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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 entire organism
• Multicellular organisms depend on
cell division for:
– Development from a fertilized cell
– Growth
– Repair of injured tissues.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
DNA is the genetic material of the cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Most of the DNA of the Eukaryotic Cell is Located in the Nucleus
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
What is a Chromosome?
• One long DNA molecule and a number of protein molecules attached to it form one chromosome.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
DNA Replication
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The The Cell Cell CycleCycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
G2 of interphase → Prophase → Prometaphase
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Metaphase → Anaphase →Telophase & Cytokinesis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Cytokinesis in Animal Cells
Cleavage furrow100 µm
Contractile ring ofmicrofilaments
Daughter cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
MitosisMitosis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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