Newly synthesized RNA molecules undergo splicing to produce mRNA. Which one of the following best describes the process of splicing?
A. In splicing, the non-coding introns (intervening codons) are removed and exons are pasted together, producing a compact mRNA. This takes place in the nucleus. The splicing is carried out by small nuclear RNAs and protein complexes, which together constitute spliceosomes. This is an irreversible process as normally hnRNAs (heterogeneous nuclear RNAs) cannot be reassembled from mRNAs.
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Which one of the following stages of the cell cycle is dominant in nondividing cells such as neurones?
C. Each cell undergoes a natural cycle in terms of its replication and nucleic acid synthesis activity. The cell cycle consists of four separate phases: G1, S, G2, and M. G1 stands for growth phase 1, S for synthetic phase, G2 for growth phase 2 and M for mitosis phase. In mitosis the cellular material, including chromosomes, is divided between two daughter cells. Cells can leave G1 phase to enter a G0 phase, also called the quiescent phase as no replicatory activity takes place here. Most of these cells have temporarily or reversibly stopped dividing, for example liver parenchyma, in which case they enter G1 phase on stimulation. Cells such as neurones enter G0 phase indefinitely, but note that this dogma of absolute neuronal cell cycle dormancy is increasingly being challenged. A number of neurodegenerative diseases in humans, such as Pick’s disease, intractable temporal lobe epilepsy, progressive supranuclear palsy, Lewy body disease, and Parkinson’s disease, are thought to be associated with a few neurones retaining the ability to re-enter mitosis, thus disrupting the normal cell cycle.
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Which one of the following nitrogenous bases is present in RNA but not DNA?
E. DNA and RNA are the most important nucleic acids in the cellular machinery. These nucleic acids are composed of many nucleotides. Nucleotides are phosphorylated versions of nucleosides. Each nucleoside consists of two components: a nitrogenous base and a pentose sugar. There are two types of nitrogenous bases that can constitute a nucleoside—purines and pyrimidines. Purines include adenine and guanine. Pyrimidines include cytosine, uracil, and thymine. Thymine is usually found only in DNA while uracil is specific to RNA. DNA is double stranded with hydrogen-bonded base pairs. In DNA adenine always bonds with thymine (two hydrogen bonds) while cytosine bonds with guanine (three hydrogen bonds). As a result of this specific pairing, the amount of total purines is always equal to the total pyrimidines in normal DNA (Chargaff’s rule).
Gene cloning is the process of insertion of foreign DNA into a replicating sequence such as a plasmid.
Which of the following is essential for successful cloning?
A. Cloning is the process of copying; cloning laboratory animals refers to making identical genetic copies of the organisms while cloning a gene refers to producing identical copies of the gene. Gene cloning involves the insertion of foreign DNA into vectors such as bacterial plasmids or phages. Replication of these vectors then produces numerous identical copies of the cloned gene. In order to carry out successful cloning, a method of cutting DNA at specific sites to obtain the necessary genetic element is crucial. This is possible using restriction enzymes. DNA ligase (not RNA ligase) is used to paste the cut genetic element with plasmid DNA. A stem cell or ovum is not necessary for gene cloning. Active mitosis is sufficient to carry out cloning, thus meiosis is not necessary for gene cloning.
Polymerase chain reaction (PCR) was used in a study to search for various viruses in hippocampal tissue and CSF of patients with schizophrenia.
PCR is the preferred method for the above study due to which of the following properties?
A. PCR stands for polymerase chain reaction. It is an amplification process wherein a small amount of DNA sample is amplified many times to provide a supply for diagnostic analyses. The polymerization requires heat-stable DNA polymerase, obtained from Thermus aquaticus. Just one copy of a DNA sequence is sufficient to undertake PCR (at least theoretically). As it is extremely sensitive, contamination from other DNA present in the lab. environment (from bacteria, viruses, and DNA of lab. personnel) presents significant difficulties. As PCR requires the hybridization of primers to known sequences at either side of the region of interest (i.e. fl anking regions), completely unknown sequences cannot be polymerized. DNA cloning by PCR can be performed in a few hours, using relatively unsophisticated equipment. Typically, a PCR reaction consists of 30 cycles containing a denaturation, synthesis and reannealing step, with an individual cycle typically taking 3–5 min in an automated thermal cycler. This compares favourably with the time required for cell-based DNA cloning, which may take weeks. PCR is not error free. The DNA polymerases used for PCR usually have no error correction mechanisms such as exonuclease activity. So, if an error is made, initially it may get amplified uncorrected, but this is less of a problem now with the availability of high-fidelity DNA polymerases.
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