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Chapter 10:

How Cells Divide

I. Why do cells Reproduce?

II. Cell Division in Prokaryotes

III. Structure of Chromosomes

IV. Mitosis

V. Cell Cycle Control

I. Why do cells reproduce?I. Why do cells reproduce?

A. Single celled organisms – reproduction of species

B. Multicellular organisms

1. Growth – increase number of cells

2. Maintenance of existing cells

3. Repair of damaged cells

What is Cellular Reproduction?

Enables parent cell to pass on genes & cell components to

daughter cells

Process = cell division

! Different organisms reproduce by different means

! Prokaryotes divide far more simply than Eukaryotes

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II. Cell Division in Prokaryotes

1. Genetic information = single, circular DNA

2. Prokaryotic cell division = Binary Fission

3. DNA copying is first.

4. Protein ring forms.

5. Septum = cross wall forms.

6. One genome goes to each daughter cell.

7. End Result: 2 genetically identical daughter

cells

1. Replication of DNA

2. Elongation of Cell

3. Formation of FtsZProtein ring

4. Septum Formation byInward growth

5. Cell pinches in two

Daughter Cells

Binary Fission in Prokaryotes

Prokaryote Cell

E. coli ! 20 minutes!

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DNA: Organized in chromosomes

Accurate passage of genetic info is difficult.

Chromosomes help organize the process.

Remember:

Genes = Code for proteins

Genes = Sequences of nucleotides

= segments of DNA double helix

Chromosome Contains:

DNA double helix

Centromere

Histones: Proteins packaged with

DNA (Chromatin)

centromere

III. Structure of Chromosomes (Eukaryotes)

DNA Organization:

• Chromosomes occur in homologous pairs

• Each homologue is same length and contains same genes

in same order

Diploid: Cells with pairs of homologous

chromosomes (2n)

Haploid: Cells with only one of each type

of chromosome (n = Haploid number)

• Found in sex cells (e.g. sperm / egg)

(Gametes)

• Found in most cells of human body

Karyotype: Entire set of chromosomes

from a single cell

Gene for hair color

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Eukaryotic chromosomes occur in homologous pairsEukaryotic chromosomes occur in homologous pairs

Humans body cells are

Diploid (2n)

We have 2 of each of 23 types of

chromosomes (n = 23)

= 46 total

Human female Karyotype

Sex

Chromosomes:

X and Y

Female = XX

Male = XY

Organism:

Human

Gorilla

Dog

Cat

Shrimp

Fruitfly

Potato

Ophioglossum

Haploid Number

(n)

23

24

39

19

127

4

24

631

Diploid Number

(2n)

46

48

78

38

254

8

48

1262

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Replication

Chromosome Chromosome

Replication produces:

Duplicated chromosome

• with 2 chromatids

= sister chromatids

• copies

• remain attached at

centromere

• Packaged with proteins:

Histones

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Sister Chromatids

Centromere

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Differing genetic information on homologous chromosomes

Eye-color

genes

Coat-color

genes

ec

EC

White Pink

BlackBrown

HomologousPair

ALLELES: one of two or more alternative states of a gene

LOCUS: the position on a chromosome where a gene is

located

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1. DNA~25 X106 nucleotidesper chromosome

7. Chromosome

6. Rosettes ofchromatin loops

5. Chromatin loop

Histone

2. Nucleosome(200 nucleotides)

DNA

3. Solenoid30 nm

4. Scaffoldprotein

Eukaryotes:

Two types of cell division:

1. Mitotic Cell Division

2. Meiotic Cell division

• Daughter cells

Identical to parental cell

• Growth, repair &

asexual reproduction

• Daughter cells have !

genetic info of parent

cell

• Sexual reproduction

(Egg & sperm production)

IV. Mitosis

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Mitotic Cell Division is used for:

1) Growth

IV. Mitosis

2) Maintenance: skin cells

life span = ~ 2 weeks

3) Repair: organ regeneration

liver donation…

Mitosis occurs here !

cells

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4) Asexual reproduction (by mitotic division):

Offspring are formed from a single parent

clones

• Common in multicellulareukaryotes too

Chandelier

plant

Hydra

•Typical of unicellular eukaryotesParamecium

Eukaryotic cell cycleEukaryotic cell cycle

2 major stages:

1) Interphase (in yellow)

G1: Growth phase 1

(Acquire nutrients, grow)

G1! sensitive to signals

Go to S! Or Go to G0!

Go: Non-dividing

(resting; expression of cell fate)

S: Synthesis of DNA

(chromosomes replicate)

G2: Growth phase 2

(chromosomes condense,

organelles replicate)

*

How does a cell go about

mitotic cell division?

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Eukaryotic cell cycleEukaryotic cell cycle

2) Cell division

Separation of chromosomes,

Nucleus divides = mitosis

Cytoplasm divides=cytokinesis

Mitosis has 4 main “stages”

1) Prophase

2) Metaphase

3) Anaphase

4) Telophase

2 major stages:

1) Interphase

Eukaryotic cell cycleEukaryotic cell cycle

2 major stages:

1) Interphase (in yellow)

2) Cell division (in blue)

! The cell cycle can vary in

length from minutes to

years

! At any given point, most

cells are in the G0 phase

! Some cells do not divide

(i.e. some muscle, and

nerve cells) and thus stay

in G0 indefinetly

How does a cell go about

mitotic cell division?

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Mitosis in animal cellsMitosis in animal cells

INTERPHASE (G2)

1. DNA has replicated; started to condense

2. Centrioles replicate (animals only):

microtubule organizing centers

CentriolesAster

Nuclear membraneChromatin (replicated)

Nucleus

First: How do things look at the end of interphase?

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• Nuclear membrane disintegrates

• Nucleolus disappears

• Chromosomes finish condensing

• Mitotic spindle begins to form and is complete at end of prophase

• Kinetochores form at centromeres and attach to spindle

Centromere and kinetochoreMitotic spindle beginning to form

Chromosomes condensing

MITOSIS: MITOSIS: ProphaseProphase

Kinetochore: a disk of protein

Metaphasechromosome

Kinetochore

Kinetochoremicrotubules

Centromereregion ofchromosome

Chromatid

Spindle Apparatus

Attachment to

Chromosomes

Microtubles are

hollow cylinders

composed of

Tubulin subunits

MITOSIS: MITOSIS: MetaphaseMetaphase

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ChromosomeCentrioles

Metaphaseplate

Aster

microtubules

Spindle fibers

MITOSIS:MITOSIS:MetaphaseMetaphase

KinetochoreMicrotubules

Polar

Microtubules

Aster

Microtubules

Aster

Microtubules

• The mitotic spindle aligns the chromosomes up at the

metaphase plate (an imaginary plane)

MITOSIS:MITOSIS:AnaphaseAnaphase

Polar microtubules elongate

Kinetochoremicrotubulesshorten,separating sisterchromatids toopposite poles Ana.Ana.

Meta.Meta.

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• Polar microtubules continue to elongate• Chromosomes reach poles of cell• Kinetochores disappear• Nuclear membrane re-forms• Nucleolus reappears• Chromosomes decondense

MITOSIS:MITOSIS:TelophaseTelophase

Polar

Microtubules

of Spindle

ApparatusChromosomes

Cell plate in plant cells Animal Cells form a

Cleavage furrow

Plant cells: cell plate forms

Animal cells: cleavage furrow forms

CytokinesisCytokinesis

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Cytokinesis differs between plants and animals

Animals:

• ring of microfilaments “cinches” waist (Cleavage Furrow)

Plants:

• cell plate forms; cellulose delivered via vesicles--> new cell wall

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

Cytokinesis in an animal cell

Cleavage

furrow

forms at

the equator

SE

M 1

40!

Daughter cells

Cleavage furrowContracting ring of

microfilaments

Actin Filaments!

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Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin CummingsT

EM

7,5

00!

Cell plate

Daughter

nucleus

Cell wall Cell wall material is deposited

Vesicles containing

cell membrane materialCell plate Daughter cells

Cytokinesis in a plant cell

Fig. 10.15

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Mitosis in Plant Cells

G2

S G1

C

Spindle checkpointG2 / M checkpoint

G1 / S checkpoint(Start or Restriction Point)

M

Control of the Cell Cycle

V. Cell Cycle Control

At the checkpoints a cell

assesses its internal

state and integrates

external signals

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Chromosomesattached atmetaphase plate

Replication completed; DNA integrity

Growth factors; Nutritional state of cell; Size of cell

Cdk / G1cyclin

Cdk / G2cyclin (MPF)

G2

S

G1

CM

Spindle checkpoint

G2 / M checkpoint

G1 / S checkpoint

APC

Anaphase Promoting

Complex

Control of the Cell Cycle: Cdk’s

• Kinases are a class of enzyme that

phosphorylate other molecules

• Cdk’s consist of an enzymatic

subunit partnered w/ the protein

cyclin

• Cyclin is a regulatory protein

required to activate Cdk

• Cdk is controlled by the pattern of

phosphorylation (red=off; green=on)

• Cdk’s phosphorylate a number of

targets that result in the synthesis of

proteins required in the cell cycle

• Phosphorylation/Dephosphorylation

is a common theme in cellular

pathways

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Signal transduction pathway. Example from the G1/S checkpoint.

Fig. 10.22

Growth factor receptor: more per cell in many breast cancers.

Ras protein: activated by mutations in 20–30% of all cancers.

Src kinase: activated by mutations in 2–5% of all cancers.

Rb protein: mutated in 40% of all cancers.

p53 protein: mutated in 50% of all cancers.

Rasprotein

Srckinase

Continue PastCell cycle

checkpoints

Cytoplasm

Nucleus

Rbprotein

p53protein

PROTO-ONCOGENES

TUMOR-SUPPRESSOR GENES

Signal

transduction

pathway

Cyclins & Cyclin-Dependent Kinases

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Tumor

Single cancer

cell develops

into a tumor

Invade

Neighboring

Tissue

Metastasize

Lymph

vessels

Blood

vessel

Tumors & Metastasis

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Taxus brevifolia

Interfering with Cell Division

Radiation

Chemotherapy

Pacific Yew - Taxol

Periwinkle - Vinblastin

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END Mitosis

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1. What is the difference between chromatin and

chromatids?

2. If a diploid mother cell had 24 chromosomes, how

many chromosomes would each daughter cell have

after mitosis?

After meiosis?

3. What is the product of “replication”?