(a) Cell-cell adhesion mechanism 10 (b) Structure and functions of cytoskeleton 10 (c) Characteristics of triplet codon 10 (d)…

(a) Cell-cell adhesion mechanism

Cells in animal tissues bind each other through cell adhesion molecules (CAMs) of four main families.

• Cadherins: Ca²⁺-dependent, binding homophilically (like to like); linked through catenins to actin in adherens junctions, and to intermediate filaments in desmosomes.
• Selectins: Ca²⁺-dependent lectins that bind carbohydrates, giving transient adhesion, e.g. rolling of leukocytes on endothelium.
• Immunoglobulin superfamily (e.g. NCAM): Ca²⁺-independent, mostly homophilic, important in the nervous system.
• Integrins: bind mainly to extracellular matrix, and some to other cells.

Tight junctions (claudins, occludins) seal the space between cells, and gap junctions (connexons) allow direct passage of small molecules. Loss of E-cadherin is linked to tumour invasion. In plants, cells adhere through the middle lamella of pectin, with plasmodesmata providing cytoplasmic connection.

(b) Structure and functions of the cytoskeleton

The cytoskeleton is a dynamic protein network of three filament types.

• Microfilaments (about 7 nm): polymers of actin; they form the cortex under the membrane, and function in cell shape, motility, cytoplasmic streaming and cytokinesis.
• Intermediate filaments (about 10 nm): rope-like fibrous proteins such as keratin and vimentin, giving tensile strength; found in animals and absent in plants.
• Microtubules (about 25 nm): hollow tubes of α- and β-tubulin dimers, from microtubule organising centres; they form the spindle, cilia and flagella, and tracks for kinesin and dynein; in plants cortical microtubules guide cellulose microfibril deposition and the phragmoplast.

The cytoskeleton also positions organelles and supports intracellular transport.

(c) Characteristics of the triplet codon

The genetic code is read as nucleotide triplets on mRNA.

• Triplet: three bases code for one amino acid; 64 codons in all.
• Degenerate: 61 sense codons for 20 amino acids, with synonymous codons often differing at the third base (wobble).
• Non-overlapping and commaless: read continuously from a fixed start, without gaps.
• Unambiguous: each codon specifies one amino acid.
• Start and stop: AUG (methionine) begins translation, and UAA, UAG and UGA terminate it.
• Nearly universal: with small exceptions in mitochondria and some organisms.
• Colinear: the codon order matches amino-acid order.

(d) Crossing over and its significance

Crossing over is the reciprocal exchange of segments between non-sister chromatids of homologous chromosomes at pachytene of prophase I. The sequence is synapsis, recombination nodules and breakage and reunion, and the chiasmata seen at diplotene mark the exchange points. It is brought about by enzymes such as Spo11 (double-strand breaks) and recombinases.

Significance: it makes new allele combinations and so genetic variability; it helps the proper segregation of homologues at anaphase I through chiasmata; the recombination frequency gives a genetic map and the order of linked genes (1% = 1 map unit); and it supplies variation for evolution and plant breeding, by breaking linkage between good and bad alleles.

(e) Correlation, its types and significance

Correlation measures the strength and direction of the association between two variables. The coefficient r ranges from −1 to +1.

Types:

• Positive, negative and zero correlation: both variables rise together, one rises as the other falls, or no relation.
• Simple, partial and multiple: between two variables, between two with others held constant, and between one variable and a group.
• Linear and non-linear.

Significance: it shows which characters (e.g. grain weight and yield) go together, guides indirect selection in breeding, and in genetics, genotypic, phenotypic and environmental correlations can be separated. It does not prove cause and effect.