Q101. Cohesin and Condensin proteins of eukaryotes belong to which one of the
following groups?
(A) Structural maintenance of chromosomes (SMC) proteins
(B) Histones
(C) DNA polymerases
(D) Topoisomerases
Cohesin and condensin proteins in eukaryotes belong to the structural maintenance of chromosomes (SMC) proteins group. The correct answer is (A). These proteins form key complexes essential for chromosome dynamics during cell division.
Option Analysis
(A) Structural maintenance of chromosomes (SMC) proteins
Cohesin consists of SMC1-SMC3 heterodimers, while condensin uses SMC2-SMC4, both core SMC family members critical for sister chromatid cohesion and chromosome condensation, respectively. SMC proteins feature ATPase heads, hinges, and coiled-coils that enable DNA interactions. This classification fits perfectly as both complexes require these SMC subunits.
(B) Histones
Histones are basic proteins forming nucleosomes by wrapping DNA, aiding packaging but unrelated to cohesin or condensin functions or structures. No evidence links cohesin/condensin to histones; they operate at higher-order chromatin levels.
(C) DNA polymerases
DNA polymerases synthesize new strands during replication, a distinct enzymatic role without involvement in cohesion or condensation. Cohesin and condensin lack polymerase activity and focus on post-replication chromosome organization.
(D) Topoisomerases
Topoisomerases relieve DNA supercoiling and decatenate chromatids, often cooperating with condensin but not sharing protein family membership. They are enzymes, unlike the structural SMC complexes.
Cohesin and condensin proteins of eukaryotes play vital roles in chromosome segregation, belonging to structural maintenance of chromosomes (SMC) proteins. These complexes ensure accurate mitosis, a key topic for CSIR NET aspirants in molecular biology.
Core Functions
Cohesin holds sister chromatids together from S-phase until anaphase, preventing premature separation. Condensin compacts chromosomes into defined structures during mitosis, facilitating segregation. Both rely on ATP-dependent mechanisms for DNA looping.
Structural Features
SMC proteins form V-shaped heterodimers with ATPase heads for DNA binding and hinges for topology modulation. Cohesin includes kleisin (RAD21) to encircle DNA, while condensin has non-SMC subunits like CAP-H for compaction.
Exam Relevance
For CSIR NET, recognize SMC as the group for cohesin (SMC1/3) and condensin (SMC2/4), distinguishing from histones (packaging), polymerases (replication), or topoisomerases (untangling). Mutations disrupt segregation, linking to cancer studies.
