NEET Biology · NCERT Class 12
Molecular Basis of Inheritance mind map
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Molecular Basis of Inheritance: what the chapter covers
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1Chapter Overview
Molecular Basis of Inheritance explains how genetic information is stored, copied, expressed, regulated and used in biotechnology. DNA is the hereditary material in most organisms because it can replicate, mutate and express information through RNA and proteins. The chapter connects DNA structure with semi-conservative replication, transcription of RNA, genetic code, translation into proteins and regulation of gene expression using the lac operon. It also covers modern applications such as the Human Genome Project and DNA fingerprinting. For NEET, this chapter is highly conceptual and diagram-based; repeated questions come from DNA structure, enzymes of replication, RNA processing, codon features, lac operon and VNTR-based DNA profiling.
- The chapter moves from molecular structure to biological function: DNA structure → replication → transcription → translation → regulation.
- NCERT emphasizes experimental evidence for DNA as genetic material: Griffith, Avery-MacLeod-McCarty and Hershey-Chase.
- Replication, transcription and translation are directional processes, and 5' to 3' polarity is a frequent NEET trap.
2DNA & RNA
DNA and RNA are nucleic acids made of nucleotides, but DNA is the main hereditary material in most organisms. The proof came through classical experiments: Griffith showed transformation, Avery-MacLeod-McCarty identified DNA as the transforming principle, and Hershey-Chase confirmed DNA as genetic material in bacteriophages. Watson and Crick proposed the double helix model using Chargaff’s rules and X-ray diffraction evidence. DNA has antiparallel strands, complementary base pairing and a sugar-phosphate backbone. RNA is usually single-stranded and functions as mRNA, tRNA and rRNA. In cells, DNA is highly compacted by proteins: histones in eukaryotes and non-histone basic proteins in prokaryotes.
- A molecule must replicate, be stable, allow mutation and express information to act as genetic material.
- RNA can act as genetic material in some viruses but is generally less stable and more reactive than DNA.
- Chargaff’s rule states that in double-stranded DNA, A = T and G = C.
3DNA Replication
DNA replication is the process by which a cell copies its DNA before cell division. It is semi-conservative: each daughter DNA molecule contains one parental strand and one newly synthesized strand. Meselson and Stahl proved this using 15N and 14N isotopes in E. coli. Replication starts at an origin and proceeds through a replication fork where helicase unwinds DNA, primase adds RNA primers and DNA polymerase extends the new strand only in the 5' to 3' direction. Because strands are antiparallel, one strand is synthesized continuously as the leading strand, while the lagging strand forms Okazaki fragments that are joined by ligase. Proofreading ensures high fidelity.
- Meselson-Stahl experiment used density gradient centrifugation to prove semi-conservative replication.
- The replication fork is Y-shaped because parental strands separate and act as templates.
- Deoxyribonucleoside triphosphates provide both building blocks and energy for polymerization.
4Transcription
Transcription is the synthesis of RNA from a DNA template. Only one strand of DNA, called the template strand, is copied; the other is the coding strand because its sequence resembles RNA except T is replaced by U. A transcription unit contains a promoter, structural gene and terminator. RNA polymerase binds the promoter, initiates RNA synthesis, elongates the RNA chain in the 5' to 3' direction and stops at the terminator. In prokaryotes, transcription and translation can be coupled because there is no nuclear membrane. In eukaryotes, primary RNA transcripts undergo processing: capping, tailing and splicing to produce mature mRNA.
- RNA polymerase does not require a primer, unlike DNA polymerase.
- Promoter defines the template strand and direction of transcription.
- In bacteria, a single RNA polymerase transcribes all RNA types.
5Genetic Code & Translation
The genetic code is the relationship between nucleotide triplets in mRNA and amino acids in a polypeptide. Since four bases are read in groups of three, there are 64 codons. AUG codes for methionine and acts as the start codon, while UAA, UAG and UGA are stop codons. The code is triplet, degenerate, unambiguous, nearly universal, non-overlapping and comma-less. Translation is protein synthesis on ribosomes. mRNA provides codons, tRNA brings amino acids using anticodons, and rRNA forms the catalytic ribosome. Translation includes activation of amino acids, initiation, elongation and termination. Proteins may later undergo folding, cleavage or chemical modifications.
- 64 codons include 61 sense codons and 3 stop codons.
- The code is unambiguous: one codon specifies only one amino acid.
- Wobble at the third codon position contributes to degeneracy.
6Gene Regulation
Gene regulation means controlling when, where and how much a gene is expressed. It saves energy and allows cells to respond to environmental changes. In prokaryotes, genes involved in a common function may be organized into an operon, containing structural genes, promoter, operator and regulator gene. The lac operon of E. coli is an inducible operon that controls lactose metabolism. In the absence of lactose, a repressor binds the operator and blocks transcription. In the presence of lactose, allolactose binds the repressor, inactivates it and allows transcription of lacZ, lacY and lacA. Eukaryotic regulation is more complex and occurs at chromatin, transcriptional, post-transcriptional, translational and post-translational levels.
- Jacob and Monod proposed the operon model for regulation of gene expression.
- lacZ codes beta-galactosidase, lacY codes permease and lacA codes transacetylase.
- When lactose is absent, the repressor remains active and binds the operator.
7Human Genome Project & DNA Fingerprinting
The Human Genome Project was an international effort to sequence and map the entire human genome. It aimed to identify all human genes, determine the sequence of approximately 3.2 billion base pairs, store data in databases, improve analysis tools and address ethical, legal and social issues. NCERT highlights that only a small percentage of the genome codes for proteins, most genes have unknown functions, chromosome 1 has the most genes and chromosome Y has the fewest. DNA fingerprinting is a technique for individual identification based on variation in repetitive DNA sequences, especially VNTRs. It is used in forensics, paternity disputes, population genetics, biodiversity studies and medical diagnosis.
- HGP used sequencing, genetic mapping, physical mapping and bioinformatics.
- A major goal was to transfer related technologies to industry and improve biological research.
- Chromosome 1 has maximum genes and Y chromosome has minimum genes according to NCERT.
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