The Cell CycleIntroduction to Biology

The Key Roles of Cell Division

• The ability to reproduce is one of the key features that separates life from non-life.

• All cells have the ability to reproduce, by making exact copies of themselves.

• In unicellular organisms, division of one cell reproduces the entire organism

• In multicellular organisms, cell division is needed for:o Development of an embryo from a sperm/eggo Growtho Repair

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Reproduction

100 µm

Tissue renewalGrowth and development

20 µm200 µm

Asexual Reproduction• Asexual reproduction is reproduction that

involves a single parent producing an offspring.o The offspring produced are, in most cases,

genetically identical to the single cell that produced them.

o Asexual reproduction is a simple, efficient, and effective way for an organism to produce a large number of offspring.

o Prokaryotic organisms (like bacteria) reproduce asexually, as do some eukaryotes (like sponges)

Sexual Reproduction• In sexual reproduction, offspring are

produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow.o The offspring produced inherit some genetic

information from both parents.o Most animals and plants, and many single-celled

organisms, reproduce sexually.

Contrasting Reproduction Types

Cell Division• Cells duplicate their genetic material before

they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA.

• A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells.

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 genetic information that is passed on from one generation of cells to the next is carried by chromosomes.

• Every cell must copy its genetic information before cell division begins.

• Each daughter cell gets its own copy of that genetic information.

• Cells of every organism have a specific number of chromosomes.

Chromosomes

Prokaryotic Chromosomes

• Prokaryotic cells lack nuclei. Instead, their DNA molecules are found in the cytoplasm.

• Most prokaryotes contain a single, circular DNA molecule, or chromosome, that contains most of the cell’s genetic information.

Eukaryotic Chromosomes

• In eukaryotic cells, chromosomes are located in the nucleus, and are made up of chromatin.

• Chromatin is composed of DNA and histone proteins.

• DNA coils around histone proteins to form nucleosomes.

• The nucleosomes interact with one another to form coils and supercoils that make up chromosomes.

Chromosomes During Cell Division

• In preparation for cell division, DNA is replicated and the chromosomes condense

• Each duplicated chromosome has two sister chromatids, which separate during cell division

• The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

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Chromosomeduplication(including DNAsynthesis)

0.5 µm

Centromere

Sisterchromatids

Separationof sister

chromatids

Centromeres Sister chromatids

Phases of the Cell Cycle

• The cell cycle consists ofo Mitotic (M) phase (mitosis and cytokinesis)o Interphase (cell growth and copying of

chromosomes in preparation for cell division)

• Interphase (about 90% of the cell cycle) can be divided into subphases:o G1 phase (“first gap”)o S phase (“synthesis”)o G2 phase (“second gap”)

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G1

G2

S(DNA synthesis)

INTERPHASE

Cytokin

esis

MITOTIC(M) PHASE

Mito

sis

G1 Phase: Cell Growth• In the G1 phase, cells

increase in size and synthesize new proteins and organelles.

S Phase: DNA Replication

• In the S (or synthesis) phase, new DNA is synthesized when the chromosomes are replicated.

G2 Phase: Preparing for Cell Division

• In the G2 phase, many of the organelles and molecules required for cell division are produced.

M Phase: Cell Division• In eukaryotes, cell

division occurs in two stages: mitosis and cytokinesis.o Mitosis is the

division of the cell nucleus.

o Cytokinesis is the division of the cytoplasm.

Important Cell Structures Involved in

Mitosis• Chromatid – each strand of a duplicated

chromosome• Centromere – the area where each pair of

chromatids is joined• Centrioles – tiny structures located in the

cytoplasm of animal cells that help organize the spindle

• Spindle – long proteins (part of the cytoskeleton) that the centrioles produceo Helps move the chromosomes into place.

Prophase• During prophase, the

first phase of mitosis, the duplicated chromosome condenses and becomes visible.

Prophase• The centrioles move

to opposite sides of nucleus and help organize the spindle.

Prophase• The spindle forms

and DNA strands attach at a point called their centromere.

Prophase• The nucleolus

disappears and nuclear envelope breaks down.

Metaphase• During metaphase,

the second phase of mitosis, the centromeres of the duplicated chromosomes line up across the center of the cell.

Metaphase• The spindle fibers

connect the centromere of each chromosome to the two poles of the spindle.

Anaphase• During anaphase, the

third phase of mitosis, the centromeres are pulled apart and the chromatids separate to become individual chromosomes.

Anaphase• The chromosomes

separate into two groups near the poles of the spindle.

Telophase• During telophase, the

fourth and final phase of mitosis, the chromosomes spread out into a tangle of chromatin.

Telophase• A nuclear envelope re-

forms around each cluster of chromosomes.

Telophase• The spindle breaks apart,

and a nucleolus becomes visible in each daughter nucleus.

• Cytokinesis is the division of the cytoplasm.

• The process of cytokinesis is different in animal and plant cells.

Cytokinesis

Cytokinesis in Animal Cells

• The cell membrane is drawn in until the cytoplasm is pinched into two equal parts.

• Each part contains its own nucleus and organelles.

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Cleavage furrow100 µm

Contractile ring ofmicrofilaments

Daughter cells

Cleavage of an animal cell (SEM)

Cytokinesis in Animal Cells

• In plants, the cell membrane is not flexible enough to draw inward because of the rigid cell wall.

• Instead, a cell plate forms between the divided nuclei that develops into cell membranes.

• A cell wall then forms in between the two new membranes.

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1 µm

Daughter cells

Cell plate formation in a plant cell (TEM)

New cell wallCell plate

Wall ofparent cell

Vesiclesformingcell plate

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NucleusCell plateChromosomesNucleolus

Chromatincondensing 10 µm

Prophase. The chromatin is condensing.The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is starting to form.

Prometaphase. Wenow see discrete chromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelope will fragment.

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

Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore micro- tubules shorten.

Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.

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INTERPHASE PROPHASE PROMETAPHASE

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INTERPHASE PROPHASE PROMETAPHASE

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INTERPHASE PROPHASE PROMETAPHASE

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METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

10

µm

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METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

10

µm

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METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

10

µm

Virtual Onion Root Tip Mitosis Lab

Click here to start

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

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.

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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

LE 12-11_3Origin 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.

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 and mitosis

LE 12-12Bacterialchromosome

Chromosomes

Microtubules

Prokaryotes

Dinoflagellates (Type of plankton)

Intact nuclearenvelope

Kinetochoremicrotubules

Kinetochoremicrotubules

Intact nuclearenvelope

Diatoms (Type of Algae)

Centrosome

Most eukaryotes

Fragments ofnuclear envelope

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

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G1

S

M

M checkpoint

G2 checkpoint

G2

Controlsystem

• 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 and divide

• If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

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G1

G1 checkpoint

G1

G0

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

If a cell does not receive a go-ahead signal at the G1 checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.

• An 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 (connective tissue) in order to divide

LE 12-18aCells anchor to dish surface anddivide (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 µmNormal mammalian cells

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Cancer cells do not exhibitanchorage dependenceor density-dependent inhibition.

Cancer cells25 µm

Loss of Cell Cycle Controls in Cancer

Cells• Cancer cells do not respond normally to the

body’s control mechanisms• 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

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Cancer cell

Bloodvessel

LymphvesselTumor

Glandulartissue

Metastatictumor

A tumor grows from asingle cancer cell.

Cancer cells invadeneighboring tissue.

Cancer cells spreadthrough lymph andblood vessels toother parts of thebody.

A small percentageof cancer cells maysurvive and establisha new tumor in anotherpart of the body.