Cell cycle with reference of mitosis Cell cycle

Cell cycle with reference of mitosis

Cell cycle is the series of events from time a cell is produced until it completes mitosis & produce new cells Cell cycle consists of two major phases: § Interphase § Mitotic phase(M phase)

Mitotic phase § Short period of cell cycle § Alternates with longer interphase, where cell prepare itself for division Interphase § Time where cell metabolic activity is very high Divided into three phases • G 1 phase • S phase • G 2 phase

G 1 phase § Cell starts its cell cycle in g 1 phase § Cell increases its supply of protein § Increases number of its organelles § Grows in size § This phase is also marked by synthesis of various enzymes that required in next phase

S phase § Cell duplicate its chromosomes § Each chromosomes consists of two sister chromatids G 2 phase § Cell prepare proteins that are essential for mitosis, for production of spindle fibre § After G 2 phase, cell enter the division phase(M phase) § In this cell divides into two daughter cells

Mitosis § In 1980 s, a German biologist Walter Flemming abserved that in a dividing cell, nucleus passes through series of changes which he called it mitosis § Mitosis is type of cell division in which cell divide into two daughter cells, each with same number of chromosomes as present in parent cell

§ Mitosis occur in eukaryotic cells § In multicellular organisms, the sometic cells undergo mitosis Phases of mitosis • Karyokinesis(Division of nucleus) • Cytokinesis(Division of cytoplasm)

Karyokinesis Division of nucleus is divided into 4 phases: § Prophase § Metaphase § Anaphase § Telophase

Prophase § Genetic material in nucleus is in a loose thread like form called chromatin § Chromatin condense into highly ordered structures called chromosomes § Chromosomes are made up of two sister chromatids, bound together at the same centromere § Each chromosomes has kinetochore at centromere

§ Kinetochore is complex protein structure that is point where spindle fibers attach § Two centrioles close to nucleus § Each centrioles duplicate & two daughter centrosomes are formed § Centrosomes migrate to opposite poles, give rise to microtubules § Microtubules formed are called spindle fibers

Metaphase § When spindle fibers grown to sufficient length, some spindle fibers known as kinetochore fibers § Two kinetochore fibers from opposite poles attach with each chromosomes § Chromosomes arrange themselves along aquator of cell forming a metaphase plate

Anaphase § Kinetochore spindle fiber connects with kinetochore of chromosomes, it starts to pull toward originating centrosomes § Pulling force divide centrosome’s sister chromatids & they separate § Sister chromatids are now sister chromosomes § They are pulled apart toward the respective centrosomes

§ The other spindle fiber also elongate § At the end of anaphase, cell has succeeded in separating identical copies of chromosomes into two groups at the opposite poles

Telophase § It is a reversal of prophase § A new nuclear envelope forms around each set of separated chromosomes § Both set of chromosomes, now surrounded by new nuclear envelopes, unfold back into chromatin § Nuclear division is completed

Cytokinesis § It is the division of cytoplasm § In animal cells, cytokinesis occurred by a process known as cleavage § A cleavage furrow develops where metaphase plate used to be § The furrow deepens & eventually pinches the parent cell into two daughter cells

Cytokinesis in plant cells § Cytokinesis in plant cells occurs differently § Vesicles derived from the Golgi apparatus move to middle of cell § They fuse to form a membrane-bounded disc called cell plate or phargmoplast § The plate grows outward & more vesicles fuse with it

§ Finally membranes of cell plate fuse with plasma membrane & its content join with the parental cell wall § The result is two daughter cells, each bounded by its own plasma membrane & cell wall

Reasons for mitosis There are three main reasons: • Growth • Repair/healing • Asexual reproduction

Characteristics of mitosis § Chromosomes number remains the same § The DNA remains identicllay the same § One cell(N) give rise to two cells(2 N)

Significance of Mitosis § Importance of mitosis is the maintenance of chromosomal set § For example: each daughter cell receives chromosomes that are alike in composition & equall in number to chromosomes of parent cell

Development & growth § The number of cells within an organism increase by mitosis § This is the basis of development of a multicellular body from a single cell § For example: zygote is also the basis of growth of multicellular body

Cell replacement § In some parts of body, e. g. skin & digestive tract, cells are constantly sloughed off & replaced by new ones § New cells are formed by mitosis § They are exact copies of cells being replaced § Similarly, red blood cells have short life span § New red blood cells have are formed by mitosis

Regeneration § Some organisms can regenerate parts of their bodies § The production of new cells is achieved by mitosis § For example: sea star regenerates its lost arm through mitosis

Asexual reproduction Some organisms produce genetically similar offspring through asexual reproduction Mitosis is a means of asexual reproduction For example: Hydra reproduce asexualy by budding The cells at the surface of hydra undergo mitosis & form a mass called bud

§ Mitosis continues in the cells of bud & it grows into a new individual § The same division happens during asexual reproduction

Errors in Mitosis § Errors in control of mitosis may cause cancer § All cells have genes that control timing & number of mitosis § Mutations occur in such genes & cells continue to divide § It results in growth of abnormal cells called tumors

§ As long as these tumors remains in their original location, they are called benign tumors § If they invade other tissues, they are called malignant(cancerous)tumors § Their cells are called cencer cells § Such tumors scan send cancer cells to other parts in body where new tumors may form § This phenomenon is called metastasis(spreading of disease)