E-CONTENT MODULE MOLECULAR BIOLOGY (Code: ZLG0600204) NUCLEIC ACIDS: Structure, Types of DNA & RNA, and the Watson & Crick Model For B.Sc. Zoology Students (Undergraduate Level) Prepared By Dr Chandralekha Deka Assistant Professor Department of Zoology, PDUAM, Amjonga, Goalpara |
Subject Molecular Biology | Course Level B.Sc. Zoology (UG) 6th Semester |
Module Nucleic Acids – Structure & Types | Framework UGC Four-Quadrant e-Content Model |
Q1 e-Tutorial | Q2 e-Content | Q3 Resources | Q4 Assessment |
QUADRANT 1 e-Tutorial: Core Conceptual Content |
LEARNING OBJECTIVES |
After studying this module, you will be able to: |
1. Define nucleic acids and explain their biological significance |
2. Describe the structure of nucleotides and how they polymerize |
3. Distinguish between the A, B, and Z forms of DNA |
4. Classify and explain the types and roles of RNA |
5. Explain the Watson-Crick double helix model with all structural parameters |
6. State Chargaff's rules and justify the antiparallel arrangement of DNA strands |
Nucleic acids are large biological macromolecules that serve as the molecular basis of heredity and gene expression in all living organisms. The term 'nucleic acid' was coined by Friedrich Miescher in 1869, who first isolated a phosphorus-rich substance from the nuclei of white blood cells, which he called 'nuclein'. This substance was later identified as DNA (Deoxyribonucleic Acid).
There are two major types of nucleic acids found in cells:
• DNA (Deoxyribonucleic Acid): The primary genetic material that stores hereditary information.
• RNA (Ribonucleic Acid): Involved in transcription, translation, and regulation of gene expression.
Nucleic acids are found in every living cell — in prokaryotes, they are present in the cytoplasm; in eukaryotes, DNA is predominantly localized in the nucleus, while RNA is found in both the nucleus and cytoplasm.
Structure of Nucleotides and Nucleic Acids
The Nucleotide – The Monomer Unit
A nucleotide is the fundamental monomer (building block) of nucleic acids. Each nucleotide is composed of three components covalently linked together: Pentose Sugar — A 5-carbon (pentose) sugar, Deoxyribose (2'-deoxyribose) Found in DNA it lacks an –OH group at the 2' carbon position and Ribose: Found in RNAwhich has a hydroxyl (–OH) group at the 2' carbon position.
1. Nitrogenous Base: An organic nitrogen-containing ring compound. There are two categories:
a. Purines: Adenine (A) and Guanine (G) — double-ring structures (bicyclic).
b. Pyrimidines: Cytosine (C), Thymine (T) — found in DNA; Uracil (U) replaces Thymine in RNA. These are single-ring structures.
2. Phosphate Group (PO₄³⁻): One or more phosphate groups attached to the 5' carbon of the sugar. The phosphate group gives nucleic acids their acidic nature.
 |
| Nucleotide |
Nucleoside vs. Nucleotide
Term | Components | Example |
Nucleoside | Pentose sugar + Nitrogenous base | Adenosine, Deoxyguanosine |
Nucleotide | Pentose sugar + Nitrogenous base + Phosphate group | AMP, dGTP, ATP |
Phosphodiester Bonds and the Polynucleotide Chain
Nucleotides are linked together through phosphodiester bonds to form a long polynucleotide chain. This bond forms between:
• The 3'–OH group of one nucleotide's sugar
• The 5'–phosphate group of the next nucleotide's sugar
This creates a sugar–phosphate backbone that runs continuously, with nitrogenous bases projecting outward (or inward, as in double-stranded DNA). The polymer has directionality, running from the 5' end (free phosphate) to the 3' end (free hydroxyl). This is the 5'→3' direction.
Types of DNA — Structural Conformations (A, B, Z Forms)
DNA is a highly dynamic molecule that can adopt different helical conformations depending on the environmental conditions (humidity, ionic strength, base sequence). The three major forms are A-DNA, B-DNA, and Z-DNA.
B-DNA (The Physiological Form)
B-DNA is the most common and biologically relevant form of DNA found in living cells under physiological (normal cellular) conditions. It was the form described by Watson and Crick in 1953.
• Right-handed double helix
• 10 base pairs (bp) per complete helical turn
• Helical pitch (rise per turn): 34 Å (3.4 nm)
• Rise per base pair: 3.4 Å
• Diameter: ~20 Å (2.0 nm)
• Has two grooves: a wide major groove (~22 Å) and a narrow minor groove (~12 Å)
• Bases are nearly perpendicular to the helix axis
• Exists under high humidity (~92%)
A-DNA
A-DNA is formed when B-DNA is dehydrated (exposed to ~75% relative humidity). It is also found in RNA:DNA hybrid duplexes and in double-stranded RNA.
• Right-handed double helix
• 11 base pairs per turn
• Helical pitch: 28 Å
• Rise per base pair: 2.56 Å
• Diameter: ~26 Å — wider and shorter than B-DNA
• Bases are tilted ~19° from the helix axis
• Major groove is narrow and deep; minor groove is wide and shallow (opposite to B-DNA)
Z-DNA
Z-DNA was discovered by Alexander Rich and colleagues in 1979. The 'Z' stands for the zigzag path of the phosphodiester backbone.
• Left-handed double helix — the key distinguishing feature
• 12 base pairs per turn
• Helical pitch: 45 Å
• Rise per base pair: 3.7 Å
• Diameter: ~18 Å — slender and elongated
• Backbone has a characteristic zigzag appearance
• Typically forms in sequences with alternating purine-pyrimidine (especially GC) repeats
• Found transiently during active gene transcription; may play a role in gene regulation
Comparison Table — A, B, and Z Forms of DNA
Parameter | A-DNA | B-DNA | Z-DNA |
Helix handedness | Right-handed | Right-handed | Left-handed |
Base pairs per turn | 11 | 10 | 12 |
Helical pitch | 28 Å | 34 Å | 45 Å |
Rise per bp | 2.56 Å | 3.4 Å | 3.7 Å |
Diameter | ~26 Å | ~20 Å | ~18 Å |
Major groove | Narrow, deep | Wide, deep | Flat |
Minor groove | Wide, shallow | Narrow, deep | Narrow, deep |
Base tilt | ~19° | ~1° | ~9° |
Humidity | ~75% | ~92% | Low (high salt) |
Biological role | RNA:DNA hybrids, dsRNA | Primary genomic DNA | Gene regulation (transient) |
Types of RNA
RNA (Ribonucleic Acid) is a single-stranded nucleic acid that performs multiple essential functions in gene expression, protein synthesis, and regulation. While all RNA is synthesized using DNA as a template (transcription), different types of RNA serve distinct roles.
Messenger RNA (mRNA)
• Function: Carries the genetic information (genetic code) from DNA to ribosomes for translation into protein.
• Structure: Linear, single-stranded, with a 5'-cap (7-methylguanosine) and a 3'-poly-A tail in eukaryotes.
• Key features: Contains codons (triplets of bases) that specify amino acids. The reading frame starts at the AUG start codon.
• Amount: Constitutes ~5% of total cellular RNA. Short-lived — rapidly degraded after translation.
• In prokaryotes: mRNA is polycistronic (codes for multiple proteins). In eukaryotes, it is monocistronic.
Transfer RNA (tRNA)
• Function: Acts as an adaptor molecule that carries specific amino acids to the ribosome during translation.
• Structure: ~73–93 nucleotides long. Folds into a cloverleaf secondary structure due to intramolecular base pairing. In 3D, it adopts an L-shaped tertiary structure.
• Key structural features:
- Acceptor stem (3' end): bears the universal CCA-3'-OH sequence where amino acid is attached
- Anticodon loop: recognizes and base-pairs with the complementary mRNA codon
- D-loop: contains unusual base dihydrouridine (D)
- TΨC loop: contains pseudouridine — involved in ribosome binding
- Variable loop: size varies among tRNA species
- Charging: Aminoacyl-tRNA synthetase enzymes covalently attach the correct amino acid to tRNA (aminoacylation). One enzyme exists per amino acid.
- Amount: Constitutes ~15% of total cellular RNA.
 |
| RNA cloverleaf secondary structure |
• Function: Structural and catalytic component of ribosomes. rRNA acts as a ribozyme — it catalyzes peptide bond formation between amino acids (peptidyl transferase activity).
• Types in Prokaryotes (70S ribosome):
◦ 23S rRNA — found in the 50S large subunit
◦ 16S rRNA — found in the 30S small subunit
◦ 5S rRNA — found in the 50S large subunit
• Types in Eukaryotes (80S ribosome):
◦ 28S, 5.8S, and 5S rRNA — found in the 60S large subunit
◦ 18S rRNA — found in the 40S small subunit
Amount: Most abundant RNA — constitutes ~80% of total cellular RNA.
Other Important Types of RNA
Type | Full Name | Function |
hnRNA | Heterogeneous Nuclear RNA | Pre-mRNA in eukaryotes; undergoes processing (capping, polyadenylation, splicing) |
snRNA | Small Nuclear RNA | Component of spliceosomes; involved in pre-mRNA splicing (removal of introns) |
snoRNA | Small Nucleolar RNA | Guides chemical modification (methylation, pseudouridylation) of rRNA in the nucleolus |
miRNA | MicroRNA | Small regulatory RNA (~22 nt); silences gene expression by degrading mRNA or blocking translation |
siRNA | Small Interfering RNA | ~21–23 nt; mediates RNA interference (RNAi); used in gene silencing |
lncRNA | Long Non-coding RNA | >200 nt; regulates gene expression at transcriptional and post-transcriptional levels |
piRNA | PIWI-interacting RNA | Silences transposons in germ cells; protects genome integrity |
Ribozyme | Catalytic RNA | RNA with enzymatic activity; e.g., self-splicing introns, RNase P |
Watson and Crick Model of DNA
Historical Background |
In 1953, James D. Watson and Francis H.C. Crick proposed the double helix model of DNA, published in the journal Nature. This landmark discovery was based on: |
• X-ray crystallography data by Rosalind Franklin and Maurice Wilkins (Photo 51) |
• Chargaff's base equivalence rules (A = T; G = C) |
• Chemical composition data from Erwin Chargaff |
Watson, Crick, and Wilkins received the Nobel Prize in Physiology or Medicine in 1962. |
The Double Helix Structure
The Watson-Crick model describes B-DNA as a right-handed double-stranded helical structure with the following key features:
1. Two Antiparallel Polynucleotide Strands: DNA consists of two polynucleotide chains wound around a common central axis in a right-handed helical manner. The two strands run antiparallel to each other — one runs 5'→3' and the other runs 3'→5'.
2. Sugar-Phosphate Backbone on the Outside: The deoxyribose sugars and phosphate groups form the outer hydrophilic backbone of the helix, exposed to the aqueous environment.
3. Nitrogenous Bases on the Inside: The bases project inward toward the helical axis and face each other. Complementary bases form hydrogen bonds across the two strands.
4. Base Stacking: Adjacent base pairs stack upon one another through hydrophobic and van der Waals interactions. Base stacking contributes significantly to the thermodynamic stability of the double helix.
Major and Minor Grooves: The helical winding of the two strands creates two grooves of unequal size — a wider major groove (~22 Å) and a narrower minor groove (~12 Å). These grooves are important sites of protein–DNA interactions (e.g., transcription factors bind the major groove).
 |
| DNA Double Helix Strucutre |
Chargaff's Rules and Complementary Base Pairing
Erwin Chargaff analyzed DNA from many different organisms and established the following fundamental rules (Chargaff's Rules / Equivalence Rules):
Chargaff's Rules: |
[A] = [T] (Adenine equals Thymine) |
[G] = [C] (Guanine equals Cytosine) |
[Purines] = [Pyrimidines] i.e., [A + G] = [T + C] |
The ratio (A+T)/(G+C) varies between species — this is the species-specific ratio. |
Watson and Crick used Chargaff's rules to propose specific hydrogen-bonded base pairs:
Base Pair | Type | Number of H-bonds | Size (Purine-Pyrimidine) |
Adenine (A) — Thymine (T) | A:T | 2 hydrogen bonds | Purine — Pyrimidine |
Guanine (G) — Cytosine (C) | G:C | 3 hydrogen bonds | Purine — Pyrimidine |
The pairing of a purine with a pyrimidine in each base pair ensures a uniform width of the double helix throughout its length (~20 Å). If two purines paired, the helix would be too wide; two pyrimidines would make it too narrow.
Exam Alert: G:C base pair has 3 H-bonds → more stable, requires more energy to denature. A:T pair has only 2 H-bonds. DNA with higher G+C content has a higher melting temperature (Tm). |
Antiparallel Orientation of Strands
The two strands of the DNA double helix run antiparallel to each other — this means they run in opposite directions:
• Strand 1 runs in the 5'→3' direction (from top to bottom)
• Strand 2 runs in the 3'→5' direction (from top to bottom), or equivalently 5'→3' from bottom to top
This antiparallel arrangement is a consequence of the geometry of the phosphodiester bond linkage and is essential for:
• Correct complementary base pairing via hydrogen bonds
• Proper function of DNA polymerase, which only synthesizes DNA in the 5'→3' direction
• The mechanism of DNA replication — leading strand vs. lagging strand synthesis
Helical Parameters of B-DNA
Parameter | Value | Significance |
Helical pitch (rise per turn) | 34 Å (3.4 nm) | Distance for one complete helical turn |
Number of base pairs per turn | 10 bp | ~36° rotation per base pair |
Rise per base pair | 3.4 Å | Axial spacing between adjacent bases |
Diameter of helix | ~20 Å (2.0 nm) | Uniform width maintained by purine-pyrimidine pairing |
Width of major groove | ~22 Å | Primary site for protein-DNA interaction |
Width of minor groove | ~12 Å | Secondary binding site for small molecules |
Helix sense | Right-handed | Clockwise turn when viewed from above |
Base pair inclination | ~1° from perpendicular | Bases nearly perpendicular to helix axis |
Stability of the DNA Double Helix
The double helix is maintained by several types of non-covalent interactions:
• Hydrogen bonds: Specific A:T (2 H-bonds) and G:C (3 H-bonds) base pairing holds the two strands together.
• Base stacking interactions: Hydrophobic interactions and van der Waals forces between adjacent stacked base pairs. These are actually the primary contributors to helix stability.
• Electrostatic interactions: Positively charged histone proteins (in eukaryotes) and polyamines (spermine, spermidine) neutralize the negatively charged phosphate groups, reducing repulsion between strands.
• Hydrophobic effect: Bases (hydrophobic) are shielded from water by being stacked in the interior of the helix.
Central Dogma of Molecular Biology (Francis Crick, 1958) |
DNA ──(Replication)──► DNA |
DNA ──(Transcription)──► RNA ──(Translation)──► Protein |
Special flows: RNA ──(Reverse Transcription)──► DNA (retroviruses); RNA → RNA (RNA replication in RNA viruses) |
QUADRANT 2 e-Content: Self-Learning Materials |
Key Points and Summary
# | Key Point |
1 | Nucleic acids (DNA and RNA) are polymers of nucleotides, composed of a sugar, a nitrogenous base, and a phosphate group. |
2 | DNA uses deoxyribose sugar; RNA uses ribose sugar. DNA has Thymine; RNA has Uracil instead. |
3 | Nucleotides are linked by 3'→5' phosphodiester bonds; the polymer has 5'→3' polarity. |
4 | B-DNA is the most common form — right-handed, 10 bp/turn, 34 Å pitch, ~20 Å diameter. |
5 | A-DNA is right-handed, more compact (11 bp/turn, 28 Å pitch), formed under low humidity. |
6 | Z-DNA is left-handed (12 bp/turn, 45 Å pitch), found in alternating GC sequences. |
7 | Chargaff's rules: A=T (2 H-bonds) and G=C (3 H-bonds) in any DNA molecule. |
8 | Watson-Crick model: right-handed double helix, antiparallel strands, bases inside, backbone outside. |
9 | mRNA codes for protein (~5% of RNA), tRNA brings amino acids (~15%), rRNA is most abundant (~80%). |
10 | DNA stability = hydrogen bonds + base stacking + electrostatic neutralization by histones. |
Definitions and Important Terms
Term | Definition |
Nucleotide | Monomer of nucleic acids; consists of pentose sugar + nitrogenous base + phosphate group. |
Nucleoside | Pentose sugar + nitrogenous base only (no phosphate). |
Phosphodiester bond | Covalent bond linking the 3'-OH of one nucleotide to the 5'-phosphate of the next. |
Double helix | The structural form of DNA; two antiparallel polynucleotide strands wound around a central axis. |
Chargaff's rules | In any DNA: [A]=[T]; [G]=[C]; [purines]=[pyrimidines]. |
Antiparallel | The two DNA strands run in opposite 5'→3' directions. |
Complementary bases | Bases that form specific H-bonds: A pairs with T (or U in RNA); G pairs with C. |
Melting temperature (Tm) | Temperature at which 50% of the double-stranded DNA is denatured into single strands. |
Transcription | Synthesis of RNA from a DNA template by RNA polymerase. |
Translation | Synthesis of protein from mRNA codons at the ribosome. |
Codon | A triplet of mRNA bases that codes for a specific amino acid. |
Anticodon | A triplet of bases in tRNA that is complementary to a mRNA codon. |
Ribozyme | An RNA molecule with catalytic (enzymatic) activity. |
RNAi | RNA interference — gene silencing mechanism mediated by small RNA molecules (siRNA/miRNA). |
Aminoacylation | Attachment of the correct amino acid to its cognate tRNA by aminoacyl-tRNA synthetase. |
Comparison Tables
DNA vs. RNA — Comprehensive Comparison
Feature | DNA | RNA |
Full name | Deoxyribonucleic Acid | Ribonucleic Acid |
Sugar | 2'-Deoxyribose | Ribose |
Bases | A, G, C, T (Thymine) | A, G, C, U (Uracil) |
Strandedness | Usually double-stranded | Usually single-stranded |
Location | Nucleus (eukaryotes), cytoplasm (prokaryotes) | Nucleus and cytoplasm |
Stability | More stable (no 2'-OH) | Less stable (2'-OH susceptible to hydrolysis) |
Function | Storage & transmission of genetic info | Gene expression (transcription, translation, regulation) |
Types | A, B, Z forms | mRNA, tRNA, rRNA, snRNA, siRNA, miRNA, etc. |
Pyrimidine bases | Cytosine, Thymine | Cytosine, Uracil |
Amount | Constant in a cell (mostly) | Variable, depending on metabolic activity |
Replication | Self-replicating (semi-conservative) | Not self-replicating (transcribed from DNA) |
Grooves | Major and minor grooves (B-form) | Single-stranded, no clear grooves |
Helical pitch | 34 Å (B-form) | Variable (A-form geometry in dsRNA) |
Types of RNA — Summary Table
Type | Size | % of Total RNA | Key Function |
mRNA | Variable (200–thousands nt) | ~5% | Encodes protein sequence; template for translation |
tRNA | ~73–93 nt | ~15% | Carries amino acids; anticodon-codon recognition |
rRNA | 5S, 16S, 23S (prokaryotes); 5S, 5.8S, 18S, 28S (eukaryotes) | ~80% | Ribosome structure; peptidyl transferase activity |
snRNA | ~100–300 nt | <1% | Pre-mRNA splicing via spliceosome |
snoRNA | ~60–300 nt | <1% | rRNA modification (methylation, pseudouridylation) |
miRNA | ~22 nt | <1% | Gene silencing post-transcriptionally |
siRNA | ~21–23 nt | <1% | RNA interference; sequence-specific mRNA degradation |
lncRNA | >200 nt | Variable | Epigenetic regulation; chromatin remodeling |
Flowcharts
Flowchart 1: Nucleotide Structure — Organization
Flowchart 2: Central Dogma of Molecular Biology
QUADRANT 3 Supplementary Resources |
Recommended Textbooks and References
Primary Textbooks
1. Lehninger Principles of Biochemistry — David L. Nelson & Michael M. Cox (W.H. Freeman). Chapters 8 & 12. [Most comprehensive for nucleic acid structure]
2. Biochemistry — Jeremy M. Berg, John L. Tymoczko & Gregory J. Gatto (W.H. Freeman). Chapter 4 & 5.
3. Molecular Biology of the Cell — Alberts, Johnson, Lewis, Morgan, Raff, Roberts & Walter (Garland Science). Chapters 4 & 6.
4. Molecular Biology of the Gene — Watson, Baker, Bell, Gann, Levine & Losick (Cold Spring Harbor Laboratory Press). Chapters 4 & 6.
5. Cell and Molecular Biology — Gerald Karp (Wiley). Chapter 9.
Reference Books for Indian Students
6. Biochemistry — U. Satyanarayana & U. Chakrapani (Books & Allied). A highly accessible reference for Indian undergraduate students.
7. Molecular Biology and Genetics — S. Bisen & A. Sharma (Agrobios India).
8. Genetics and Molecular Biology — B.D. Singh (Kalyani Publishers).
Landmark Research Articles
9. Watson J.D. & Crick F.H.C. (1953). 'A Structure for Deoxyribose Nucleic Acid.' Nature, 171, 737–738. [The original double helix paper]
10. Chargaff E. (1950). 'Chemical Specificity of Nucleic Acids and Mechanism of Their Enzymatic Degradation.' Experientia, 6, 201–209.
11. Franklin R.E. & Gosling R.G. (1953). 'Molecular Configuration in Sodium Thymonucleate.' Nature, 171, 740–741. [X-ray diffraction data]
12. Rich A. et al. (1979). 'Molecular Structure of a Left-Handed Double Helical DNA Fragment at Atomic Resolution.' Nature, 282, 680–686. [Discovery of Z-DNA]
13. Holley R.W. et al. (1965). 'Structure of a Ribonucleic Acid.' Science, 147, 1462–1465. [First tRNA sequence]
Online Educational Resources
Resource | Platform | Link/Description |
Nucleic Acid Structure (NCBI) | NCBI Bookshelf | Molecular Biology of the Cell — available free online at NCBI Bookshelf |
Khan Academy — DNA Structure | Khan Academy | Free interactive lessons on nucleotide structure and DNA double helix |
iBiology Video Lectures | iBiology.org | Expert video lectures on molecular biology topics including nucleic acids |
RCSB Protein Data Bank | rcsb.org | 3D structure visualization of DNA, RNA, and protein–DNA complexes |
Nature Education — Scitable | nature.com/scitable | Peer-reviewed genetics and molecular biology articles for students |
CSIR-UGC NET Study Material | nta.ac.in | Official resources for UGC NET Life Sciences preparation |
MIT OpenCourseWare — 7.01 | ocw.mit.edu | Free MIT Biology lecture notes on molecular biology fundamentals |
Additional Reading Topics
To deepen understanding and prepare for competitive examinations, students are encouraged to explore the following advanced topics:
• DNA supercoiling — positive and negative supercoiling, role of topoisomerases
• Nucleosome and chromatin organization in eukaryotes
• DNA replication — semi-conservative mechanism (Meselson-Stahl experiment)
• Transcription — promoters, RNA polymerase, initiation, elongation, termination
• Post-transcriptional modifications of eukaryotic mRNA — 5'-capping, 3'-polyadenylation, splicing
• The genetic code — codons, wobble hypothesis (Crick, 1966)
• Riboswitches — RNA-based regulatory elements in mRNA
• CRISPR-Cas9 — uses guide RNA (gRNA) for targeted genome editing
• Telomeres and telomerase — role of RNA in chromosome end replication
• Epigenetics — DNA methylation and its effect on gene expression
QUADRANT 4 Assessment & Evaluation |
Section A: Multiple Choice Questions (MCQs)
Choose the most appropriate answer for each question.
1. The sugar present in DNA is:
(a) Ribose
(b) Deoxyribose
(c) Fructose
(d) Galactose
Answer: (b) Deoxyribose
2. Which of the following bases is found in RNA but NOT in DNA?
(a) Thymine
(b) Cytosine
(c) Uracil
(d) Guanine
Answer: (c) Uracil
3. According to Chargaff's rules, which of the following is correct?
(a) A = G
(b) A = C
(c) A = T
(d) G = T
Answer: (c) A = T
4. How many hydrogen bonds are present between a G:C base pair?
(a) 1
(b) 2
(c) 3
(d) 4
Answer: (c) 3
5. The helical pitch of B-DNA is:
(a) 20 Å
(b) 28 Å
(c) 34 Å
(d) 45 Å
Answer: (c) 34 Å
6. The number of base pairs per turn in B-DNA is:
(a) 8
(b) 10
(c) 11
(d) 12
Answer: (b) 10
7. Z-DNA is characterized by:
(a) Right-handed helix
(b) 10 bp per turn
(c) Left-handed helix
(d) Wide major groove
Answer: (c) Left-handed helix
8. Which RNA constitutes approximately 80% of total cellular RNA?
(a) mRNA
(b) tRNA
(c) snRNA
(d) rRNA
Answer: (d) rRNA
9. The anticodon is found in:
(a) mRNA
(b) rRNA
(c) tRNA
(d) snRNA
Answer: (c) tRNA
10. The type of bond linking adjacent nucleotides in a polynucleotide chain is:
(a) Peptide bond
(b) Glycosidic bond
(c) Phosphodiester bond
(d) Disulfide bond
Answer: (c) Phosphodiester bond
11. The Watson and Crick model of DNA was published in:
(a) 1944
(b) 1950
(c) 1953
(d) 1962
Answer: (c) 1953
12. X-ray crystallography data crucial for the Watson-Crick model was provided by:
(a) Erwin Chargaff
(b) Friedrich Miescher
(c) Rosalind Franklin
(d) Linus Pauling
Answer: (c) Rosalind Franklin
13. The 3' end of a tRNA molecule carries the sequence:
(a) AUG
(b) CCA-OH
(c) UAA
(d) GGC
Answer: (b) CCA-OH
14. A-DNA is formed under:
(a) High humidity conditions
(b) Low humidity/dehydrating conditions
(c) Physiological conditions
(d) High temperature
Answer: (b) Low humidity/dehydrating conditions
15. Which enzyme charges a tRNA with its specific amino acid?
(a) RNA polymerase
(b) Ribonuclease
(c) Aminoacyl-tRNA synthetase
(d) Ligase
Answer: (c) Aminoacyl-tRNA synthetase
Section B: Short Answer Questions
Answer in 3–5 sentences or 50–100 words. (2–5 marks each)
Q.No. | Question |
1. | Define a nucleotide. Name its three components. How is it different from a nucleoside? |
2. | State Chargaff's rules and explain their significance in the Watson-Crick model. |
3. | Distinguish between A-DNA and B-DNA with respect to three structural parameters. |
4. | What is the antiparallel nature of DNA strands? Why is it important for DNA replication? |
5. | Write a note on the cloverleaf structure of tRNA. |
6. | What is a phosphodiester bond? Draw a simplified diagram showing two nucleotides linked by a phosphodiester bond. |
7. | Differentiate between mRNA and rRNA with respect to function and abundance. |
8. | What are ribozymes? Give one example and state their significance. |
9. | Define the melting temperature (Tm) of DNA. Why does DNA with higher G+C content have a higher Tm? |
10. | What is Z-DNA? Under what conditions does it form and what is its biological significance? |
Section C: Long Answer / Descriptive Questions
Answer in detail with diagrams wherever applicable. (10–15 marks each)
14. Describe the Watson and Crick double helix model of DNA in detail. Include:
◦ All structural features of the model
◦ Chargaff's rules and their significance
◦ Helical parameters (pitch, diameter, bp per turn)
◦ Forces that stabilize the double helix
◦ A labeled diagram description
15. Describe the structure and functions of the three major types of RNA. Discuss their roles in protein synthesis.
16. Compare and contrast A-DNA, B-DNA, and Z-DNA with a comprehensive table. Describe the conditions that favour the formation of each and their biological relevance.
17. Write an essay on the biological significance of nucleic acids. Discuss the central dogma of molecular biology and the roles of DNA and various types of RNA in gene expression.
18. Describe the structure of a nucleotide and explain how nucleotides are joined to form a polynucleotide chain. Include: types of nitrogenous bases, the nature of the phosphodiester bond, and the polarity (5'→3' directionality) of the chain.
Section D: Diagram-Based Questions
Study the following diagram descriptions and answer the questions. (5 marks each)
Q.No. | Diagram-Based Question |
1. | Draw and label a nucleotide of DNA. Show the position of all three components. Mark the bond between the sugar and the base (N-glycosidic bond) and between the sugar and phosphate. |
2. | Sketch the cloverleaf structure of tRNA. Label: acceptor stem, anticodon loop, D-loop, TΨC loop, variable loop, and 3'-CCA terminus. |
3. | Draw the Watson-Crick double helix model of B-DNA. Label: sugar-phosphate backbone, base pairs (A:T and G:C), major groove, minor groove, helical pitch (34 Å), diameter (20 Å), and indicate the antiparallel orientation (5'→3' and 3'→5'). |
4. | Draw a diagram showing two nucleotides connected by a phosphodiester bond. Label: 5' end, 3' end, the phosphodiester bond, and the deoxyribose sugars. |
5. | Draw a flowchart/diagram representing the Central Dogma of Molecular Biology. Show all major and minor (special) information flows. |
Section E: True or False / Fill in the Blanks
True or False
Statement | Answer |
1. DNA contains Uracil as one of its nitrogenous bases. | False (DNA has Thymine; RNA has Uracil) |
2. The two strands of DNA run antiparallel to each other. | True |
3. G:C base pair is stabilized by 2 hydrogen bonds. | False (G:C has 3 H-bonds; A:T has 2) |
4. Z-DNA is a left-handed double helix. | True |
5. rRNA constitutes approximately 80% of total cellular RNA. | True |
6. The tRNA anticodon pairs with the mRNA codon. | True |
7. A-DNA is the most common form of DNA in living cells. | False (B-DNA is the most common) |
8. The phosphodiester bond links the 3'-OH to the 5'-phosphate of adjacent nucleotides. | True |
Fill in the Blanks (with Answers)
Statement | Answer |
1. The sugar present in RNA is ______. | Ribose |
2. In B-DNA, there are ______ base pairs per helical turn. | 10 |
3. According to Chargaff's rules, the molar amount of Adenine equals ______. | Thymine (T) |
4. The ______ loop of tRNA contains the anticodon. | Anticodon |
5. ______ discovered the left-handed Z-DNA. | Alexander Rich (1979) |
6. The helical pitch of B-DNA is ______ Å. | 34 |
7. The universal 3' terminus of tRNA is ______. | CCA-3'-OH |
8. The most abundant RNA in a cell is ______. | rRNA |
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