🌸 Chapter 5
Molecular Basis Of Inheritance
( 3 Marks)
1. Differentiate between DNA and
RNA.
Answer:
|
Feature |
DNA |
RNA |
|
Sugar |
Deoxyribose |
Ribose |
|
Bases |
A, T, G, C |
A, U, G, C |
|
Strands |
Double-stranded (mostly) |
Single-stranded (mostly) |
|
Stability |
More stable |
Less stable |
|
Function |
Genetic material |
Protein synthesis, gene regulation |
2. Describe the structure of a
nucleotide.
Answer:
A nucleotide has three components:
1.
Nitrogenous
base – Purine (A, G) or Pyrimidine (C,
T, U).
2.
Pentose
sugar –
Deoxyribose (DNA) or Ribose (RNA).
3.
Phosphate
group –
Attached to 5′ carbon of sugar.
Nucleotides join via phosphodiester bonds to form nucleic acids.
3. Explain the salient features of
the Watson and Crick model of DNA.
Answer:
- DNA is a double helix
with two antiparallel strands.
- The strands are held by hydrogen
bonds between complementary bases (A–T, G–C).
- One turn of the helix = 10
base pairs = 3.4 nm.
- Sugar-phosphate backbone lies outside; bases face
inside.
- The two strands are complementary.
4. Why is DNA considered a better
genetic material than RNA?
Answer:
- DNA is chemically more
stable (lacks 2′–OH group).
- Thymine in DNA provides protection
from mutations (instead of uracil).
- DNA is double-stranded,
allowing error correction.
Hence, DNA is more suitable for long-term genetic storage.
5. Explain the packaging of DNA in
eukaryotes.
Answer:
- DNA is wrapped around histone
octamers forming nucleosomes.
- Nucleosomes coil to form chromatin
fibers.
- Chromatin further condenses to
form chromosomes during cell division.
Each nucleosome has about 200 bp of DNA wrapped around histones.
6. Explain Griffith’s experiment and
its conclusion.
Answer:
- Worked with Streptococcus
pneumoniae:
- S strain (smooth, virulent)
- R strain (rough, non-virulent)
- Mice injected with:
- Live S → died
- Live R → lived
- Heat-killed S → lived
- Heat-killed S + live R → died
Conclusion: Something from dead S transformed R → S → called transforming principle.
7. What was proved by Avery,
MacLeod, and McCarty’s experiment?
Answer:
They used enzymes to destroy DNA, RNA, and proteins in Griffith’s transforming
extract.
Transformation stopped only when DNA was destroyed.
Thus, they proved that DNA is the transforming principle.
8. Describe Hershey and Chase
experiment.
Answer:
- Used T2 bacteriophage
labeled with:
- ³²P (DNA) and ³⁵S (protein)
- Infected bacteria; only ³²P
entered cells → progeny had radioactive DNA.
Conclusion: DNA, not protein, is the genetic material.
9. What is meant by semiconservative
DNA replication?
Answer:
In replication, each daughter DNA has one old (parental) and one new
strand.
Proved by Meselson and Stahl using E. coli grown in ¹⁵N → ¹⁴N
media → intermediate DNA band after one generation confirmed semiconservative
nature.
10. Explain the role of enzymes in
DNA replication.
Answer:
- Helicase: Unwinds DNA helix.
- DNA polymerase: Adds nucleotides (5′→3′
direction).
- Primase: Synthesizes RNA primer.
- Ligase: Joins Okazaki fragments on
lagging strand.
- Topoisomerase: Removes supercoiling ahead of
replication fork.
11. What are Okazaki fragments? How
are they joined?
Answer:
Short DNA fragments synthesized on the lagging strand during
replication.
They are joined by DNA ligase to form a continuous strand.
12. Explain the Meselson and Stahl
experiment.
Answer:
- Grew E. coli in ¹⁵N
medium, then shifted to ¹⁴N medium.
- DNA after 1 generation →
intermediate density.
- After 2 generations → half
light, half intermediate.
Conclusion: Replication is semiconservative.
13. Differentiate between leading
and lagging strand.
Answer:
|
Feature |
Leading Strand |
Lagging Strand |
|
Direction |
5′ → 3′ (continuous) |
3′ → 5′ (discontinuous) |
|
Synthesis |
Continuous |
In Okazaki fragments |
|
Enzyme |
DNA polymerase |
DNA polymerase + Ligase |
14. What is transcription? Describe
briefly its steps.
Answer:
Transcription = DNA → RNA
Steps:
1.
Initiation: RNA polymerase binds promoter.
2.
Elongation: RNA chain elongates complementary
to DNA.
3.
Termination: RNA polymerase detaches at
terminator sequence.
15. Name three types of RNA
polymerases in eukaryotes and their functions.
Answer:
1.
RNA
Pol I:
Synthesizes rRNA.
2.
RNA
Pol II:
Synthesizes mRNA.
3.
RNA
Pol III:
Synthesizes tRNA, snRNA.
16. What are exons and introns? What
happens to them during transcription?
Answer:
- Exons: Coding sequences.
- Introns: Non-coding sequences.
During RNA processing, introns are spliced out, and exons are joined to form mature mRNA.
17. What is the role of tRNA in
protein synthesis?
Answer:
tRNA carries specific amino acids to the ribosome and has anticodon
complementary to mRNA codon.
It helps decode the message and align amino acids during translation.
18. What is the significance of
genetic code being universal?
Answer:
It means the same codon codes for the same amino acid in all organisms —
e.g., AUG codes for methionine in all species.
This universality supports common ancestry and enables genetic
engineering.
19. List the salient features of the
genetic code.
Answer:
- Triplet codon.
- Degenerate (more than one codon
for an amino acid).
- Unambiguous (one codon = one
amino acid).
- Universal.
- Start codon: AUG; Stop codons:
UAA, UAG, UGA.
20. Explain the process of
translation.
Answer:
Steps:
1.
Initiation: Ribosome binds to mRNA at AUG
codon.
2.
Elongation: tRNA brings amino acids, peptide
bonds form.
3.
Termination: Stop codon triggers release of
polypeptide.
21. What are the major types of RNA
and their functions?
Answer:
- mRNA: Carries message from DNA.
- tRNA: Brings amino acids to
ribosome.
- rRNA: Structural and catalytic part
of ribosome.
22. What is the function of ribosome
in protein synthesis?
Answer:
Ribosome binds mRNA and tRNA, catalyzes peptide bond formation (via rRNA), and
moves along mRNA to elongate the polypeptide chain.
23. Explain the lac operon in E.
coli.
Answer:
- Genes: Z, Y, A (structural)
- Regulator gene (i): Makes repressor protein.
- Lactose absent: Repressor binds operator →
transcription off.
- Lactose present: Repressor inactivated →
transcription on.
→ Example of inducible operon.
24. What is an operon? Mention its
components.
Answer:
An operon is a functional unit of DNA containing genes regulated
together.
Components: Promoter, Operator, Structural genes, and Regulator gene.
25. Explain the significance of
regulatory genes.
Answer:
Regulatory genes produce repressor proteins that can inhibit
transcription by binding to operator regions → control gene expression.
26. What is the Human Genome Project
(HGP)?
Answer:
A scientific project to map the entire human DNA sequence (~3 billion
base pairs) and identify all human genes (~20,000–25,000).
27. Mention two major findings of
the Human Genome Project.
Answer:
- Only <2% of genome
codes for proteins.
- Many DNA sequences are repetitive.
- Genes are unevenly
distributed.
28. What is DNA fingerprinting?
Answer:
A technique to identify individuals based on unique DNA polymorphisms
(VNTRs).
Used in forensics, paternity testing, and evolutionary studies.
29. Name the scientist who developed
DNA fingerprinting and state its principle.
Answer:
Alec Jeffreys (1985) — based on VNTRs (Variable Number Tandem
Repeats).
30. Differentiate between coding and
non-coding DNA.
Answer:
|
Type |
Function |
Example |
|
Coding DNA |
Codes for proteins |
Exons |
|
Non-coding DNA |
Regulatory/repetitive |
Introns, VNTRs |
31. What is the central dogma of
molecular biology?
Answer:
Proposed by Francis Crick – It explains information flow:
DNA → RNA → Protein
In retroviruses, it is RNA → DNA → RNA → Protein.
32. Define mutation and mention its
significance.
Answer:
Mutation = Sudden change in DNA sequence.
Significance:
- Creates genetic variation.
- May cause diseases or
evolution.
- Useful in breeding and
research.
33. What is meant by genetic
polymorphism?
Answer:
It refers to the occurrence of multiple forms of DNA sequences among
individuals in a population — used in DNA fingerprinting.
34. Why is RNA called a catalyst in
some reactions?
Answer:
Certain RNAs (rRNA) act as ribozymes, catalyzing peptide bond formation
during translation.
35. Mention differences between
euchromatin and heterochromatin.
Answer:
|
Feature |
Euchromatin |
Heterochromatin |
|
Structure |
Loosely packed |
Densely packed |
|
Activity |
Transcriptionally active |
Inactive |
|
Staining |
Lightly stained |
Darkly stained |
36. What are repetitive DNA
sequences?
Answer:
DNA sequences repeated multiple times in the genome without coding for proteins
(e.g., VNTRs, microsatellites).
37. Describe the role of hydrogen
bonds in DNA stability.
Answer:
Hydrogen bonds between complementary bases (A–T = 2, G–C = 3) stabilize the DNA
helix while allowing strand separation during replication and transcription.
38. What are the differences between
replication and transcription?
Answer:
|
Feature |
Replication |
Transcription |
|
Template |
DNA → DNA |
DNA → RNA |
|
Enzyme |
DNA polymerase |
RNA polymerase |
|
Product |
2 DNA molecules |
mRNA, tRNA, rRNA |
39. What are the
post-transcriptional modifications in eukaryotic mRNA?
Answer:
1.
Capping (5′ methyl guanosine cap)
2.
Polyadenylation (Poly-A tail at 3′ end)
3.
Splicing (Removal of introns)
40. Explain the term “replication fork.”
Answer:
A Y-shaped region where DNA helicase unwinds the double helix and new
strands are synthesized during replication.
41. What is the function of DNA
ligase?
Answer:
It joins Okazaki fragments on the lagging strand to make a continuous
DNA strand.
42. What is the role of the promoter
and operator in the lac operon?
Answer:
- Promoter: Binding site for RNA
polymerase.
- Operator: Binding site for repressor
protein; regulates gene transcription.
43. What is the importance of
complementary base pairing?
Answer:
Ensures accurate replication and transcription and maintains DNA’s
double-helical structure.
44. Differentiate between
prokaryotic and eukaryotic transcription.
Answer:
|
Feature |
Prokaryotic |
Eukaryotic |
|
RNA Polymerase |
One type |
Three types |
|
RNA Processing |
Absent |
Present (splicing, capping) |
|
Location |
Cytoplasm |
Nucleus |
45. What is the importance of the
start codon AUG?
Answer:
AUG codes for methionine, initiating translation and setting the reading
frame for mRNA.
46. How does lactose regulate the
lac operon?
Answer:
When lactose is present, it binds to the repressor → changes its shape → cannot
bind operator → genes Z, Y, A are transcribed → lactose metabolized.
47. What is the importance of
repetitive DNA in a genome?
Answer:
Helps in chromosome structure, centromere formation, gene
regulation, and DNA fingerprinting.
48. How are DNA fragments separated
in DNA fingerprinting?
Answer:
Using gel electrophoresis, fragments are separated based on size and
charge — smaller fragments move faster.
49. What is the function of the
enzyme reverse transcriptase?
Answer:
It synthesizes DNA from RNA template — found in retroviruses
(e.g., HIV).
50. Why is replication said to be
bidirectional in prokaryotes?
Answer:
Because replication begins at a single origin of replication and
proceeds in both directions, forming two replication forks.
