Molecular Basis of Inheritance: Key Concepts and Principles

Molecular Basis of Inheritance

1. Which enzyme transcribes hnRNA?

Answer: RNA polymerase II.

2. Why is the genetic code considered 'degenerate'?

Answer: Because more than one codon can code for the same amino acid.

3. What are Okazaki fragments?

Answer: Short DNA fragments synthesized discontinuously on the lagging strand during DNA replication.

4. Which two basic amino acids are abundant in histone proteins?

Answer: Lysine and Arginine.

5(a). What is the function of mitochondria in sperm?

Answer: It provides energy (ATP) for sperm motility.

5(b). Which codon is the starting codon?

Answer: AUG.

5(c). What is the genetic code?

Answer: The genetic code is the sequence of codons in mRNA that determines the sequence of amino acids in a protein.

5(d). What is the AUG codon called?

Answer: Start codon (Initiation codon).

6. What is a 'Template strand'?

Answer: The DNA strand that serves as a pattern for the synthesis of RNA during transcription.

7. Differences between Homozygous and Heterozygous

• Homozygous: Same alleles for a trait.
• Heterozygous: Different alleles for a trait.

9. Which enzyme helps unwind the DNA helix?

Answer: DNA helicase.

10. Who first discovered DNA fingerprinting?

Answer: Alec Jeffreys.

11. What is DNA fingerprinting?

Answer: A technique used to identify individuals based on unique patterns in their DNA.

12. What is the human genome?

Answer: The complete set of genetic material (DNA) present in a human cell.

13. Structure of a nucleosome

Answer: A nucleosome is the basic unit of chromatin. It consists of a segment of DNA wrapped around a histone protein core. The core contains eight histone proteins (two each of H2A, H2B, H3, and H4). About 146 base pairs of DNA coil around this core. Histone H1 acts as a linker and helps in further packing.

14. Goals of the Human Genome Project

• Identify all human genes.
• Determine the sequence of 3 billion base pairs.
• Store data in databases.
• Develop tools for data analysis.
• Address ethical, legal, and social issues.

15. Four features of genetic material

Answer: Genetic material must:

• Replicate accurately.
• Be chemically and structurally stable.
• Express itself in the form of traits.
• Allow mutations for evolution.

16. The Lac Operon model

Answer: The lac operon in E. coli regulates lactose metabolism. It consists of structural genes (lac Z, lac Y, lac A), a promoter, an operator, and a regulator gene. In the absence of lactose, the repressor binds to the operator and blocks transcription. When lactose is present, it inactivates the repressor, allowing transcription.

17. Double helical structure of DNA

Answer: DNA consists of two antiparallel strands forming a double helix. Each strand has a sugar-phosphate backbone with nitrogen bases inside. Bases pair specifically: Adenine–Thymine (2 H-bonds) and Guanine–Cytosine (3 H-bonds). The helix has major and minor grooves.

18. Hershey and Chase experiment

Answer: Alfred Hershey and Martha Chase used bacteriophages labeled with radioactive sulfur (protein) and phosphorus (DNA). Only radioactive DNA entered bacteria and directed virus formation, proving DNA is the genetic material.

19. Applications of DNA fingerprinting

• Criminal identification
• Paternity testing
• Identifying dead bodies
• Wildlife conservation
• Genetic disorder studies

20. Why does hnRNA undergo splicing?

Answer: hnRNA contains introns (non-coding sequences). Splicing removes introns and joins exons to form functional mRNA for protein synthesis.

21. Termination of mRNA translation

Answer: Translation stops when a stop codon (UAA, UAG, UGA) appears on mRNA. Release factors bind to the ribosome, causing the release of the polypeptide chain.

22. Comparison: DNA vs. RNA

23. Linkage and Crossing Over

• Linkage: Genes located on the same chromosome tend to be inherited together.
• Crossing over: Exchange of genetic material between homologous chromosomes during meiosis, producing variation.

24. Plasmid vs. Chromosomal DNA

Plasmid DNA: Small, circular, double-stranded; found in bacteria; replicates independently; carries non-essential genes (e.g., antibiotic resistance).

Chromosomal DNA: Large, linear; present in the nucleus (eukaryotes) or nucleoid (prokaryotes); contains essential genes; controls cellular functions.

25. Nucleotide vs. Nucleoside

• Nucleotide: Nitrogen base + sugar + phosphate group (e.g., ATP).
• Nucleoside: Nitrogen base + sugar only (e.g., Adenosine).

26. Gene Cloning

Answer: Gene cloning is the process of making multiple copies of a desired gene. The gene is inserted into a vector (like a plasmid) and introduced into a host cell where it replicates.

27. UTR in mRNA

Answer: UTRs (Untranslated Regions) are present at both ends of mRNA (5′ UTR before the start codon; 3′ UTR after the stop codon). They regulate translation, maintain mRNA stability, and help ribosome binding.

28. Transcription in eukaryotes

Steps:

• Initiation: RNA polymerase binds to the promoter with transcription factors.
• Elongation: RNA strand synthesized using template DNA.
• Termination: RNA transcript released.
• Processing: hnRNA → splicing, capping, poly-A tail → mRNA.

29. Types of RNA polymerases

• RNA polymerase I: Synthesizes rRNA.
• RNA polymerase II: Synthesizes mRNA (hnRNA).
• RNA polymerase III: Synthesizes tRNA and small rRNA.

30. Central Dogma of Molecular Biology

Answer: Proposed by Francis Crick, it explains the flow of genetic information: DNA → RNA → Protein. DNA replicates, transcribes into RNA, which translates into protein.

31. Replicating fork

Answer: The replication fork is the Y-shaped region where DNA unwinds. The leading strand undergoes continuous synthesis, while the lagging strand undergoes discontinuous synthesis (Okazaki fragments). Helicase unwinds DNA, and DNA polymerase synthesizes new strands.

32. Semiconservative DNA replication

Answer: Each new DNA molecule contains one parental strand and one newly synthesized strand. The double helix unwinds, and each parental strand acts as a template for a new complementary strand.

33. Transcription unit

Answer: Consists of a promoter (starting site), structural gene (codes for RNA), and terminator (stop signal).

34. Enzymes in DNA replication

• Helicase: Unwinds the DNA helix.
• Topoisomerase: Relieves strain during unwinding.
• Primase: Synthesizes RNA primer.
• DNA polymerase: Adds nucleotides to form a new strand.
• DNA ligase: Joins Okazaki fragments.

35. Mutation: Deletion vs. Duplication

• Deletion: Loss of a DNA segment; reduces genetic material; often harmful.
• Duplication: Repetition of a DNA segment; increases genetic material; may cause gene redundancy.

36. Meselson and Stahl Experiment

Answer: Conducted in 1958, they grew E. coli in heavy nitrogen (¹⁵N) and shifted it to normal nitrogen (¹⁴N). After one generation, DNA showed intermediate density (hybrid), and after two generations, one light and one hybrid band appeared, proving semiconservative replication.

37. Features of the genetic code

• Triplet: Each codon consists of three nucleotides.
• Degenerate: More than one codon codes for the same amino acid.
• Universal: Same codons specify the same amino acids in most organisms.
• Non-overlapping: Each base is read only once.
• Commaless: Codons are read continuously.

38. Southern Blotting

Answer: A technique developed by E.M. Southern in 1975 to detect specific DNA sequences in a mixture. Steps include DNA extraction and digestion, gel electrophoresis, denaturation, blotting, hybridization, and detection.