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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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 in its packing in eukaryotic cells are called histones.

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

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

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

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.

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

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

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

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

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 synthesized continuously and are essential for basic cellular metabolism.

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

• 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 in expression and transmission of genetic information through protein synthesis.

1. DNA structure was first studied by Rosalind Franklin (1953); later explained by Watson and Crick, who proposed the double helix model (Nobel Prize, 1962).

2. DNA is a macromolecule made of two complementary strands twisted into a double helix.

3. Each strand is made up of nucleotides, which include phosphate, sugar (pentose), and a nitrogenous base.

4. There are four nitrogenous bases:

• Adenine (A) pairs with Thymine (T) (2 hydrogen bonds)
• Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds)

5. The two strands form a ladder-like structure, with bases as rungs and sugar-phosphate as the backbone.

• Nature of RNA: RNA is usually a single-stranded nucleic acid molecule.
• Composition: RNA is composed of nucleotides containing ribose sugar, phosphate group, and a nitrogenous base.
• Nitrogenous Bases: RNA contains four bases—adenine (A), guanine (G), cytosine (C), and uracil (U); uracil replaces thymine.
• Base Pairing: Adenine pairs with uracil, while guanine pairs with cytosine.
• Backbone Structure: The RNA backbone consists of alternating ribose sugar and phosphate groups.

• 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).

• Transfer RNA (tRNA) acts as an adapter molecule that transports specific amino acids to ribosomes during protein synthesis.
• It is a small RNA molecule of about 80 nucleotides and has a cloverleaf secondary structure and an inverted L-shaped tertiary structure.
• tRNA contains an anticodon that pairs with the complementary codon on mRNA.
• The 3′ end of tRNA has the sequence CCA, which serves as the amino acid attachment site.
• Each tRNA is charged with its specific amino acid by a specific enzyme called aminoacyl-tRNA synthetase.

• Initiation at Origin: DNA replication begins at a specific site called the origin of replication; prokaryotes have a single origin, while eukaryotes have multiple origins (replicons).
• Unwinding of DNA: The DNA double helix is unwound and strands are separated by helicase, while topoisomerase (DNA gyrase) relieves supercoiling, forming replication forks.
• Primer Formation: A short RNA primer is synthesized by the enzyme primase to provide a free 3′-OH end for DNA synthesis.
• Elongation of New Strands: DNA polymerase adds nucleotides in the 5′ → 3′ direction using parental strands as templates; synthesis is continuous on the leading strand and discontinuous on the lagging strand forming Okazaki fragments.
• Completion and Ligation: RNA primers are removed, gaps are filled with DNA, and Okazaki fragments are joined by DNA ligase to form complete daughter DNA molecules.

• The genetic code is the specific sequence of nitrogenous bases in DNA or mRNA that determines the order of amino acids in a protein.
• It is a triplet code, where a sequence of three nucleotides called a codon codes for one amino acid.
• There are 64 codons (4³), of which 61 code for amino acids and 3 act as stop codons (UAA, UAG, UGA).
• AUG functions as the start codon and codes for the amino acid methionine.
• The genetic code is degenerate, meaning more than one codon can code for the same amino acid, showing the wobble effect at the third base.

• In eukaryotes, chromosomes are gene carriers, and each chromosome consists of a single long DNA molecule associated with histone proteins (unineme model).
• DNA is packaged into repeating units called nucleosomes, where DNA is wrapped around a histone octamer made of H₂A, H₂B, H₃, and H₄ proteins.
• Nucleosomes form a 10 nm beaded fibre, which coils further into a 30 nm solenoid structure with the help of histone H₁.
• Higher-level folding condenses chromatin into looped domains attached to a protein scaffold, finally forming metaphase chromosomes with the help of non-histone chromosomal proteins.
• Chromatin exists as euchromatin (loosely packed, transcriptionally active) and heterochromatin (densely packed, transcriptionally inactive).

• The operon model, proposed by Jacob and Monod (1961), explains coordinated regulation of gene expression in prokaryotes at the transcriptional level.
• An operon consists of a promoter, operator, and a group of structural genes that are regulated together and transcribed as a single polycistronic mRNA.
• In the lac operon of E. coli, three structural genes—z (β-galactosidase), y (permease), and a (transacetylase)—are involved in lactose metabolism.
• The regulator gene (lac i) produces a repressor protein that binds to the operator and prevents transcription in the absence of lactose.
• When lactose is present, it inactivates the repressor, allowing RNA polymerase to transcribe the structural genes (induction).
• Catabolite repression occurs when glucose is present; lac operon expression is suppressed and later activated by CAP–cAMP when glucose is depleted.

• The Human Genome Project (HGP) was proposed in 1986 to map and sequence the entire human genome.
• It was an international effort involving about 100 laboratories from nearly 18 countries.
• The main goals included identifying all human genes, sequencing three billion base pairs, and storing data in databases.
• Two major approaches were used: Expressed Sequence Tags (ESTs) and whole genome sequencing with annotation.
• DNA fragments were cloned using BAC and YAC vectors in bacteria and yeast for sequencing.
• The human genome contains about 3.2 billion nucleotide bases and approximately 20,000–30,000 genes.
• Less than 2% of the genome codes for proteins, while repetitive DNA forms a large portion.
• About 99.9% of DNA sequences are identical among all humans; variations include SNPs and CNVs.
• HGP has applications in disease diagnosis, genetic counselling, gene therapy, and understanding human evolution.
• Ethical concerns include misuse of genetic data for discrimination, insurance denial, and attempts at genetic manipulation.

• DNA fingerprinting is a technique used to identify individuals based on unique patterns in their DNA sequences.
• It is based on repetitive DNA sequences called VNTRs (Variable Number Tandem Repeats), which show high polymorphism among individuals.
• These repetitive sequences are part of satellite DNA, including minisatellites and microsatellites.
• The technique was developed by Alec Jeffreys, and even a very small DNA sample can be used.
• The main steps include DNA extraction, PCR amplification, restriction digestion, gel electrophoresis, Southern blotting, hybridisation, and autoradiography.
• The resulting banding pattern is unique for each individual, except in identical (monozygotic) twins.
• DNA fingerprinting is widely used in forensic science, paternity testing, and personal identification.
• It is also useful in studying genetic diversity, population structure, and diagnosing certain inherited diseases.

• 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 rice genome provides insights into genome organization 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.