(a) Describe the structure of mitochondrion. Why is it known as powerhouse of the cell? (20 marks) (b) Explain the mechanism of…

Mitochondrion. The mitochondrion is a double-membrane-bound organelle. The outer membrane is smooth and permeable to small molecules, while the inner membrane is folded into cristae, greatly increasing surface area. The cristae bear elementary particles, also called F1 particles, which are the visible heads of ATP synthase. The enclosed matrix contains mitochondrial DNA, 70S ribosomes, tRNA, and enzymes of the Krebs cycle and oxidative phosphorylation, including NADH dehydrogenase and cytochrome oxidase. During respiration, NADH and FADH2 deliver electrons to the electron transport chain embedded in the inner membrane. As electrons pass through complexes, protons are pumped from the matrix to the intermembrane space, creating an electrochemical proton gradient. ATP synthase uses the return of protons down this gradient to phosphorylate ADP, a process explained by the chemiosmotic mechanism. Because mitochondria generate most of the cell’s ATP through this coupling of electron transport and oxidative phosphorylation, they are called the powerhouse of the cell.

Sex determination in humans. Human sex determination follows the XX-XY system. Females have two X chromosomes, while males have one X and one Y. The Y chromosome carries the SRY gene, whose product initiates testis differentiation from the indifferent gonad. In the absence of a functional SRY, the gonad develops into an ovary. Once testes form, Sertoli cells secrete anti-Mullerian hormone, causing regression of Mullerian ducts, while Leydig cells secrete testosterone, promoting Wolffian duct development and male structures. In females, absence of testicular hormones allows Mullerian ducts to persist, and estrogen supports ovarian and female development. Dosage compensation is achieved by X-inactivation, in which one X chromosome in each female cell is condensed into a Barr body, equalising X-linked gene expression. The system is supported by sex-chromosome aneuploidies: Klinefelter syndrome (47,XXY) usually gives a male phenotype because SRY is present, while Turner syndrome (45,X) gives a female phenotype with ovarian failure. Modern genetics also recognises genes beyond SRY, such as SOX9 and DAX1, which refine gonadal development and show that sex determination is a pathway rather than a single switch.

Changes in gene frequencies. In an ideal population, Hardy-Weinberg equilibrium predicts that allele and genotype frequencies remain constant when mating is random, population size is infinite, and there is no mutation, migration or selection. Real populations deviate from this baseline. The processes usually considered are mutation, genetic drift, gene flow, natural selection, and non-random mating. Mutation is the ultimate source of new alleles; although individual mutation rates are low, it supplies raw variation for evolution. Genetic drift changes allele frequencies by chance, especially in small populations; founder effects occur when a few individuals colonise a new area, and bottleneck effects follow a sharp reduction in population size. In India, isolated tribal groups, such as some Andaman populations, can show pronounced drift because of small effective population sizes and limited gene exchange. Gene flow, or migration, introduces or removes alleles when individuals move between populations, tending to reduce differences among them. Natural selection changes frequencies when alleles affect survival or reproduction; directional selection favours one extreme, stabilizing selection favours the mean, and disruptive selection favours extremes. A classic example is the sickle-cell trait, where heterozygotes have an advantage in malaria-endemic regions, including parts of India, maintaining the allele despite the cost of homozygous disease. Non-random mating, such as assortative mating or consanguinity, changes genotype frequencies by increasing homozygosity; it does not directly alter allele frequencies, though it can make selection on recessive alleles more effective.

Together, these topics show that inheritance operates at several levels: mitochondrial inheritance demonstrates cytoplasmic and organelle genetics, human sex determination shows chromosomal and gene-regulatory control, and population genetics shows how allele frequencies respond to mutation, drift, migration and selection.