Chapter Summary
Molecular genetics explains how DNA stores, replicates and expresses information. Topics include DNA/RNA structure, classic experiments (Griffith, Avery–MacLeod–McCarty, Hershey–Chase, Meselson–Stahl), DNA packaging in chromosomes, replication (enzymes and directionality), transcription (promoters, RNA polymerases, capping–tailing–splicing), genetic code, translation (initiation–elongation–termination), and gene regulation (lac operon, chromatin modifications). Applications include DNA fingerprinting, Human Genome Project and bioinformatics.
DNA & RNA
Replication
Transcription
Translation
Genetic Code
Gene Regulation
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50 Questions & Answers
1) Who proposed the double helix model of DNA?
Watson and Crick (1953) based on X-ray data of Franklin & Wilkins.
2) State Chargaff’s rules.
%A = %T, %G = %C; purines equal pyrimidines; base composition varies between species.
3) What bonds stabilize the DNA double helix?
Hydrogen bonds between complementary bases and hydrophobic base stacking interactions.
4) What is the pitch and base pairs per turn in B‑DNA?
Pitch ≈ 3.4 nm with ~10 bp per turn; distance between adjacent bases ≈ 0.34 nm.
5) Differentiate purines and pyrimidines.
Purines: double-ring (A, G); Pyrimidines: single-ring (C, T, U).
6) What is a nucleosome?
~146 bp DNA wrapped 1.65 turns around histone octamer (2 each of H2A, H2B, H3, H4); H1 links nucleosomes.
7) Define euchromatin and heterochromatin.
Euchromatin: lightly packed, transcriptionally active; Heterochromatin: tightly packed, inactive.
8) What are satellite DNA/ VNTRs?
Highly repetitive non‑coding sequences varying in number; used in DNA fingerprinting.
9) What is a gene?
Functional unit of heredity; a DNA segment coding for a polypeptide/functional RNA.
10) Define cistron, intron and exon.
Cistron: coding sequence; Exons: expressed sequences; Introns: intervening non‑coding segments removed by splicing.
11) Describe Griffith’s experiment.
Transformation in Pneumococcus: heat‑killed virulent S strain + live R strain → mice died; suggested transforming principle.
12) What did Avery–MacLeod–McCarty conclude?
DNA is the transforming principle; DNase destroyed activity whereas RNase/protease did not.
13) What did Hershey–Chase show?
Using 35S (protein) and 32P (DNA) labeled phages: DNA entered bacteria and directed phage production.
14) What did Meselson–Stahl demonstrate?
Semiconservative DNA replication using 15N→14N shift in E. coli; intermediate then light bands.
15) What is the significance of semiconservative replication?
Each daughter DNA has one parental and one new strand ensuring fidelity.
16) What is denaturation and renaturation of DNA?
Separation of strands by heat/alkali; reassociation on cooling depending on complementarity.
17) What is Tm (melting temperature)?
Temperature at which half of DNA becomes single‑stranded; increases with GC content and salt.
18) Define supercoiling and the enzyme involved.
Over/under‑winding of DNA; controlled by topoisomerases (gyrase in prokaryotes).
19) Name key enzymes in DNA replication.
Helicase, SSBP, primase, DNA polymerase (III in prokaryotes), DNA polymerase I (primer removal), ligase, topoisomerase.
20) Why is replication 5′→3′?
DNA polymerase adds dNTPs to free 3′‑OH; pyrophosphate release drives reaction.
21) Distinguish leading and lagging strands.
Leading synthesized continuously; lagging discontinuously as Okazaki fragments.
22) What are Okazaki fragments?
Short DNA segments (~1000–2000 nt in prokaryotes; ~100–200 nt in eukaryotes) on lagging strand later ligated.
23) Role of DNA polymerase I in bacteria?
Removes RNA primers via 5′→3′ exonuclease and fills gaps with DNA.
24) What is the origin of replication (ori)?
Specific sequence where replication initiates; eukaryotes have multiple ori per chromosome.
25) What is telomerase?
RNA‑dependent DNA polymerase that extends 3′ ends of linear chromosomes to maintain telomeres (active in germ/stem/tumor cells).
26) Define proofreading.
3′→5′ exonuclease activity of DNA polymerases that corrects misincorporated nucleotides.
27) Why are primers required?
DNA polymerases cannot initiate synthesis de novo; primase lays RNA primers providing 3′‑OH.
28) Which strand acts as template during transcription?
Template (antisense) strand is read 3′→5′ to synthesize RNA 5′→3′; coding (sense) strand matches RNA (U instead of T).
29) What is a promoter? Give example.
DNA sequence where RNA polymerase binds to initiate transcription; e.g., TATA box in eukaryotes, −10/−35 in prokaryotes.
30) Types of eukaryotic RNA polymerases.
Pol I: rRNA (28S, 18S, 5.8S); Pol II: mRNA, some snRNA; Pol III: tRNA, 5S rRNA.
31) What are the post‑transcriptional modifications?
5′ capping (m⁷G), 3′ poly‑A tailing, and splicing of introns by spliceosome.
32) What are snRNPs?
Small nuclear ribonucleoproteins (U1, U2, etc.) that form spliceosome for intron removal.
33) Define hnRNA and mRNA.
hnRNA is primary transcript with introns; processed to mature mRNA after capping, splicing, tailing.
34) What is polycistronic vs monocistronic mRNA?
Prokaryotic mRNA often polycistronic (multiple ORFs); eukaryotic mRNA usually monocistronic.
35) What is an operon?
Cluster of genes under one promoter/operator producing polycistronic mRNA (e.g., lac operon).
36) Define UTRs (5′‑UTR/3′‑UTR).
Untranslated regions flanking coding sequence; regulate translation, stability, localization.
37) List properties of the genetic code.
Triplet, degenerate, non‑overlapping, comma‑less, nearly universal, unambiguous, wobble.
38) What is the start and stop codons?
Start: AUG (Met); Stops: UAA, UAG, UGA.
39) What is wobble hypothesis?
Flexibility at 3rd base of codon allows one tRNA to pair with multiple codons.
40) Outline translation steps.
Initiation (assembly at start codon), elongation (peptidyl transferase forms peptide bonds; translocation), termination (release factor at stop codon).
41) What are A, P, E sites?
tRNA binding sites on ribosome: Aminoacyl (A), Peptidyl (P), Exit (E).
42) Role of rRNA in translation.
rRNA forms ribozyme peptidyl transferase activity in large subunit; structural and catalytic roles.
43) What is Shine–Dalgarno sequence?
Purine‑rich sequence in prokaryotic 5′‑UTR aligning mRNA with 16S rRNA to start translation.
44) What is post‑translational modification?
Processing of polypeptides after translation: cleavage, folding, glycosylation, phosphorylation, etc.
45) Define polyribosome (polysome).
Complex of multiple ribosomes translating a single mRNA simultaneously.
46) Describe regulation of the lac operon.
In absence of lactose: repressor binds operator, transcription off. In presence of allolactose: repressor inactivated, transcription on; cAMP‑CAP enhances under low glucose.
47) What is epigenetic regulation?
Heritable changes in gene expression without DNA sequence change, e.g., DNA methylation, histone acetylation/deacetylation.
48) What is DNA fingerprinting?
Identification based on VNTR/STR profiles using techniques like Southern blot/PCR.
49) What is the Human Genome Project (HGP)?
International effort to determine entire human DNA sequence, map genes, and store data; enabled genomics/bioinformatics.
50) Define mutation types relevant here.
Point mutations (transition/transversion), frameshift (insertion/deletion), nonsense, missense, silent; can affect gene expression.