Cell Cycle

The replication of the genome and the subsequent segregation of chromosomes into daughter cells are controlled by a series of events collectively known as the <b>cell cycle</b>. DNA replication is carried out during a discrete temporal period known as the S (synthesis)-phase, and chromosome segregation occurs during a massive reorganization to cellular architecture at mitosis. Two gap-phases separate these major cell cycle events: G1 between mitosis and S-phase, and G2 between S-phase and mitosis. In the development of the human body, cells can exit the cell cycle for a period and enter a quiescent state known as G0, or terminally differentiate into cells that will not divide again, but undergo morphological development to carry out the wide variety of specialized functions of individual tissues.<p>A family of protein serine/threonine kinases known as the cyclin-dependent kinases (CDKs) controls progression through the cell cycle. As the name suggests, the activity of the catalytic subunit is dependent on binding to a cyclin partner. The human genome encodes several cyclins and several CDKs, with their names largely derived from the order in which they were identified. The oscillation of cyclin abundance is one important mechanism by which these enzymes phosphorylate key substrates to promote events at the relevant time and place. Additional post-translational modifications and interactions with regulatory proteins ensure that CDK activity is precisely regulated, frequently confined to a narrow window of activity.<p>In addition, genome integrity in the cell cycle is maintained by the action of a number of signal transduction pathways, known as <b>cell cycle checkpoints</b>, which monitor the accuracy and completeness of DNA replication during S phase and the orderly chromosomal condensation, pairing and partition into daughter cells during mitosis.<p>Replication of telomeric DNA at the ends of human chromosomes and packaging of their centromeres into chromatin are two aspects of <b>chromosome maintenance</b> that are integral parts of the cell cycle.<p><b>Meiosis</b> is the specialized form of cell division that generates haploid gametes from diploid germ cells, associated with recombination (exchange of genetic material between chromosomal homologs).

基因组复制及其随后在子细胞中染色体分离的过程，共同构成了所谓的细胞周期。DNA复制在一段被称为S（合成）期的离散时间段内进行，而染色体分离则发生在有丝分裂期间细胞结构的大规模重组过程中。两个间隙期分隔了这些主要的细胞周期事件：G1期位于有丝分裂与S期之间，G2期位于S期与有丝分裂之间。在人体发育过程中，细胞可以暂时退出细胞周期，进入一个被称为G0期的静息状态，或者最终分化为不再分裂但执行个体组织广泛多样化功能的细胞。<p>一类被称为细胞周期蛋白依赖性激酶（CDKs）的蛋白质丝氨酸/苏氨酸激酶家族，控制着细胞周期的进展。正如其名所示，催化亚基的活性依赖于与细胞周期蛋白伙伴的结合。人类基因组编码了多种细胞周期蛋白和多种CDKs，其名称在很大程度上来源于它们被发现的顺序。细胞周期蛋白丰度的振荡是这些酶通过磷酸化关键底物来促进相关时间和地点事件的一个重要机制。此外，翻译后修饰以及与调节蛋白的相互作用确保了CDK活性的精确调控，通常被限制在一个狭窄的活动窗口内。<p>此外，细胞周期中维持基因组完整性是通过多种信号转导途径的作用实现的，这些途径被称为细胞周期检验点，它们监控S期DNA复制的准确性和完整性，以及有丝分裂期间染色体的有序凝聚、配对和分配到子细胞中。<p>端粒DNA的复制以及它们的着丝粒包装进入染色质，是细胞周期中染色体维护的两个不可或缺的方面。<p>减数分裂是细胞分裂的特殊形式，它从二倍体的生殖细胞中产生单倍体的配子，与重组（同源染色体之间的遗传物质交换）相关。