(a) Write about different ways by which biodiversity is being threatened. (10 marks) (b) How is hn RNA (primary transcript)…

(a) Ways in which biodiversity is threatened

Biodiversity is lost mainly through human activities, summarised as HIPPO. Habitat loss and fragmentation: forests, wetlands and grasslands are cleared for farms, towns, dams and roads, so populations shrink and are isolated. It is the major cause of extinction. Invasive species: alien species such as Lantana camara, Parthenium and water hyacinth displace native species, as Nile perch did in Lake Victoria. Pollution: pesticides and industrial wastes poison species and are magnified up the food chain, as with DDT in birds of prey. Nutrient runoff causes eutrophication and oxygen loss in water bodies. Population growth: it raises the demand for land, water and resources. Overexploitation: overfishing, hunting and illegal trade remove individuals faster than they can be replaced, as in tigers, rhinos and some marine fish. Climate change adds to these pressures by shifting the ranges of species and bleaching coral. Species threatened with extinction are listed as vulnerable, endangered or critically endangered in the IUCN Red List.

(b) Processing of hnRNA into mRNA in eukaryotes

hnRNA is the primary transcript made by RNA polymerase II in the nucleus. It is converted into functional mRNA by three processing steps. Capping: a 7-methylguanosine cap is added to the 5′ end. It protects the RNA from exonucleases and helps its export and ribosome binding. Polyadenylation: the 3′ end is cut at a signal sequence, and poly-A polymerase adds a tail of about 200 adenines, which stabilises the mRNA and aids translation. Splicing: non-coding introns are cut out and exons joined by the spliceosome, made of snRNPs U1, U2, U4, U5 and U6. Introns start with GU and end with AG, and the removal takes two transesterification steps with a lariat intermediate. Alternative splicing can join exons in different combinations so that one gene yields several proteins. The mature mRNA is then exported through the nuclear pores to the cytoplasm for translation.

(c) Continental drift and discontinuous distribution of alligators and lungfishes

The breakup of the supercontinents Pangaea, Laurasia and Gondwana separated populations that were once continuous, and the isolated groups then evolved on their own (vicariance). Alligators: the American alligator, Alligator mississippiensis, in south-eastern North America and the Chinese alligator, A. sinensis, in the Yangtze valley are close relatives separated by the Pacific and Atlantic. Their ancestors lived across Laurasia, and the opening of the north Atlantic and loss of land links divided them. Lungfishes: Neoceratodus in Australia, Lepidosiren in South America and Protopterus in Africa are freshwater fishes of Gondwana origin, as shown by fossil lungfishes in these continents. They cannot cross salt water, so separation of the Gondwana fragments explains their present positions. Thus their discontinuous distribution is a consequence of drift and not of dispersal across oceans.

(d) RFLP: principle and applications

Principle: restriction enzymes cut DNA at specific recognition sites. A mutation, insertion or deletion can create or remove a site, so the lengths of the restriction fragments differ between individuals. These differences are restriction fragment length polymorphisms and are inherited in a Mendelian way. Procedure: DNA is digested with a restriction enzyme, the fragments are separated by gel electrophoresis, transferred to a membrane by Southern blotting, and detected with a labelled probe. Applications: DNA fingerprinting in forensic work and paternity testing. Genetic mapping and marker-assisted selection. Diagnosis of genetic disease such as sickle-cell anaemia. Study of population genetics and evolutionary relationships. Limits: it needs a large amount of DNA and is slow, so PCR-based STR typing is now preferred.

(e) Receptor-mediated endocytosis

It is the selective uptake of particular macromolecules after binding to specific receptors on the cell surface. Steps, using LDL: Step 1: LDL particles bind LDL receptors, which gather in clathrin-coated pits. Step 2: The pit invaginates and dynamin pinches it off as a coated vesicle. Step 3: The clathrin coat is shed and the vesicle fuses with an early endosome. Step 4: The acidic pH of the endosome frees LDL from its receptor, and the receptors return to the membrane in recycling vesicles. Step 5: LDL goes to the lysosome, where enzymes release cholesterol for the cell's use. Other examples are transferrin carrying iron, insulin and viruses. Advantages: it is specific, takes up ligands efficiently even when they are scarce outside, and allows receptor recycling. It keeps cellular cholesterol under control, and failure of LDL receptors causes familial hypercholesterolaemia.