MitosisAND Cell DiVISION

Cell Division• Characteristic of living things: ability to reproduce

their own kind.

• Cell division purpose:• When unicellular organisms such as amoeba divide

to form offspring –reproduction• The process of creating a multicellular organism

from a single celled zygote – growth• The replace existing cells that die from normal wear

and tear - repair

Cell Cycle• From the time a cell is first formed from a dividing

parent cell

• Until its own division into two cells.

• The purpose is to pass identical genetic information to cellular offspring.

DNA• The entire amount of DNA within a cell is called its’

genome.

• Prokaryotes – single long DNA molecule

• Eukaryotes – many linear DNA molecules

• Before cell can divide to form genetically identical daughter cells, all of this DNA must be copied, and separated so that each daughter cell ends up with a genome.

• DNA is packaged into chromosomes.

• Every eukaryotic species has a characteristic number of chromosomes.

• Ex: nuclei of human somatic (all body cells except reproductive cells) cells contain 46 chromosomes, made of 2 sets of 23 pairs, one set inherited from each parent.

• Eukaryotic chromosomes are made of chromatin, which is a combination of DNA and proteins.

• Each single chromosome contains one very long molecule of DNA.

• It carries several hundred to a few thousand genes

• When a cell is carrying out normal activities and duplicating its DNA it is in the form of chromatin

• After DNA duplicates, and a cell prepares for division the chromatin fiber becomes condensed to form chromosomes that can be seen under a microscope.

Sister chromatids

• Each duplicated chromosome has two sister chromatids.

• The two chromatids each contain an identical DNA molecule.

• The duplicated chromosome has a narrowing called the centromere where the chromatids are attached.

• Mitosis is the division of the nucleus, it is followed by cytokinesis, the division of the cytoplasm.

• Mitosis is only one small part of the cell cycle. (mitotic M phase)

• Cell division alternates with a much longer stage called interphase. (90% of the cell cycle)

• During interphase a cell grows and copies its chromosomes in preparation for cell division.

• Interphase has subphases:• G1 phase • S phase – when DNA is replicated• G2 phase

• During all three, the cell grows, and makes more organelles, but it only duplicates DNA during S phase.

• Mitosis is broken down into five stages:• Prophase, prometaphase (early prophase, late

prophase), metaphase, anaphase, and telophase.

Interphase (G2)• Notice:

• A nuclear membrane envelopes the nucleus.

• The nucleus contains one or more nucleoli.

• Two centrosomes have formed (from one)

• Chromosomes are duplicated, but cannot be seen individually.

Prophase (early prophase)• Chromatin fibers become

more tightly coiled.

• Nucleoli disappear.

• The mitotic spindle begins to form – composed of centrosomes and microtubules that extend from them

• Centrosomes move away from each other

Prometaphase (late prophase)

• The nuclear membrane breaks up.

• Microtubules extending from centrosomes reach into nucleus

• Some microtubules begin attaching to centromere of sister chromatids.

Metaphase• The centrosomes are now at

opposite poles of the cell.

• The chromosomes align at the metaphase plate.

• For each chromosome, the sister chromatids are attached to microtubules from opposite poles.

Anaphase• Anaphase begins when the

centromeres split, allowing the two sister chromatids to separate into single chromosomes.

• The daughter chromosomes begin moving to opposite poles of the cell, as the microtubules shorten.

• The cell elongates.

Telophase • Two daughter nuclei form in the

cell.

• Nuclear membranes reform.

• Nucleoli reappear

• Chromosomes become less condensed

• Mitosis is complete

Cytokinesis in Animal Cells• The division of cytoplasm begins in

telophase.

• In animal cells cleavage occurs• A cleavage furrow (shallow groove)

occurs near the middle of the cell.• The furrow has a contractile ring

that slowly contracts smaller until the parent cell is pinched in two.

Cytokinesis in Plant Cells• Instead of a cleavage furrow, during telophase vesicles

from the Golgi apparatus move to the middle of the cell.

• The vesicles group together to form a cell plate.

• Cell wall materials carried in the vesicles collect in the cell plate and it grows.

• Eventually the membrane fuses with the cell membrane, forming daughter cells.