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3.2 Mitosis and cytokinesis
3.2
Mitosis and cytokinesis
KEY IDEAS
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
Eukaryotic cells mostly exist in the first gap phase (G1) of interphase. During this phase, the cells undergo everyday metabolic activities. G1 phase
apoptosis
programmed cell The G1 phase is usually the longest phase of the cell cycle. During the G1 phase, the cell death becomes larger and synthesises the proteins and organelles that it needs to stay alive. How chromatin long a cell stays in this phase depends on the type of cell. Some cells can be forced to remain the DNA and proteins in this phase if they are deprived of nutrients or resources. When this occurs, the cell is that can be wound described as entering the G0 phase. tightly to form a chromosome 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. 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. semi-conservative replication Each single strand of the DNA molecule is then used to form a new complementary a process where each strand of DNA, which results in two daughter molecules. Each daughter molecule contains strand of the previous one strand from the old DNA molecule and one new strand. This is known as semiDNA is used to form a complementary conservative replication (half the molecule is conserved from the old molecule) (Figure 2). new strand This effectively doubles all DNA from the G1 phase.
In this topic, you will learn that: ✚ 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. Study tip 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. G1 checkpoint 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 the phase cells undergo in their everyday lifecycles quiescent in a period of rest or dormancy DRAFT ONLY - NOT FOR SALE
Double helix
Single strand
Each daughter molecule of DNA winds itself around proteins called histones to prevent any tangling of the now double amounts of DNA. The two daughter molecules remain centromere connected at a central point called a centromere . The cell is now ready to begin the Gap 2 the centre of a chromosome (G2) phase of interphase.
FIGURE 1 During DNA replication, the double-stranded helix unwinds and separates into single strands. FIGURE 2 DNA replication is described as semi-conservative. In each daughter molecule, half the DNA comes from the original molecule and half is newly formed.
New strandOriginal strand G2 phase This final stage of interphase is when the cell starts rapidly growing and synthesising proteins for the start of mitosis. This includes proteins responsible for spindle formation. Not all cells undergo G2 phase; instead they move straight from S phase to mitosis. As in G1, there is an important checkpoint – the G2 checkpoint . The DNA molecules must be checked to make sure the synthesis of the new strands was correct. Any mistakes need to be repaired or the cell will apoptose (die). If the checkpoint is cleared, the cell G2 checkpoint the point in the cell cycle when a cell is assessed to see if the new DNA strands DRAFT ONLY - NOT FOR SALE were made correctly starts mitosis.
Mitosis
Mitosis is the organised division of the nucleus of the cell. There are four stages in mitosis: prophase, metaphase, anaphase and telophase (Figure 3).
S y n t h e s is IN E R PHASE Mitosis
Checkpoint Daughter cells Gap 1 Gap 2 S G1 G2 Relative time taken M PROPHASE METAPHASE ANAPHASE TELOPHASE CYTOKINESIS FIGURE 3 The cell cycle. Interphase is the period between two episodes of mitosis. Prophase In prophase , chromatin continues to wind into a condensed form of DNA called a bivalent chromosome . Each chromosome has a distinctive ‘H’ shape in which two parallel ‘sister’ chromatids are joined at the centromere (Figure 4).
prophase
a stage of mitosis when DNA coils into chromosomes
bivalent chromosome
a pair of chromatids connected by a centromere
chromatid
one strand of a bivalent chromosome
Sister chromatids
Centromere Study tip In metaphase, chromosomes are linked down the middle of a T DRAFT ONLY - NOT FOR SALE cell. In anaphase,Sister chromosomes are chromatids moving away from each other.
FIGURE 4 A bivalent chromosome is made up of two sister chromatids joined at the centromere.
spindle
thread-like proteins that attach to the centromere of chromosomes in order to move them during mitosis
While the DNA molecule is condensing into a chromosome, the nuclear membrane breaks down and a number of protein threads spread from one end of the cell to the other. These protein threads link together at each cell pole to form a spindle (Figure 6).
Metaphase
During metaphase, the newly formed chromosomes migrate to the centre (equator) of the Study tip cell. One of the protein threads that form the spindle then attaches to the centromere of each chromosome. At this point, checkpoint M checks that the spindle fibre is correctly attached Anaphase and telophase are the to each centromere before the cell progresses to the next stage. only phases when the number of Anaphase chromosomes in a human cell is 92. During anaphase, the sister chromatids of each bivalent chromosome separate at the In all of the other centromere. This breaks apart the distinctive ‘H’ shape into two individual (identical) DNA phases, the bivalent molecules and each chromatid becomes a single, independent chromosome. The spindle chromosome contains two chromatids. fibres pull each identical chromosome to opposite poles of the cell. This occurs for each of the 46 chromosomes in a human cell. The result is 46 individual chromosomes at each pole of metaphase the cell. a stage of mitosis when chromosomes Telophase move to the centre of the cell After the 46 chromosomes reach each end of the cell, they gather together, and a new nuclear membrane is formed and the spindle fibres break down. Once protected by the nuclear checkpoint M membrane, the chromosomes unravel to form chromatin. The new daughter nuclei are the point in the cell cycle just before genetically identical to each other. anaphase when all sister chromatids are checked that they are Cytokinesis attached correctly to the spindle During the end of mitosis, cytokinesis begins. In animal cells, this involves a constriction microtubules at the centre of the cell called cleavage. This is similar to a belt tightening around a waist. Eventually the membrane meets in the middle (cleaves) and forms two genetically identical anaphase daughter cells.a stage of mitosis when the In plant cells, cytokinesis involves the formation of the beginning of a new plasma chromosomes membrane and cell wall between the new nuclei. This cell plate spreads to the outside of the separate at the centromere and move cell until the two new daughter cells are separated from each other. to opposite sides of the cell cytokinesis a c d e a stage in mitosis and meiosis when the cell divides into two daughter cells
cleavage
the process of splitting of a cell to form two daughter Interphase Metaphase Anaphase Telophase Cytokinesis cells FIGURE 5 These animal cells are undergoing mitosis. They have been stained with a fluorescent stain to show the separation of DNA. a The cell is at the end of interphase. b The blue chromosomes line up along the middle of the cell and attach to the yellow spindle during metaphase. c Yellow spindles are contracting and separating two chromatids at the centromere during anaphase. d The nuclear membrane re-forms around the two sets of DNA in telophase. e Cytokinesis occurs when the plasma membrane divides in two.
b DRAFT ONLY - NOT FOR SALE
Centrioles
Chromosomes Mitotic spindle Prophase
a Interphase Daughter cells b Metaphase
e Cytokinesis
c Anaphase d Telophase Late anaphase
Nucleolus Nucleus Chromatin
Early prophase DRAFT ONLY - NOT FOR SALE
FIGURE 6 The phases of mitosis and cytokinesis occurring in an animal cell (diagrams) and a plant cell (electron micrographs)
CASE STUDY 3.2
Variations in the cell cycle
Although cytokinesis normally follows mitosis, there are exceptions. For example, the formation of skeletal muscle fibres in vertebrate animals results from mitosis without cytokinesis. These cells often have many nuclei in a single cell (Figure 7). Other cells constantly go through the cell cycle of growth, mitosis and cytokinesis for their entire existence. For example, red blood cells do not have a nucleus and live for about 120 days. Bone marrow cells need to constantly divide so that there are enough red blood cells in the body. A third category of cells only divides to produce the exact number of cells required for growth and replacement. For example, human liver cells rarely divide after the liver has reached a certain size. If some of the liver is surgically removed, this triggers the existing cells to divide until the liver regains its original size.
Nuclei FIGURE 7 Skeletal muscle cells often have many nuclei.
Describe and explain
1 Describe each stage of mitosis. 2 Explain the difference between the S phase and
G2 phase in interphase. 3 Describe the three different checkpoints that ensure that the cell cycle progresses correctly. 4 Identify the phase of mitosis shown in Figure 8.
CHECK YOUR LEARNING 3.2 Apply, analyse and compare
5 Compare cytokinesis in a plant cell and an animal cell. 6 Read Case study 3.2 and compare the process of mitosis and cytokinesis in muscle fibres to the process in standard eukaryotic cells.
Design and discuss
7 Design a picture showing each stage of mitosis.
Discuss what occurs at each stage. 8 A student claimed that a cell at metaphase and anaphase was the easiest to identify. Suggest why prophase might be more difficult to identify than metaphase and anaphase. FIGURE 8 A stage of mitosis
