MITOSIS DR. A. TARAB DEPT. OF BIOCHEMISTRY HKMU

Transcription

1 MITOSIS DR. A. TARAB DEPT. OF BIOCHEMISTRY HKMU

2 Mitosis is the process by which a cell duplicates In mitosis, chromosomes in the cell nucleus are separated into two identical sets of chromosomes, each in its own nucleus It is a form of nuclear division Mitosis generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components

3 Mitosis divides the chromosomes in a cell nucleus

4 Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell This accounts for approximately 20% of the cell cycle Mitosis occurs only in eukaryotic cells and the process varies in different species

5 For example, animals undergo an open mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae (yeast) undergo a closed mitosis, where chromosomes divide within an intact cell nucleus Prokaryotic cells, which lack a nucleus, divide by a process called binary fission

6 The process of mitosis is fast and highly complex The sequence of events is divided into stages corresponding to the completion of one set of activities and the start of the next These stages are prophase, prometaphase, metaphase, anaphase and telophase During mitosis the pairs of chromatids condense and attach to fibers that pull the sister chromotids to opposite sides of the cell

7 The cell then divides in cytokinesis, to produce two daughter cells Because cytokinesis often occurs in conjunction with mitosis, mitosis is often used interchangeably with mitotic phase However, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei

8 This occurs most notably among the fungi and slime molds, but is found in various groups Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development Errors in mitosis can either kill a cell through apoptosis or cause mutations Certain types of cancer can arise from such mutations

9 Phases of mitosis Prophase: Normally, the genetic material in the nucleus is in a loosely bundled coil called chromatin At the onset of prophase, chromatin fibers become tightly coiled, condensing into discrete chromosomes Since the genetic material has already been duplicated earlier in S phase, the replicated chromosomes have two sister chromatids, bound together at the centromere by the cohesin protein complex

10 Chromosomal components 1. Chromatid 2. Centromere (part of chromosome that links sister chromatids) 3. Short arm 4. Long arm

11 Chromosomes are typically visible at high magnification through a light microscope Also inside the nucleus, the nucleolus in the nucleus disappears from view This is noteworthy because the cell does not need to divide the nucleolus right away It will later reform when the nucleus divides completely

12 Close to the nucleus are structures called centrosomes, consisting of a pair of centrioles and actin, a halo of microtubule fragments, centrioles are found in most eukaryotic animal cells The centrosome is the coordinating centre for the cell s microtubules A cell inherits a single centrosome at cell division, which is replicated by the cell with the help of the nucleus before a new mitosis begins, giving a pair of centrosomes

13 The structure of the centrosome Organelle that serves as the main microtubule organizing centre of the animal cell

14 The two centrosomes nucleate microtubules (which may be thought of as cellular ropes or poles) to form the spindle by polymerizing soluble tubulin Molecular motor proteins then push the centrosomes along these microtubules to opposite sides of the cell

15 Although centrioles help organize microtubule assembly, they are not essential for the formation of the spindle, since they are absent from plants, and centrosomes are not always used in mitosis

16 Prophase The two round objects above the nucleus are the centrosomes The chromatin is condensing into chromosomes

17 Prometaphase During early prometaphase, the nuclear membrane disintegrates and the microtubules invade the nuclear space This is called open mitosis, and it occurs in most multicellular organisms Fungi and some protists, such as algae or trichomonads, undergo a variation called closed mitosis where the spindle forms inside the nucleus, or its microtubules are able to penetrate an intact nuclear membrane, which stays intact

18 In late prometaphase, each chromosome forms two kinetochores at its centromere, one attached at each chromatid A kinetochore is a complex protein structure that is analogous to a ring for the microtubule hook, it is the point where microtubules attach themselves to the chromosome Although the kinetochore structure and function are not fully understood, it is known that it contains some form of molecular motor

19 Image of a human cell showing microtubules in green, chromosomes (DNA) in blue and kinetochores in pink Kinetochere protein structure on chromatids where the spindle fibres attach during cell division to pull sister chromatids apart

20 When a microtubule connects with the kinetochore, the motor activates, using energy from ATP to crawl up the tube towards the originating centrosome This motor activity, coupled with polymerisation and depolymerisation of microtubules, provides the pulling force necessary to later separate the chromosome s two chromatids

21 When the spindle grows to sufficient length, kinetochore microtubules begin searching for kinetochore to attach to In the fishing pole analogy, the kinetochore would be the hook that catches a sister chromatid or fish The centrosome acts as the reel that draws in the spindle fibers or fishing line It is also one of the main phases of mitosis because without it cytokinesis would not be able to occur

22 Prometaphase The nuclear membrane disintegrates, and microtubules have invaded the nuclear space These microtubules can attach to kinetochores or they can interact with opposing microtubules

23 Structure and main features of a spindle apparatus

24 Structure of an eukaryotic chromosome

25 Metaphase After the microtubules have found and attached to the kinetochores in prometaphase, the two centrosomes start pulling the chromosomes through their attached centromeres towards the two ends of the cell As a result, the chromosomes come under longitudinal tension from the two ends of the cell

26 The centomeres of the chromosomes, in some sense, convene along the metaphase plate or equatorial plane, an imaginary line that is right in between the two centrosome poles This line is called the spindle equator This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochores, analogous to a tug-of-war between people of equal strength

27 In certain types of cells, chromosomes do not line up at the metaphase plate and instead move back and forth between the poles randomly, only roughly lining up along the midline

28 The chromosomes align at the metaphase plate Metaphase

29 A cell in late metaphase All chromosomes (blue) but one have arrived at the metaphase plate

30 Anaphase When every kinetochore is attached to a cluster of microtubules and the chromosomes have lined up along the metaphase plate, the cell proceeds to anaphase Two events occur: first the proteins that bind sister chromatids together are cleaved

31 These sister chromatids now become separate daughter chromosomes, and are pulled apart by shortening kinetochore microtubules and move towards the respective centrosomes to which they are attached The cleaved centromeres go first while the chromatids trail behind Next, the polar microtubules elongate, pulling the centrosomes (and the set of chromosomes to which they are attached) apart to opposite ends of the cell

32 The force that causes the centrosomes to move towards the ends of the cell is still unknown, although there is a theory that suggests that the rapid assembly and breakdown of microtubules may cause this movement At the end of anaphase the kinetochore microtubules all degrade

33 The chromosomes split and the kinetochore microtubules shorten Anaphase

34 Cell cycle progressing between prometaphase and anaphase

35 Telophase Telophase is a reversal of prophase and prometaphase events It cleans up the after effects of mitosis At telophase, the polar microtubules continue to lengthen, elongating the cell even more Corresponding daughter chromosomes attach at opposite ends of the cell

36 A new nuclear membrane, using the membrane vesicles of the parent cell s old nuclear membrane, forms around each set of separated daughter chromosomes (though the membrane does not enclose the centrosomes) The nucleoli reappear, too Both sets of chromosomes, now surrounded by new nuclei, begin to relax or decondense back into chromatin

37 Mitosis is complete, but cell division is not

38 Telophase The decondensing chromosomes are surrounded by nuclear membranes Cytokinesis has already begun, the pinched area is known as the cleavage furrow

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40 Cytokinesis Cytokinesis is often mistakenly thought to be the final part of telophase; however, cytokinesis is a separate process that begins at the same time as telophase Cytokinesis is technically not even a phase of mitosis, but rather a separate process, necessary for completing cell division In animal cells, a cleavage furrow (pinch) containing a contractile ring develops where the metaphase plate used to be, pinching off the separated nuclei

41 In both animal and plant cells, cell division is also driven by vesicles derived from the Golgi apparatus, which move along microtubules to the middle of the cell In plants this structure coalesces into a cell plate at the centre of the phragmoplast and develops into a cell wall, separating the two nuclei The phragmoplast is a microtubule structure typical for higher plants

42 Each daughter cell has a complete copy of the genome of its parent cell The end of cytokinesis marks the end of the M-phase

43 Anion (Allium) cells in different phases of the cell cycle, some in mitosis

44 Cilliate undergoing cytokinesis, with the cleavage furrow being clearly visible Cytokinesis

45 Significance Mitosis is important for maintenance of the chromosomal set, each cell formed receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell Mitosis occurs in the following circumstances: Development and growth The number of cells within an organism increases by mitosis

46 This is the basis of the development of a multicellular body from a single cell i.e., zygote and also the basis of the growth of a multicellular body Cell replacement In some parts of body e.g., skin and digestive tract, cells are constantly sloughed off and replaced by new ones

47 New cells are formed by mitosis and so are exact copies of the cells being replaced Similarly, red blood cells (RBCs) have short life span (only about 4 months) and new RBCs are formed by mitosis Regeneration Some organisms can regenerate body parts The production of new cells in such instances is achieved by mitosis

48 For example, starfish regenerate lost arms through mitosis; lizards regenerate lost tails by mitosis Asexual reproduction Some organisms produce genetically similar offspring through asexual reproduction For example, the hydra reproduces asexually by budding

49 The cells at the surface of hydra undergo mitosis and form a mass called a bud Mitosis continues in the cells of the bud and this grows into a new individual The same division happens during asexual reproduction or vegetative propagation in plants