πΈ Chapter 5
Molecular Basis Of Inheritance
( 4 Marks)
1. Explain the structure of a
nucleotide with a labeled diagram.
Answer:
Each nucleotide is composed of:
- A pentose sugar
(deoxyribose in DNA, ribose in RNA),
- A nitrogenous base (A,
T, G, C, or U),
- A phosphate group
attached to the 5′ carbon of the sugar.
The nitrogenous base attaches to the 1′ carbon, and the phosphate group links to the 3′ carbon of the next sugar via a phosphodiester bond, forming a sugar-phosphate backbone.
2. Describe the double-helical model
of DNA proposed by Watson and Crick.
Answer:
- DNA is a double-stranded
helix of two polynucleotide chains.
- The strands are antiparallel
(one runs 5′→3′, the other 3′→5′).
- Nitrogen bases pair by hydrogen
bonds (A–T with 2 bonds, G–C with 3).
- The helix pitch is 3.4
nm with 10 base pairs per turn.
- Sugar-phosphate forms the
backbone, and bases face inward.
3. Explain Griffith’s experiment and
its conclusion.
Answer:
- Griffith (1928) worked on Streptococcus
pneumoniae.
- He found that heat-killed S
strain + live R strain = death in mice, showing that a “transforming
principle” from dead S converted R into virulent form.
- Conclusion: Some hereditary
material (later found to be DNA) can transfer genetic information between
cells.
4. What did Avery, MacLeod, and
McCarty’s experiment prove?
Answer:
- They isolated DNA, RNA, and
protein from S. pneumoniae and tested their transforming ability.
- Only DNA from S strain
transformed R strain into S type.
- When DNA was destroyed by DNase,
transformation did not occur.
- Conclusion: DNA is the
genetic material responsible for heredity.
5. Explain Hershey and Chase’s
experiment.
Answer:
- Used bacteriophage T2 labeled
with ³²P (DNA) and ³⁵S (protein).
- After infection of E. coli,
only ³²P entered bacterial cells.
- Progeny phages contained
³²P-labeled DNA, not ³⁵S protein.
- Conclusion: DNA, not
protein, is the genetic material.
6. What are the differences between
DNA and RNA?
Answer:
|
Feature |
DNA |
RNA |
|
Sugar |
Deoxyribose |
Ribose |
|
Bases |
A, T, G, C |
A, U, G, C |
|
Structure |
Double-stranded |
Usually single-stranded |
|
Stability |
More stable |
Less stable |
|
Function |
Genetic material |
Protein synthesis and catalytic roles |
7. Explain the semiconservative
nature of DNA replication with Meselson and Stahl’s experiment.
Answer:
- E. coli grown in ¹⁵N medium
(heavy DNA), then shifted to ¹⁴N medium.
- After one generation: hybrid
DNA (¹⁵N–¹⁴N).
- After two generations: half
hybrid, half light DNA.
- Conclusion: Each new DNA
molecule has one parental and one new strand — semiconservative
replication.
8. What are the enzymes involved in
DNA replication and their functions?
Answer:
- Helicase: Unwinds DNA strands.
- Primase: Synthesizes RNA primer.
- DNA polymerase: Adds nucleotides in 5′→3′
direction.
- Ligase: Joins Okazaki fragments.
- Topoisomerase: Removes supercoils ahead of
replication fork.
9. Differentiate between leading and
lagging strands in DNA replication.
Answer:
- Leading strand: Synthesized continuously
toward the replication fork.
- Lagging strand: Synthesized discontinuously as
Okazaki fragments away from the fork.
- DNA ligase later joins
fragments to form a continuous strand.
10. Describe the structure of
nucleosome.
Answer:
- DNA wraps around a histone
octamer (2 molecules each of H2A, H2B, H3, H4).
- About 200 bp of DNA
wraps around the octamer.
- H1 histone seals DNA entry/exit.
- Nucleosomes coil to form chromatin
fibers, leading to chromosome structure.
11. What are the stages of
transcription in prokaryotes?
Answer:
1.
Initiation: RNA polymerase binds to the
promoter with sigma factor.
2.
Elongation: RNA chain grows in the 5′→3′
direction.
3.
Termination: RNA polymerase reaches terminator;
rho protein helps release RNA transcript.
12. Explain the structure of RNA
polymerase in prokaryotes.
Answer:
- RNA polymerase = core enzyme
(Ξ±₂Ξ²Ξ²′) + Ο factor.
- Core enzyme synthesizes RNA.
- Ο factor recognizes promoter
and initiates transcription.
- Once transcription starts, Ο
detaches.
13. How is transcription in
eukaryotes different from prokaryotes?
Answer:
- RNA Polymerases: Three types in eukaryotes (I,
II, III); one in prokaryotes.
- mRNA Processing: Eukaryotic mRNA undergoes capping,
splicing, and polyadenylation.
- Compartment: In eukaryotes, transcription
occurs in the nucleus; translation in the cytoplasm.
14. What is RNA splicing? Why is it
necessary?
Answer:
- Process of removing introns
(non-coding regions) and joining exons (coding regions) in
pre-mRNA.
- Required to produce a mature
mRNA capable of translation.
- Ensures proper coding sequence
for protein synthesis.
15. What are the main features of
the genetic code?
Answer:
1.
Triplet: 3 bases = 1 codon = 1 amino acid.
2.
Degenerate: More than one codon for some amino
acids.
3.
Universal: Same in all organisms.
4.
Non-overlapping
& comma-less:
Read continuously.
5.
Start
codon: AUG; Stop
codons: UAA, UAG, UGA.
16. What is the role of tRNA in
translation?
Answer:
- tRNA acts as an adapter
molecule.
- Has anticodon loop that
pairs with codon on mRNA.
- Carries specific amino acid
to the ribosome.
- Ensures correct sequence of
amino acids during protein synthesis.
17. Describe the process of translation
in prokaryotes.
Answer:
1.
Initiation: mRNA binds to small ribosomal
subunit; tRNA carrying methionine binds to AUG.
2.
Elongation: Ribosome moves along mRNA, adding
amino acids.
3.
Termination: Stop codon reached; release factors
free polypeptide.
4.
Polypeptide
folding forms
functional protein.
18. Explain the structure and
function of ribosome.
Answer:
- Ribosomes are ribonucleoprotein
complexes with large and small subunits.
- Prokaryotic: 70S (50S + 30S), Eukaryotic:
80S (60S + 40S).
- Sites: A (Aminoacyl), P
(Peptidyl), and E (Exit).
- Function: Translation of mRNA
into polypeptides.
19. What is the central dogma of
molecular biology? Explain with a diagram.
Answer:
Francis Crick proposed the central dogma:
DNA → RNA → Protein
Information flows from DNA to RNA (transcription) and from RNA to protein
(translation).
In some viruses, RNA → DNA (reverse transcription).
20. Describe the lac operon model of
gene regulation in prokaryotes.
Answer:
- Genes: lacZ, lacY, lacA;
regulated by promoter, operator, and regulator gene.
- Without lactose → repressor
binds operator → no transcription.
- With lactose → lactose binds
repressor → repressor inactivated → genes transcribed for lactose
metabolism.
Result: Inducible system.
21. What are exons and introns?
Explain their role.
Answer:
- Exons: Coding sequences; joined to
form mature mRNA.
- Introns: Non-coding sequences; removed
by splicing.
- Exon–intron arrangement allows alternative
splicing, leading to multiple proteins from one gene.
22. Explain the significance of
Human Genome Project.
Answer:
- Aimed to sequence the entire
human genome (~3 × 10⁹ bp).
- Identified about 30,000
genes.
- Revealed that less than 2%
of genome codes for proteins.
- Helped in studying genetic
diseases, evolution, and drug development.
23. What are the main findings of
the Human Genome Project (HGP)?
Answer:
- Human genome: ~3.2 billion base
pairs.
- About 30,000 genes.
- Only 2% codes for proteins.
- Repetitive sequences and
non-coding DNA abundant.
- Genes distributed unevenly
across chromosomes.
24. Define DNA fingerprinting.
Describe its basis.
Answer:
- Technique to identify
individuals based on unique DNA patterns.
- Based on VNTRs (Variable
Number Tandem Repeats).
- DNA is extracted, cut with
restriction enzymes, separated by gel electrophoresis, and detected
by radioactive probes.
- Used in forensic science,
paternity tests, etc.
25. What are VNTRs and what is their
significance?
Answer:
- VNTRs = DNA segments repeated
in varying numbers among individuals.
- Located in non-coding regions.
- Serve as genetic markers
for identification and population studies.
26. Why is DNA more stable than RNA?
Answer:
- DNA has deoxyribose (no
2’-OH group) — less reactive.
- Thymine replaces uracil,
preventing mutations.
- Double-stranded structure
allows repair mechanisms.
27. What are Okazaki fragments?
Explain their formation.
Answer:
- Short DNA fragments formed on
the lagging strand during replication.
- Synthesized discontinuously by
DNA polymerase.
- Joined by DNA ligase to
form continuous strand.
28. Differentiate between
prokaryotic and eukaryotic transcription.
Answer:
|
Feature |
Prokaryotes |
Eukaryotes |
|
RNA Polymerase |
Single |
Three (I, II, III) |
|
mRNA Processing |
Absent |
Capping, splicing, poly-A tail |
|
Location |
Cytoplasm |
Nucleus |
|
Coupling with Translation |
Simultaneous |
Separate |
29. Explain the importance of
non-coding DNA.
Answer:
- Regulates gene expression.
- Contains promoters,
enhancers, and repetitive sequences.
- Used in DNA fingerprinting and
evolution studies.
30. What is a chromatin? Explain its
types.
Answer:
- Complex of DNA and histone
proteins in the nucleus.
- Euchromatin: Lightly packed,
transcriptionally active.
- Heterochromatin: Densely packed, inactive
region.
