Revision: Genetics and Evolution >> Molecular Basis of Inheritance Biology (Theory) ISC (Science) ISC Class 12 CISCE

A nucleoside consists of a nitrogenous base linked to a pentose sugar without a phosphate group.

RNA is a single-stranded nucleic acid that helps in protein synthesis and information transfer.

A nitrogenous base is an organic molecule (purine or pyrimidine) that carries genetic information in nucleic acids.

Nucleic acids are large biological macromolecules that store and transmit genetic information in living organisms.

The unit formed by joining the anomeric carbon of the furanose (sugar) with a nitrogen of a base is called nucleoside.

DNA is a double-stranded nucleic acid that stores and transmits hereditary information and can replicate itself.

A nucleotide is the basic structural unit of nucleic acids, composed of a nitrogenous base, a pentose sugar, and a phosphate group.

A polynucleotide is a long chain formed by the joining of many nucleotide monomers.

When DNA directs its own synthesis, this function is called the autocatalytic function. Eg., replication.

When DNA directs the synthesis of chemical molecules other than itself, such functions of DNA are called heterocatalytic functions. Eg, Synthesis of RNA (transcription), synthesis of protein (Translation), etc.

The process by which DNA duplicates itself is called replication. 

Conservative replication is a mode of DNA replication in which the original parental DNA molecule remains intact, and a completely new DNA molecule is synthesised.

Conservative Replication

Dispersive replication is a mode of DNA replication in which the parental DNA is broken into fragments, and each daughter DNA molecule contains a mixture of old and new DNA segments.

Dispersive replication

Semi-conservative replication is a mode of DNA replication in which each daughter DNA molecule consists of one parental (old) strand and one newly synthesised strand.

Semi-Conservative Replication

The movement of the ribosome from one end of the mRNA to the other end by the distance of one triplet codon during translation is known as translocation.

Translation is the process by which tRNA having anticodon to the codon on the mRNA, supplies amino acids, as per the message on mRNA.

A sequence of three adjacent nucleotides in mRNA that codes for one amino acid is known as a codon.

A sudden change that occurs in the nucleotide sequence of a gene, causing either a minor or considerable change in the characters of an individual is known as mutation.

The central dogma is the principle that genetic information flows in one direction in a cell, from DNA to RNA to protein.

or

Central Dogma is the process by which genetic information flows from DNA to RNA to protein, controlling cellular functions and body structure.

Reverse transcription is the process by which DNA is synthesised from an RNA template.

\[\mathrm{RNA}\quad\xrightarrow{\text{reverse transcription}}\quad\mathrm{DNA}\quad\xrightarrow{\text{transcription}}\quad\mathrm{mRNA}\quad\xrightarrow{\text{translation}}\quad\mathrm{Protein}\]

hnRNA stands for heterogeneous nuclear RNA, which is the freshly synthesised RNA formed in the nucleus of eukaryotic cells before maturation. It contains both coding and non-coding regions and therefore needs further processing.

Post-transcriptional processing is the modification of a primary RNA transcript after transcription and before translation or final functional use. In eukaryotes, the first transcript formed is often called 'hnRNA' or 'primary transcript', and it is not fully ready for protein synthesis.

A sequence of three nucleotides on mRNA that codes for a specific amino acid is called a triplet codon.

The nucleoid is the region in prokaryotic cells where DNA is organised and associated with proteins, despite the absence of a true nucleus.

The basic repeating unit of chromatin formed by DNA wrapped around a histone octamer is called a nucleosome.

The thread-like complex of DNA and proteins present in the nucleus of eukaryotic cells is called chromatin.

The process by which a very long DNA molecule is compactly organised inside the cell nucleus so that it fits within the limited nuclear space and remains functional is called DNA packaging.

Positively charged basic proteins rich in lysine and arginine that associate with DNA to help pack it in eukaryotic cells are called histones.

A structural unit composed of eight histone protein molecules around which DNA is wrapped is called a histone octamer.

Transfection is the process of inserting a vector into eukaryotic cells.

Proteins other than histones that are associated with chromatin and help in higher-order DNA packaging and regulation are called non-histone chromosomal (NHC) proteins.

A co-repressor is a substance which, when present in excess, binds with a repressor to switch off gene expression.

Constitutive genes are genes that are continuously expressed irrespective of environmental conditions.

Constitutive enzymes are enzymes that are synthesised continuously and are essential for basic cellular metabolism.

Inducible proteins are enzymes that are synthesised only in the presence of a specific inducer or substrate.

An inducer is a small molecule that activates gene expression by initiating the synthesis of specific enzymes.

The technique of identifying an individual by analysing the unique DNA sequence present in each person, similar to fingerprints, is called DNA fingerprinting.

• Genes are hereditary DNA units arranged linearly on chromosomes that transmit traits from parents to offspring.
• They code for proteins or RNAs and can contain both coding (exons) and non-coding (introns) regions.
• A gene consists of functional subunits: cistrons (coding), recons (recombination), and mutons (mutation).
• They can self-duplicate, exist in alternate forms (alleles), and undergo heritable changes (mutations).
• Genes control all cellular functions and physical characteristics, primarily through protein synthesis.

• Friedrich Miescher (1869) first isolated an organic compound from cell nuclei called "nuclein", which Altmann (1889) later renamed nucleic acid.
• By 1920, it was discovered that chromosomes are made up of two distinct macromolecules: proteins and DNA.
• Biologists initially believed proteins were the genetic carriers because of their vast complexity and diversity compared to the limited types of nucleic acids.
• It was eventually proven beyond doubt that DNA (nucleic acid), rather than protein, is the actual genetic material responsible for storing hereditary information.

1. Frederick Griffith's 1928 experiments on Streptococcus pneumoniae shifted from developing a pneumonia vaccine to investigating the transmission of bacterial virulence.
2. The study compared two distinct bacterial variants: the virulent, encapsulated S (Smooth) strain and the harmless, non-encapsulated R (Rough) strain.
3. Baseline experimental controls established that mice survived injections of either the live R strain or the heat-killed S strain independently, but perished when injected with the live S strain.
4. The pivotal final experiment revealed that injecting a mixture of live R strain and heat-killed S strain unexpectedly caused fatal pneumonia, resulting in the recovery of live S strain bacteria.
5. Griffith concluded that a heritable "Transforming Principle" transferred from the dead S strain and assimilated by the live R strain, converting it into a virulent phenotype.
6. While the experiment successfully demonstrated genetic transformation, it was limited by its inability to biochemically identify the transforming substance or confirm it as DNA.

• In 1952, Alfred Hershey and Martha Chase proved that DNA is the genetic material using bacteriophages and E. coli bacteria.
• They used radioactive isotopes: ³²P to label DNA and ³⁵S to label proteins.
• Viruses grown in ³²P medium had radioactive DNA, while those grown in ³⁵S medium had radioactive protein.
• These labelled viruses were allowed to infect E. coli, and then blending and centrifugation were done to separate viral coats.
• Only bacteria infected with ³²P-labelled viruses became radioactive, showing that DNA entered the bacterial cells.
• Bacteria infected with ³⁵S-labelled viruses were not radioactive, proving proteins did not enter the cells; hence, DNA is the genetic material.

• DNA is the primary genetic material in most organisms, while RNA acts as genetic material in some viruses.
• A genetic material must be capable of replication, which both DNA and RNA can achieve through base pairing.
• DNA is chemically and structurally more stable than RNA because it lacks the reactive 2′-OH group and contains thymine instead of uracil.
• Both DNA and RNA can undergo mutations, but RNA mutates faster due to its unstable nature, leading to rapid evolution in RNA viruses.
• DNA stores genetic information efficiently, whereas RNA helps express and transmit it through protein synthesis.

• DNA is universally located on chromosomes, and its composition is similar within a single species but varies across different species.
• The amount of DNA in a diploid cell is approximately twice the amount found in a haploid germ cell.
• Chemical and physical agents (like specific UV light wavelengths) that alter DNA structure directly correspond to those that cause mutations.
• DNA acts as a highly stable cellular molecule that replicates exactly and exhibits specific functions.
• Except for certain viruses (such as TMV and QB bacteriophage), DNA is the genetic material for all living organisms.
• The Feulgen reaction confirms the presence of DNA (and chromatin) by producing a pink colour when reacted with weak acids and Schiff's reagent.

• Fraenkel-Conrat and Singer (1957) proved RNA is the genetic material in Tobacco Mosaic Virus (TMV).
• TMV lacks DNA; it consists of 6% RNA surrounded by a hollow protein coat.
• They created hybrid viruses by mixing the RNA of one mutant strain with the protein coat of another.
• When infecting tobacco leaves, the viral progeny always matched the parental RNA type, never the protein type.
• Infection occurs with intact TMV, isolated RNA, and hybrid viruses, but isolated protein subunits cannot cause infection.

• Discovery of Ribozymes - Sidney Altman and Thomas Cech independently discovered that RNA can act as a biocatalyst.
• RNA World hypothesis - The RNA World hypothesis suggests that early life was based exclusively on nucleic acids, most probably RNA, and was first proposed by Carl Woese, Francis Crick, and Leslie Orgel in 1960.
• Evidence for RNA World - RNA is found abundantly in all living cells, structurally related to DNA, and can evolve, replicate, and catalyse reactions.
• Formation of primitive cells - RNA molecules underwent replication, mutation, and developed their own machinery to form primitive cells.
• Formation of DNA - Double-stranded DNA formed eventually, resulting in rich biodiversity.

• Nucleic acids are biomacromolecules present in the acid-insoluble fraction and are responsible for the storage and transmission of genetic information (DNA and RNA).
• They are polynucleotides, formed by repeated units called nucleotides.
• Each nucleotide consists of three components: a nitrogenous base, a pentose sugar, and a phosphate group.
• Nitrogenous bases are of two types: purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil).
• The sugar present is either ribose (in RNA) or 2′-deoxyribose (in DNA).
• DNA is double-stranded and contains bases A, T, G, and C, while RNA is single-stranded and contains A, U, G, and C.
• DNA stores genetic information, while RNA plays a key role in protein synthesis and the expression of genetic information.

• DNA was established as the primary genetic material and formally modelled as a double helix by Watson and Crick in 1953.
• The structural blueprint relied heavily on Erwin Chargaff’s chemical base equivalence rules and Rosalind Franklin’s X-ray diffraction data.
• The fundamental building block of DNA is a nucleotide, which comprises a five-carbon deoxyribose sugar, a phosphate group, and a nitrogenous base.
• A nucleotide is distinct from a nucleoside, as a nucleoside contains only the nitrogenous base and pentose sugar without the attached phosphate group.
• The double helix is formed by two antiparallel polynucleotide chains running in opposite directions (5′→3′ and 3′→5′) coiled in a clockwise, right-handed fashion.
• The physical architecture places the hydrophilic sugar-phosphate backbone on the exterior, while the information-carrying nitrogenous bases stack flat on the interior.
• Structural stability is maintained horizontally by complementary base pairing (A = T via two hydrogen bonds; G = C via three hydrogen bonds) and vertically by strong covalent phosphodiester bonds.

• DNA base composition is unique to each species, consistently maintaining an equal overall amount of purines and pyrimidines (A + G = T + C).
• Adenine exclusively pairs with thymine in equal amounts (A = T), and cytosine exclusively pairs with guanine in equal amounts (G = C).
• The ratio of (A + T) to (G + C) acts as a constant identifier for a given species, even though the total amounts of these pairs are not necessarily equal.
• The length of a DNA molecule is determined by its total number of nucleotide base pairs, and these adjacent nucleotides are connected by phosphodiester bonds.
• Nucleotide chains are classified by their length, ranging from dinucleotides (two) to oligonucleotides (up to twenty) and polynucleotides (more than twenty).

• A nucleotide has three parts: a nitrogenous base, a pentose sugar and a phosphate group.
• Sugars differ → RNA has ribose; DNA has deoxyribose.
• Two types of bases:
  • Purines → Adenine (A), Guanine (G)
  • Pyrimidines → Cytosine (C), Thymine (T), Uracil (U)
• Cytosine is common to both DNA and RNA; thymine is in DNA, and uracil is in RNA.
• Nucleoside = base + sugar; nucleotide = nucleoside + phosphate group.
• Nucleotides join by 3′–5′ phosphodiester bonds to form a polynucleotide chain.
• The chain has a 5′ end (free phosphate), a 3′ end (free OH), and a backbone made of sugar and phosphate.

• RNA is a nucleic acid that is usually single-stranded and made of ribonucleotides linked by phosphodiester bonds.
• RNA contains ribose sugar and the bases adenine, guanine, cytosine, and uracil, with uracil present instead of thymine.
• The backbone of RNA consists of alternating ribose and phosphate groups, forming a flexible single-stranded chain.
• RNA plays a central role in gene expression by acting as an intermediate between DNA and proteins.
• Major types of RNA are mRNA, tRNA, and rRNA, each with specific roles in protein synthesis.

• Ribosomal RNA (rRNA) is the most abundant RNA, constituting about 80% of the total cellular RNA.
• It is single-stranded and the most stable form of RNA with the highest molecular weight.
• rRNA is a major structural and functional component of ribosomes.
• It helps bind mRNA and tRNA to ribosomes during protein synthesis, possibly through Mg²⁺ linkages.
• rRNA is present in the cytoplasm of both prokaryotic and eukaryotic cells.

• Messenger RNA (mRNA) is the most variable RNA in size and stability and is generally short-lived.
• It is single-stranded and has a primary structure complementary to a specific DNA segment.
• mRNA is formed by transcription, where uracil replaces thymine and ribose replaces deoxyribose.
• It carries genetic information from DNA to ribosomes and acts as a template for protein synthesis.
• mRNA may be monocistronic (coding for one protein) or polycistronic (coding for multiple proteins).

• tRNA is the adapter molecule (Crick) that picks up amino acids and matches them to the correct mRNA codon.
• Robert Holley proposed the cloverleaf model (2D); in 3D, tRNA looks like an inverted L.
• It has three arms - DHU (binds aminoacyl-tRNA synthetase), anticodon (pairs with codon), TΨC (binds ribosome) - plus a 3′ CCA end for amino acid attachment.
• The amino acid is attached by aminoacyl-tRNA synthetase in a process called aminoacylation (charging), which needs energy.
• Each amino acid has a specific tRNA, with a special initiator tRNA for translation start; no tRNAs exist for stop codons.

• DNA replication is the process by which a DNA molecule makes exact copies of itself, each parental strand acting as a template for a new complementary strand, giving two identical daughter molecules with one old and one new strand each (semi-conservative).
• It occurs during the S-phase (Synthesis phase) of interphase in the cell cycle.
• Proposed by Watson and Crick (1953) and experimentally confirmed as semi-conservative by Meselson and Stahl (1958) in E. coli.
• Of the three proposed models, the semi-conservative model was proved correct, while the conservative and dispersive models were disproved.
• Replication is an autocatalytic function (DNA making DNA), unlike heterocatalytic functions, in which DNA directs the synthesis of other molecules, such as RNA (transcription) or protein (translation).

• The experiment was performed by Meselson and Stahl in 1958 using E. coli, which divides every 20 minutes and is easy to track across generations.
• Bacteria were grown in heavy nitrogen (¹⁵N) medium, then shifted to light nitrogen (¹⁴N) medium, and their DNA was separated by CsCl density gradient centrifugation.
• After the first replication, a single hybrid band appeared, which ruled out the conservative model.
• After the second replication, one hybrid and one light band appeared, which ruled out the dispersive model.
• These results proved that DNA replication is semi-conservative, where each new DNA molecule has one old strand and one new strand.

• Replication starts at the origin (ori), where helicase unwinds the DNA helix to form a replication fork, topoisomerase relieves supercoiling, and SSBPs keep the strands apart.
• Primase lays down a short RNA primer, giving DNA polymerase a free 3′-OH end to start adding nucleotides.
• The leading strand is made continuously (one primer); the lagging strand is made discontinuously as Okazaki fragments (many primers).
• DNA Pol I removes the primers and fills the gaps, and DNA ligase seals the nicks into a continuous strand.
• Termination occurs when forks meet at the Ter site (prokaryotes) or when replicons fuse (eukaryotes).

• While the initial DNA replication error rate is roughly 3 in 100,000 base pairs, proofreading reduces the final error rate to 1 in 1 billion.
• Repair enzymes within the DNA polymerase complex immediately proofread each new base pair as it forms to remove and replace mistakes.
• Defective DNA segments are cut out by nucleases, and the correct replacement segments are joined together by the enzyme ligase.
• Specialised enzymes continuously monitor and repair DNA damage that occurs between replication cycles.
• Environmental damage, such as pyrimidine dimers caused by ultraviolet light, is corrected through a process called excision repair.

• Protein synthesis is the process by which cells produce proteins, which act as structural components, enzymes, and hormones.
• It involves two main steps: transcription (DNA → RNA) and translation (RNA → protein).
• In transcription, genetic information from DNA is copied into mRNA, where uracil (U) replaces thymine (T).
• Central dogma, proposed by Francis Crick (1958), states that information flows from DNA → RNA → protein.
• In retroviruses, reverse transcription occurs (RNA → DNA), as explained by Temin and Baltimore (1970) using RNA-dependent DNA polymerase.

• Reverse transcription is the synthesis of complementary DNA (cDNA) from an RNA template by the enzyme reverse transcriptase (RNA-dependent DNA polymerase).
• It is also called Teminism, after Howard Temin, and occurs only in retroviruses.
• It contradicts the Central Dogma, since the flow here is RNA → DNA (not DNA → RNA).
• Discovered independently in 1970 by Temin and Baltimore in retroviruses.
• They won the 1975 Nobel Prize (with Dulbecco); it laid the foundation for retrovirology.

• Transcription is the process by which genetic information from one DNA strand (the template strand) is copied into RNA using RNA polymerase.
• It occurs in the nucleoid in prokaryotes and in the nucleus in eukaryotes; mRNA then moves to the cytoplasm for translation.
• A transcription unit has three parts: promoter (start site), structural gene, and terminator (stop site).
• The process occurs in three stages: initiation (RNA polymerase binds the promoter), elongation (the RNA chain is formed), and termination (RNA polymerase detaches).
• Only one DNA strand acts as a template (3′→5′), while the other is the coding strand (5′→3′).
• In eukaryotes, primary RNA (hnRNA) is processed by capping, tailing, and splicing to form mature mRNA.

• Primary transcripts or hnRNA in eukaryotes are biologically inactive and contain both exons and introns, so they need post-transcriptional processing.
• In prokaryotes, the newly formed mRNA has a continuous coding region and is mature enough to undergo translation immediately in the same region where it is transcribed.
• Eukaryotic genes are split genes; their coding exons are interrupted by non-coding introns, making the initial hnRNA heterogeneous and unstable.
• Eukaryotic hnRNA is processed by 5′ capping, splicing (removal of introns and joining of exons by spliceosomes), and 3′ tailing (polyadenylation) to form stable mature mRNA with a 5′ G-cap and a 3′ poly A tail.
• Mature eukaryotic mRNA, after these processing steps, is transported from the nucleus to the cytoplasm, where it participates in translation and protein synthesis.

• Translation is the process by which the codon sequence on mRNA is decoded with the help of tRNA at the ribosome to form a specific sequence of amino acids in a protein.
• It needs mRNA (template), tRNA (adapter), ribosome (with A, P, and E sites), amino acids, aminoacyl-tRNA synthetase, ATP/GTP for energy, and Mg²⁺ ions.
• Before translation, amino acids are activated and linked to their specific tRNAs (charging) by aminoacyl-tRNA synthetase.
• Initiation: the small ribosomal subunit binds mRNA at the start codon (AUG), the initiator tRNA carrying methionine attaches, and the large subunit joins to form the initiation complex.
• Elongation: amino acids are added one by one through codon–anticodon pairing; peptide bonds form between them, and the ribosome moves forward by one codon at a time (translocation).
• Termination occurs at a stop codon (UAA, UAG, UGA), where release factors free the polypeptide and the ribosomal subunits separate.

• The genetic code is the information encoded in the base sequence of DNA/mRNA that determines the amino acid sequence of a protein.
• It is a triplet code - three consecutive bases form one codon, proposed by George Gamow (1954).
• There are 64 codons in total: 61 code for amino acids and 3 are stop codons (UAA, UAG, UGA).
• AUG is the start codon and codes for methionine.
• It was deciphered primarily by Nirenberg, Khorana, and Ochoa (poly-U mRNA showed that UUU encodes phenylalanine).
• It is degenerate (one amino acid can have several codons, usually differing in the third base - the wobble effect) and nearly universal.
• A change in the base sequence alters the amino acid sequence, so the code directly controls protein synthesis.

• Mutation is a heritable, permanent change in the DNA sequence that may alter gene function.
• DNA replication is highly accurate but not perfect; errors and mutagens (UV rays, X‑rays, and chemicals) can produce mutations.
• Point mutations change single base pairs, while insertions and deletions can cause frame‑shift mutations that disrupt many downstream codons.
• Because the genetic code is degenerate, some point mutations are silent, but others can significantly change protein structure and function.
• Sickle‑cell anaemia is a key example where a single base change in the beta chain of haemoglobin alters an amino acid and produces disease.

Prokaryote vs Eukaryote Packaging

• Gene regulation = switching genes ON or OFF based on the cell's requirements and developmental stage.
• In eukaryotes, regulation occurs at 4 levels: Transcriptional (primary transcript), Processing (splicing), Transport (mRNA from nucleus to cytoplasm), and Translational.
• In prokaryotes, control of the transcriptional initiation rate is the primary mechanism of gene expression control.
• E. coli produces β-galactosidase to break lactose → galactose + glucose. If lactose is absent, the enzyme is not produced, proving the environment regulates gene expression.
• Enzymes synthesised in response to substrate availability are called inducible enzymes. The process is induction; the triggering molecule is the inducer. This is a positive control.
• Feedback repression = when the end product (e.g., amino acid) is already available, genes for its production are switched OFF. This is a negative control.

• Gene regulation turns genes ON or OFF so cells save energy and respond to conditions.
• Induction: genes are normally OFF, but a substrate/inducer makes them switch ON to produce needed enzymes (e.g., lactose in lac operon).
• Repression: genes are normally ON, but excess end product/co-repressor switches them OFF to stop overproduction (e.g., tryptophan in trp operon).
• Inducible operons are linked to catabolic (breakdown) pathways; repressible operons are linked to anabolic (synthesis) pathways.
• Constitutive genes are expressed all the time for basic functions, while regulated genes use induction or repression to control expression only when needed.

• The operon concept (Jacob & Monod, 1961) explains how related genes are regulated together as one unit, based on the lac operon in E. coli.
• An operon has four parts: regulator (i), promoter (p), operator (o), and structural genes (z, y, a) – coding for β-galactosidase, permease, and transacetylase.
• The operator acts as an ON/OFF switch; when active, all three genes are transcribed into a single polycistronic mRNA (each gene = one cistron).
• The repressor (from the regulator gene) controls the operator: bound = OFF; unbound = ON.
• Inducible system (lac operon): the inducer (lactose) inactivates the repressor → transcription ON.
• Repressible system: a co-repressor activates the repressor → transcription OFF.

• The lac operon is an inducible operon in E. coli, proposed by Jacob and Monod (1961), that controls lactose metabolism.
• It consists of a regulator gene (i), promoter (P), operator (O), and three structural genes - lac Z, lac Y, lac A - coding for β-galactosidase, permease, and transacetylase, respectively.
• When lactose is absent, the regulator gene produces an active repressor that binds the operator and blocks RNA polymerase, so the operon remains switched OFF.
• When lactose is present, lactose is converted into allolactose (the inducer), which binds the repressor and inactivates it, leaving the operator free.
• RNA polymerase then transcribes the structural genes into a single polycistronic mRNA, producing the enzymes that break down lactose – the operon is now switched ON.

• The Human Genome Project (HGP) was an international mega-project launched in 1990 and completed in 2003, coordinated mainly by the U.S. DOE and NIH.
• Its aim was to identify all human genes and to sequence the entire human genome, about 3 billion base pairs.
• The main goals were to identify genes, sequence the genome, store the data, develop analysis tools, transfer technologies, and address ethical issues.
• Methodology: DNA was isolated, fragmented, cloned into vectors like BACs and YACs, sequenced by automated methods, and assembled using computers.
• Salient features: the genome has ~3 billion base pairs and 20,000–25,000 genes; less than 2% of the genome codes for proteins, and humans are 99.9% identical.
• Most genetic variation between individuals is due to single-nucleotide polymorphisms (SNPs).
• Applications: disease gene mapping, early diagnosis, personalised medicine, evolutionary studies, and advances in biotechnology.
• ELSI (Ethical, Legal, Social Issues): genome data must be kept confidential to prevent misuse and discrimination.

• DNA fingerprinting is a technique used to identify an individual by analysing the unique DNA pattern present in every person (except identical twins).
• It is based on satellite DNA, especially VNTRs (Variable Number Tandem Repeats) - short sequences repeated in tandem, whose number varies among individuals and creates DNA polymorphism.
• Principle: the differences in VNTR repeat number produce DNA fragments of different lengths, which appear as a unique banding pattern.
• Steps: DNA isolation → PCR amplification → restriction digestion → gel electrophoresis → Southern blotting → probe hybridisation → autoradiography → comparison of band patterns.
• Applications: forensic identification, paternity/maternity testing, pedigree studies, medical research, conservation biology, and evolutionary/anthropological studies.

• The Rice Genome Project was initiated because rice (Oryza sativa) is a major global food crop and has the smallest genome among major cereals (400–430 Mb).
• The International Rice Genome Sequencing Project (IRGSP) began in 1997 as a multinational collaboration involving 11 countries, including Japan, India, China, the USA, and the UK.
• The rice genome was sequenced mainly using the shotgun sequencing approach with BAC and PAC clones.
• The rice genome is estimated to contain about 37,500 genes, with nearly 50% consisting of repetitive DNA.
• Sequencing the rice genome provides insights into genome organisation and helps in understanding cereal crop genetics.
• Knowledge from the rice genome aids in developing improved rice varieties with high yield, disease resistance, and stress tolerance through modern breeding techniques.