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Mitosis (/ m aɪ ˈ t oʊ s ɪ s /) is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis is an equational division which gives rise to genetically identical cells in which the total number of chromosomes is maintained. [1]
In humans, the number of NORs is equal to the number of nucleoli, which is ten. However, not all secondary constrictions are NORs. The formations of nucleoli takes place around the NOR region. The secondary constriction also contains the genes for rRNA synthesis (18S rRNA, 5.8S rRNA, and 28S rRNA). Genes for 5S rRNA are present on chromosome 1.
Although the interior of the nucleus does not contain any membrane-bound subcompartments, a number of nuclear bodies exist, made up of unique proteins, RNA molecules, and particular parts of the chromosomes. The best-known of these is the nucleolus, involved in the assembly of ribosomes.
Little was known about the function of the nucleolus until 1964, when a study [10] of nucleoli by John Gurdon and Donald Brown in the African clawed frog Xenopus laevis generated increasing interest in the function and detailed structure of the nucleolus. They found that 25% of the frog eggs had no nucleolus, and that such eggs were not capable ...
NADs are associated with nucleolus function. The nucleolus is the largest sub-organelle within the nucleus and is the principal site for rRNA transcription. It also acts in signal recognition particle biosynthesis, protein sequestration, and viral replication. [36] The nucleolus forms around rDNA genes from different chromosomes.
In the nucleolus, rRNA is synthesized by RNA polymerase I using the specialty genes that encode for it, which are found repeatedly throughout the genome. [34] The genes coding for 18S, 28S and 5.8S rRNA are located in the nucleolus organizer region and are transcribed into large precursor rRNA (pre-rRNA) molecules by RNA polymerase I.
The location of NORs and the nucleolar cycle in human cells. Nucleolus organizer regions (NORs) are chromosomal regions crucial for the formation of the nucleolus.In humans, the NORs are located on the short arms of the acrocentric chromosomes 13, 14, 15, 21 and 22, the genes RNR1, RNR2, RNR3, RNR4, and RNR5 respectively. [1]
The thick lines are chromosomes, and the thin blue lines are fibers pulling on the chromosomes and pushing the ends of the cell apart. The cell cycle in eukaryotes: I = Interphase, M = Mitosis, G 0 = Gap 0, G 1 = Gap 1, G 2 = Gap 2, S = Synthesis, G 3 = Gap 3. Cell division is the process by which a parent cell divides into two daughter cells. [1]