DNA stores genetic information and copies itself.
DNA is a double helix composed of repeating nucleotides.
Nucleotide = phosphate + deoxyribose (sugar) + nitrogenous base (A, T, C, G)
Nucleoside = base + sugar (no phosphate)
Phosphodiester bonds link nucleotides together (strong covalent bonds).
Griffith's experiment: transformation in S. pneumoniae
Avery, MacLeod, McCarty: DNA is the genetic material
Watson & Crick: used Franklin's X-ray crystallography and Chargaff’s rules to model DNA as a right-handed double helix
A pairs with T (2 H-bonds), G pairs with C (3 H-bonds)
Explains DNA’s replication fidelity and mutation resistance
DNA strands are antiparallel: 5′ → 3′ and 3′ → 5′
Feature | DNA | RNA |
---|---|---|
Sugar | Deoxyribose | Ribose |
Bases | A, T, C, G | A, U, C, G |
Structure | Double-stranded | Single-stranded |
Stability | Stable | Less stable |
Function | Long-term storage | Many roles (messenger, enzymatic, etc.) |
🧪 RNA World Hypothesis: RNA may have been the original molecule of life due to its catalytic ability and role in early evolution.
mermaid
CopyEdit
graph LR A[DNA] -->|Transcription| B[mRNA] B -->|Translation| C[Protein]
Information flows from gene to protein
Transcription: DNA → RNA
Translation: RNA → Protein
Prokaryotes: cytoplasm
Eukaryotes: nucleus (due to nuclear membrane)
RNA Polymerase: catalyzes RNA synthesis
Promoter: DNA sequence signaling start (e.g., TATA box)
Template Strand: strand RNA is copied from
Coding Strand: same as RNA sequence, but with T instead of U
Initiation:
RNA polymerase + transcription factors bind promoter
DNA unwinds locally (transcription bubble)
Elongation:
RNA is synthesized 5′ → 3′
Complementary to template strand
Termination:
RNA polymerase releases RNA
In prokaryotes: directly terminates
In eukaryotes: coupled to mRNA processing
Modification | Purpose |
---|---|
5′ Cap | Ribosome binding, stability |
Poly-A Tail (3′) | Export, stability, translation |
Splicing | Removes introns, joins exons |
Alternative splicing | Creates protein diversity |
DNA wraps around histone proteins forming nucleosomes
“Beads on a string” appearance
Compacts DNA ~50,000x
Type | Description | Gene Expression |
---|---|---|
Euchromatin | Loosely packed | Active |
Heterochromatin | Densely packed | Inactive |
Eukaryotes: linear chromosomes, histones, nucleus
Prokaryotes: circular genome, no histones, cytoplasm
Template strand: 3′-TACCGGATA-5′
mRNA transcript: 5′-AUGGCCUAU-3′
How does histone binding influence gene expression?
What would happen if the 5′ cap wasn't added to mRNA?
How does base-pairing ensure accurate transcription and replication?
What’s the functional advantage of alternative splicing for a multicellular organism?
How is RNA stability influenced by both ends of the transcript?
Nucleotide / Nucleoside
Phosphodiester bond
Base-pairing rule
Antiparallel strands
RNA Polymerase
Promoter
Template / coding strand
Transcription factors
5′ cap / poly-A tail
Intron / exon / splicing
Euchromatin / heterochromatin
Nucleosome / histone
Why does euchromatin show higher gene expression than heterochromatin?
A. It's more tightly packed
B. It has more histones
C. It's more accessible to RNA polymerase ✅
D. It's located outside the nucleus
Given the template strand 3′-TACGGTACG-5′, what is the RNA transcript?
A. 5′-AUGCCAUGC-3′ ✅
B. 3′-AUGCCAUGC-5′
C. 5′-TACGGTACG-3′
D. 5′-UACGGUACG-3′
Which of the following is NOT a post-transcriptional modification?
A. Polyadenylation
B. Exon splicing
C. Addition of 5′ cap
D. DNA methylation ✅
If RNA polymerase II continuously binds to a mutated enhancer, what would be the likely effect?
A. No transcription
B. Sporadic transcription
C. Continuous transcription ✅
D. RNA degradation
Which component provides the energy for RNA polymerization?
A. ATP from mitochondria
B. GTP binding proteins
C. The phosphate bonds in incoming NTPs ✅
D. RNA helicase
Understand how DNA is compacted into chromosomes and how chromatin structure affects gene expression.
Definition: The basic unit of DNA packaging in eukaryotes.
DNA wraps around histone proteins (a histone octamer) forming nucleosomes.
This "beads on a string" structure allows long DNA molecules to fit inside the nucleus.
Nucleosomes are further coiled and supercoiled into condensed chromosomes.
This hierarchical structure enables efficient storage and regulation of DNA.
Chromatin Type | Structure | Gene Activity |
---|---|---|
Euchromatin | Loosely packed | Transcriptionally active |
Heterochromatin | Densely packed | Transcriptionally silent |
The degree of DNA packing influences gene expression: less compact = more accessible to transcription machinery.
Histones: Small, positively charged proteins that DNA wraps around. Their modifications can regulate gene accessibility.
DNA with histones forms chromatin.
DNA without histones (rare in eukaryotes) is unstructured and not protected.
Feature | Eukaryotes | Prokaryotes |
---|---|---|
DNA shape | Linear | Circular |
Location | Nucleus | Cytoplasm |
Packaging | Wrapped around histones (nucleosomes) | No histones, supercoiling only |
Organization | Chromosomes | Single circular DNA molecule |
✅ Key distinction: "DNA wraps around histones in eukaryotes" — not in prokaryotes.
What is one key difference between eukaryotic and prokaryotic DNA packaging?
Correct answer:
C. DNA wraps around histones in eukaryotes.
Nucleosome: DNA + histone complex
Chromatin: General term for DNA-protein complex
Histone: Protein core for nucleosome
Euchromatin: Loose, gene-active DNA
Heterochromatin: Tight, gene-inactive DNA