Write short notes on the following in about 150 words each : 10×5=50 (a) Cell-cell adhesion mechanism 10 (b) Structure and…

(a) Cell-cell adhesion mechanism Cell-cell adhesion is the process by which cells bind to one another, mediated by specific transmembrane proteins. The primary mechanisms involve cadherins, which are calcium-dependent glycoproteins that form homophilic bonds, creating adherens junctions and desmosomes. These structures provide mechanical strength and organize the actin cytoskeleton. Selectins mediate transient, calcium-dependent adhesion, crucial for leukocyte rolling in blood vessels. Integrins facilitate cell-matrix and cell-cell interactions, linking the extracellular matrix to the intracellular cytoskeleton. Members of the immunoglobulin superfamily, such as NCAM, also contribute to adhesion, particularly in the nervous system. Tight junctions, formed by claudins and occludins, seal the paracellular space, creating a barrier. These adhesion mechanisms are vital for tissue integrity, morphogenesis, and immune response. Disruptions in cadherin expression, for instance, are linked to metastasis in cancers like breast and gastric carcinoma, highlighting the role of adhesion in maintaining tissue architecture and preventing aberrant cell migration.

(b) Structure and functions of cytoskeleton The cytoskeleton is a dynamic network of protein filaments that provides structural support and facilitates movement. It comprises three main components: microfilaments, intermediate filaments, and microtubules. Microfilaments, composed of actin monomers, have a diameter of approximately 7 nm. They are involved in muscle contraction, cell motility, and cytokinesis. Intermediate filaments, such as keratin, vimentin, and neurofilaments, are 10 nm in diameter and provide tensile strength, anchoring the nucleus and organelles. Microtubules, made of tubulin dimers, are 25 nm in diameter and form the tracks for intracellular transport via motor proteins like kinesin and dynein. They are essential for chromosome segregation during mitosis and meiosis. The cytoskeleton maintains cell shape, enables cell division, and facilitates intracellular signaling. In plant cells, the cytoskeleton interacts with the cell wall to regulate growth and division. Its dynamic nature allows rapid reorganization in response to environmental stimuli, ensuring cellular homeostasis and function.

(c) Characteristics of triplet codon The genetic code is triplet, meaning three nucleotides (a codon) specify one amino acid. There are 64 possible codons, of which 61 encode the 20 standard amino acids, and 3 are stop codons (UAA, UAG, UGA) that signal termination of translation. The code is degenerate, as most amino acids are encoded by more than one codon, often differing only in the third position, explained by the wobble hypothesis. It is non-overlapping, meaning each nucleotide is read only once in a triplet frame. The code is unambiguous, with each codon specifying only one amino acid. It is nearly universal, with minor exceptions in mitochondrial genomes and some protists. The start codon is usually AUG, which also codes for methionine. This triplet nature ensures efficient and accurate protein synthesis. The degeneracy provides a buffer against mutations, as changes in the third base often do not alter the amino acid sequence, preserving protein function.

(d) Crossing over and its significance Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis, specifically at the pachytene stage. This process results in the formation of chiasmata, the physical manifestations of crossover events. The mechanism involves the formation of a Holliday junction, a four-stranded DNA structure, which is resolved to produce recombinant chromatids. Crossing over is significant for generating genetic diversity by creating new combinations of alleles. It is the basis for linkage mapping, where recombination frequency is used to estimate the distance between genes. However, recombination frequency is only approximately proportional to physical distance for short intervals; for larger distances, it plateaus at 50% due to multiple crossovers and interference. Crossing over ensures proper chromosome segregation during meiosis I. It is a key driver of evolution, providing the raw material for natural selection. In plant breeding, understanding recombination patterns helps in marker-assisted selection for desirable traits.

(e) Correlation, its types and significance Correlation is a statistical measure that quantifies the degree of association between two variables. It is denoted by Pearson’s coefficient of correlation, r, which ranges from -1 to +1, inclusive. A value of +1 indicates perfect positive correlation, -1 indicates perfect negative correlation, and 0 indicates no linear correlation. Types of correlation include simple (between two variables), partial (between two variables controlling for a third), and multiple (between one variable and a set of independent variables). Correlation can be linear or non-linear. In plant breeding, correlation analysis is used to identify traits associated with yield, aiding in the development of selection indices. For example, in wheat, positive correlation between grain yield and thousand-kernel weight helps in selecting high-yielding varieties. In ecology, correlation studies reveal relationships between species and environmental factors. However, correlation does not imply causation; further experimental validation is required. It is a fundamental tool in biometrical genetics and agricultural research for optimizing crop improvement strategies.