Cell divisionmitosis

Muthu bharathiI Msc., Biotechnology

• Cell cycle M phase

Inter phaseM phase

MitosisCytokinesis

InterphaseG1, S phase( DNA synthesis),G2 phases

DNA synthesis was first proposed by alma howard,Stephen pelc in 1953

Check points• Crucial protective response• Leland hartwell,Ted weinert 1988• As a part of cell cycle• Surveillence mechanism that halt the cell cycle when• 1. Any of the chromosomalDNA is damaged• 2.Certain critical processes such as DNAreplication during S phase or

chromosomal alignment during M phase do not occur properly• Several proteins of checkpoint machinery are work during abnormal

condition that are not taking role in normal cell cycle• Check points activated throughout the cell cycle• Check point proteins first identified in yeast cells

Importance of check point• If the DNA is damaged beyond repair, the checkpoint mechanism can

transmit a signal that leads either to • (1) the death of the cell or • (2) its conversion to a state of permanent cell cycle arrest (known as

senescence).

Mitosis • Mitosis ( Greek) thread • Term coined by walther flemming 1882• Thread represents chromosome• Process of nuclear division• DNA molecule of each chromosome segregated into two nuclei• maintains the chromosome number and generates new cells• Cytokinesis– splitting of dividing cell

partitioning of cytoplasm into two cellular packages• possess a genetic content identical to each other and to

the mother cell from which they arose.

• Mitosis can take place in either haploid or diploid cells. • Haploid mitotic cells are found in fungi, plant gametophytes, and a few

animals (including male bees known as drones).

• Mitosis is a stage of the cell cycle when the cell devotes virtually all of its energy to a single activity—chromosome segregation.

PROPHASEduplicated chromosomes are segregated Mitotic machinery is assembled

Chromosomal compaction (condensation) occurs in early prophaseConverting chromosome into shorter, thicker molecule for segregationBecause, interphase chromosomes are responsible for transcription

and replication and not suitable for segregationProtein for chromosome condensation is condensin

• Chromatin of interphase is fiber like structure nearly 30 nm in dia

• Chromosome scaffold occur in interphase stage• Histone proteins are removed• Loops formed by non histone proteins• chromosome scaffold are dispersed within the nucleus, possibly forming part of the nuclear matrix cohesion • Form ring around sister chromosome• interphase replication G2 phase cohesion cohesion mitotic phase

Centromere & kinetochore• Centromere - repeated DNA sequence ,act as a binding site for specific

proteins• Kinetochore – proteinaceous, button like structure, outer surface of

centromere at each chromatid• Assemble during prophase

(1) the site of attachment of the chromosome to the dynamic microtubules of the mitotic spindle

(2) the residence of several motor proteins involved in chromosome motility (3) a key component in the signaling pathway of an important mitotic checkpoint

Kinetochore protein•Ringshaped protein complex called Dam1 whose inner

diameter of 32 nm is large enough to comfortably surround a microtubule

• Floating grip• Maintain the Attachment of kinetochore to microtubule• 2 motor proteins Ndc80, CENP-E FORMATION OF THE MITOTIC SPINDLE ( bundle of microtubule)

Centrosome cycle occur ( separation of right angled centrosome by enzyme seperase) ie., they lose their close associationDuplication of centrosome occur prior to mitosis (G1 –S phase)Which become a Pair of mother – daughter centrosome by Cdk2 proteins

Step1 - sunburst arrangement / aster around each centrosome (early prophase)Step2 – separation and movement of centrosome towards opposite pole of cellStpe3 – stretching of microtubule and increase in numberBipolar spindle

The Dissolution of the Nuclear Envelope and Partitioning of Cytoplasmic Organelles• Interaction between the spindle and chromosomes is made possible

by the breakdown of the nuclear envelope at the end of prophase• Major components of the nuclear envelope—the nuclear pore complexes, nuclear lamina, and nuclear membranes which are initiated by phosphorylation of mitotic kinases cyclin B-Cdk1 Dynein molecules are responsible for torning nuclear pore in nuclear envelope

Prometaphase• Mitotic spindle assembly completed• Chromosomes scattered in Center of cell• Free (plus) ends of the microtubules are seen to grow and shrink in a

dynamic fashion • Microtubule catches kinetochorecongression chromosome toward the center of mitotic spindle, midway between the poles

Microtubule behaviour• When attached to kinetochore, elongated microtubule become

shortened whereas the shortened tubules become elongated to the sister kinetochore• This is due to the pulling force ( tension )• Spindle fiber – each kinetochore attached

to 20-30 microtubule

Metaphase • 3 groups of microtubule in metaphase stage• Astral• Chromosomal • polar

• Astral microtubuleradiate outward from centrosomehelp position of spindle apparatus in the celldetermine the plan of cytokinesis

Chromosomal microtubuleextend between centrosome and kinetochorepulling force is due to spindle fiber

Polar microtubuleextend from centrosome to chromosomemicrotubule of opposite centrosome may overlapform a structural basketmaintain the mechanical integrity of spindle

Microtubule Flux in the Metaphase Spindle• Microtubules are in dynamic state

Anaphase • Sister chromatids split apart• Move towards opposite polesAPC Cdc20 activated prior to metaphase ubiquitinates anaphase Inhibitor protein ,securin ( secure the attachment of sister chromatids)Activate seperase cleave cohesion(hold sister chromatids)

Thus anaphase starts

Complexes of anaphase• SCF & APC – add ubiquitinin to protein at different stages of cell cycle,

targeting them for destruction by proteasomes • SCF activates during interphase• APC ( Anaphase promoting complex) activates during metaphase

contains dozen of core proteinsadapter protein Cdc20, Cdh1determining substrate selection of APCend of mitosis Cdc20 inactivatedCdh1 take part for substrate selection of APC

• SAC – spindle assembly checkpoint, prevents the binding of APC till metaphase

• APCCdh1 activity during early G1 helps maintain the low cyclin–Cdk activity

Events of anaphase• Movement of chromosomes to opposite poles• They move slowly (1µm for 1 minute)• Metaphase (subunits added to plus end, keeping the length constant)• Anaphase (subunits lost from plus end, shortening of chromosomal length)• Anaphase A – movement of chromosome toward poles• Anaphase B – movement of spindle fibres

for elongation of microtubule, subunits are added to +end

Forces for anaphase• Depolymerization of microtubule

occur at both +, - ends + end “chew up chromosome”- end “transport of chromosome towards poles”

Depolymerizing kinesin protein,present at both +,- ends

• “ATPdependent, kinesin-mediated depolymerization forms the basis for chromosome segregation during mitosis.”

Spindle assembly checkpoints(SAC)• Operates at transition between metaphase and anaphase• Reveal when chromosome fails to align properly• There may be delay for onset of anaphase