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Inside cells, the process of folding often begins co-translationally, so that the N-terminus of the protein begins to fold while the C-terminal portion of the protein is still being synthesized by the ribosome; however, a protein molecule may fold spontaneously during or after biosynthesis. [18]
The unfolded protein response (UPR) is a cellular stress response related to the endoplasmic reticulum (ER) stress. [1] It has been found to be conserved between mammalian species, [2] as well as yeast [1] [3] and worm organisms. The UPR is activated in response to an accumulation of unfolded or misfolded proteins in the lumen of the
Misfolded proteins can form protein aggregates or amyloid fibrils, get degraded, or refold back to its native structure. In molecular biology, protein aggregation is a phenomenon in which intrinsically-disordered or mis-folded proteins aggregate (i.e., accumulate and clump together) either intra- or extracellularly.
It is referred to as the mitotic spindle during mitosis, a process that produces genetically identical daughter cells, or the meiotic spindle during meiosis, a process that produces gametes with half the number of chromosomes of the parent cell. Besides chromosomes, the spindle apparatus is composed of hundreds of proteins.
Kinetochore proteins can be grouped according to their concentration at kinetochores during mitosis: some proteins remain bound throughout cell division, whereas some others change in concentration. Furthermore, they can be recycled in their binding site on kinetochores either slowly (they are rather stable) or rapidly (dynamic).
Condensins are large protein complexes that play a central role in chromosome assembly and segregation during mitosis and meiosis (Figure 1). [ 1 ] [ 2 ] Their subunits were originally identified as major components of mitotic chromosomes assembled in Xenopus egg extracts .
During G 1 and S phase, the CDK1 subunit of MPF is inactive due to an inhibitory enzyme, Wee1. Wee1 phosphorylates the Tyr-15 residue of CDK1, rendering MPF inactive. During the transition of G 2 to M phase, cdk1 is de-phosphorylated by CDC25. The CDK1 subunit is now free and can bind to cyclin B, activate MPF, and make the cell enter mitosis.
Three types of cell division: binary fission (taking place in prokaryotes), mitosis and meiosis (taking place in eukaryotes).. When cells are ready to divide, because cell size is big enough or because they receive the appropriate stimulus, [20] they activate the mechanism to enter into the cell cycle, and they duplicate most organelles during S (synthesis) phase, including their centrosome.