Medical Science Test 2 Nddc Scholarship

Nitrogenous bases are divided into two structural groups: pyrimidines and purines. Pyrimidines (Cytosine, Thymine, and Uracil) are characterized by a single-ring structure. Guanine (G) and Adenine (A) are purines, which possess a larger, double-ring structure. A helpful mnemonic for remembering the pyrimidines is 'CUT the PY' (Cytosine, Uracil, and Thymine are PYrimidines).

In both DNA and RNA, Cytosine (C) always pairs with Guanine (G) via three hydrogen bonds. In RNA specifically, Adenine (A) pairs with Uracil (U), while in DNA, Adenine (A) pairs with Thymine (T). Pairing a purine with another purine (A-G) or incorrectly matching a pyrimidine (G-T) would disrupt the structural integrity and uniform width of the nucleic acid double helix.

The Western blot (also known as a protein immunoblot) is a technique used to identify specific proteins. The process involves separating proteins by size via gel electrophoresis, transferring them to a membrane, and then using labelled antibodies that specifically bind to the target protein. This 'hybridization' between the antibody and the protein allows for the confirmation of both the presence and quantity of a specific protein, unlike Southern or Northern blots which target DNA or RNA respectively.

Cystic fibrosis is one of the most common fatal genetic diseases, particularly prevalent among Caucasian populations where the average incidence is approximately 1 in 2,000 to 3,000 live births. It is an autosomal recessive disorder caused by mutations in the CFTR gene. These mutations lead to the production of thick, sticky mucus that causes chronic damage to the lungs, pancreas, and other organs.

The cell cycle consists of Interphase (comprising G1, S, and G2) and the Mitotic phase. The G2 phase (Gap 2) follows DNA replication (S phase) and serves as the final period of growth and 'quality control'. During G2, the cell checks for DNA replication errors and ensures all proteins required for division are synthesized before officially entering Mitosis. G1 is the initial growth phase, S is for synthesis, and G0 is a quiescent resting state.

Down syndrome is also known as Trisomy 21. It is caused by the presence of a third copy of chromosome 21 (XXI). This extra genetic material disrupts the normal course of development and leads to the characteristic cognitive and physical traits associated with the syndrome. It is most commonly the result of 'nondisjunction', an event where the chromosome pair fails to separate properly during the formation of an egg or sperm cell.

The Hardy-Weinberg law describes a theoretical state where allele and genotype frequencies in a population remain constant from generation to generation. For this equilibrium to hold, several strict criteria must be met: the population must be infinitely large, mating must be random, and there must be no mutation, no migration (gene flow), and no natural selection. Therefore, a key characteristic is that genotype selection does not occur at the locus; every genotype must have an equal chance of survival and reproduction. If selection were to occur, certain alleles would become more frequent, breaking the equilibrium.

Hurler's syndrome is a severe lysosomal storage disorder caused by a deficiency of the enzyme alpha-L-iduronidase. It is inherited in an autosomal recessive pattern. Clinical hallmarks include skeletal abnormalities (dysostosis multiplex), coarse facial features, corneal clouding (deficits), and progressive intellectual disability or delayed mental development. Spasticity, which refers to stiff or rigid muscles with exaggerated reflexes, is more characteristic of other neurological conditions like cerebral palsy or certain leukodystrophies, rather than the primary skeletal and storage manifestations of Hurler's syndrome.

Angelman's syndrome is a genetic disorder that primarily affects the nervous system. It is caused by a loss of function of a specific gene (UBE3A) on the maternally inherited copy of chromosome 15. This is a classic example of genomic imprinting, where the manifestation of the syndrome depends on whether the genetic defect was inherited from the mother. Symptoms include severe intellectual disability, speech impairment, sleep disturbance, and a characteristically happy, excitable demeanour with frequent smiling and laughter.

DiGeorge's syndrome (22q11.2 deletion syndrome) results from the defective development of the third and fourth pharyngeal pouches during embryonic growth. Because these pouches normally give rise to the thymus and the parathyroid glands, affected individuals often suffer from T-cell immunodeficiency and hypocalcaemia. The syndrome is also frequently associated with congenital heart defects (specifically conotruncal anomalies) and characteristic facial features.