Chapter 5 - Molecular Basis of Inheritance

Molecular Basis of Inheritance explains how genetic information is stored, copied, expressed, regulated and inherited at the molecular level. NCERT Class 12 Biology Chapter 5 covers DNA structure and packaging, evidence for genetic material, RNA, replication, transcription, the genetic code, translation, gene regulation, the Human Genome Project and DNA fingerprinting.

Teachoo’s detailed chapter resources include process diagrams, NCERT solutions, experiment summaries, important questions, sequence-based problems, MCQs and case-based questions for CBSE and NEET.

Structure of DNA and RNA

DNA and RNA are polynucleotides. Each nucleotide contains a nitrogenous base, a pentose sugar and a phosphate group. Nucleotides join through 3′–5′ phosphodiester bonds, creating a chain with polarity.

Watson and Crick proposed the double-helix model using available chemical and X-ray diffraction evidence. Important features include:

• Two antiparallel polynucleotide strands

• Complementary base pairing

• A–T pairing through two hydrogen bonds

• G–C pairing through three hydrogen bonds

• A right-handed helix

• Approximately ten base pairs per turn

• A pitch of 3.4 nm

• A distance of 0.34 nm between successive base pairs

Chargaff’s observations support complementary base pairing. Students should use these relationships in percentage-composition problems.

RNA contains ribose and uracil and is usually single-stranded. Messenger RNA, transfer RNA and ribosomal RNA perform different roles in protein synthesis. Some RNA molecules can also act as catalysts.

Packaging of DNA

Prokaryotic DNA is organised in the nucleoid with associated proteins. In eukaryotes, negatively charged DNA wraps around positively charged histone octamers to form nucleosomes. Nucleosomes produce a beads-on-string appearance and undergo higher levels of folding to form chromatin and chromosomes.

Euchromatin is relatively loosely packed and transcriptionally active, while heterochromatin is more densely packed and generally inactive.

Search for the genetic material

Several experiments established DNA as the genetic material:

• Griffith demonstrated bacterial transformation

• Avery, MacLeod and McCarty identified DNA as the transforming substance

• Hershey and Chase used radioactive labelling to distinguish DNA from protein in bacteriophages

The chapter also compares the properties required of genetic material and explains why DNA is generally more stable than RNA, while RNA may have supported early catalytic and genetic functions.

DNA replication

DNA replication is semiconservative: each daughter DNA molecule contains one parental strand and one newly synthesised strand. Meselson and Stahl demonstrated this principle using nitrogen isotopes and density-gradient centrifugation.

DNA polymerase synthesises DNA only in the 5′ to 3′ direction. This produces continuous synthesis on one template and discontinuous synthesis through fragments on the other. Ligase joins the fragments.

Students should understand template polarity, complementary sequence formation and experimental-generation patterns.

Transcription

Transcription copies information from a DNA template into RNA. A transcription unit includes a promoter, structural gene and terminator. Only one DNA strand normally acts as the template for a particular transcription unit.

In prokaryotes, transcription and translation can be coupled. Eukaryotic transcription involves different RNA polymerases and processing of the primary transcript through capping, tailing and splicing.

Genetic code and translation

The genetic code is a set of triplet codons in mRNA. It is degenerate, nearly universal, unambiguous and read continuously. AUG acts as an initiation codon and codes for methionine, while UAA, UAG and UGA act as stop codons.

Transfer RNA functions as an adapter between codons and amino acids. Translation includes amino-acid activation, initiation, elongation and termination and takes place on ribosomes.

Sequence questions may require students to determine complementary DNA, RNA codons, anticodons or amino-acid sequences. Direction must always be checked before answering.

Regulation of gene expression

The lac operon explains inducible gene regulation in bacteria. In the absence of lactose, a repressor prevents transcription of structural genes. When lactose is available under appropriate conditions, the inducer inactivates the repressor and permits transcription.

Students should understand the roles of the regulator gene, promoter, operator, structural genes, repressor and inducer.

Human Genome Project and DNA fingerprinting

The Human Genome Project aimed to sequence the human genome, identify genes, store and analyse data and address related ethical and social questions.

DNA fingerprinting uses highly variable DNA sequences to distinguish individuals. Variable-number tandem repeats and DNA polymorphism form the conceptual basis. Applications include forensic analysis, biological relationship testing and population studies.

Important diagrams and experiments

• Polynucleotide chain and DNA double helix

• Nucleosome and chromatin packaging

• Griffith experiment

• Hershey–Chase experiment

• Meselson–Stahl experiment

• Replication fork

• Transcription unit

• tRNA structure

• Translation

• Lac operon

• DNA fingerprinting pattern

How should students study this chapter?

Divide the chapter into information flow: storage in DNA → replication → transcription → translation → regulation. Learn each experiment using question, method, observation and conclusion. Practise sequence direction and base-pair calculations rather than only reading solved examples.

Frequently Asked Questions

Why are DNA strands called antiparallel?

One strand runs from 5′ to 3′, while the complementary strand runs from 3′ to 5′.

What proved that DNA replication is semiconservative?

The Meselson–Stahl experiment demonstrated semiconservative replication through isotope labelling and DNA-density patterns across generations.

Why is one DNA strand synthesised discontinuously?

DNA polymerase works only in the 5′ to 3′ direction. Because the templates are antiparallel, one new strand must be formed as separate fragments.

What is meant by degeneracy of the genetic code?

More than one codon can specify the same amino acid.

Is the lac operon normally on or off when lactose is absent?

It is off because the active repressor binds the operator and blocks transcription.

What is the basis of DNA fingerprinting?

It is based on polymorphism in repetitive DNA sequences that vary among individuals.