Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings INTERPHASE G1G1 S (DNA synthesis) G2G2 Cytokinesis Mitosis MITOTIC (M) PHASE Ch

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

INTERPHASE

G1

S(DNA synthesis)

G2

Cytokin

esis

Mito

sisM

ITOTIC

(M) PHASE

Ch. 12 The Cell Cycle

• Interphase

• M phase

– Mitosis

– Cytokinesis


20 µm100 µm 200 µm

(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).

(b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).

(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

Reproduction – unicellular organisms – binary fission in bacteria

Growth & development from fertilized egg

Repair (& replacement) of damaged cells

Cell Division - Purpose


Figure 12.4 Chromosome duplication and distribution during cell division

0.5 µm

Chromosomeduplication(including DNA synthesis)

Centromere

Separation of sister

chromatids

Sisterchromatids

Centrometers Sister chromatids

A eukaryotic cell has multiplechromosomes, one of which is

represented here. Before duplication, each chromosome

has a single DNA molecule.

Once duplicated, a chromosomeconsists of two sister chromatids

connected at the centromere. Eachchromatid contains a copy of the

DNA molecule.

Mechanical processes separate the sister chromatids into two chromosomes and distribute

them to two daughter cells.


Figure 12.5 The cell cycle

INTERPHASE

G1

S(DNA synthesis)

G2

Cytokin

esis

Mito

sisM

ITOTIC

(M) PHASE

5-6hrs. 10-12hrs.

4-6hrs.

1hr.

growth

growth

90% of cell’s life


INTERPHASE

G1

S(DNA synthesis)

G2

Cytokin

esis

Mito

sisM

ITOTIC

(M) PHASE

An organism’s genome:

- Total genes in the cell of a species

- Nuclear & extra-nuclear (mito & chloro)

Human chromosome number:

- 46 – somatic cell (body cells)

- 2n

- diploid

- 23 – gamete (sex cells – sperm & egg)

- n

- haploid

Chromosomes:

- Chromatin

- DNA & proteins


Interphase

Centromsomes – with 2 centrioles

Nucleus – with mass of chromatin

Nucleolus in nucleus


INTERPHASE

G1

S(DNA synthesis)

G2

Cytokin

esis

Mito

sisM

ITOTIC

(M) PHASE

Phases of Mitosis

• Prophase Prometaphase – chromosomes are in Pairs

• Metaphase – chromosomes in Middle

• Anaphase – chromosomes Apart

• Telophase – Two cells


Figure 12.6 Exploring The Mitotic Division of an Animal Cell

G2 OF INTERPHASE PROPHASE PROMETAPHASE

Centrosomes(with centriole pairs) Chromatin

(duplicated)

Early mitoticspindle

Aster

CentromereFragmentsof nuclearenvelope

Kinetochore

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Kinetochore microtubule

Nonkinetochoremicrotubules


METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate Nucleolus

forming

Cleavagefurrow

Nuclear envelopeformingCentrosome at

one spindle poleDaughter chromosomes

Figure 12.6 Exploring The Mitotic Division of an Animal Cell


Cytokinesis:

• division of the cytoplasm

• animal cells

– Forms cleavage furrow

• plant cells

– Forms a cell plate


Figure 12.9 Cytokinesis in animal and plant cells

Daughter cells

Cleavage furrow

Contractile ring of microfilaments

Daughter cells

100 µm

1 µmVesiclesforming cell plate

Wall of patent cell Cell plate

New cell wall

(a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM)


Control system

G2 checkpoint – cyclin dependent complex CDKcomplex

(go ahead signal)

M checkpoint

G1 checkpoint

G1

S

G2M

Molecular Control of Cell Cycle

• regulated at checkpoints

• kinases - cyclin-dependent kinase (CDK)

• cyclins

- MPF

- maturation promoting factor

- Cyclin + CDK = MPF

- (mitosis promoting factor)


Figure 12.15 The G1 checkpoint

G1 checkpoint

G1G1

G0

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

(b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.


Figure 12.16 Molecular control of the cell cycle at the G2 checkpoint

Accumulated cyclin moleculescombine with recycled Cdk mol-ecules, producing enough molecules of MPF to pass the G2 checkpoint and initiate the events of mitosis.

MPF promotes mitosis by phosphorylating various proteins. MPF‘s activity peaks during metaphase. (breaking down nuclear envelop, etc.)

3

During G1, conditions in the cell favor degradation of cyclin, and the Cdk component of MPF is recycled.

5

During anaphase, the cyclin component of MPF is degraded, terminating the M phase. The cell enters the G1 phase.

4

2

Synthesis of cyclin begins in late S phase and continues through G2. Because cyclin is protected from degradation during this stage, it accumulates.

1

Cdk

CdkG2

checkpoint

CyclinMPF

Cyclin is degraded

DegradedCyclin

G 1

G 2

S

M

G1G1 S G2 G2SM M

MPF activity

Cyclin

Time

(a) Fluctuation of MPF activity and cyclin concentration during the cell cycle

(b) Molecular mechanisms that help regulate the cell cycle

Rel

ativ

e C

on

cen

tra

tion

MPF – maturation

promoting factor


Control of Cell Cycle

• Cyclin accumulates - late S phase – Binds to/activating CDK

• Producing MPF

• MPF at G2 initiates mitosis

• Anaphase - cyclin component of MPF degrades

• M phase terminates - cell -> G1


Kinetochore microtubules shorten at their kinetochore ends

CONCLUSION This experiment demonstrated that during anaphase, kinetochore microtubules shorten

at their kinetochore ends, not at their spindle pole ends. This is just one of the experiments supporting the

hypothesis that during anaphase, a chromosome tracks along a microtubule as the microtubule

depolymerizes at its kinetochore end, releasing tubulin subunits

Chromosomemovement

Microtubule Motorprotein

Chromosome

Kinetochore

Tubulin

subunits


Prophase:

• chromatin - condenses

• nucleoli disappear.

• sister chromatids appear

• mitotic spindle forms

• centrosomes move away from each other


Prometaphase:

• nuclear envelope fragments

• microtubules join chromosomes

• 2 chromatids/chromosome

– kinetochore center

• “kinetochore microtubules.” - jerk the chromosomes back and forth.

• Nonkinetochore microtubules interact with opposite poles


Metaphase:

• Metaphase - longest stage

– about 20 min.

• centrosomes at opposite ends.

• chromosomes convene on metaphase plate,

– at centromeres

• Sister chromatid kinetochores

– attached to kinetochore microtubules

• spindle - apparatus of microtubules


Anaphase:

• shortest stage - a few minutes.

• 2 sister chromatids part

– chromatid becomes chromosome

• move toward opposite ends of the cell,

– kinetochore microtubules shorten

• cell elongates

– nonkinetochore microtubules lengthen

• finally the ends have equivalent—complete—collections of chromosomes


Telophase:

• 2 daughter nuclei form

• Nuclear envelopes form

• chromosomes -less condensed

• Mitosis

– the division of 1 nucleus into 2 genetically identical nuclei - complete


Figure 12.7 The mitotic spindle at metaphase

Sisterchromatids

MetaphasePlate

Kinetochores

Overlappingnonkinetochoremicrotubules

Kinetochores microtubules

Centrosome

ChromosomesMicrotubules0.5 µm

CentrosomeAster

1 µm


Lack of Cell Cyle Controls - Cancer Cells

25 µm

Cells anchor to dish surface anddivide (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 do not exhibitanchorage dependence or density-dependent inhibition.

25 µm

Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells.

Normal mammalian cells. The availability of nutrients, growth factors, and a substratum for attachment limits cell density to a single layer.

(a)

(b)

Figure 12.18 Density-dependent inhibition and anchorage dependence of cell division


Figure 12.19 The growth and metastasis of a malignant breast tumor

A tumor grows from a single cancer cell.

Cancer cells invade neighboring tissue.

Cancer cells spread through lymph and blood vessels to other parts of the body.

A small percentage of cancer cells may survive and establish a new tumor in another part of the body.

2 431

Tumor

Glandulartissue

Cancer cell

Bloodvessel

Lymphvessel

MetastaticTumor


Lab: Figure 12.10 Mitosis in a plant cell

1 Prophase. The chromatinis condensing. The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is staring to from.

Prometaphase.We now see discretechromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelop will fragment.

Metaphase. The spindle is complete,and the chromosomes,attached to microtubulesat their kinetochores, are all at the metaphase plate.

Anaphase. Thechromatids of each chromosome have separated, and the daughter chromosomesare moving to the ends of cell as their kinetochoremicrotubles shorten.

Telophase. Daughternuclei are forming. Meanwhile, cytokinesishas started: The cellplate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell.

2 3 4 5

NucleusNucleolus

ChromosomeChromatinecondensing


Plant Cells – meristematic tissue (root tip)


Bacterial cell division (binary fission)Origin ofreplication

E. coli cellBacterialChromosome

Cell wall

Plasma Membrane

Two copiesof origin

OriginOrigin

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

1

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

2

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

3

Two daughter cells result.4

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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings INTERPHASE G1G1 S (DNA synthesis) G2G2 Cytokinesis Mitosis MITOTIC (M) PHASE Ch