(a) Cell adhesion molecules 10 (b) Ribosomal RNA processing in nucleolus 10 (c) Genetic consequences of Inversion 10 (d) Gene…

(a) Cell Adhesion Molecules Cell adhesion molecules (CAMs) are cell-surface glycoproteins (transmembrane proteins) that bind cells to one another or to the extracellular matrix (ECM).

Features: they have an extracellular binding domain, a transmembrane segment and a cytoplasmic tail linked to the cytoskeleton (actin through catenins, or intermediate filaments). Binding may be homophilic (like to like) or heterophilic. Many need Ca²⁺ (cadherins, selectins), others do not (immunoglobulin-superfamily CAMs, such as NCAM). They also transmit signals.

Main families: cadherins (E-cadherin, N-cadherin), which form adherens junctions and desmosomes; integrins, α/β heterodimers that bind ECM fibronectin and collagen at focal adhesions and hemidesmosomes; selectins, lectin-like proteins that make leukocytes roll on endothelium; and the immunoglobulin superfamily (NCAM, ICAM).

Example: E-cadherin binds epithelial cells in a homophilic, Ca²⁺-dependent manner.

Significance: they maintain tissue integrity, guide morphogenesis, wound healing and immune responses, and loss of E-cadherin is linked with cancer metastasis. (Plant cells are instead joined by the middle lamella and plasmodesmata.)

(b) Ribosomal RNA Processing in the Nucleolus The nucleolus is the site of ribosomal DNA (rDNA) transcription and ribosome subunit assembly. Tandem rDNA repeats are transcribed by RNA polymerase I into a long 45S precursor rRNA.

Features:

• The 45S pre-rRNA is cleaved by endonucleases and exonucleases, removing external and internal transcribed spacers (ETS, ITS), to give 18S (for the small subunit), 5.8S and 28S (25S in plants) rRNA.
• Small nucleolar RNAs, as snoRNPs (box C/D for 2'-O-methylation, H/ACA for pseudouridylation), guide nucleotide modifications.
• Processing is co-transcriptional, with ribosomal proteins binding in the 90S pre-ribosome and SSU processome.
• 5S rRNA is made by RNA polymerase III outside the nucleolus and imported.

Example: in human cells 45S (about 13 kb) gives 18S, 5.8S and 28S.

Significance: the pre-ribosomal subunits are exported through nuclear pores to form ribosomes for protein synthesis; defects cause ribosomopathies.

(c) Genetic Consequences of Inversion An inversion is a chromosome segment that is broken at two points, turned 180° and rejoined. It is paracentric (not including the centromere) or pericentric (including it).

Consequences in heterozygotes: at pachytene the chromosomes form an inversion loop for pairing. Crossing over within the loop in a paracentric inversion gives a dicentric chromatid, forming an anaphase bridge, and an acentric fragment, both lost; so gametes are nonviable. In a pericentric inversion, crossover chromatids carry duplications and deficiencies, with no bridge. Thus fertility falls, and recombinants are not recovered, so crossing over appears suppressed within the inverted region, making inversions act as crossover suppressors (as the CIB balancer chromosome in Drosophila). Inversions can also cause position effects, and in homozygotes cause no pairing problem.

Significance: inversions protect gene combinations, reduce gene flow and aid evolution and speciation, as in Drosophila pseudoobscura inversion polymorphisms; they appear in McClintock's maize bridge-breakage-fusion studies.

(d) Gene Silencing Gene silencing is the regulation by which gene expression is switched off or reduced without changing the DNA sequence, at the transcriptional or post-transcriptional level.

Features:

• Transcriptional gene silencing (TGS): DNA methylation, histone modifications and heterochromatin formation (RNA-directed DNA methylation) block transcription.
• Post-transcriptional gene silencing (PTGS) or RNA interference (RNAi): double-stranded RNA is cut by Dicer into 21-24 nt siRNAs, which are loaded into RISC with Argonaute to guide cleavage of complementary mRNA. MicroRNAs (miRNA) repress translation.
• It is heritable (epigenetic) and can spread systemically in plants.
• It defends against viruses and transposons.

Examples: co-suppression in transgenic petunia (flower pigment, Napoli and Jorgensen, 1990); RNAi in C. elegans (Fire and Mello, 1998; Nobel 2006).

Significance: it regulates genes naturally and is used to produce virus-resistant plants and crops with altered traits (antisense Flavr Savr tomato), and in functional genomics.

(e) Use of Apomixis in Plant Breeding Apomixis is asexual seed formation: embryos develop without meiosis and fertilisation, so seed progeny are genetically identical to the mother plant.

Features:

• Types include diplospory (Taraxacum, the dandelion), apospory (Hieracium, Pennisetum), parthenogenesis, and adventitious embryony from nucellus (Citrus, mango).
• It gives uniform, true-to-type progeny, generation after generation.
• Often facultative, and associated with polyploidy.

Use in breeding: the main advantage is fixing hybrid vigour (heterosis): an F₁ hybrid that is apomictic would breed true, so farmers could save seed, cutting the cost of hybrid seed production. It also provides rapid multiplication of elite genotypes, with virus-free nucellar seedlings in citrus. Genes from apomictic Pennisetum and Tripsacum are being transferred into maize, pearl millet and rice, but this is not yet routine.

Limitation: apomixis limits genetic variation, making it difficult to breed apomictic species.