BIOL 101 General Biology I

Chp 12The Cell Cycle

Rob SwatskiAssociate Professor of Biology

HACC – York Campus

Overview: The Key Roles of Cell Division

• The ability of organisms to reproduce best

distinguishes living things from nonliving things

• The continuation of life is based on cell

reproduction (cell division)

Fig. 12-1

• In unicellular organisms, division of 1 cell

reproduces the entire organism

• Multicellular organisms use cell division for:

– Development from a fertilized cell

– Growth

– Repair

• Cell division is a vital part of the cell cycle, the

life of a cell from formation to its own division

Fig. 12-2a

100 µm

(a) Reproduction

Fig. 12-2b

200 µm

(b) Growth and development

Fig. 12-2c

20 µm

(c) Tissue renewal

Concept 12.1: Cell division results in geneticallyidentical daughter cells

• Most cell division

results in daughter cells

with identical genetic

information, DNA

• A special type of

division (...?...)

produces nonidentical

daughter cells

(gametes)

- sperm & egg cells

Cellular Organization of the Genetic Material

• All the DNA in a cell makes up the cell’s

genome

• A genome can consist of one DNA molecule

(common in prokaryotes) or many DNA

molecules (common in eukaryotes)

• DNA molecules in a cell are packaged into

chromosomes

Fig. 12-3

• Every eukaryotic species has a characteristic

chromosome number in each cell nucleus

• Somatic cells (nonreproductive body cells)

have 2 sets of chromosomes

• Gametes (reproductive cells: sperm & eggs)

have half the number of chromosomes as

somatic cells

• Eukaryotic chromosomes consist of

chromatin, a complex of DNA & protein that

condenses during cell division

Distribution of Chromosomes During Eukaryotic Cell Division

• In preparation for cell division, DNA is

replicated & the chromosomes condense

• Each duplicated chromosome has 2 sister

chromatids, which separate during cell

division

• The centromere is the narrow “waist” of the

duplicated chromosome, where the 2

chromatids are most closely attached

Fig. 12-40.5 µm Chromosomes

Chromosomeduplication(including DNAsynthesis)

Chromo-some arm

Centromere

Sisterchromatids

DNA molecules

Separation ofsister chromatids

Centromere

Sister chromatids

• Eukaryotic cell division consists of:

– Mitosis: division of the nucleus

– Cytokinesis: division of the cytoplasm

• Gametes are produced by another type of cell

division called meiosis

• Meiosis produces nonidentical daughter cells

that have only 1 set of chromosomes (half as

many as the parent cell)

Phases of the Cell Cycle

• The cell cycle consists of:

– Interphase: cell growth & copying of

chromosomes in preparation for cell division

– Mitotic (M) phase: mitosis & cytokinesis

Fig. 12-5

S(DNA synthesis)

G1

G2

Fig. 12-UN1

Telophase andCytokinesis

Anaphase

Metaphase

Prometaphase

Prophase

MITOTIC (M) PHASE

Cytokinesis

Mitosis

SG1

G2

• Interphase (90% of the cell cycle) is divided

into subphases:

– G1 phase (“first gap” or “growth”)

– S phase (“synthesis”)

– G2 phase (“second gap” or “growth”)

• The cell grows during all 3 phases, but

chromosomes are copied only during the S

phase

• Mitosis is divided into 5 phases:

– Prophase

– Prometaphase

– Metaphase

– Anaphase

– Telophase

• Cytokinesis is well underway by late telophase

BioFlix: Mitosis

Fig. 12-6a

Prophase PrometaphaseG2 of Interphase

Fig. 12-6b

PrometaphaseProphaseG2 of Interphase

Nonkinetochoremicrotubules

Fragmentsof nuclearenvelope

Aster CentromereEarly mitoticspindle

Chromatin(duplicated)

Centrosomes(with centriolepairs)

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Kinetochore Kinetochoremicrotubule

Fig. 12-6c

Metaphase Anaphase Telophase and Cytokinesis

Fig. 12-6d

Metaphase Anaphase Telophase and Cytokinesis

Cleavage

furrow

Nucleolus

forming

Metaphase

plate

Centrosome at

one spindle poleSpindle

Daughter

chromosomesNuclear

envelope

forming

The Mitotic Spindle: A Closer Look

• The mitotic spindle is made of microtubules

(MT’s) that control chromosome movement

during mitosis

• During prophase, assembly of spindle MT’s

begins in the centrosome, the MT organizing

center

• The centrosome replicates & then each

migrates to opposite ends of the cell, as

spindle MT’s grow out from them

• An aster (a star-like array of short MT’s)

extends from each centrosome

• The spindle includes the centrosomes, the

spindle MT’s, & the asters

• During

Prometaphase, some

spindle MT’s attach to

the kinetochores of

chromosomes & start

to move the

chromosomes

• At Metaphase, the

chromosomes are all

lined up at the

metaphase plate, the

midway point between

the spindle’s 2 poles

Fig. 12-7

Microtubules Chromosomes

Sisterchromatids

Aster

Metaphaseplate

Centrosome

Kineto-chores

Kinetochoremicrotubules

Overlappingnonkinetochoremicrotubules

Centrosome 1 µm

0.5 µm

Fig. 12-UN4

• In Anaphase, sister chromatids separate & move

along the kinetochore MT’s toward opposite ends

of the cell

• The MT’s shorten by depolymerizing at their

kinetochore ends

Fig. 12-8a

Kinetochore

Spindlepole

Mark

EXPERIMENT

RESULTS

Fig. 12-8b

Kinetochore

MicrotubuleTubulinSubunits

Chromosome

Chromosomemovement

Motorprotein

CONCLUSION

• Nonkinetochore MT’s from opposite poles

overlap & push against each other, which

elongates the cell

• In Telophase, genetically identical daughter

nuclei form at opposite ends of the cell

- opposite of Prophase

Cytokinesis: A Closer Look

• In animal cells, Cytokinesis occurs by a

process known as cleavage

- forms a cleavage furrow

• In plant cells, a cell plate forms during

cytokinesis

Animation: Cytokinesis

Cleavage furrow

Fig. 12-9a

100 µm

Daughter cells

(a) Cleavage of an animal cell (SEM)

Contractile ring ofmicrofilaments

Rat Kangaroo

kidney cells

Fig. 12-9b

Daughter cells

(b) Cell plate formation in a plant cell (TEM)

Vesiclesformingcell plate

Wall ofparent cell

New cell wallCell plate

1 µm

Fig. 12-10

Chromatincondensing

Metaphase Anaphase TelophasePrometaphase

Nucleus

Prophase1 2 3 54

Nucleolus Chromosomes Cell plate10 µm

Video: Sea Urchin (Time Lapse)

Video: Animal Mitosis

Plant Mitosis

Fig. 12-10a

Nucleus

Prophase1

Nucleolus

Chromatin

condensing

Fig. 12-10b

Prometaphase2

Chromosomes

Fig. 12-10c

Metaphase3

Fig. 12-10d

Anaphase4

Fig. 12-10e

Telophase5

Cell plate10 µm

Fig. 12-UN2

Binary Fission

• Prokaryotes (bacteria & archaea) reproduce by

another type of cell division called binary

fission

• In binary fission, the single chromosome

replicates (beginning at the origin of

replication), & the 2 daughter chromosomes

actively move apart

Fig. 12-11-1

Origin ofreplication

Two copiesof origin

E. coli cellBacterialchromosome

Plasmamembrane

Cell wall

Fig. 12-11-2

Origin ofreplication

Two copiesof origin

E. coli cellBacterialchromosome

Plasmamembrane

Cell wall

Origin Origin

Fig. 12-11-3

Origin ofreplication

Two copiesof origin

E. coli cellBacterialchromosome

Plasmamembrane

Cell wall

Origin Origin

Fig. 12-11-4

Origin ofreplication

Two copiesof origin

E. coli cellBacterialchromosome

Plasmamembrane

Cell wall

Origin Origin

The Evolution of Mitosis

• Since prokaryotes evolved before eukaryotes,

mitosis probably evolved from binary fission

• Some protists display types of cell division that

appear intermediate between binary fission &

mitosis

Fig. 12-12ab

Bacterial

chromosome

Chromosomes

Microtubules

(a) Bacteria

(b) Dinoflagellates

Intact nuclearenvelope

Fig. 12-12cd

Kinetochoremicrotubule

(c) Diatoms and yeasts

Kinetochoremicrotubule

(d) Most eukaryotes

Fragments of nuclear envelope

Intact nuclearenvelope

Concept 12.3: The eukaryotic cell cycle is regulated by a molecular control system

• The frequency of cell division varies with the

type of cell

• These cell cycle differences result from

regulation at the molecular level

Evidence for Cytoplasmic Signals

• The cell cycle appears to be driven by specific

chemical signals present in the cytoplasm

• Some evidence for this hypothesis comes from

experiments in which cultured mammalian cells

at different phases of the cell cycle were fused

to form a single cell with two nuclei

Fig. 12-13

Experiment 1 Experiment 2

EXPERIMENT

RESULTS

S G1 M G1

M MSS

When a cell in theS phase was fused with a cell in G1, the G1

nucleus immediatelyentered the Sphase—DNA was synthesized.

When a cell in theM phase was fused with a cell in G1, the G1

nucleus immediatelybegan mitosis—aspindle formed andchromatin condensed,even though the

chromosome had notbeen duplicated.

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 cell cycle control system is regulated by

both internal & external controls

• The clock has specific checkpoints where the

cell cycle stops until a go-ahead signal is

received

Fig. 12-14

SG1

M checkpoint

G2M

Controlsystem

G1 checkpoint

G2 checkpoint

• For many cells, the G1 checkpoint seems to

be the most important one

• If a cell receives a go-ahead signal at the G1

checkpoint, it will usually complete the S, G2,

and M phases & divide

• If the cell does not receive the go-ahead signal,

it will exit the cycle & switch into a nondividing

state called the G0 phase

Fig. 12-15

G1

G0

G1 checkpoint

(a) Cell receives a go-aheadsignal

G1

(b) Cell does not receive ago-ahead signal

The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases

• Two types of regulatory proteins are involved in

cell cycle control: cyclins & cyclin-dependent

kinases (Cdks)

• The activity of cyclins & Cdks fluctuates during

the cell cycle

• MPF (maturation-promoting factor) is a

cyclin-Cdk complex that triggers a cell’s

passage past the G2 checkpoint into the M

phase

Fig. 12-17a

Time

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

Cyclinconcentration

MPF activity

M M MSSG1 G1 G1G2 G2

Fig. 12-17b

Cyclin isdegraded

Cdk

MPF

CdkG2

checkpoint

Degradedcyclin

Cyclin

(b) Molecular mechanisms that help regulate the cell cycle

Cyclin

accu

mu

latio

n

Stop and Go Signs: Internal and External Signals at the Checkpoints

• An example of an internal signal is that

kinetochores not attached to spindle

microtubules send a molecular signal that

delays anaphase

• Some external signals are growth factors,

proteins released by certain cells that stimulate

other cells to divide

• For example, platelet-derived growth factor

(PDGF) stimulates the division of human

fibroblast cells in culture

Fig. 12-18

Petriplate

Scalpels

Cultured fibroblasts

Without PDGFcells fail to divide

With PDGFcells prolifer-ate

10 µm

• Another example of external signals is density-

dependent inhibition, in which crowded cells

stop dividing

• Most animal cells also exhibit anchorage

dependence, in which they must be attached

to a substratum in order to divide

Fig. 12-19

Anchorage dependence

Density-dependent inhibition

Density-dependent inhibition

(a) Normal mammalian cells (b) Cancer cells

25 µm25 µm

Loss of Cell Cycle Controls in Cancer Cells

• Cancer cells do not respond normally to the

body’s control mechanisms

- they do not display either density-dependent

inhibition nor anchorage dependence

• Cancer cells may not need growth factors to grow

& divide:

– They may make their own growth factor

– They may convey a growth factor’s signal without the

presence of the growth factor

– They may have an abnormal cell cycle control system

Loss of Cell Cycle Controls in Cancer Cells

Dividing Prostate Cancer Cells

• A normal cell is converted to a cancerous cell

by a process called transformation

• Cancer cells form tumors, masses of abnormal

cells within otherwise normal tissue

• If abnormal cells remain at the original site, the

lump is called a benign tumor

• Malignant tumors invade surrounding tissues

and can metastasize, exporting cancer cells to

other parts of the body, where they may form

secondary tumors

Schwannoma Tumor (benign)

Fig. 12-20

Tumor

A tumor grows

from a single

cancer cell.

Glandulartissue

Lymphvessel

Bloodvessel

Metastatictumor

Cancercell

Cancer cells

invade neigh-

boring tissue.

Cancer cells spreadto other parts ofthe body.

Cancer cells maysurvive and

establish a new

tumor in another

part of the body.

1 2 3 4