1.Mitosis

Chapter 12

Purpose of Cell Division

• _{In unicellular organisms, division of one cell}

reproduces the entire organism

• _{Multicellular organisms depend on cell}

division for:

1. Development from a fertilized cell 2. Growth

Cell division results in genetically identical daughter cells

• A dividing cell

1. duplicates its DNA,

2. allocates the two copies to opposite ends of the cell,

3. then splits into daughter cells

Cellular Organization of the Genetic Material

• _{A cell’s endowment of DNA (its genetic information) is}

called its genome

• _{DNA molecules in a cell are packaged into chromosomes}

– _{Eukaryotic chromosomes consist of chromatin: DNA and protein}

• _{Somatic (nonreproductive) cells usually have two sets of}

chromosomes

• _{Gametes (reproductive cells: sperm and eggs) have one set}

of chromosomes

Structure of Chromosomes

• _{Each duplicated}

chromosome is made of two sister chromatids

– _{These separate during cell}

division

• _{Centromere: where the two}

chromatids are most closely attached Chromosome duplication (including DNA synthesis) 0.5 µm Centromere Sister chromatids Separation of sister chromatids

The Cell Cycle

• _{The cell cycle is}

divided into:

1. Mitotic (M) phase: mitosis and

cytokinesis

2. Interphase: cell

growth and copying of chromosomes

A. G₁ phase

B. S phase (“synthesis”) C. G₂ phase

G₁

G₂

Mitosis

• _{Mitosis is conventionally divided into four or five}

phases:

1. Prophase

2. Prometaphase 3. Metaphase

4. Anaphase 5. Telophase

A Handy Mnemonic

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p

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http://www.youtube.com/watch?v=VlN7

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G₂ OF INTERPHASE PROPHASE PROMETAPHASE

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

10

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Mitosis: The mitotic spindle

• _{Mitotic spindle: microtubules that control chromosome}

movement during mitosis

• _{During cell division, the centrosome replicates, moves to}

opposite ends of the cell as microtubules grow out from them

• Some spindle microtubules attach to the kinetochores of

chromosomes and move the chromosomes to the metaphase plate

Whats in a centrosome in animal cells?

What are microtubules? What are they made of?

Mitosis: The mitotic spindle

• _{In anaphase, sister chromatids separate and move}

along the kinetochore microtubules

• _{The microtubules shorten by depolymerizing at their}

kinetochore ends

• _{Nonkinetochore microtubules from opposite poles}

overlap and push against each other

Chromosome movement

Microtubule Motor protein

Chromosome

Kinetochore

• _{Arts and crafts with students, yarn, brooms}

Cleavage furrow

Contractile ring of

microfilaments Daughter cells

Cytokinesis in animal cells

• _{In animal cells,}

1 µm

Daughter cells Cell plate formation in a plant cell (TEM)

New cell wall Cell plate

Wall of parent cell Vesicles

forming cell plate

Cytokinesis in plant cells

• _{In plant cells, a cell}

plate forms during cytokinesis

• _{http://www.cellsalive.com/mitosis.htm}

• _{plant cells dividing}

• _{Lung cells dividing}

• _{Cancer cells dividing (skip a blk)}

Binary Fission

• _{Prokaryotes (bacteria and archaea) reproduce}

by a type of cell division called binary fission

LE 12-11_3 Origin of replication Cell wall Plasma membrane Bacterial chromosome

E. coli cell

Two copies of origin Chromosome

replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.

Replication continues. One copy of the origin is now at each end of the cell.

Origin Origin

Replication finishes. The plasma membrane grows inward, and new cell wall is deposited.

The Evolution of Mitosis

• _{Mitosis probably}

evolved from binary fission

• _{Certain protists}

Regulation of the cell cycle

• _{The frequency of cell}

division varies with the type of cell

• _{The sequential events}

of the cell cycle are directed by a distinct cell cycle control

system

– _{specific checkpoints}

where the cell cycle stops until a go-ahead signal is received

G₁ checkpoint

G₁ S

M

M checkpoint

Experiment 1 Experiment 2

S

S S

G₁ M G₁

M M

When a cell in the S phase was fused with a

cell in G₁, the G₁ cell

immediately entered the S phase—DNA was synthesized.

When a cell in the M phase

was fused with a cell in

G₁,

the G₁ cell immediately

began

mitosis—a spindle formed and chromatin

condensed, even though the chromosome had not been duplicated.

Evidence for Chemical Control of Cell Cycle

• _{Experiments in}

which cultured

mammalian cells at different phases of the cell cycle were fused to form a

Cell Cycle Control

• The G₁ checkpoint seems to be the

most important one

• At the G₁

checkpoint, If the

cell does not receive the go-ahead signal, it will switch into a nondividing state called the G₀ phase

G₁ G₁ checkpoint

G₁ G₀

If a cell receives a go-ahead signal at the G₁ checkpoint, the cell continues on in the cell cycle.

If a cell does not receive a go-ahead signal at the G₁

checkpoint, the cell exits the cell cycle and goes into G₀, a nondividing state.

The Cell Cycle Clock

• _{Cyclins and cyclin-dependent kinases (Cdks):}

regulatory proteins are involved in cell cycle control

MPF activity

G₁ S G₂ M

S G₂ M

G₁ M

Cyclin

Time

LE 12-16b Degraded cyclin _G 2 checkpoint S M G2 G 1 Cdk Cyclin is degraded MPF Cyclin Cdk

Molecular mechanisms that help regulate the cell cycle

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• _{Hungry Tafties need cookies…..}

Internal and External Signals Regulate Cell Cycle

• _{Internal signal:}

kinetochores not attached to spindle microtubules

send a molecular signal that delays anaphase

• _{External signals: growth}

factors; proteins released by certain cells that

Internal and External Signals Regulate Cell Cycle

• _{External signal:}

density-dependent inhibition; crowded cells stop dividing

• _{External signal:}

anchorage

dependence; they

must be attached to a substratum in order to divide

Cells anchor to dish surface and divide (anchorage dependence).

When cells have formed a complete single layer, they stop dividing (density-dependent inhibition).

If some cells are scraped away, the remaining cells divide to fill the gap and then stop (density-dependent inhibition).

Cancer Cells & The Cell Cycle

• _{Cancer cells do not respond normally to the}

body’s control mechanisms

• _{Cancer cells form tumors, masses of abnormal}

cells within otherwise normal tissue

Cancer cells do not exhibit anchorage dependence

or density-dependent inhibition.

Cancer cells

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