How to approach USMLE questions ? ( Lesch–Nyhan syndrome )

Signs and Symptoms Associated with Down Syndrome

Down syndrome is the most common autosomal trisomy identified in liveborn infants. As many as 95% of Down syndrome cases arise due to chromosomal nondisjunction during maternal meiosis (47 XX, +21) an abnormality that positively correlates with increasing maternal age. Two of the more prominent and consistent lectures of Down syndrome are mental retardation and facial dysmorphism. Almost every organ and system, however, is affected

Signs and Symptoms Associated with Down Syndrome

Trisomy 21 (Down syndrome) is characterized by mental retardation, facial dysmorphism, single palmar crease, endocardial cushion defects, and duodenal atresia. Affected individuals have an increased risk of AML-M7 and ALL in childhood and early Alzheimer disease in adulthood. 

Homocystinuria

Homocystinuria is caused by cystathionine synthetase deficiency. Affected individuals manifest with skeletal abnormalities resembling those of Marfan syndrome. In addition, they are also at high risk of developing thromboembolism. About 50% of affected patients respond to high doses of vitamin B6 (pyridoxine).

Homocystinuria is the most common inborn error of methionine metabolism and is caused by cystathionine synthetase deficiency. Thromboembolic episodes involving both large and small vessels, especially those of the brain, are classically associated with this condition and may occur at any age. Other clinical manifestations resemble those of Marfan syndrome. These include ectopia lentis, elongated limbs, arachnodactyly, and scoliosis.

Glycogen storage diseases

Polycistronic mRNA (Bacterial lac operon)

Bacterial mRNA can be polycistronic, meaning that one mRNA codes for several proteins. An example of polycistronic mRNA is the bacterial lac operon, which codes for the proteins necessary for lactose metabolism by E. coil; the transcription and translation of these bacterial proteins is regulated by a single promoter, operator, and set of regulatory elements. 

Multiple origins of replication (DNA replication)

The process of DNA replication is similar in eukaryotes and prokaryotes. The key steps involved in DNA replication are:
1. Unwinding of double stranded DNA (dsDNA) by helicase to produce single stranded DNA (ssDNA)
2. Formation of a replication fork
3. Formation of an RNA primer by the action of the enzyme primase
4. Synthesis and concurrent proofreading of daughter DNA strands by DNA polymerases
5. Ligation of Okazaki fragments on lagging strands by ligase and removal and replacement of RNA primers with DNA by DNA polymerase I
6. Reconstitution of chromatin and ligation of daughter strands.

In E. coil, a prokaryote, the three major types of DNA polymerase are DNA polymerase I, II and III. In eukaryotes there are five major DNA polymerases: alpha, beta, gamma, delta and epsilon. Though the eukaryotic genome is much larger and more complex than the prokaryotic genome, interestingly the size of the eukaryotic genome is not the source of its complexity. Its complexity results from the presence of a large number of non-coding DNA regions between coding regions. Within genes there are introns (Non-coding regions - Think “IN” between) separating exons (Coding regions - Think “EX” pressed). Prokaryotes rarely have introns within their genes.
In contrast to prokaryotes which typically have a single origin of replication eukaryotes have multiple origins of replication? With multiple origins of replication, the genome can be copied much more quickly because multiple regions are being replicated at once. 

Bilirubin metabolism