Nimrit Palan, Mumbai University
Cell division is the process by which a cell divides to produce new daughter cells by distributing its genetic material and cytoplasm. It is a component of the larger cell cycle and is involved in cell reproduction directly. Mitosis and meiosis are the two kinds of cell division seen in fully grown organisms. Cytokinesis and cytoplasm division are the common last stages in both processes. This topic will cover both types of cell division. One cell divides into two genetically identical daughter cells in this type of cell division. Mitosis is responsible for the vast majority of cell divisions in our bodies. In every dividing cell, the two copies of the genome must be split at some point. The enzyme separase, as its name suggests, jumps into action and gets the job done. It is harmful to release separase at any other time during cell life, which is why it is kept in check. Not one, but three mutually exclusive inhibitors keep human separase in check.
Role of cohesin:
The ring-shaped cohesin complex holds the duplicated chromosomes together during early mitosis. During development, the processes of mitosis and meiosis are crucial. Cohesion is essential throughout these processes to keep the sister chromatids together until they separate at anaphase. Cohesion is formed by cohesins, which are multiprotein component complexes.
Role of separase and its inhibitors:
Separase, also known as separin, is a cysteine protease that causes anaphase to begin by hydrolyzing cohesin, the protein that binds sister chromatids during the early stages of anaphase. One of the inhibitors snakes its way along the separase until it reaches the active site. In an entangled embrace, the other pushes separase to inhibit itself while this inhibitor is also inhibited by separase. Separase has functions other than cohesin cleavage in cell division, and various inhibitors may allow for geographical or temporal modulation of separase activity. The inhibitors are as follows:
Securin:
The protein securin is the best-studied inhibitor, as it begins binding to separate while the enzyme is still being produced, and even helps separase synthesis. The notion that securin mimics the cohesin substrate and binds to the active site of separase was originally suggested by genetic and biochemical investigations. Securin is an APC (antigen-presenting cell) substrate that is known for inactivating separase, a cohesin-cleaving enzyme, until the metaphase to anaphase transition.
Cyclin-dependent kinase 1 (CDK1) complex:
Cks1, CDK1, and cyclin B are the proteins that make up this complex. This complex is a key factor in cell division, as it phosphorylates (adds phosphate groups too) hundreds, if not thousands, of distinct proteins to cause the cellular alterations essential for division. The autoinhibitory components of separase identified in this critical binding region are non-contiguous and are made up of three loops called autoinhibitory loops (AIL1, AIL2 and AIL3).CDK1–cyclin B1 directs one autoinhibitory loop to block the catalytic sites of both separase and CDK1.AIL3 inhibits CDK1 via attaching to its active site, as well as autoinhibiting separase. This loop most likely binds to all of the proteins in the complex. The CDK1 complex is inhibited by a fourth separase loop that wraps around cyclin B.
Also read: Early animals probably survived the Neoproterozoic era
Reference: Hauf, S. (2021b). Two giants of cell division in an oppressive embrace. Nature, d41586-021-01944–01946. https://doi.org/10.1038/d41586-021-01944-6
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Congratulations Nimrit !!