Lecture 2, Ch 12, Cell Cycle

8/3/2019 Lecture 2, Ch 12, Cell Cycle

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Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures forBiology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 12

The Cell Cycle


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Overview: The Key Roles of Cell Division

The ability of organisms to reproduce bestdistinguishes living things from non-living matter

The continuity of life is based upon thereproduction of cells, or cell division

Cell division is integral part of cell cycle


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Types of cell division

Prokaryotes

Binary fission

Eukaryotes

Mitosis:

Growth, development & repair

Asexual reproduction (yields genetically identical cells)

Occurs in somatic (body) cells

Meiosis:

Sexual reproduction (yields genetically different cells with halfthe # of chromosomes)

Occurs in specific reproductive cells

Yields gametes (e.g., eggs & sperm) or spores


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Eukaryotic cell division consists of:

Mitosis, the division of the nucleus

Cytokinesis, the division of the cytoplasm


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Concept 12.1: Mitotic cell division results ingenetically identical daughter cells

Cells duplicate their genetic material before theydivide, ensuring that each daughter cell receivesan exact copy of the genetic material, DNA

A dividing cell duplicates its DNA, allocates thetwo copies to opposite ends of the cell, and onlythen splits into daughter cells


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Cellular Organization of the Genetic Material

A cells endowment of DNA (its genetic

information) is called its genome

DNA molecules* in a cell are packaged intochromosomes

*Prokaryotes-circular DNA

Eukaryotes-linear DNA



Every eukaryotic species has a characteristicnumber of chromosomes in each cell nucleus

Somatic (non-reproductive) cells (normally) havetwo sets of chromosomes

Gametes (reproductive cells: sperm and eggs)(and spores) have half as many chromosomes assomatic cells

Eukaryotic chromosomes consist of chromatin, acomplex of DNA and protein that condensesduring cell division



DNA associates with special proteins to form more stable

structure called chromosomes (different proteins inprokaryotes and eukaryotes, so chromosomes built different)

Chromosomes are found inside nucleus in eukaryotes

Human - 46 chromosomes, 23 pairs (1 set of 23 from egg, 1set of 23 from sperm)

Each chromosome contains many genes

Gene is a segment of DNA that is responsible for controllinga trait (e.g., coding for a specific protein)



Human female karyotype

H l k



Human male karyotype



Phases of the Cell Cycle

The cell cycle consists of

Mitotic (M) phase (mitosis and cytokinesis)

Interphase (cell growth and copying of

chromosomes in preparation for cell division)

Interphase (about 90% of the cell cycle) can bedivided into subphases:

G1phase (first gap) S phase (synthesis)

G2phase (second gap)

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LE 12-5

G1

G2

S(DNA synthesis)

INTERPHASE


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Distribution of Chromosomes During Cell Division

In preparation for cell division, DNA is replicated

and the chromosomes condense

Each duplicated chromosome has two sisterchromatids, which separate during cell division

The centromere is the narrow waist of the

duplicated chromosome, where the twochromatids are most closely attached

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LE 12-4

Chromosomeduplication(including DNAsynthesis)

0.5 m

Centromere

Sisterchromatids

Separation

of sisterchromatids

Centromeres Sister chromatids


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Mitosis is conventionally divided into five phases:

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

Cytokinesis is well underway by late telophase


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1. Prophase

- Chromatin condenses, this causes the chromosomes tobegin to become visible

- Centrosomes separate, moving to opposite ends of thenucleus

- The centrosomes start to form a framework used toseparate the two sister chromatids called the mitoticspindle, that is made of microtubules

- Nucleolus disappears


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2. Prometaphase

- Nuclear envelope fragments

- Chromosomes become more condensed

- A kinetochore is formed at the centromere, the point wherethe sister chromatids are attached

- Microtubules attach at the kinetochores


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3. Metaphase

- Chromosomes align on an axis called the metaphase plate

- Note: the spindle consists of microtubules, one attached toeach chromosome


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4. Anaphase

- Each centromere splits making two chromatids free

- Each chromatid moves toward a pole

- Cell begins to elongate, caused by microtubules notassociated with the kinetochore


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5. Telophase

Formation of nuclear membrane and nucleolus

Short and thick chromosomes begin to elongate to formlong and thin chromatin

Formation of the cleavage furrow - a shallow groove inthe cell near the old metaphase plate

Cytokinesis = division of the cytoplasm


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Mitosis in an onion root


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Th Mit ti S i dl A Cl L k


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The Mitotic Spindle:A Closer Look

The mitotic spindle is an apparatus of

microtubules that controls chromosome movementduring mitosis

Assembly of spindle microtubules begins in the

centrosome, the microtubule organizing center

The centrosome replicates, forming twocentrosomes that migrate to opposite ends of the

cell, as spindle microtubules grow out from them

An aster (a radial array of short microtubules)extends from each centrosome


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The spindle includes the centrosomes, the spindle

microtubules, and the asters

Some spindle microtubules attach to thekinetochores of chromosomes and move the

chromosomes to the metaphase plate

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Microtubules ChromosomesSisterchromatids

AsterCentrosome

Metaphaseplate

Kineto-chores

Kinetochoremicrotubules

0.5 m

Overlapping

nonkinetochoremicrotubules

1 mCentrosome


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In anaphase, sister chromatids separate and

move along the kinetochore microtubules towardopposite ends of the cell

The microtubules shorten by depolymerizing at

their kinetochore ends

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Chromosomemovement

Microtubule Motorprotein

Chromosome

Kinetochore

Tubulinsubunits


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Nonkinetochore microtubules from opposite polesoverlap and push against each other, elongatingthe cell

In telophase, genetically identical daughter nuclei

form at opposite ends of the cell

Cytokinesis: A Closer Look


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Cytokinesis:A Closer Look

In animal cells, cytokinesis occurs by a process

known as cleavage, forming a cleavage furrow

In plant cells, a cell plate forms during cytokinesis

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Cleavage furrow

100 m

Contractile ring ofmicrofilaments

Daughter cells

Cleavage of an animal cell (SEM)

Mitosis


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1 m

Daughter cells

Cell plate formation in a plant cell (TEM)

New cell wallCell plate

Wall ofparent cell

Vesiclesformingcell plate

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Nucleus

Cell plateChromosomesNucleolus

Chromatin

condensing 10 m

Prophase. Thechromatin is condensing.The nucleolus isbeginning to disappear.Although not yet visiblein the micrograph, the

mitotic spindle is startingto form.

Prometaphase. Wenow see discretechromosomes; eachconsists of two identicalsister chromatids. Laterin prometaphase, the

nuclear envelope willfragment.

Metaphase. The spindle iscomplete, and thechromosomes, attachedto microtubules at theirkinetochores, are all atthe metaphase plate.

Anaphase. Thechromatids of eachchromosome haveseparated, and thedaughter chromosomesare moving to the ends of

the cell as theirkinetochore micro-tubules shorten.

Telophase. Daughternuclei are forming.Meanwhile, cytokinesishas started: The cellplate, which will dividethe cytoplasm in two, is

growing toward theperimeter of the parentcell.

Binary Fission


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Binary Fission

Prokaryotes (bacteria and archaea) reproduce by

a type of cell division called binary fission

In binary fission, the chromosome replicates(beginning at the origin of replication), and the two

daughter chromosomes actively move apart

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Origin ofreplication

Cell wall

Plasmamembrane

Bacterial

chromosome

E. colicell

Two copiesof origin

Chromosomereplication begins.Soon thereafter,one copy of the originmoves rapidly toward

the other end of the cell.

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Cell wall

Plasma

membraneBacterialchromosome

E. colicell

Two copiesof origin

Chromosomereplication begins.Soon thereafter,

one copy of the originmoves rapidly towardthe other end of the cell.

Replication continues.One copy of the originis now at each end ofthe cell.

Origin Origin

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O i i fCell wall


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Cell wall

Plasmamembrane

Bacterialchromosome

E. colicell

Two copies

of origin

Chromosome

replication begins.Soon thereafter,one copy of the originmoves rapidly towardthe other end of the cell.

Replication continues.

One copy of the originis now at each end ofthe cell.

Origin Origin

Replication finishes.The plasma membrane

grows inward, andnew cell wall isdeposited.

Two daughtercells result.

The Evolution of Mitosis


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The Evolution of Mitosis

Since prokaryotes evolved before eukaryotes,mitosis probably evolved from binary fission

Certain protists exhibit types of cell division that

seem intermediate between binary fission andmitosis

LE 12-12Bacterial


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chromosome

Chromosomes

Microtubules

Prokaryotes

Dinoflagellates

Intact nuclearenvelope



Intact nuclearenvelope

Diatoms

Centrosome

Most eukaryotes

Fragments ofnuclear envelope

Concept 12.3: The cell cycle is regulated by a


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Concept 12.3: The cell cycle is regulated by amolecular control system

The frequency of cell division varies with the typeof cell and with cell happiness

These cell cycle differences result from regulationat the molecular level

The cell cycle appears to be driven by specificchemical signals present in the cytoplasm

The levels of these chemical signals are

influenced by biotic & abiotic factors

The Cell Cycle Control System


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The Cell Cycle Control System

The sequential events of the cell cycle are

directed by a distinct cell cycle control system,which is similar to a clock

The clock has specific checkpoints where the cell

cycle stops until a go-ahead signal is received

For many cells, the G1 checkpoint seems to be themost important one

LE 12-14G1 checkpoint


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G1 checkpoint

G1S

M

M checkpoint

G2 checkpoint

G2

Controlsystem

LE 12-15


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G1

G1 checkpoint

G1

G0

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

If a cell does not receive ago-ahead signal at the G1checkpoint, the cell exits thecell cycle and goes into G0, anondividing state.

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Degradedcyclin G2

checkpoint

Cdk

Cyclin isdegraded

MPFCyclin

Cdk

Molecular mechanisms that help regulate the cell cycle

Stop and Go Signs: Internal and External Signals at


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p g gthe Checkpoints An example of an internal signal is that

kinetochores not attached to spindlemicrotubules send a molecular signal thatdelays anaphase

Some external signals are growth factors,proteins released by certain cells that stimulateother cells to divide

For example, platelet-derived growth factor(PDGF) stimulates the division of humanfibroblast cells in culture

LE 12-17Scalpels


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Petriplate

p

Without PDGF

With PDGF

Without PDGF

With PDGF

10 mm


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Another example of external signals is density-

dependent inhibition, in which crowded cells stopdividing

Most animal cells also exhibit anchorage

dependence, in which they must be attached to asubstratum in order to divide

LE 12-18aCells anchor to dish surface and


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divide (anchorage dependence).

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

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

25 mNormal mammalian cells


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Cancer cells exhibit neither density-dependentinhibition nor anchorage dependence

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Cancer cells do not exhibit

anchorage dependenceor density-dependent inhibition.

Cancer cells25 m

Loss of Cell Cycle Controls in Cancer Cells


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y

Cancer cells do not respond normally to the

bodys control mechanisms

Cancer cells form tumors, masses of abnormalcells within otherwise normal tissue

If abnormal cells remain at the original site, thelump is called a benign tumor

Malignant tumors invade surrounding tissues andcan metastasize, exporting cancer cells to otherparts of the body, where they may form secondarytumors

Documents

Lecture 2, Ch 12, Cell Cycle