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Cellular Cellular DivisionDivision

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When do Cells Divide?When do Cells Divide? Cells obtain nutrients and eliminate wastes through

the cell membrane. There must be enough surface area for all of the exchanges to take place with the environment.

It is more difficult for cells with a larger volume to transport, so cells remain relatively small to survive.

When a cell reaches its maximum size, the nucleus initiates cell division.

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Why do Cells Divide?Why do Cells Divide? Cell division accounts for 3 essential life processes:

Growth - increase in size

Differentiation - cells develop specialized shapes and functions.

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Why do Cells Divide?Why do Cells Divide? Cell division accounts for 3 essential life processes:

Repair - heal injuries by cell regeneration at the injury site

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Why do Cells Divide?Why do Cells Divide? Cell division accounts for 3 essential life processes:

Reproduction - making new organisms

Asexual - offspring produced by one parent; the result of cell division

Sexual - offspring have a combination of genetic material from two parent organisms; requires meiosis, a specialized form of cell division

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How do Cells Divide?How do Cells Divide?

Phase - a defined period within a cycle of change

Cell cycle - the sequence of phases in the life cycle of a cell

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The Cell The Cell CycleCycle

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Interphase• Interphase - growth and preparation

– Occurs between divisions

• 3 stages– G1 (gap 1) - growth and development– S (synthesis - chromosomes replicate; the

cell commits to cell division– G2 (gap 2) - cell synthesizes other

organelles and other materials needed for division

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Interphase• During interphase, the chromosomes

are in chromatin form - a tangled bundle

• During S phase, each chromosome is replicated, creating sister chromatids

• As cell division begins (during prophase), the chromatin coils and condenses to chromosomes and the sister chromatids are clearly visible, joined at the centromere.

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• Chromatin - uncoiled DNA

• Chromosome - neatly coiled DNA

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Cell Division - Mitosis and Cytokinesis

• There are 2 stages in cell division:– 1. Mitosis

» The cell’s nucleus divides into 2 nuclei.

– 2. Cytokinesis» The cell’s cytoplasm divides to make 2

daughter cells

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Mitosis• Mitosis is the splitting of the cell’s

nucleus

• It has 4 parts:– Prophase– Metaphase– Anaphase– Telophase

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Mitosis - Prophase• Chromosomes condense• In animal cells, a pair of centrioles

migrates to each side of the cell• The mitotic spindle forms from the

centrioles• The nuclear membrane and nucleolus

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Mitosis - Prophase

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Mitosis - Metaphase• Spindle arranges chromosomes along

“metaphase plate”

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Mitosis - Anaphase• Centromeres divide and sister

chromatids are pulled apart, so that each daughter cell has an identical set of chromosomes

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Mitosis - Telophase• Nuclear membrane reforms around each

set of chromosomes• Two nucleoli form• Chromosomes uncoil into chromatin• Cleavage furrow starts to form (for

cytokinesis)

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Cytokinesis• The cell’s cytoplasm divides to make 2

daughter cells• The cleavage furrow pinches cell in half

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Four Mitotic StagesFour Mitotic Stages

ProProphasephaseMetaMetaphasephaseAnaAnaphasephaseTeloTelophasephase

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Identical Daughter Identical Daughter CellsCells

Parent CellParent Cell

Two Two identical identical daughter daughter

cellscells

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Mitosis in Animal Cells• Rat epithelial cells

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Mitosis in Plant Cells• Plant cells have a cell wall.• During mitosis, a cell plate forms a new

cell membrane and cell wall between the two daughter cells.

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History• Mitosis was first observed

microscopically in the 1870s by the German biologist Walter Flemming

– He coined the term mitosis– from the Greek word mitos, – meaning thread QuickTime™ and a

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The Cell CycleInterphase may be up to 90% of the cell cycle.

The time varies for different cell types.

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The Cell Cycle

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MeiosisMeiosisFormation of Formation of

GametesGametes (Eggs & (Eggs & Sperm)Sperm)

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Facts About MeiosisFacts About Meiosis

Produces sex cells called Produces sex cells called gametesgametes (eggs & sperm) (eggs & sperm)

Occurs in the testes in males Occurs in the testes in males ((SpermatogenesisSpermatogenesis))

Occurs in the ovaries in Occurs in the ovaries in females (females (OogenesisOogenesis))

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Facts About MeiosisFacts About Meiosis

The gametes (eggs and sperm) The gametes (eggs and sperm) are are haploidhaploid - which means they - which means they have 1 set of chromosomes, not have 1 set of chromosomes, not 2 like all of our diploid body 2 like all of our diploid body cells.cells.

Why we need meiosis --> this is Why we need meiosis --> this is critical for sexual reproduction.critical for sexual reproduction.

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It starts with a diploid cell It starts with a diploid cell that has 46 chromosomes (2N).that has 46 chromosomes (2N).

The DNA is replicated, so The DNA is replicated, so there are 92 chromosomes. there are 92 chromosomes. There are There are 44 copies of each, copies of each, making a making a tetradtetrad..

The Process of MeiosisThe Process of Meiosis

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The first division, very similar The first division, very similar to mitosis, occurs, producing to mitosis, occurs, producing two cells with 46 chromosomes two cells with 46 chromosomes each.each.

A second division occurs, A second division occurs, producing 4 haploid daughter producing 4 haploid daughter cells that each have 23 cells that each have 23 chromosomes (N)chromosomes (N)

The Process of MeiosisThe Process of Meiosis

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Meiosis: Two Part Cell Meiosis: Two Part Cell DivisionDivision

DiploidDiploid

MeiosisMeiosis I I

MeiosisMeiosisIIII

Diploid Diploid at startat start

HaploiHaploidd

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Meiosis must reduce the chromosome number by halfMeiosis must reduce the chromosome number by half Fertilization then restores the 2n numberFertilization then restores the 2n number

from mom from dad child

meiosis reducesgenetic content

toomuch!

The right number!

Meiosis Forms Haploid Meiosis Forms Haploid GametesGametes

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Pieces of chromosomes are Pieces of chromosomes are exchanged, which produces genetic exchanged, which produces genetic recombination in the offpsringrecombination in the offpsring

Crossing-OverCrossing-Over

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Crossing-over multiplies the number Crossing-over multiplies the number of different gamete typesof different gamete types

Crossing-OverCrossing-Over

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FertilizationFertilization

1N = 31N = 3

2N = 62N = 6

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• Humans– Haploid number (1N) = 23– Diploid number (2N) = 46

• Somatic cells are diploid.• Gametes are haploid.

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