Course Information

Mitosis Department of AGB Veterinary College Bangalore

Mitotic Cell Division Functions: • Growth, maintenance, repair of body tissues • Forms the basis of Asexual Reproduction

What is mitosis? • Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during prophase, prometaphase, metaphase, anaphase, and telophase. • Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle.

Mitosis • Somatic cells divide by mitosis – Involves 1 cell cycle / division

• Parental and 2 daughter cells are genetically identical • Parental cells are diploid (46 chromosomes) • 2 daughter cells are diploid (46 chromosomes)

Interphase The cell is engaged in metabolic activity and prepare for mitosis (the next four phases that lead up to and include nuclear division).

Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible.

Mitosis 4 phases: 1st – Prophase 2nd – Metaphase

3rd – Anaphase 4th – Telophase and Cytokinesis

Prophase • 3 major events i) chromosomes condense

ii) spindle fibers form iii) chromosomes are captured by spindle

Chromosomes Condense • Each chromosome consists of 2 sister chromatids attached to each other at the centromere

Mitotic Spindle Forms • Spindle fibers are specialized microtubules • Spindle fibers radiate out from centrioles, forming the “aster” • Centrioles occur in pairs, and are duplicated during interphase

• One pair of centrioles migrates to one pole of cell, the other pair migrates to opposite pole of cell chromatin

nucleus nucleolus

centrioles

condensing chromosomes

Spindle Captures Chromosomes • When spindle fibers are fully formed nuclear envelope disintegrates and nucleolus disappears

• Spindle fibers attach to chromosomes at the kinetochore, a structure located at the centromere

• Function of spindle fibers is to organize division of sister chromatids into daughter cells

chromatin

nucleus nucleolus

centrioles

condensing chromosomes

Prometaphase • The nuclear membrane dissolves, marking the beginning of prometaphase. Proteins attach to the centromeres creating the kinetochores. Microtubules attach at the kinetochores and the chromosomes begin moving.

Metaphase • Chromosomes align along equator of the cell, with one kinetochore facing each pole centrioles chromosomes

spindle fibers

Metaphase •Spindle fibers align the chromosomes along the middle of the cell nucleus.

•This line is referred to as the metaphase plate.

•This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.

Anaphase • Sister chromatids separate • Spindle fibers attached to kinetochores shorten and pull chromatids poleward

• Free spindle fibers lengthen and push poles of cell apart

Anaphase V-shaped chromatid

free spindle fibers

Anaphase • The paired chromosomes separate at the kinetochores and move to opposite sides of the cell. • Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules.

Telophase • Spindle fibers disintegrate • Nuclear envelopes form around both groups of chromosomes • Chromosomes revert to their extended state

• Nucleoli reappear

Telophase

Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei.

The chromosomes disperse and are no longer visible under the light microscope.

The spindle fibers disperse, and cytokinesis or the partitioning of the cell may also begin during this stage.

• Cytokinesis occurs, enclosing each daughter nucleus into a separate cell chromosomes decondensing pinching of cell membrane at equator nuclear envelope reforming nucleolus reappears

cytokinesis

Cytokinesis In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus.

In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.

Cytokinesis • Animal cells: – microfilaments attached to plasma membrane form a ring around equator of cell – ring contracts, like a drawstring, dividing the cytoplasm