氨基酸残基的磷酸化和区磷酸化。然后被降解。CDKs只有与 cyclin结合才具有活 性。大部分的 cyclin能很快转换。 Cyclin在细胞周期的特殊转变位点被合成,然后 被降解。由Gl-到-S需要 Gl cyclins来激活CDKs:由G2-到M需要G2 cyclins来激 活CDKs。CDKs有两个酪氨酸的磷酸化位点:其中一个引起该酶的活化,另一个 引起该酶的失活。激活和抑制CDKs酶活的磷酸化均由特异的蛋白激酶催化。与 CDKs磷酸化类似,蛋白磷酸酶能使CDKs去磷酸化,磷酸所处的位置决定去磷酸 化使CDKs激活还是失活。CDKs的磷酸化和去磷酸化是控制细胞周期进程的重要 调控机制。 Regulation of cell cycle by protein kinases The key enzymes that control the transitions between the different states of the cell cycle are the cyclin-dependent protein kinases, or CDKs(Figure). Protein kinases are enzymes that phosphorylate proteins using ATP. Cells use several protein kinases which activate different phases of the cell cycle depending on regulatory subunits called cyclins for their activities. The regulated activity of CDKs is essential for the transitions from Gl to S and from g2 to M. cdk activity can be regulated in various ways, but two of the most important mechanisms are(1)cyclin synthesis and destruction and (2)the phosphorylation and dephosphorylation of key amino acid residues within the CDK protein CDKs are inactive unless they are associated with a cyclin. Most cyclins turn over rapidly. They are synthesized and then actively degraded at specific points in the cell cycle. The transition from Gl to S requires G1 cyclins to activate the CDKs; the transition from G2 to mitosis requires another set of cylins(known as mitotic cyclins)to activate CDKs(see Figure). CDKs possess two tyrosine phosphorylation sites: One causes activation of the enzyme, the other causes inactivation. Specific kinases carry out both the stimulatory and the inhibitory phosphorylations. Similarly, protein phosphatases can remove phosphate from CDKs, either stimulating or inhibiting their activity, depending on the position of the phosphate. The addition or removal of phosphate groups from CDKs is highly regulated and an important mechanism for the control of cell cycle progression(see Figure) 激素对细胞分裂的调控 激素能显著影响细胞分裂,细胞分裂素对维持分生组织的分裂具有重要的作 用。GA能促进G1到S的过程,CTK能促进S期中DNA的合成,所以CTK和 GA都对细胞分裂起促进作用 Regulation of cell cycle by hormones Certain hormones can affect cell cycle significantly. Ga plays important role in maintaining cell division capability of meristem. GA stimulates the process from Gl to S; CTK promotes DNA synthesis during S phase, both CTK and Ga stimulate cell division 2.细胞生长 细胞生长的概念 体积的增大是细胞生长的主要因素。而水分是细胞体积构成的主要成份,所 以细胞必须吸水才能使体积增大。细胞体积增大的促进因素是细胞吸水。细胞的 水势主要是通过膨压的变化,也就是不断增大的原生质对细胞壁产生的压力的变 化来调节的。细胞能产生很大的膨压。为了抵拒这种压力,细胞壁必须足够坚固。 细胞体积增大最关键的限制因素是由细胞壁的强度和刚性。所以,细胞壁的存在 阻碍着细胞体积的增长。克服这种阻碍有两种方式:一种是增加膨压,因为只有氨基酸残基的磷酸化和区磷酸化。然后被降解。CDKs只有与cyclin结合才具有活 性。大部分的cyclin能很快转换。Cyclin在细胞周期的特殊转变位点被合成，然后 被降解。由G1-到-S需要G1 cyclins来激活CDKs；由G2-到-M需要G2 cyclins来激 活CDKs。CDKs有两个酪氨酸的磷酸化位点：其中一个引起该酶的活化，另一个 引起该酶的失活。激活和抑制CDKs酶活的磷酸化均由特异的蛋白激酶催化。与 CDKs磷酸化类似，蛋白磷酸酶能使CDKs去磷酸化，磷酸所处的位置决定去磷酸 化使CDKs激活还是失活。CDKs的磷酸化和去磷酸化是控制细胞周期进程的重要 调控机制。 Regulation of cell cycle by protein kinases The key enzymes that control the transitions between the different states of the cell cycle are the cyclin-dependent protein kinases, or CDKs (Figure). Protein kinases are enzymes that phosphorylate proteins using ATP. Cells use several protein kinases; which activate different phases of the cell cycle depending on regulatory subunits called cyclins for their activities. The regulated activity of CDKs is essential for the transitions from G1 to S and from G2 to M. CDK activity can be regulated in various ways, but two of the most important mechanisms are (1) cyclin synthesis and destruction and (2) the phosphorylation and dephosphorylation of key amino acid residues within the CDK protein. CDKs are inactive unless they are associated with a cyclin. Most cyclins turn over rapidly. They are synthesized and then actively degraded at specific points in the cell cycle. The transition from G1 to S requires G1 cyclins to activate the CDKs; the transition from G2 to mitosis requires another set of cylins (known as mitotic cyclins) to activate CDKs (see Figure). CDKs possess two tyrosine phosphorylation sites: One causes activation of the enzyme; the other causes inactivation. Specific kinases carry out both the stimulatory and the inhibitory phosphorylations. Similarly, protein phosphatases can remove phosphate from CDKs, either stimulating or inhibiting their activity, depending on the position of the phosphate. The addition or removal of phosphate groups from CDKs is highly regulated and an important mechanism for the control of cell cycle progression (see Figure). 激素对细胞分裂的调控 激素能显著影响细胞分裂，细胞分裂素对维持分生组织的分裂具有重要的作 用。GA 能促进 G1到 S 的过程，CTK 能促进 S 期中 DNA 的合成，所以 CTK 和 GA 都对细胞分裂起促进作用。 Regulation of cell cycle by hormones Certain hormones can affect cell cycle significantly. GA plays important role in maintaining cell division capability of meristem. GA stimulates the process from G1 to S; CTK promotes DNA synthesis during S phase, both CTK and GA stimulate cell division. 2. 细胞生长 细胞生长的概念 体积的增大是细胞生长的主要因素。而水分是细胞体积构成的主要成份, 所 以细胞必须吸水才能使体积增大。细胞体积增大的促进因素是细胞吸水。细胞的 水势主要是通过膨压的变化，也就是不断增大的原生质对细胞壁产生的压力的变 化来调节的。细胞能产生很大的膨压。为了抵拒这种压力，细胞壁必须足够坚固。 细胞体积增大最关键的限制因素是由细胞壁的强度和刚性。所以，细胞壁的存在 阻碍着细胞体积的增长。克服这种阻碍有两种方式：一种是增加膨压，因为只有