Cell DivisionCell DivisionBinary Fission, Mitosis & MeiosisBinary Fission, Mitosis & Meiosis

Binary FissionBinary Fission• Most cells reproduce through Most cells reproduce through

some sort of some sort of Cell DivisionCell Division• Prokaryotic cells divide through a Prokaryotic cells divide through a

simple form of division calledsimple form of division called Binary FissionBinary Fission

• 3 step process3 step process• Single “naked” strand splits and Single “naked” strand splits and

forms a duplicate of itself.forms a duplicate of itself.• The two copies move to opposite The two copies move to opposite

sides of the cellsides of the cell• Cell “pinches” into two new and Cell “pinches” into two new and

identical cells called identical cells called ""daughter daughter cellscells". (Cell wall then forms if ". (Cell wall then forms if applicable)applicable)

Mitosis

• Eukaryotes divide by a more complicated system called Eukaryotes divide by a more complicated system called MitosisMitosis• This is because:This is because:

1.1. They have a nucleus which must be broken up and then They have a nucleus which must be broken up and then reformedreformed

2.2. They have their DNA “packaged” in the form of They have their DNA “packaged” in the form of ChromosomesChromosomes3.3. Chromosomes are composed of Chromosomes are composed of ChromatinChromatin

1.1. Made of DNA Strands & ProteinsMade of DNA Strands & Proteins4.4. Also contain Also contain NucleosomesNucleosomes containing containing HistonesHistones - Proteins the - Proteins the

DNA is wrapped around Name for the DNA/Protein complex is DNA is wrapped around Name for the DNA/Protein complex is ChromatinChromatin

5.5. They usually have more than 1 chromosome (Humans have 23 They usually have more than 1 chromosome (Humans have 23 pairs)pairs)

6.6. They have numerous organelles to equally shareThey have numerous organelles to equally share

Chromatin / Chromosomes

The Cell CycleThe Cell Cycle•Most of the cell's life is spent doing Most of the cell's life is spent doing its regular function.its regular function.•Cells divide along a rough time frame Cells divide along a rough time frame called its called its Cell CycleCell Cycle..•The Cell cycle consists of the The Cell cycle consists of the folowing steps:folowing steps:•G1 (Gap 1)G1 (Gap 1) Phase - Cell performs its Phase - Cell performs its normal function (cells which do not normal function (cells which do not divide stay in this stage for their entire divide stay in this stage for their entire life span)life span)•S (Synthesis) PhaseS (Synthesis) Phase - Here the cell - Here the cell actively duplicates its DNA in actively duplicates its DNA in preparation for divisionpreparation for division•G2 (Gap 2) PhaseG2 (Gap 2) Phase - Amount of - Amount of cytoplasm (including organelles) cytoplasm (including organelles) increases in preparation for division.increases in preparation for division.•MitosisMitosis - Actual division occurs - Actual division occurs

InterphaseInterphase

• Cell Replicates its DNA/Chromosomes Cell Replicates its DNA/Chromosomes in preparation of upcoming division in preparation of upcoming division

Animal Cell Plant cell

ProphaseProphase

1.1.ChromosomesChromosomes Shorten Shorten and become visible. and become visible.

2. 2. CentriolesCentrioles move to move to opposite sides of the cellopposite sides of the cell

3. Nuclear envelope 3. Nuclear envelope disappearsdisappears

4. 4. Spindle FibersSpindle Fibers & Astral & Astral FibersFibers both together are both together are known as the known as the Spindle Spindle ApparatusApparatus begin to form begin to form

Animal Cell

Plant Cell

MetaphaseMetaphase

• Chromosomes line up Chromosomes line up along center of cell along center of cell called the called the Metaphase Metaphase PlatePlate

• Chromosomes attach Chromosomes attach to spindle fibersto spindle fibers

• Spindle & Astral fibers Spindle & Astral fibers are now clearly visible are now clearly visible

Animal CellAnimal Cell

Plant CellPlant Cell

AnaphaseAnaphase

• Centromeres break up separating chromosome copies

• Chromosomes are pulled apart to opposite sides of cell

• Spindle & Astral fibers begin to break down

Animal Cell

Plant Cell

Telophase (cytokenesis)Telophase (cytokenesis)• 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 – CytokenesisCytokenesis happens with happens with

most (but not all) cellsmost (but not all) cells– Cytoplasm & organelles Cytoplasm & organelles

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

Animal Cell

PlantCell

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 Cytokenesis• With Plants, a cell wall must be formed between

the 2 daughter cells.• Vessicles containing Cellulose form and fuse

between the tow daughter cells, eventually forming a complete cell wall.

Overview of MitosisOverview of Mitosis

MeiosisMeiosis

• Similar in many ways to mitosisSimilar in many ways to mitosis

• Several differencesSeveral differences

• Involves 2 cell divisionsInvolves 2 cell divisions

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

VocabularyVocabulary• Diploid (2N)Diploid (2N) - Normal amount - Normal amount

of genetic materialof genetic material• Haploid (N)Haploid (N) - 1/2 the genetic - 1/2 the genetic

material.material.• Meiosis results in the formation Meiosis results in the formation

of haploid cells.of haploid cells.• In Humans, these are the In Humans, these are the OvaOva

(egg) and (egg) and spermsperm..• Ova are produced in the Ova are produced in the

ovariesovaries in females in females• Process is called Process is called ooogenesisogenesis• Sperm are produced in the Sperm are produced in the

testestestes of males. of males.• Process is called Process is called

spermatogenesisspermatogenesis

Meiosis Phases

• Meiosis occurs in 2 phases; Meiosis I, & Meiosis II.

• Meiosis I.– Prior to division,

amount of DNA doubles

Crossing OverCrossing Over

• During metaphase 1 During metaphase 1 homologous homologous chromosomes line-up chromosomes line-up along the metaphase along the metaphase plateplate

• Areas of homologous Areas of homologous chromosomes chromosomes connect at areas connect at areas called called chiasmatachiasmata

Crossing over contd.Crossing over contd.

• Crossing OverCrossing Over of genes of genes occurs now occurs now – Segments of homologous Segments of homologous

chromosomes break and chromosomes break and reform at similar locations.reform at similar locations.

– Results in new genetic Results in new genetic combinations of offspring.combinations of offspring.

– This is the main This is the main advantage of sexual advantage of sexual reproductionreproduction

Chromosome reductionChromosome reduction

• During anaphase 1, During anaphase 1, each homologous each homologous chromosome is chromosome is pulled to opposite pulled to opposite sides of the cell. sides of the cell. Unlike mitosis, Unlike mitosis, THE THE CENTROMERES CENTROMERES DO NOT BREAKDO NOT BREAK..

Meiosis I continuedMeiosis I continued

• Nuclei may or Nuclei may or may not reform may not reform following following division.division.

• Cytokenesis may Cytokenesis may or may not occur or may not occur

Meiosis II

• DNA does not double• Chromosomes randomly

line-up along metaphase plate like regular mitosis.

• During anaphase 2, CENTROMERES BREAK and each chromosome is pulled to opposite sides of the cell.

• Nuclei reform and cytokenesis usually occurs (although it is often unequal).

Overview of MeiosisOverview of Meiosis

Comparison of Mitosis & MeiosisComparison of Mitosis & Meiosis