Question

Indicate whether each of the following, when active, tends to activate (A) or inactivate (I) M-Cdks. The correct answer will be a four-letter string composed of letters A and I only, e.g. IIAI.
( A ) M-Cdk
( I ) Cdc20–APC/C
( I ) Cdh1–APC/C
( I ) Wee1

Answer 1

AIII

Explanation:

• Cell cycle is mainly divided into two phases-Interphase and Mitotic phase
• Interphase is the preparatory phase whereas mitotic phase is the dividing phase
• Various cyclins and Cdks are regulated during these phases
• M-Cdks stands for Mitotic Cyclin Dependent Kinases which helps in cell cycle regulation
• Cdks are the catalytic subunit bound to the regulatory subunit(cyclins)
• Mitosis includes four stage namely Prophase,Metaphase,Anaphase and Telophase
• During the Anaphase stage separation of sister chromatids occurs due to the destruction of a protein called cohesin
• Destruction of cohesin is catalysed by an enzyme called separase
• Prior to anaphase securin binds to separase and inhibits its activity
• The APC(anaphase promoting complex) with specificity factor Cdc20 tragets securin for polyubiquitination which then degraded by proteasome
• The separase released becomes active and breaks the bridge between sister chromatids
• Movement of sister chromatids occurs towards their respective poles
• In late anaphase,specific phopshates called Cdc14 removes the regulatory phosphate group from cdh1 specificity factor and therefore Cdh1 becomes active
• Active Cdh1 specificity factor binds to and activate APC(anaphase promoting complex)
• Cdh1 bound APC catalyse ubiquitination of mitotic cyclins
• As the mitotic cyclins are degraded by the proteasome,onset of telophase starts
• Wee1 is a kinase enzyme which comes into consideration in case of fission yeast
• Wee 1 phosphorylates the inhibitory site of M-Cdk which further leads the cells to enter into mitosis