2. Cycle progression depends on discrete control points

27.1 Introduction

Key terms defined in this section
Cell cycle is the period from one division to the next.
Interphase is the period between mitotic cell divisions; divided into G1, S, and G2.
S phase is the restricted part of the eukaryotic cell cycle during which synthesis of DNA occurs.
Spindle describes the reorganized structure of a eukaryotic cell passing through division; the nucleus has been dissolved and chromosomes are attached to the spindle by microtubules.

The act of division is the culmination of a series of events that have occurred since the last time a cell divided. The period between two mitotic divisions defines the somatic cell cycle. The time from the end of one mitosis to the start of the next is called interphase. The period of actual division, corresponding to the visible mitosis, is called M phase.


In order to divide, a eukaryotic somatic cell must double its mass and then apportion its components equally between the two daughter cells. Doubling of size is a continuous process, resulting from transcription and translation of the genes that code for the proteins constituting the particular cell phenotype. By contrast, reproduction of the genome occurs only during a specific period of DNA synthesis.




Figure 27.1 Overview: interphase is divided into the G1, S, and G2 periods. One cell cycle is separated from the next by mitosis (M). Cells may withdraw from the cycle into G0 or reenter from it.

Mitosis of a somatic cell generates two identical daughter cells, each bearing a diploid complement of chromosomes. Interphase is divided into periods that are defined by reference to the timing of DNA synthesis, as summarized in Figure 27.1:



  • Cells are released from mitosis into G1 phase, during which RNAs and proteins are synthesized, but there is no DNA replication.
  • The initiation of DNA replication marks the transition from G1 phase to the period of S phase. S phase is defined as lasting until all of the DNA has been replicated. During S phase, the total content of DNA increases from the diploid value of 2n to the fully replicated value of 4n.
  • The period from the end of S phase until mitosis is called G2 phase; during this period, the cell has two complete diploid sets of chromosomes.

(S phase was so called as the synthetic period when DNA is replicated, G1 and G2 standing for the two "gaps" in the cell cycle when there is no DNA synthesis. (Howard and Pelc, 1953))




Figure 27.2 Synthesis of RNA and proteins occurs continuously, but DNA synthesis occurs only in the discrete period of S phase. The units of mass are arbitrary.

The changes in cellular components are summarized in Figure 27.2. During interphase, there is little visible change in the appearance of the cell. The more or less continuous increase of RNA and protein contrasts with the discrete doubling of DNA. The nucleus increases in size predominantly during S phase, when proteins accumulate to match the production of DNA. Chromatin remains a compact mass in which no change of state is visible.


Mitosis segregates one diploid set of chromosomes to each daughter cell. Individual chromosomes become visible only during this period, when the nuclear envelope dissolves, and the cell is reorganized on a spindle. The mechanism for specific segregation of material applies only to chromosomes, and other components are apportioned essentially by the flow of cytoplasm into the two daughter cells. Virtually all synthetic activities come to a halt during mitosis.


In a cycling somatic animal cell, this sequence of events is repeated every 18 V24 hours. Figure 27.1 shows that G1 phase usually occupies the bulk of this period, varying from ~6 h in a fairly rapidly growing animal cell to ~12 h in a more slowly growing cell. The duration of S phase is determined by the length of time required to replicate all the genome, and a period of 6 V8 h is typical. G2 phase is usually the shortest part of interphase, possibly comprising the preparations for mitosis. M phase (mitosis) is a brief interlude in the cell cycle, usually <1 h in duration.



Research
Howard, A. and Pelc, S. (1953). Synthesis of DNA in normal and irradiated cells and its relation to chromosome breakage. Heredity Suppl. 6, 261-273.



Genes VII
Genes VII
ISBN: B000R0CSVM
EAN: N/A
Year: 2005
Pages: 382

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