Following statements were made about cell cycle regulation: A. De novo synthesis and destruction of Cyclin B are essential for cell cycle progression in yeast. B. De novo synthesis and destruction of Cyclin B and the related Cyclin dependent Kinase (CDK) are essential for cell cycle progression. C. CDK activity is regulated by both activating and inhibitory phosphorylation. D. Retinoblastoma (Rb) functions as an inhibitor of G2 to M transition. E. Inactivation of Sic 1 is essential for transition into S phase. Which one of the following represents the combination of the correct statements?
A, C, E
Let's analyze each statement about cell cycle regulation to determine which ones are correct.
Cell Cycle Regulation Statements Analysis
The cell cycle is a tightly regulated process involving a series of events that lead to cell division. This regulation is controlled by various proteins, notably cyclins and cyclin-dependent kinases (CDKs).
Cyclin Synthesis and Destruction
Statement A: De novo synthesis and destruction of Cyclin B are essential for cell cycle progression in yeast.
Cyclin B, also known as mitotic cyclin, binds to CDK1 to form the Maturation Promoting Factor (MPF), which drives the cell into mitosis (M phase). The level of Cyclin B increases during G2 phase and peaks in M phase. Its rapid destruction via ubiquitination is crucial for exiting mitosis and completing the cell cycle. In yeast, this dynamic synthesis and destruction are indeed essential for proper cell cycle progression, particularly the transition into and out of M phase.
This statement is correct.
Cyclin and CDK Regulation
Statement B: De novo synthesis and destruction of Cyclin B and the related Cyclin dependent Kinase (CDK) are essential for cell cycle progression.
While Cyclin B synthesis and destruction are essential, the concentration of CDKs typically remains relatively constant throughout the cell cycle. CDK activity is primarily regulated by the availability of cyclins, phosphorylation status, and binding of inhibitory proteins, not by their own de novo synthesis and destruction.
This statement is incorrect.
CDK Activity Regulation
Statement C: CDK activity is regulated by both activating and inhibitory phosphorylation.
CDKs are regulated by phosphorylation. For example, activating phosphorylation occurs on a threonine residue by CDK-activating kinase (CAK), which is necessary for full CDK activity. Inhibitory phosphorylation occurs on threonine and tyrosine residues near the active site, typically by kinases like Wee1, which blocks CDK activity. These phosphorylation events are critical control points in the cell cycle.
This statement is correct.
Retinoblastoma (Rb) Function
Statement D: Retinoblastoma (Rb) functions as an inhibitor of G2 to M transition.
The Retinoblastoma protein (Rb) is a tumor suppressor that primarily regulates the G1 to S phase transition. Unphosphorylated Rb binds to and inhibits the E2F transcription factor, preventing the expression of genes required for DNA replication and S phase entry. Phosphorylation of Rb by G1-CDKs (like Cyclin D-CDK4/6 and Cyclin E-CDK2) releases E2F, allowing the cell to progress into S phase. Rb's role is mainly in G1, not G2 to M.
This statement is incorrect.
Sic1 Role in S Phase Transition
Statement E: Inactivation of Sic 1 is essential for transition into S phase.
Sic1 is a CDK inhibitor (CKI) found in budding yeast that specifically inhibits Clb-CDK complexes (which are equivalent to mammalian S-phase CDKs). High levels of Sic1 in late G1 prevent premature activation of these CDKs. For the cell to enter S phase and begin DNA replication, Sic1 must be destroyed, typically through ubiquitination mediated by the SCF complex (specifically, the Cdc4 subunit). Inactivation (destruction) of Sic1 allows the Clb-CDKs to become active, driving the cell into S phase.
This statement is correct.
Summary of Correct Statements
Based on the analysis:
- Statement A is correct: Cyclin B synthesis and destruction are essential for cell cycle progression, especially in yeast.
- Statement B is incorrect: CDK levels are generally stable; their activity is regulated by other factors, not synthesis/destruction.
- Statement C is correct: CDK activity is regulated by both activating and inhibitory phosphorylation.
- Statement D is incorrect: Retinoblastoma (Rb) inhibits G1 to S transition, not G2 to M.
- Statement E is correct: Inactivation of Sic1 is essential for S phase transition in yeast.
Therefore, the combination of correct statements is A, C, and E.
Important Questions from Cell Division and Cell Cycle
During cell cycle, entry in the S-phase is tightly regulated. This is possible because:
A. APC/C promotes ubiquitination of S-phase cyclins and mitotic cyclins, marking them for proteolyses at the mitotic exit.
B. Cyclin B1 helps in the activation of S-phase CDKs only in late G1.
C. As mitotic CDK activity declines in late mitosis, cdc14 phosphatase activates APC/C by dephosphorylating Cdh1, thus promoting formation of APC/CCdh1
D. Securin keeps S-phase cyclins in inactive state till late G1.
Which one of the options represents all correct statements?
The table below lists cell cycle regulatory proteins and their known functions
Which one of the following options represents the correct match between cell cycle regulatory proteins with their known functions?Cell Cycle regu latory proteins Function A Cdk-activating kinase (CAK) (i) Suppresses G1/S-Cdk and S-Cdk activation in G1; helps cells withdraw from cell cycle when they terminally differentiate; phosphorylation by Cdk2 triggers its ubiquitylation by SCF. B Wee1 kinase (ii) Suppresses G1/S-Cdk and S-Cdk activities following DNA damage C p27 (mammals) (iii) Phosphorylates inhibitory sites in Cdks: primarily involved in suppressing Cdk1 activity before mitosis D p21 (mammals) (iv) Phosphorylates an activating site in Cdks Following statements were made about the characteristics of cyclin proteins:
A. Synthesis of M-cyclin is dependent on the cyclin mRNA that is newly transcribed after every cycle.
B. Destruction of M-cyclin toward the end of mitosis is driven by ubiquitin independent proteolytic system.
C. G1 cyclins can be activated by mitogenic factors.
D. Retinoblastoma (Rb) is a key target of the activated cyclin D - Cdk 4/6 complex.
E. While cyclin A1 expression is ubiquitous, cyclin A2 expression is restricted to the germ cell lineages.
Which one of the following options contains a combination of all correct statements?
To test the impact of cAMP on protein kinase A conformation in cells, an investigator made FRET biosensor by fusing two fluorescent proteins at the N-and C-terminus of protein kinase A. In the absence of cAMP in the cellular milieu, no FRET signal was detected. However, upon cAMP addition, a strong emission at 530 nm was observed. What could be the best configuration of fluorophores that were used by the investigator?
Given below are a few steps in clathrin‐coated vesicle formation in the secretory pathway.
(A) Receptor‐ligand recognition and binding
(B) Recruitment of adapter protein and clathrin
(C) Vesicle formation
(D) Uncoating of clathrin coats
Choose the option that correctly identifies the sequence of events in making a clathrin‐coated vesicle.
