21. The structure of chromosomes Difficulty: 2 A condensed eukaryotic chromosome

Question

21. The structure of chromosomes Difficulty: 2 A condensed eukaryotic chromosome is known to be associated with all of the following proteins except for Ans: D Pages: 942-943 A) Core histones H2A, H2B, H3, and H4. B) Histone HI C) SMC proteins D) Topoisomerase I E) Topoisomerase II 22. The structure of chromosomes Pages: 943-944 Difficulty: 2 Ans: C The SMC proteins (for structural maintenance of chromosomes) include cohesins and condensins, and are known to have all of the following properties except: CA) A complete ATP binding site. B) A hinge region C) Topoisomerase activity to produce positive supercoils D) The ability to condense DNA E) Two coiled-coil domains

Explanation / Answer

21. Let’s discuss different options one by one-

Option (C)- SMC stands for structural maintenance of chromosomes. SMC proteins participate in many aspects of higher-order chromosome organization and dynamics, which also includes chromosome condensation. A pair of SMC proteins SMC2 and SMC4 acts as the core of the condensin complexes that take part in chromosome condensation.

Options ( A) and (B) - We know all five families of histones i.e. four core histones (H2A, H2B, H3, and H4) and the linker histone H1 participate in packaging and ordering of DNA in structural unit called nucleosomes. While core histones form the core of a nucleosome, linker histone binds the nucleosome at the entry and exit sites of the DNA, thus locking the DNA into place. During mitosis and meiosis, these nucleosomes and other regulatory proteins assemble to make the condensed chromosome.

Options (D) and (E) – We know Topoisomerases are enzymes responsible for overwinding or underwinding of DNA. While Topoisomerase I is primarily responsible for relaxing positively supercoiled (over-wound) and/or negatively supercoiled (under-wound) DNA, Topoisomerase II (gyrase) introduce tight supercoils into the DNA helix in order to condense them in chromosome.

Thus we see all of SMC proteins, Histones (core and linker) proteins, and Topoisomerase II protein are directly associated with condensation of chromosome in one way or the other but Topoisomerase I protein is not related to condensation of chromosome in a eukaryotic cell.

Therefore, your answer is (D).

22. Let’s discuss different options one by one –

Option (E)- SMC proteins in their primary structure have two coiled – coil regions namely coiled - coil region I and coiled - coil region II.

Option (A) and (B)- SMC proteins in their secondary structure form a V-shaped dimer. To make this V-shaped structure, an SMC protomer is self-folded through anti-parallel coiled-coil interactions, forming a rod-shaped molecule. At one end of the molecule, the N-terminal and C-terminal domains together form an ATP-binding site, while the other end (which is called a hinge region) remains open. Two self-folded protomers then dimerize through their hinge regions and make a V-shaped dimer.

Option (D) As discussed in option (C ) of question 21 above, A pair of SMC proteins SMC2 and SMC4 acts as the core of the condensin complexes that take part in chromosome (DNA) condensation.

Option (C ) – SMC proteins do not have Topoisomerase activity to produce positive coils, they take part in compaction or condensation of DNA but not in super coiling.

Therefore, your answer is ( C).

Thanks!

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