Cell Biology, Genetics & Biotechnology

The cell membrane (plasma membrane) is present in every cell and is selectively permeable. Its accepted structure is the fluid-mosaic model given by Singer and Nicolson (1972) — a fluid double layer of phospholipids in which proteins float about like tiles in a mosaic. The cell wall, by contrast, is found only in plants, fungi and bacteria: it is rigid, fully permeable and gives shape and protection.

A cell does not divide whenever it likes. It passes through a fixed cell cycle: a long interphase (G1 → S → G2, in which the cell grows and DNA is copied during the S phase) followed by a short M phase (division). Interphase takes up most of the cycle — the cell spends far more time preparing than dividing.

• The four phases of mitosis in order: Prophase → Metaphase → Anaphase → Telophase, followed by cytokinesis (splitting of the cytoplasm).
• In metaphase the chromosomes line up at the equator — this is when they are most clearly visible, so it is the stage used for karyotyping (counting chromosomes).
• Meiosis I is the reduction division (homologous chromosomes separate); Meiosis II resembles mitosis (sister chromatids separate). Prophase I has the famous five sub-stages: leptotene, zygotene, pachytene, diplotene, diakinesis — crossing over happens in pachytene.

The two strands are held together by hydrogen bonds between bases, and the pairing is strictly complementary: adenine pairs with thymine (A–T, two hydrogen bonds) and guanine pairs with cytosine (G–C, three hydrogen bonds). Hence Chargaff's rule: in any DNA, A = T and G = C. In RNA, uracil (U) replaces thymine, so the pairing is A–U.

1. Replication — DNA makes an exact copy of itself (semi-conservative: each new molecule keeps one old strand). This happens before cell division.
2. Transcription — one strand of DNA is copied into mRNA inside the nucleus.
3. Translation — the mRNA travels to a ribosome, where each codon (a triplet of three bases) is read and the matching amino acid is added, building a protein. There are 64 codons for 20 amino acids; AUG is the start codon.

This one-way flow DNA → RNA → protein is called the central dogma, given by Francis Crick. The famous exception is the retrovirus (e.g. HIV), which uses the enzyme reverse transcriptase to go backwards from RNA to DNA.

• A gene is a segment of DNA that codes for one product; genes sit in order along chromosomes.
• Humans have 46 chromosomes = 23 pairs: 22 pairs of autosomes + 1 pair of sex chromosomes. XX = female, XY = male — so the sex of the child is determined by the father, who alone can contribute a Y.
• A mutation is a sudden change in the DNA sequence. Point mutations change a single base (this is what causes sickle-cell anaemia); mutagens include X-rays, UV rays and certain chemicals. The term "mutation" was coined by Hugo de Vries.

1. Law of Dominance — in a pair of contrasting alleles, only the dominant one shows in the hybrid; the recessive one stays hidden.
2. Law of Segregation (law of purity of gametes) — the two alleles separate during gamete formation, so each gamete carries only one allele. This law has no exception.
3. Law of Independent Assortment — two different characters are inherited independently of each other. This law comes from the dihybrid cross (the first two come from the monohybrid cross).

Work the monohybrid cross once and you will never forget the ratios. Cross a pure tall plant (TT) with a pure dwarf (tt). All F1 plants are Tt and tall — dominance. Now self-pollinate the F1: the offspring are TT, Tt, Tt, tt. So the F2 phenotypic ratio is 3 tall : 1 dwarf (3:1) and the genotypic ratio is 1 TT : 2 Tt : 1 tt (1:2:1). In a dihybrid cross (two characters together) the F2 phenotypic ratio is 9:3:3:1.

• Test cross — cross the individual of unknown genotype with a homozygous recessive (tt). If all offspring are tall, the parent was TT; if you get a 1:1 ratio, it was Tt. (A back cross is a cross with either parent.)
• Incomplete dominance — neither allele fully dominates, so the hybrid is intermediate. In Mirabilis jalapa (four o'clock plant) and snapdragon, red × white gives pink, and the F2 ratio is 1:2:1 for both phenotype and genotype.
• Co-dominance — both alleles express fully and together. The best human example is the AB blood group, where both A and B antigens appear.
• Sex-linked inheritance — genes on the X chromosome. Haemophilia (blood does not clot) and red-green colour blindness are X-linked recessive, and therefore far more common in males: a male has only one X, so a single defective allele shows up. A female needs the defect on both X chromosomes; with one, she is only a carrier.

Human ABO blood groups are the textbook case of multiple alleles: a single gene has three alleles — I^A, I^B and i. I^A and I^B are co-dominant to each other and both are dominant over i. So four phenotypes come from six genotypes: A (I^A I^A or I^A i), B (I^B I^B or I^B i), AB (I^A I^B) and O (ii only). Separately, the Rh factor is inherited as a simple dominant: Rh-positive is dominant over Rh-negative.

Once we could read DNA, the next step was to cut, paste and copy it. That is recombinant DNA (rDNA) technology: restriction enzymes act as "molecular scissors" and cut DNA at fixed recognition sequences leaving sticky ends; DNA ligase is the "molecular glue" that joins the desired gene into a vector (usually a bacterial plasmid); the vector is then put into a host cell, which multiplies and manufactures the product. The first restriction enzyme identified was EcoRI, from E. coli. Human insulin was the first widely used rDNA medicine.

• PCR (Polymerase Chain Reaction) — invented by Kary Mullis, it makes millions of copies of a tiny piece of DNA through repeated cycles of denaturation → annealing → extension, using heat-stable Taq polymerase. It powers RT-PCR tests, forensic DNA matching and paternity testing.
• Tissue culture & micropropagation — because plant cells are totipotent (one cell can regenerate a whole plant), a small explant grown on a nutrient medium forms a callus and then thousands of identical, disease-free saplings. This is how banana, sugarcane, orchid and potato planting material is mass-produced — including for Bihar's banana-growing districts such as Vaishali and Katihar.
• Cloning — Dolly the sheep (1996, Roslin Institute, Scotland) was the first mammal cloned from an adult somatic cell, by somatic cell nuclear transfer: the nucleus of a body cell was placed into an egg emptied of its own nucleus. India's NDRI, Karnal produced cloned buffalo calves in 2009.
• Stem cells — unspecialised cells that can become other cell types. Embryonic stem cells are pluripotent (can form almost any tissue) but raise ethical objections; adult stem cells (bone marrow, umbilical cord blood) are multipotent and ethically uncontroversial. The oldest established stem-cell therapy is the bone-marrow transplant.
• Gene therapy — inserting a correct gene to make up for a defective one. The first gene therapy (1990) was given to a young girl with ADA (adenosine deaminase) deficiency.
• CRISPR-Cas9 — gene editing, not just gene transfer. A guide RNA carries the Cas9 enzyme to one exact address on the DNA and cuts it there, so a faulty stretch can be removed or replaced — a "find and replace" for the genome. Borrowed from a bacterial defence system; Emmanuelle Charpentier and Jennifer Doudna won the Nobel Prize in Chemistry, 2020.

A genetically modified organism (GMO) carries a gene deliberately introduced from another species. Bt cotton — carrying cry genes from the bacterium *Bacillus thuringiensis* for resistance to bollworm — is the only GM crop approved for commercial cultivation in India (since 2002). Bt brinjal was cleared by the regulator but placed under a moratorium in 2010 and is still not permitted. Golden Rice, engineered to be rich in beta-carotene (pro-vitamin A), is the famous nutritional GM example but is not commercially grown in India.

• Vaccines teach the immune system to recognise a germ in advance. Traditional types: live attenuated (BCG, oral polio, MMR), inactivated/killed (injectable polio), toxoid (tetanus) and recombinant subunit (Hepatitis B). mRNA vaccines are newer — they deliver mRNA that instructs our own cells to make a harmless viral protein, which then triggers immunity.
• India's vaccine capacity is a genuine strength: the Serum Institute of India (Pune) is the world's largest vaccine manufacturer by number of doses, and Covaxin, developed by Bharat Biotech with ICMR–NIV, was India's first indigenous COVID-19 vaccine. India also has an indigenous HPV vaccine (CERVAVAC).
• Regulation in India is three-tiered: the IBSC at the institution level, the RCGM under the Department of Biotechnology, and at the top the GEAC (Genetic Engineering Appraisal Committee) under the Ministry of Environment, Forest and Climate Change — GEAC is the apex body that appraises the environmental release of GM organisms.
• Bioethics asks what we should do, not only what we can: consent and privacy in genetic testing, the risk of "designer babies" from germline editing, biopiracy of India's genetic resources (guarded by the Biological Diversity Act, 2002), and the fear of gene flow from GM crops to wild relatives.

Cells need energy, and in nature that energy travels along a food chain: producers → herbivores → carnivores, with decomposers recycling everything at the end. Each step is a trophic level. Green plants (producers) always form the first trophic level, herbivores the second, and so on.

Energy transfer is wasteful. By Lindeman's 10% law, only about 10% of the energy at one trophic level passes to the next; the remaining 90% is lost mainly as heat and in the organism's own life processes. Energy flow through an ecosystem is therefore unidirectional — it never cycles back.