1. DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | 2-deoxyribose | Ribose |
| Base unique to it | Thymine | Uracil |
| Usual structure | Double-stranded antiparallel helix | Usually single-stranded |
| Main role | Stable genetic material in most organisms | Messenger, adapter, structural and catalytic/genetic roles |
Base-pairing cueA = T through two hydrogen bonds; G ≡ C through three.
2. Evidence for genetic material
| Scientists | Experiment | Conclusion |
|---|---|---|
| Griffith | S and R strains of Streptococcus pneumoniae | Heat-killed S cells transformed live R cells. |
| Avery, MacLeod and McCarty | Enzyme treatment of transforming principle | Only DNase prevented transformation; DNA is the transforming substance. |
| Hershey and Chase | T2 phage; ³²P labelled DNA, ³⁵S labelled protein | DNA entered bacteria and directed phage formation. |
| Fraenkel-Conrat and Singer | Tobacco mosaic virus reconstitution | RNA is genetic material in some viruses. |
3. Prokaryotic vs eukaryotic DNA packaging
| Point | Prokaryotes | Eukaryotes |
|---|---|---|
| Location | Nucleoid | Nucleus |
| DNA organisation | Usually circular; looped domains | Linear DNA associated with histones as chromatin |
| Basic unit | Nucleoid-associated proteins | Nucleosome: about 146 bp around histone octamer |
| Histones | No canonical nucleosome octamer | Two each of H2A, H2B, H3, H4; H1 binds linker DNA |
| Chromatin | Not classified as eu-/heterochromatin | Euchromatin is active and loose; heterochromatin is inactive and dense. |
4. Replication vs transcription vs translation
| Feature | Replication | Transcription | Translation |
|---|---|---|---|
| Flow | DNA → DNA | DNA → RNA | mRNA → polypeptide |
| Template | Both parental DNA strands | One DNA template strand of a gene | mRNA codons |
| Main machinery | DNA-dependent DNA polymerase | RNA polymerase | Ribosome and tRNA |
| Product | Two semiconservative DNA molecules | RNA transcript | Polypeptide chain |
5. Leading strand vs lagging strand
| Feature | Leading strand | Lagging strand |
|---|---|---|
| Template orientation | 3′ → 5′ | 5′ → 3′ |
| Synthesis | Continuous | Discontinuous |
| Fragments | No Okazaki fragments | Okazaki fragments joined by DNA ligase |
| Primers | One primer at initiation | Repeated RNA primers needed |
Helicase→Primase→DNA polymerase 5′ → 3′→Ligase
6. Prokaryotic vs eukaryotic transcription
| Point | Prokaryotes | Eukaryotes |
|---|---|---|
| Site | Cytoplasm / nucleoid region | Nucleus; translation follows in cytoplasm |
| RNA polymerases | One polymerase | Pol I: rRNA; Pol II: hnRNA; Pol III: tRNA, 5S rRNA and snRNA |
| Primary transcript | Generally functional as mRNA | hnRNA needs processing |
| Processing | Minimal in NCERT model | 5′ capping, 3′ tailing and splicing |
7. Lac operon: off vs on
| Condition | Repressor / operator | Structural genes z, y, a | Outcome |
|---|---|---|---|
| Lactose absent | Active repressor binds operator | Not transcribed | Operon is off. |
| Lactose present | Inducer inactivates repressor | Transcribed | β-galactosidase, permease and transacetylase are produced. |
8. Human Genome Project vs DNA fingerprinting
| Feature | Human Genome Project | DNA fingerprinting |
|---|---|---|
| Purpose | Identify genes and sequence the human genome | Identify individuals through DNA polymorphism |
| Material | Whole-genome sequence and maps | Highly variable repetitive DNA, especially VNTRs |
| Uses | Genome biology and disease-gene discovery | Forensics, paternity testing and identification |