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

Binary FissionBinary Fission• Most cells reproduce Most cells reproduce

through some sort of through some sort of Cell Cell DivisionDivision

• Prokaryotic cells divide Prokaryotic cells divide through a simple form of through a simple form of division called division called Binary Binary FissionFission

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

splits and forms a duplicate splits and forms a duplicate of itself.of itself.

• The two copies move to The two copies move to opposite sides of the cellopposite sides of the cell

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

3.3. Chromosomes are composed of Chromosomes are composed of ChromatinChromatin1.1. Made of DNA Strands & ProteinsMade of DNA Strands & Proteins

4.4. Also contain Also contain NucleosomesNucleosomes containing containing HistonesHistones - - Proteins the DNA is wrapped around Name for the Proteins the DNA is wrapped around Name for the DNA/Protein complex is DNA/Protein complex is ChromatinChromatin

5.5. They usually have more than 1 chromosome (Humans They usually have more than 1 chromosome (Humans have 23 pairs)have 23 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 Most of the cell's life is spent doing its regular function.doing its regular function.•Cells divide along a rough time Cells divide along a rough time frame called its frame 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 Phase - Cell performs its normal function (cells which do its normal function (cells which do not divide stay in this stage for not divide stay in this stage for their entire life span)their entire life span)•S (Synthesis) PhaseS (Synthesis) Phase - Here the - Here the cell actively duplicates its DNA in cell 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 increases in preparation for division.division.•MitosisMitosis - Actual division occurs - Actual division occurs

InterphaseInterphase

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

Animal Cell Plant cell

• Mitosis is a continuum of changes.– For description, mitosis is usually broken

into five subphases: •prophase, •prometaphase, •metaphase, •anaphase, and • telophase.

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• By late interphase, the chromosomes have been duplicated but are loosely packed.

• The centrosomes have been duplicated and begin to organize microtubules into an aster (“star”).

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Fig. 12.5a

• In prophase, the chromosomes are tightly coiled, with sister chromatids joined together.

• The nucleoli disappear.• The mitotic spindle begins

to form and appears to push the centrosomes away from each other toward opposite ends (poles) of the cell.

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Fig. 12.5b

• During prometaphase, the nuclear envelope fragments and microtubules from the spindle interact with the chromosomes.

• Microtubules from one pole attach to one of two kinetochores, special regions of the centromere, while microtubules from the other pole attach to the other kinetochore.

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Fig. 12.5c

• The spindle fibers push the sister chromatids until they are all arranged at the metaphase plate, an imaginary plane equidistant between the poles, defining metaphase.

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Fig. 12.5d

• At anaphase, the centromeres divide, separating the sister chromatids.

• Each is now pulled toward the pole to which it is attached by spindle fibers.

• By the end, the two poles have equivalent collections of chromosomes.

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Fig. 12.5e

• At telophase, the cell continues to elongate as free spindle fibers from each centrosome push off each other.

• Two nuclei begin to form, surrounded by the fragments of the parent’s nuclear envelope.

• Chromatin becomes less tightly coiled.

• Cytokinesis, division of the cytoplasm, begins.

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Fig. 12.5f

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Fig. 12.5 left

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Fig. 12.5 right

ProphaseProphase

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

2. 2. CentriolesCentrioles move to move to opposite sides of the opposite sides of the cellcell

3. Nuclear envelope 3. Nuclear envelope disappearsdisappears

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

Animal Cell

Plant Cell

MetaphaseMetaphase

• Chromosomes line Chromosomes line up along center of up along center of cell called the cell called the Metaphase PlateMetaphase Plate

• Chromosomes Chromosomes attach to spindle attach to spindle fibersfibers

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

forms around both sets forms around both sets of chromosomesof chromosomes

• DNA uncoilsDNA uncoils• Spindle & Astral fibers Spindle & Astral fibers

completely disappear completely disappear – CytokenesisCytokenesis happens happens

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

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

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Fig. 12.9

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 - Normal

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

genetic material.genetic material.• Meiosis results in the Meiosis results in the

formation of haploid cells.formation 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 During metaphase 1 homologous 1 homologous chromosomes line-chromosomes line-up along the up along the metaphase platemetaphase plate

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

Crossing over contd.Crossing over contd.

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

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

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

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

Chromosome reductionChromosome reduction

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

Meiosis I continuedMeiosis I continued

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

• Cytokenesis Cytokenesis may or may may or may not occur 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 & Comparison of Mitosis & MeiosisMeiosis