacteria and live R‑type bacteria develop pneumonia?

The live R bacteria acquired DNA from the dead S bacteria, gaining the capsule gene.

Which feature distinguishes a temperate phage from a virulent phage during its life cycle?

It can integrate its genome into the host chromosome forming a prophage.

During Sanger sequencing, why does incorporation of a dideoxynucleotide terminate DNA synthesis?

It lacks a 3'‑OH group, preventing formation of the next phosphodiester bond.

What is the primary consequence of negative supercoiling on bacterial DNA during replication?

It facilitates strand separation by reducing the energy required for unwinding.

A researcher observes that a DNA fragment migrates faster than a larger fragment on an agarose gel. Which statement best explains this observation?

Smaller fragments experience less friction and move more quickly toward the anode.

Why does a higher GC content raise the melting temperature (Tm) of a DNA duplex?

GC pairs form three hydrogen bonds, requiring more thermal energy to disrupt.

In eukaryotic cells, what determines whether a chromatin region is classified as heterochromatin or euchromatin?

The degree of compaction and associated transcriptional activity.

During meiosis I, crossing‑over occurs between homologous chromosomes. What is the main genetic outcome of this event?

Generation of recombinant chromosomes with new allele combinations.

Which enzyme activity is essential for relieving positive supercoils that arise ahead of a replication fork in bacteria?

DNA gyrase (type II topoisomerase) that introduces negative supercoils.

A retrovirus integrates its cDNA into the host genome. Which cellular component is required for the viral cDNA to enter the nucleus in a non‑dividing cell?

A nuclear localization signal (NLS) recognized by importin proteins.

Molecular Genetics and Cell Division | Free quiz

Understanding Molecular Genetics and Cell Division

Welcome to this comprehensive module on molecular genetics and cell division. This course is designed for students of general medicine and genetics, providing clear explanations of classic experiments, viral life cycles, DNA sequencing techniques, and the mechanics of chromosome behavior during meiosis. By the end of the lesson, you will be able to answer key quiz questions with confidence and apply the concepts to real‑world laboratory scenarios.

1. The Transforming Power of DNA – Griffith’s Experiment

In 1928, Frederick Griffith demonstrated that a "transforming" factor could convert non‑virulent bacteria into a lethal form. The classic set‑up involved two strains of Streptococcus pneumoniae:

S‑type (smooth): encapsulated, virulent, causes pneumonia.
R‑type (rough): non‑encapsulated, non‑virulent.

When mice received a mixture of heat‑killed S‑type cells and live R‑type cells, the R bacteria acquired DNA from the dead S cells. This newly obtained DNA encoded the capsule, turning the R strain into a virulent S‑type that caused pneumonia. The key takeaway is that DNA, not protein or toxin, carries the genetic information responsible for transformation.

2. Temperate vs. Virulent Bacteriophages

Bacteriophages (phages) can follow two distinct life cycles:

Virulent (lytic) cycle: The phage hijacks the host, rapidly produces progeny, and lyses the cell.
Temperate (lysogenic) cycle: After infection, the phage genome integrates into the bacterial chromosome, forming a prophage. The host cell continues to divide, replicating the prophage DNA along with its own genome.

The defining feature of a temperate phage is its ability to become a prophage, allowing the viral DNA to persist silently within the host until induction triggers the lytic cycle.

3. Sanger Sequencing – Why dideoxynucleotides Stop DNA Synthesis

Sanger’s chain‑termination method relies on the incorporation of dideoxynucleotides (ddNTPs). Unlike normal deoxynucleotides, ddNTPs lack a 3'‑OH group on the ribose sugar. Without this hydroxyl, the DNA polymerase cannot form the phosphodiester bond required for the addition of the next nucleotide, effectively halting elongation. This property enables the generation of DNA fragments of varying lengths that can be separated by electrophoresis to read the sequence.

4. The Role of Negative Supercoiling in Bacterial DNA Replication

Bacterial chromosomes are circular and highly supercoiled. Negative supercoiling introduces under‑winding, which stores energy that facilitates strand separation. During replication, the helicase does not need to expend as much energy to unwind the double helix because the negative supercoils already predispose the DNA to open up. This makes the replication process more efficient and reduces the requirement for additional topoisomerase activity.

5. Interpreting Agarose Gel Electrophoresis Results

When DNA fragments are loaded onto an agarose gel, they migrate toward the anode due to their negative charge. However, migration speed is primarily governed by fragment size. Smaller fragments encounter less friction within the gel matrix, allowing them to travel faster than larger fragments. This principle underlies the common laboratory practice of estimating fragment size by comparing migration distances to a DNA ladder.

6. GC Content and DNA Melting Temperature (Tm)

The melting temperature of a DNA duplex is the point at which half of the helix becomes single‑stranded. GC base pairs form three hydrogen bonds, compared with two for AT pairs, providing greater thermal stability. Consequently, DNA regions with higher GC content require more heat energy to disrupt these bonds, resulting in a higher Tm.

7. Chromatin Structure: Heterochromatin vs. Euchromatin

In eukaryotic nuclei, DNA is packaged into chromatin, which can exist in two functional states:

Heterochromatin: Highly compacted, transcriptionally silent, and often enriched with repetitive sequences.
Euchromatin: Loosely packed, transcriptionally active, and associated with gene‑rich regions.

The primary determinant of these states is the degree of compaction and the presence of specific histone modifications that either promote or inhibit transcription.

8. Genetic Consequences of Crossing‑Over in Meiosis I

During prophase I of meiosis, homologous chromosomes pair and exchange genetic material through crossing‑over. This recombination creates recombinant chromosomes that contain new combinations of alleles, increasing genetic diversity in the resulting gametes. Unlike the segregation of sister chromatids (which occurs in meiosis II), crossing‑over reshuffles alleles between homologs, a crucial step for evolution and adaptation.

9. Summary of Key Concepts

Griffith’s experiment proved that DNA is the transforming factor.
Temperate phages can integrate into the host genome as prophages.
Sanger sequencing terminates DNA synthesis because ddNTPs lack a 3'‑OH group.
Negative supercoiling eases strand separation during bacterial replication.
Smaller DNA fragments migrate faster in agarose gels due to reduced friction.
Higher GC content raises the melting temperature of DNA.
Chromatin compaction determines heterochromatin (silent) vs. euchromatin (active).
Crossing‑over generates recombinant chromosomes, enhancing genetic variation.

10. Frequently Asked Questions (FAQ)

Q: Can a virulent phage become temperate under certain conditions?

A: Generally, phage life cycles are genetically determined. Some phages can switch between lytic and lysogenic cycles, but true virulent phages lack the necessary integration machinery.

Q: Why is negative supercoiling more common in prokaryotes than eukaryotes?

A: Prokaryotic chromosomes are circular and lack nucleosomes, making supercoiling an efficient way to compact DNA and facilitate replication. Eukaryotes rely on nucleosome packaging and topoisomerases for similar functions.

Molecular Genetics and Cell Division