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    cyclins are involved in cell cycle progression. why would the expression of these cyclin proteins be highly regulated?


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    D-type cyclins are the activator subunits of cyclin-dependent kinases that have RB tumour suppressor-related proteins (RBRs) as their substrates and key targets for controlling cell cycle progression.

    From: Advances in Botanical Research, 2012

    Related terms:

    PhosphorylationCyclinCyclin-Dependent KinaseMitosisCell CycleS PhaseApoptosisCell ProliferationProteinsCell Division

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    Cell Wall, Cell Division, and Cell Growth

    Lalit M. Srivastava, in Plant Growth and Development: Hormones and Environment, 2002

    2.2. Classification and Structure of Cyclins

    In contrast to the relative conservatism of CDKs, cyclins are an abundant and diverse group of regulatory proteins, each of which has affinities for particular substrates of CDK. The cyclin subunit determines which proteins are held close to the CDK and can become substrates, whereas the CDK determines where in the substrate phosphorylation will occur. Thus, while CDKs phosphorylate proteins, cyclins determine the choice of substrate proteins, as well as the timing and intracellular location of phosphorylation. The entry of quiescent cells into the cell cycle is highly regulated and modulated by environmental and hormonal factors in a species- and organ-specific manner. Active cell proliferation involves association of CDKs with cyclins, and the multiple facets of cyclin function make them one of the more complex and fascinating proteins for study. In more recent years, evidence has accumulated that cyclins serve not only to determine the choice of substrates for their cognate CDKs, but also participate in DNA replication, mitotic progress, and cytoskeletal deployment during the cell cycle.

    In fission yeast, five to six cyclins are known, although the cell cycle can be successfully completed in genetically modified cells able to express only the mitotic cyclin B (encoding cdcl3). The number of cyclins is larger in the budding yeast, and the total numbers identified from cDNA clones in multicellular organisms, including plants, run into hundreds, although an individual cell may express only a dozen or so. Cyclins are classified on the basis of their amino acid sequence and the point or period in the cell cycle during which they activate their CDK partners. The classification is based on mammalian cyclins. Two broad classes are recognized: mitotic cyclins and G1 cyclins (Fig. 2-25). Mitotic cyclins include types A and B. Type A cyclins appear in the S phase, are maximally expressed in G2, and decline in content by early metaphase. Type B cyclins appear in G2 and are maximally expressed in late G2 and M phases and disappear after metaphase, depending on the type of cyclin. G1 cyclins include types D and ɛ and are maximally expressed in early to late G1 and disappear by the end of S phase. Among these, D-type cyclins are better known; they are inducible by peptide growth factors and thus are thought to connect an extracellular signal to induction of the cell cycle. In addition to the just-named cyclins, some less characterized mitotic and G1 cyclins are known from mammals also.

    2.2.1. Structure of Cyclins

    Cyclins are 30- to 65-kDa proteins, which show considerable sequence heterogeneity commensurate with their multiple roles, but they also share some common structural motifs. Both mitotic and G1 cyclins are characterized by the presence of an approximately 100 amino acid conserved sequence, known as the cyclin box, where the CDK partner binds (Fig. 2-27). The cyclin box is part of a larger sequence known as the cyclin fold with five α helices. Two such folds occur in each protein, and the cyclin box spans over helix 1 through helix 5 of the first cyclin fold.

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    FIGURE 2-27. Schematic representations of structures of a mitotic (B type) and a G1 (D type) cyclin. Approximate locations of the two sets of five α helices (H1-H5) in the first cylin fold are indicated. The binding site for a CDK, the cyclin box, is shown for both cyclins. Mitotic cyclins have a destruction box (RxxL[x]2-4xxN, where x is a variable amino acid), which occurs near the N-terminal (hatched box). G1 cyclins have the PEST sequence, which occurs near the carboxy terminus. D-type cyclins among them also have a binding suite for retinoblastoma (Rb) protein (see Section 3.3).

    Modified from Sorrel et al. (1999).

    Cyclins are transient proteins and show a high turnover during the cell cycle. The mitotic cyclins, A and B, carry an amino acid recognition sequence at their N-terminal, called destruction box, which targets them to the ubiquitination pathway of proteolysis (for this pathway, see Chapter 22). G1-type cyclins carry a conserved amino acid sequence called PEST, which is rich in proline, glutamine, serine, and threonine residues (see Fig. 2-27). The PEST sequence occurs in proteins with a fast turnover rate and is also believed to target proteins for ubiquitination. Some cyclins have both a destruction box and a PEST sequence.

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    Functional Cell Biology

    N.H. Lents, J.J. Baldassare, in Encyclopedia of Cell Biology, 2016

    The Cyclins

    Cyclins are found in every cell throughout all eukaryotes. Given the long history and frequent gene duplication, the cyclin family is quite large, with 30 identified members to date. Different cyclins function in different phases of the cell cycle in concert with their associated CDK, and different subtypes show specific tissue-distribution patterns in multicellular organisms. Nevertheless, the proteins are highly conserved, as evidenced by the fact that human cyclins can substitute for yeast cyclins to form functional cyclin-CDK complexes (Lew et al., 1991).

    Source : www.sciencedirect.com

    Cell cycle regulators (article)

    The core control system of the cell cycle. Cyclins, cyclin-dependent kinases (Cdks), and the APC/C.

    Regulation of cell cycle

    Cell cycle regulators

    The core control system of the cell cycle. Cyclins, cyclin-dependent kinases (Cdks), and the APC/C.

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    In the article on cell cycle checkpoints, we looked at the why of cell cycle transitions: the factors that a cell considers when deciding whether or not to move forward through the cell cycle. These include both external cues (like molecular signals) and internal cues (like DNA damage).

    Cues like these act by changing the activity of core cell cycle regulators inside the cell. These core cell cycle regulators can cause key events, such as DNA replication or chromosome separation, to take place. They also make sure that cell cycle events take place in the right order and that one phase (such as G

    _1 1 ​

    start subscript, 1, end subscript

    ) triggers the onset of the next phase (such as S).

    In this article, we'll look at a few of the most important core cell cycle regulators: proteins called cyclins, enzymes called Cdks, and an enzyme complex called the APC/C.


    Cyclins are among the most important core cell cycle regulators. Cyclins are a group of related proteins, and there are four basic types found in humans and most other eukaryotes: G

    _1 1 ​

    start subscript, 1, end subscript

    cyclins, G _1 1 ​

    start subscript, 1, end subscript

    /S cyclins, S cyclins, and M cyclins.

    As the names suggest, each cyclin is associated with a particular phase, transition, or set of phases in the cell cycle and helps drive the events of that phase or period. For instance, M cyclin promotes the events of M phase, such as nuclear envelope breakdown and chromosome condensation

    ^{1,2} 1,2

    start superscript, 1, comma, 2, end superscript


    Diagram: the cyclin expression cycle. This is a graph showing how concentrations of the various cyclins change in a cell over the course of the cell cycle.

    G1 cyclin: low in G1, rising slowly to a peak in mid-S phase, then dropping slowly back down to zero at the end of M phase.

    G1/S cyclin: very low for most of the cell cycle, with a sharp, symmetrical peak at the G1/S transition.

    S cyclin: low in early G1, rising slowly through late G1 and S, peaking in early G2 and dropping sharply back to zero in early M phase.

    M cyclin: very low through all of G1, rising slowly through, peaking at the G2/M transition, and dropping sharply to zero in the middle of M phase.

    Image modified from "Control of the cell cycle: Figure 2," by OpenStax College, Biology (CC BY 3.0). Modification of original work by WikiMaMa.

    The levels of the different cyclins vary considerably across the cell cycle, as shown in the diagram at right. A typical cyclin is present at low levels for most of the cycle, but increases strongly at the stage where it's needed. M cyclin, for example, peaks dramatically at the transition from G

    _2 2 ​

    start subscript, 2, end subscript

    to M phase. G _1 1 ​

    start subscript, 1, end subscript

    cyclins are unusual in that they are needed for much of the cell cycle.

    Cyclin-dependent kinases

    In order to drive the cell cycle forward, a cyclin must activate or inactivate many target proteins inside of the cell. Cyclins drive the events of the cell cycle by partnering with a family of enzymes called the cyclin-dependent kinases (Cdks). A lone Cdk is inactive, but the binding of a cyclin activates it, making it a functional enzyme and allowing it to modify target proteins.

    How does this work? Cdks are kinases, enzymes that phosphorylate (attach phosphate groups to) specific target proteins. The attached phosphate group acts like a switch, making the target protein more or less active. When a cyclin attaches to a Cdk, it has two important effects: it activates the Cdk as a kinase, but it also directs the Cdk to a specific set of target proteins, ones appropriate to the cell cycle period controlled by the cyclin. For instance, G

    _1 1 ​

    start subscript, 1, end subscript

    /S cyclins send Cdks to S phase targets (e.g., promoting DNA replication), while M cyclins send Cdks to M phase targets (e.g., making the nuclear membrane break down).

    Simplified diagram showing how cyclins modify activity of Cdks.

    Left panel (no cyclin): no cyclin is present, Cdk is inactive, and targets specific to the G1/S transition are not phosphorylated. Nothing happens, and S phase factors remain "off."

    Right panel (+G1/S cyclin): the G1/S cyclin is present and binds to the Cdk. The Cdk is now active and phosphorylates various targets specific to the G1/S transition. The phosphorylated targets cause the activation of DNA replication enzymes, and S phase begins.

    In general, Cdk levels remain relatively constant across the cell cycle, but Cdk activity and target proteins change as levels of the various cyclins rise and fall. In addition to needing a cyclin partner, Cdks must also be phosphorylated on a particular site in order to be active (not shown in the diagrams in this article), and may also be negatively regulated by phosphorylation of other sites

    Source : www.khanacademy.org

    Chapter 11 Summative Quiz Flashcards

    Start studying Chapter 11 Summative Quiz. Learn vocabulary, terms, and more with flashcards, games, and other study tools.

    Chapter 11 Summative Quiz

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    Cyclins are involved in cell cycle progression. Why would the expression of these cyclin proteins be highly regulated?

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    If cyclins were constantly expressed, cells would be rapidly dividing rather than performing their normal metabolic activities.

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    Gene expression cannot be regulated

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    during replication.

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    1/20 Created by madisonyoungufl

    Terms in this set (20)

    Cyclins are involved in cell cycle progression. Why would the expression of these cyclin proteins be highly regulated?

    If cyclins were constantly expressed, cells would be rapidly dividing rather than performing their normal metabolic activities.

    Gene expression cannot be regulated

    during replication.

    A(n) _______ operon is turned on unless not needed.


    What kind of mutation would lead to an increase in the expression, or inappropriate expression, of a gene?

    A mutation that lowers the efficiency of a repressor of the gene

    Which statement about operons is true?

    They consist of a promoter, an operator, and structural genes.

    The expression of the lac structural genes is _______ when lactose is absent from the culture medium and is _______ when lactose is added, because lactose binds to the lac _______ and inactivates it.

    low; high; repressor

    A difference between promoters and operators is that

    RNA polymerase binds within the promoter, while a repressor binds the operator.

    Your lab group is studying the lac operon. You perform a mutagenesis and plate out the resulting bacterial colonies on medium containing only essential minerals, agar, and X-Gal. Even before the addition of lactose, you notice some of the colonies are turning light blue. It is likely that the light blue colonies have a mutation in

    gene I.

    Prokaryotes and eukaryotes differ in transcription in that

    there are three RNA polymerases in eukaryotes.

    Which is/are not involved in the construction of the basal transcription apparatus?


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