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Glossary | Linus Pauling Institute | Oregon State University
, previously known as NCC-1 for ematode ell ycle, was identified based on its close similarity to the prototypical yeast CDK (). In contrast to yeast, but similar to mammalian Cdk1, / is specifically required for G2/M progression and not for G1 or S phase (). Maternal product suffices for embryogenesis, and candidate null mutant animals arrest cell division during L1 development. Several observations indicate that the post-embryonic precursor cells in these mutants arrest in G2 phase: such cells show normal expression of the :: reporter and BrdU incorporation during S phase, but fail to proceed into mitosis (as indicated by absence of chromosome condensation and nuclear envelope breakdown). Moreover, endoreduplication cycles, which skip M phase, continue in mutants and intestinal nuclei accumulate the normal 32n DNA content. Following RNAi of in adult hermaphrodites, oocytes show delayed meiotic maturation, form an eggshell upon fertilization, but neither align nor segregate homologous chromosomes. Thus, is required for meiotic as well as mitotic M phase.
Animal development from a single-cell zygote to fertile adult requires many rounds of cell division. During each division, cells complete an ordered series of events that collectively form the "cell cycle". This cycle includes accurate duplication of the genome during the DNA synthesis phase (), and segregation of complete sets of chromosomes to each of the daughter cells in (A). The somatic cell cycle also contains "ap" phases, known as , which connects the completion of M phase to initiation of S phase in the next cycle, and , which separates the S and M phases. Dependent on environmental and developmental signals, cells in G1 may temporarily or permanently leave the cell cycle and enter a quiescent or arrested phase known as .
AP Biology Animations - Biology Junction
Endoreduplication cycles are characterized by a DNA synthesis phase that is not followed by M phase, thus doubling the DNA ploidy with each additional cycle. Such endoreduplication cycles take place in the intestine and hypodermis during development (). Fourteen of the twenty intestinal cells undergo a final nuclear division at the end of the L1 stage. Subsequently, all intestinal nuclei go through an endoreduplication cycle during each larval stage, which results in intestinal nuclei with a 32n DNA content in adult animals.
The somatic nuclei of post-embryonic precursor cells appear to contain a 2n DNA content at the time of hatching and go through a DNA synthesis phase before initiating mitosis (; ). These divisions generally depend on the function of G1/S and G2/M control genes. Measurements of the time of DNA duplication in the vulval precursor cells demonstrated that G1/G0 extends from mid L1 until shortly after the L2 molt, and that S phase is completed hours before mitosis initiates (). Similarly, S phase in the intestinal nuclei occurs between 6 and 8 hours of L1 development, approximately 4 hours before nuclear division (). Thus, the precursor cells of the post-embryonic lineages and their descendents follow canonical cell cycles in which the S and M phases are separated by G1 and G2 Gap phases. As in embryogenesis, the length of interphase varies greatly between different cell types. Divisions frequently follow each other within one hour, but some cells remain quiescent for 20 hours, before dividing again two larval stages later ().
Unit 2 Cells - Biology Junction
While somatic cells show little developmental response to genotoxic stress, DNA damage in the germline induces both cell-cycle arrest and apoptosis (). Cell-cycle arrest is restricted to the mitotically active stem-cell population in the distal region of the gonad arms, while cell death occurs only in the meiotic pachytene regions in adult hermaphrodites. As in other eukaryotes, the RAD1 and HUS1 genes are required for checkpoint-induced arrest as well as apoptosis, together with the novel checkpoint gene / (). It is currently unclear how damage triggers cell-cycle arrest in . Neither nor Cip/Kip expression is induced in response to irradiation, which rules out one of the mechanisms used in mammals (). Other conserved mechanisms involve inactivation of Cdc25 and activation of Wee1 family members, which may mediate the arrest in .
As in other metazoans, early embryonic divisions in are fast and cycle between S and M phases, apparently lacking the Gap phases G1 and G2 (). In the initial division cycles, DNA synthesis, nuclear division, and cytokinesis are completed within approximately 15-20 minutes. However, the exact division times vary, as even the first mitotic division is asymmetric and generates daughter cells that are unequal and divide asynchronously. Just a few hours into embryonic development, cells in different lineages diverge greatly in cell-cycle profiles. Certain cells continue rapid divisions, others divide after an extended interphase of two hours or more, yet other cells become quiescent or post-mitotic (). Nearly all embryonic divisions are completed during the first half of embryogenesis, within the proliferation phase that ends approximately 7 hours after fertilization.
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A Class of Environmental and Endogenous Toxins ..
The Cdk4/6 kinase and D-type cyclin genes are required for progression through G1 phase during larval development (; ). and likely act in complex, as indicated by their direct interaction and close similarity in null phenotypes (). In contrast to larval divisions, only a few very late embryonic divisions depend on / activity (; ). Possible explanations for this difference include that the early embryonic divisions lack a G1 phase and therefore will not need a G1 cyclin or CDK. Also, as one of its most important functions, and act to antagonize the transcriptional repressor Rb (see below; (). In the absence of / function, Rb may inappropriately repress cell-cycle genes, but this cannot prevent divisions that are driven by maternal products. Also, and could primarily promote growth, as in (; ), which is not incorporated in the embryonic divisions. However, larval divisions arrest in G1 while cells continue to grow in the mutants, and growth retardation occurs later (). Therefore, absence of G1 phases and maternal contribution of DNA replication components likely explain the limited requirement for / during embryogenesis
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