Mitosis Lecture delivered at KMC IC On 6th October 2011
Format of lecture: • Anaphase • Telophase • Cytokinesis • Functions of non kinetochore microtubules. • Binary Fission • The Evolution of Mitosis • Cancer cells • Significance of Mitosis
Features of Anaphase • Aanaphase is the shortest stage of mitosis lasting only a few minutes. • It begins when two sister chromatids of each pair suddenly part.. • Centromere divides and chromatids move to their respective poles • Each chromatid appears as V-shaped structure.
Continued • The two liberated chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubules shorten. Because these microtubules are attached to centromere region, the chromosomes move centromere first. • The cell elongates as nonkinetochore microtubules lengthen. • By the end of anaphase, the two end of the cells have equivalent and complete collections of chromosomes
Corelating complete mitotic spindle with its function during anaphase • Anaphase commences suddenly when proteins holding together the sister chromatids of each chromosome are inactivated. • Once the chromatids become separate, full fledged chromosomes, they move toward opposite ends of the cell.
Howkinetochore microtubules function in poleward movement of chromosomes…. • Possibility one: Chromosomes are “reeled in” by microtubules that are shortening at the spindle poles. • Experimental evidence supported hypothesis: Primary mechanism of movement involves motor proteins on the kinetochore that “walk” a chromosome along the attached microtubules toward the nearest pole. • Meanwhile, the microtubules shorten by depolymerizing at their kinetochore ends.
Anaphase • Shortest phase
Features of Telophase • Two daughter nuclei begin to form in the cell. • Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the endomembrane cell. • The chromosomes become less condensed. • The chromosomes are at the poles in telophase and become less distinguishable as individual strands
Continued: • It is as long as prophase and is characterised by reappearance of nuclear membrane and nucleolus • It is some sort of a reversal of prophase • Mitosis, the division of nucleus into two genetically identical nuclei, is now complete.
Features of Cytokinesis: • It is the division of the cytoplasm which begins in late anaphase. • The division of cytoplasm is usually well underway by late Telophase, so the two daughter cells appear shortly after the end of mitosis. • A contractile belt is formed around the equator which contracts more and more finally separating a cell into two daughter cells • In animal cells, cytokinesis involves the formation of a cleavage furrow which pinches the cell into two.
Cytokinesis: A closer look • In animal cells, cytokinesis occurs by a process known as cleavage. • First sign of cleavage is the appearance of cleavage furrow- a shallow groove in the cell surface near the old metaphase plate. • On the cytoplasmic side, the furrow is a contractile ring of actin microfilaments associated with molecules of the protein myosin.
Continued • The actin microfilaments interact with myosin molecules causing the ring to contract. • The cleavage furrow deepens until the parent cell divides into two complete cells, each with its own nucleus and share of cytosol and organelles.
Continued • Cytokinesis in plant cells, which have cell walls is markedly different. • There is no cleavage furrow. Instead, during telophase, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a cell plate. • Cell wall materials carried in the vesicles collect in the cell plate as it grows.
Continued • The cell plate enlarges until its surrounding membranes fuses with the plasma membrane along the perimeter of the cell. • Two daughter cells result each with its own plasma membrane. Meanwhile, a new cell wall arising from the contents of the cell plates has formed between the daughter cells.
Difference between cytokinesis in plant cells and cytokinesis in animal cells • In an animal cell cytokinesis occurs by cleavage, which divides the parent cell in two with a contractile ring of actin filaments. • In a plant cell , a cell plate forms in the middle of the cell and grows until its membrane fuses with the plasma membrane of the parent cell. • A new cell wall is also produced from cell plate resulting in 2 daughter cell.
Function of non-kinetochore microtubules • In a dividing animal cell these microtubules are responsible for elongating the whole cell during anaphase. • Non kinetochore microtubules overlap extensively during metaphase. • During anaphase the region of overlap is reduced as motor protein attached to microtubules. • As the microtubules push apart from each other, their spindle poles are pushed apart, elongating the cell.
Continued • At the end of anaphase duplicate groups of chromosomes have arrived at opposite ends of the elongated parent cell. • Nuclei re-form during telophase. • Cytokinesis generally begins during these later stages of mitosis and the spindle eventually disassembles.
Metaphase plates At metaphase, the centromeres of all duplicated chromosomes are on a plane between the spindle’s two poles. This imaginary plane is called the metaphase plate of the cell.
Binary Fission • Prokaryotes reproduce by a type of cell division called binary fission.(meaning literally: division in half) • Most bacterial genes are carried on a single bacterial chromosome that consists of a circular DNA molecule and associated proteins. • The problem of replicating genomes in an orderly fashion and distributing the copies equally to two daughter cells is still formidable. Eg. Chromosome of bacterium Escherichia coli when fully stretched is 500 times longer than the cell.
continued • In E. coli the process of cell division begins when the DNA of the bacterial chromosome begins to replicate at a specific place on the chromosome called origin of replication producing two origins. • As the chromosomes continue to replicate, one origin moves rapidly toward the opposite end of the cell. • While the chromosome is replicating, the cell elongates.
continued • When replication is complete and the bacterium has reached about twice its initial size the plasma membrane grows inward dividing the parent E. coli cell into two daughter cells. • Each cell inherits a complete genome.
Evolution of mitosis • Hypothesis: mitosis had its origin in simpler bacterial mechanisms of cell reproduction. • Evidences: prokaryotes preceded eukaryotes on earth by billions of years. Some of the proteins involved in bacterial binary fission are related to eukaryotic proteins. • Recent research shows two of the proteins involved in binary fission are related to eukaryotic tubulin and actin proteins.
Continued • As eukaryotes evolved, along with their larger genomes and nuclear envelopes, the ancestral process of binary fission somehow gave rise to mitosis- hypothesis for step wise evolution of mitosis. • Two examples of nuclear division where ancestral mechanisms have remained relatively unchanged over evolutionary time: • Dinoflagellates- replicated chromosomes are attached to nuclear envelope and separate as the nucleus elongates prior to cell division.
Continued 2. Diatoms- a spindle within the nucleus separates the chromosomes. In most eukaryotic cells, the nuclear envelope breaks down and a spindle separates the chromosomes. • In both cases the nuclear envelope remains intact.
Concepts on Cancer cells • Cancer cells elude normal regulations and divide out of control, forming tumors. • Malignant tumors invade surrounding tissues and can metasize, exporting cancer cells to other parts of the body where they may form secondary tumors.
Significance of mitosis • The two daughter cells formed by mitosis possess the same number of chromosomes as the parent cell. • Due to mitosis, all cells of living body possess equal number of chromosomes. • As mitosis takes place in somatic cells (all body cells except the reproductive cells) , it has significance of growth in the body.