Mitosis and cytokinesis
3.2
KEY IDEAS In this topic, you will learn that: ✚
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The eukaryotic cells that make up multicellular organisms must reproduce over the lifetime of the organism. Reproduction occurs through mitosis and cytokinesis, which must be very organised to prevent damage. There are many chromosomes in a nucleus that can easily become tangled or broken during replication. When somatic cells undergo division, the daughter cells must contain the same number of undamaged chromosomes as the parent cell. Between each cycle of replication, the somatic cell must perform the normal functions that are required by the organism. This phase of normal functioning is called interphase.
interphase
the point in the cell cycle when a cell is assessed for the health of its organelles so it can proceed to the G2 phase
Interphase
in a period of rest or dormancy
G1 phase
apoptosis
The G1 phase is usually the longest phase of the cell cycle. During the G1 phase, the cell becomes larger and synthesises the proteins and organelles that it needs to stay alive. How long a cell stays in this phase depends on the type of cell. Some cells can be forced to remain in this phase if they are deprived of nutrients or resources. When this occurs, the cell is described as entering the G0 phase. Before a cell can leave the G1 phase, the number and health of the organelles and the structure and function of the DNA are carefully checked. If a cell fails at the G1 checkpoint, a protein called p53 is produced in large quantities. This forces a cell to become quiescent (dormant) while the DNA is repaired or, if the cell cannot be repaired, undergo a process of organised cell death (apoptosis). If the cell passes the G1 checkpoint, it will be able to replicate its DNA.
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programmed cell death
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Eukaryotic cells mostly exist in the first gap phase (G1) of interphase. During this phase, the cells undergo everyday metabolic activities.
quiescent
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chromatin
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G1 checkpoint
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the phase cells undergo in their everyday lifecycles
the DNA and proteins that can be wound tightly to form a chromosome
the reproduction of eukaryotic cells occurs through mitosis and cytokinesis interphase has three phases – G1 (normal cell growth and function), S (DNA synthesis) and G2 (rapid cell growth and protein synthesis) mitosis has four sub-phases – prophase, metaphase, anaphase and telophase cytokinesis is the physical separation of a parent cell into two daughter cells.
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Interphase used to be called the resting stage of the cell cycle. However, the cell is not resting but busy making proteins and undergoing chemical reactions for an organism’s metabolism.
semi-conservative replication a process where each strand of the previous DNA is used to form a complementary new strand
OXFORD UNIVERSITY PRESS
S phase During the S phase of interphase, the cell replicates the genetic material (DNA). A human somatic cell has 46 double-helix molecules of DNA called chromatin. Before a cell can replicate, it must copy each DNA molecule. This occurs when the double-stranded DNA helix unwinds and then separates into single strands. Each single strand of the DNA molecule is then used to form a new complementary strand of DNA, which results in two daughter molecules. Each daughter molecule contains one strand from the old DNA molecule and one new strand. This is known as semiconservative replication (half the molecule is conserved from the old molecule) (Figure 2). This effectively doubles all DNA from the G1 phase. CHAPTER 3 THE CELL CYCLE
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