Chapter 6: Molecular Basis of Inheritance
Q1. What evidence supported the role of DNA as genetic material?
Solution1. Griffith's experiment (1928) - Transforming principle
2. Avery, MacLeod, McCarty (1944) - Identified DNA as transforming material
3. Hershey-Chase experiment (1952) - Confirmed DNA is genetic material
Q2. What is the Central Dogma of Molecular Biology?
SolutionDNA → RNA → Protein
Replication Transcription Translation
Flow of genetic information: DNA → (transcription) → mRNA → (translation) → Protein
Q3. What are Okazaki fragments?
SolutionOkazaki fragments are short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication. They are joined by DNA ligase to form a continuous strand.
Q4. What is the role of tRNA in protein synthesis?
SolutiontRNA (transfer RNA) carries amino acids to the ribosome during translation. It has an anticodon that base pairs with the codon on mRNA, ensuring the correct amino acid is added to the growing polypeptide chain.
Q5. What is the structure of DNA? Who proposed it?
SolutionProposed by Watson and Crick (1953)
Features: (1) Two polynucleotide chains coiled (2) Anti-parallel strands (3) Sugar-phosphate backbone outside (4) Bases face inward (5) Hydrogen bonds between bases (A=T, G≡C) (6) 3.4 nm per turn, 10 base pairs per turn
Q6. What are the properties of the genetic code?
Solution1. Triplet (3 bases for one amino acid)
2. Universal (same in all organisms)
3. Degenerate (multiple codons for same amino acid)
4. Non-overlapping and comma-less
5. Unambiguous
6. AUG = Start codon; UAA, UAG, UGA = Stop codons
Q7. What is an operon? Explain the lac operon.
SolutionLac operon: Inducible system in E. coli for lactose metabolism
Components: (1) Structural genes (z, y, a) (2) Operator (3) Promoter (4) Regulatory gene (i)
Without lactose: Repressor binds operator, blocks transcription
With lactose: Allolactose inactivates repressor, transcription occurs
Q8. What is DNA fingerprinting? What are its applications?
SolutionApplications: (1) Paternity testing (2) Criminal identification (3) Species identification (4) Solving immigration disputes (5) Identifying dead bodies (6) Evolutionary studies
Q9. What was the Human Genome Project?
SolutionHGP (1990-2003): International project to sequence human DNA
Goals: (1) Sequence 3 billion base pairs (2) Identify all genes (~20,000-25,000) (3) Store in databases (4) Improve analysis tools (5) Address ethical issues (ELSI)
Q10. What is a mutation? What are its types?
SolutionGene mutation: Point mutation (substitution), Frameshift (insertion/deletion)
Chromosomal mutation: Deletion, Duplication, Inversion, Translocation
Cause: Physical (UV, X-rays), Chemical (alkylating agents), Biological (viruses)
Q11. What is transcription? Describe the process in prokaryotes.
SolutionSteps: (1) RNA polymerase binds to promoter (2) DNA unwinds (3) Elongation - RNA chain grows 5' to 3' (4) Termination - RNA polymerase reaches terminator, releases mRNA
Q12. What are the differences between DNA replication and transcription?
SolutionReplication: Both strands copied, DNA product, DNA polymerase
Transcription: One strand copied, RNA product, RNA polymerase
Q13. What is the wobble hypothesis?
SolutionThe third base of a codon can form non-standard base pairing with the first base of the anticodon, allowing one tRNA to recognize more than one codon.
Significance: (1) Reduces number of tRNAs needed (2) Explains degeneracy of genetic code (3) Only 61 codons need about 40 tRNAs
Q14. What are the differences between prokaryotic and eukaryotic DNA?
SolutionProkaryotic: Circular, naked DNA, single chromosome, no introns
Eukaryotic: Linear DNA with histones, multiple chromosomes, introns present
Q15. What are the post-transcriptional modifications in eukaryotes?
Solution1. 5' capping (methylated guanosine cap)
2. 3' polyadenylation (poly-A tail)
3. Splicing (removal of introns, joining of exons)
Result: Mature mRNA exported to cytoplasm