DNA Organization, Replication and Repair MCQs

Werner syndrome is characterized by the dramatic, rapid appearance of features associated with normal aging. Individuals with this disorder typically grow and develop normally until they reach puberty Werner syndrome is characterised by the defective DNA Helicase Enzym, which causes short telomere and damage DNA

Ans. is 'b' i.e., Seals the single strand nick between the nascent chain and Okazaki fragments on lagging strand [Ref Harper 2501e p. 367]

Some Types of DNA Damage Caused by Radiation Option A: Formation Pyrimidine Dimer (Thymine dimer) - condensation of adjacent thymine Option B: Formation of purine dimer not seen commonly. Option C: Formation of apurinic or apyrimidinic sites by elimination of corresponding bases, Hydrolysis of N glycosidic bond which connect N base with deoxyribose sugar Option D: Formation of strand (chain) breaks or cross-linking of DNA strands Chain break - Disintegration of backbone element, can be single strand or double strand Cross linkage - Between bases in same or opposite strands, also between DNA and protein molecules (eg, histones) Reference: Harper's illustrated biochemistry 32nd edition CH 56 PN 683

Option A: RNA polymerase does not have reverse transcriptase activity, it has only transcriptase activity. Telomerase enzymes have reverse transcriptase activity. Option B: Reverse transcriptase enzyme is RNA dependent DNA polymerase, not DNA dependent RNA polymerase. Option C: Reverse transcriptase enzyme is RNA dependent DNA polymerase Option D: Present in RNA virus. There exists in one species of animal viruses (retroviruses) a class of enzymes capable of synthesizing a single-stranded and then a dsDNA molecule from a single-stranded RNA template. This polymerase, termed RNA-dependent DNA polymerase, or "reverse transcriptase," first synthesizes a DNA - RNA hybrid molecule utilizing the RNA genome as a template. Reference: Harper's illustrated biochemistry 31 st edition CH 35 PN 367

Explanation: DNA Polymerase is an essential enzyme responsible for DNA replication and repair. It reads the DNA template strand and synthesises a complementary strand by adding nucleotides in a 5' to 3' direction, ensuring the fidelity of DNA replication. Magnesium ions (Mg²+) act as crucial cofactors for DNA polymerase. Magnesium stabilises the structure of nucleotides and the active site of the enzyme, facilitating the formation of phosphodiester bonds between nucleotides. This ion is essential for both the catalytic activity and the accuracy of DNA polymerase during DNA synthesis.

Explanation: The 3′ to 5′ exonuclease activity of DNA polymerase functions in proofreading by removing incorrectly paired nucleotides from the 3′ end of the growing DNA strand and replacing them with the correct ones, thereby maintaining high fidelity of DNA replication. DNA unwinding is done by helicase, RNA synthesis by RNA polymerase, and primer removal occurs through 5′ to 3′ exonuclease activity (in prokaryotes) or RNase H and DNA polymerase δ (in eukaryotes).

In chromatin, core nucleosome particles are separated by DNA termed "linker DNA" Option A: It is a Short DNA consisting of roughly 30-bp regions. Option B: It links two nucleosomes, not present inside the nucleosome core. Option C: Contain histone H1; H2A & H2B are present in the nucleosome core. Option D: Two strands of DNA is linked by hydrogen bond, not linker DNA Reference: Harper's illustrated biochemistry 32 nd edition CH 35 PN 353

DNA polymerases only synthesize DNA in the 5' to 3' direction. Because the DNA strands are antiparallel, the polymerase functions asymmetrically. On the leading (forward) strand, the DNA is synthesized continuously. On the lagging (retrograde) strand, the DNA is synthesized in short (1-5 kb) fragments, the so-called Okazaki fragments, so named after the scientist who discovered them. Several Okazaki fragments (up to 1000) must be sequentially synthesized for each replication fork. Option A: On the lagging (retrograde) strand, the DNA is synthesized in short fragments, called Okazaki fragments. Option B: On the leading (forward) strand, the DNA is synthesized continuously. Option C & D: Any RNA does not contain Okazaki fragments. Reference: Harper's illustrated biochemistry 32 nd edition CH 35 PN 363