Revision: Molecular Genetics Zoology HSC Science Class 12 Tamil Nadu Board of Secondary Education

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

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 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.

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.

The process by which DNA duplicates itself is called replication. 

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

When DNA directs the synthesis of DNA itself, then such function of DNA is called autocatalytic function. Eg. Replication.

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

• The discovery of nuclein and the proposal of inheritance principles occurred simultaneously, yet confirming DNA as the genetic material required considerable time.
• By 1926, the scientific investigation into the mechanisms of genetic inheritance had advanced to the molecular level.
• Cumulative research by scientists such as Mendel, Sutton, and Morgan successfully narrowed the source of genetic inheritance to chromosomes within the cellular nucleus.
• Key historical milestones include Hofmeister's 1848 observation of chromosomes during mitosis and Miescher's 1869 isolation of nuclein, which Altman later renamed nucleic acid.
• By 1920, the establishment that chromosomes are composed of both proteins and DNA initiated further experimental studies to identify the exact molecular carrier of genetic information.

• Discovery of Ribozymes - Sidney Altman and Thomas Cech independently discovered that RNAs can act as biocatalysts.
• 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.

• 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.

Prokaryote vs Eukaryote Packaging

• 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, semi-conservative was proved correct, while conservative and dispersive were disproved.
• Replication is an autocatalytic function (DNA making DNA), unlike heterocatalytic functions, where DNA directs the synthesis of other molecules, like RNA (transcription) or protein (translation).

• Transcription is the process in which genetic information from one strand of DNA (template strand) is copied into RNA with the help of 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 promoter), elongation (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.

• The genetic code is the coded information in the base sequence of DNA/mRNA that determines the amino acid sequence in 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 mainly by Nirenberg, Khorana, and Ochoa (poly-U mRNA showed UUU = 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.

• 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.

• 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 sequence the entire human genome of 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% codes for proteins, and humans are 99.9% identical.
• Most genetic variation between individuals is due to SNPs (single-nucleotide polymorphisms).
• 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.