Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Why Cells divide? In unicellular organisms, division of one cell reproduces the

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Why Cells divide?• In unicellular organisms, division of one cell

reproduces the entire organism

• Multicellular organisms depend on

cell division for:

– Development from a fertilized cell

– Growth

– Repair of injured tissues.


DNA is the genetic material of the cell


Most of the DNA of the Eukaryotic Cell is Located in the Nucleus


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.

• The number of chromosomes in a eukaryotic cell depends on the species. For example, a human body cell has 46 chromosomes. A dog’s body cell has 78 chromosomes.


What is a Chromosome?

• One long DNA molecule and a number of protein molecules attached to it form one chromosome.


Chromatin Fibers& Chromosomes

• In nondividing cells, chromosomes exist as a diffuse mass of long and thin fibers called CHROMATIN.

• As a cell prepares to divide, its chromatin coils up forming compact and distinct (short and thick) chromosomes that are visible under the light microscope.


Concept 12.1: Cell division results in genetically identical daughter cells

• Cells duplicate their DNA before they divide

• So one DNA molecule in the mother cell doubles ensuring that each daughter cell receives an exact copy of the genetic material, DNA, and consequently the exact # of chromosomes as in the mother cell.


In preparation for cell division:

•DNA is replicated and

•the two new DNA molecules and associated proteins stay attached while condensing to form two sister chromatids.

•The two sister chromatids form a duplicated chromosome.

•The narrow “waist” of the duplicated chromosome, where the two sister chromatids are most closely attached, is called the CENTROMERE


DNA Replication


The Cell Cycle

• The cell cycle is an ordered sequence of events in the life of a eukaryotic cell from its origin in the division of a parent cell until its own division into two


Phases of the Cell Cycle:

– The Interphase (about 90% of the cell cycle) has three subphases:

• G1 phase (“first gap”) centers on the growth of the cell.

• S phase (“synthesis”) duplicates the chromosomes.

• G2 phase (“second gap”) completes the preparation for cell division before the mitotic phase starts.

– The Mitotic (M) phase has two subphases:

• Mitosis (the nuclear division)

• Cytokinesis (the cytoplasmic division)


Phases of the Cell Cycle

G1(Cell Growth)

S(DNA Replication)

G2(Cell Preparation for Mitosis)

M

C

Interphase

Mitotic Phase

M: Mitosis(Nuclear Division)

C: Cytokinesis(Cytoplasmic Division)


Concept 12.2: The mitotic phase alternates with interphase in the cell cycle

G1

G2

S(DNA synthesis)

INTERPHASE

Cytokin

esis

MITOTIC(M) PHASE

Mito

sis


The The Cell Cell CycleCycle


Two pairs of centrioles

Chromatin

Nucleolus

Nuclearenvelope

Plasmamembrane

Nucleus withchromatin

-Growth of the baby cell.

-Synthesis of cellular components needed for cell division, including replication of DNA and the centrosome

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Interphase


• 1- Mitosis is conventionally divided into the following phases:

– Prophase

• Early prophase

• Late prophase (Prometaphase)

– Metaphase

– Anaphase

– Telophase

• 2- Cytokinesis is well underway by late telophase

Phases of the Mitotic Phase of the Cell Cycle


Centromere

Chromosome(two sister chromatidsjoined at centromere)

Developingspindle

Nucleus withdispersed chromosomes

Sisterchromatids

- Chromosomes appear due to coiling of chromatin.

- Nucleolus breaks down.

- Spindle fibers begin to form from centrioles.

- Centrioles move toward opposing cell poles.

- Nuclear envelope breaks down at theend of this stage.

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Prophase

PROPHASE

PROMETAPHASE

G2 OF INTERPHASE


G2 of interphase → Prophase → Prometaphase


Cell Division: The Cell Cycle

• Metaphase

– Chromosomes aligned on equatorial plate of cell

– Spindle fibers extending from centriole attach at centromere of chromosome

– Total array termed mitotic spindle

Chromosomes alignedon equatorial plate Spindle fibers

Spindle fibers

Equatorial plate

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Anaphase

• Centromeres that held chromatid pairs together separate

• Spindle fibers move sister chromatids apart toward poles

• The microtubules shorten by depolymerizing at their kinetochore ends

• Each chromatid is now a chromosome of one DNA molecule

Sister chromatids being pulled apart

Sister chromatidsbeing pulled apart

Spindle fibers

(d) Anaphase

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Telophase– Arrival of new

chromosomes at each pole

– Chromosomes begin to uncoil and return to chromatin (long and thin)

– A nucleolus reforms within each nucleus.

– Spindle fibers break up and disappear.

– New nuclear envelope forms around each set of chromosomes.

Nucleolus

Cytokinesis occurring

Re-formingnuclear

envelope

Cleavage furrowof cytokinesis

Cleavage furrow

(e) Telophase

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Metaphase → Anaphase →Telophase & Cytokinesis



Cytokinesis (Cytoplasmic Division)

• In animal cells, cytokinesis occurs by a forming a cleavage furrow

• In plant cells, cytokinesis occurs by a forming a cell plate.


Cytokinesis in Animal Cells

Cleavage furrow100 µm

Contractile ring ofmicrofilaments

Daughter cells


1 µm

Daughter cells

Cell plate formation in a plant cell (TEM)

New cell wallCell plate

Wall ofparent cell

Vesiclesformingcell plate

Cytokinesis in Plant Cells

http://www.youtube.com/watch?v=mzeowbIxgwI&feature=PlayList&p=4DFFFFDF820B4C76&playnext_from=PL&playnext=1&index=7


MitosisMitosis


Binary Fission

• Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission

• In binary fission:

– the chromosome replicates

– and the two daughter chromosomes actively move apart

– The plasma membrane grows inward, and

– new cell wall is deposited resulting in two daughter cells


Origin ofreplication

Cell wall

Plasmamembrane

Bacterialchromosome

E. coli cell

Two copiesof 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.

Two daughtercells result.

Binary Fission

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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Why Cells divide? In unicellular organisms, division of one cell reproduces the