L4 Cell Division

Cell Division: Mitosis & Meiosis

Cell Division

All new cells come from previously existing cells. New cells are formed by the process of cell division, which involves both replication of the cell's nucleus (karyokinesis) and division of the cytoplasm (cytokinesis).

Cell Division

Cell division is a process by which a parent cell divides into two or more daughter cells.

It is the process by which we grow and it is how our body repairs and renews itself.

Cell division is of three types: Amitosis Mitosis Meiosis

Binary Fission/ AmitosisBinary Fission/ Amitosis

Most cells reproduce through Most cells reproduce through some sort of Cell Divisionsome sort of Cell Division Prokaryotic cells divide through a simple form of division Prokaryotic cells divide through a simple form of division

calledcalled Binary FissionBinary Fission

3 step process:3 step process:

1. Single “naked” strand splits and forms a duplicate of 1. Single “naked” strand splits and forms a duplicate of itself.itself.

2. The two copies move to opposite sides of the cell2. The two copies move to opposite sides of the cell

3. Cell “pinches” into two new and identical cells called 3. Cell “pinches” into two new and identical cells called ""daughter cells". (Cell wall then forms if applicable) (Cell wall then forms if applicable)

Eukaryotic ChromosomeEukaryotic Chromosome

Mitotic SpindleMitotic Spindle

Cell Cycle Occurs between one cell

division & next. Three main stages –

interphase, mitosis & cytokinesis.

Interphase – encompasses G1, S & G2 phases is the period of synthesis & growth.

Mitosis – contains prophase, metaphase, anaphase & telophase.

Cytokinesis – division of cytoplasm.

Interphase

Encompass stages G1, S & G2

G1 – growth stage

S – synthesis stage

G2 – time between DNA

Synthesis & Mitosis

Each chromosome exists as a

pair of chromatids joined together

by a centromere.

Mitosis Cell Division Mitosis: Two daughter cells

form,

contain same Number of chromosomes as mother cell

Contains four phases: Prophase Metaphase Anaphase Telophase

Prophase

Nuclear membrane disintegrates Nuclelous disappears Chromosomes condense Chromatids joined at centromere Centrioles begin migration to

poles Mitotic spindle begins to form

between centrioles Kinetochores begin to mature

and attach to spindle

Metaphase

Kinetochores attach chromosomes to mitotic spindle

Chromosomes are align along the metaphase plate

Spindle fibers are now clearly visible

Anaphase

Kinetochore microtubules shorten Resulting in separation of

chromosomes to opposite poles Polar microtubules elongate, preparing

cell for cytokinesis

Telophase Nuclear envelope forms Nuclear envelope forms

around both sets of around both sets of chromosomeschromosomes

DNA uncoilsDNA uncoils Spindle & Astral fibers Spindle & Astral fibers

completely disappear completely disappear

CytokinesisCytokinesis

Cytokinesis happens with most (but not all) Cytokinesis happens with most (but not all) cellscells

Cytoplasm & organelles move (mostly equally) Cytoplasm & organelles move (mostly equally) to either side of the cell. Cell Membrane to either side of the cell. Cell Membrane “pinches” to form 2 separate cells“pinches” to form 2 separate cells

Animal Cytokeneisis

Cytokenesis differs significantly between Animal & Plant cells.

With animals, the membranes pinch together to form a cleavage furrow, which eventually fuses to form two daughter cells

Plant Cytokinesis

With Plants, a cell wall must be formed between the 2 daughter cells.

Vesicles containing Cellulose form and fuse between the two daughter cells, eventually forming a complete cell wall.

Significance of Mitosis

Genetic stability (In mitosis, the two cells will each be genetically the same. )

Growth Cell replacement Regeneration Asexual reproduction

Mitosis produces genetically identical cells whereas meiosis does not.

Meiosis Cell Division

Meiosis: Reduces two sets of chromosomes to one set in

daughter cell.

Meiosis is a two-part cell division process Meiosis I – Reduction Meiosis II – Division

Results in 4 cells with 1/2 the normal genetic information

Meiosis I

Consist of four stages Prophase I Metaphase I Anaphase I Telophase I

Prophase I

Homologous chromosomes further condense and pair Crossing-over occurs Spindle fibers form between centrioles Spindle fibers move towards opposite poles

Crossing Over

Segments of homologous chromosomes break and reform at similar locations.

Metaphase I

Microtuble spindle attaches to chromosomes Homologus pairs align align equator

Anaphase I

Homologus pairs of chromosomes separate Pairs move to opposite poles

Telophase I

One set of paired chromosomes arrives at each pole Nuclear division begins Nuclear membrane re-forms

Meiosis II

Consist of four stages Prophase II Metaphase II Anaphase II Telophase II

Prophase II Chromosomes recondense Spindle fibers form between centrioles Spindle fibers move towards opposite poles

Metaphase II

Microtuble spindle apparatus attaches to chromosomes Chromosomes align along spindle

Anaphase II

Sister chromatids separate Chromatids move to opposite poles

Telophase II

Chromatids arrive at each pole

Cell division begins

Significance Of Meiosis Produce the gametes in sexually reproduce

organism. Meiosis reduces the chromosome number by half,

enabling sexual recombination to occur. Meiosis of diploid cells produces haploid

daughter cells, which may function as gametes. Gametes undergo fertilization, restoring the

diploid number of chromosomes in the zygote Restore the diploid number in genome.

produces haploid gametes so that the diploid number of the species remains constant generation after generation.

Increase genetic variation in population.

Significance Of Meiosis

Sexual reproduction Genetic variation

Comparison between Mitosis and Meiosis

Mitosis Meiosis

Occurs in somatic cells Occurs in reproductive cells.

One single division of the mother cell. Two divisions of the mother cell.

Chromosome number remains same Chromosome number become halves

A mitotic mother cell can be either diploid

A meiotic mother cell is always haploid

Two diploid daughter cells Four haploid daughter cells

Genetically identical Genetically different

Crossing over does not occur

No kiasma forms in this cell division.

Nucleus and chromosome both are divided once.

Usually simple

Occurs crossing over

Occurrence of kiasma usually takes place.

Nucleus is divided for two times but the chromosome is divided for once

Usually complex

Comparison of Mitosis & Meiosis

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