Chapter 12~ Chapter 12~ The Cell CycleThe Cell Cycle

2007-20082007-2008

Biology is the only subject Biology is the only subject

in which in which multiplicationmultiplication is is

the same thing as the same thing as

divisiondivision……

Why do cells divide?Why do cells divide?

There are functional limits to cell size, determined by:There are functional limits to cell size, determined by:

– 1.) 1.) SA/V RatioSA/V Ratio– The volume of a cell increases faster than the surface area when a The volume of a cell increases faster than the surface area when a

cell grows. WHY?cell grows. WHY?– As the cell grows, the SA/V ratio decreases. When it gets too small, As the cell grows, the SA/V ratio decreases. When it gets too small,

the cell stops growing or begins cell division.the cell stops growing or begins cell division. Key: Large SA/V ratio = good; Small SA/V ratio = badKey: Large SA/V ratio = good; Small SA/V ratio = bad

– 2.) 2.) Genome to Volume RatioGenome to Volume Ratio– If cell grows too large (G1), the DNA will not be able to control the cell If cell grows too large (G1), the DNA will not be able to control the cell

adequately.adequately.– The cell divides before the G/V ratio gets too small.The cell divides before the G/V ratio gets too small.

For reproductionFor reproduction – asexual reproductionasexual reproduction

unicellular organismsunicellular organisms

For growthFor growth– from fertilized egg to from fertilized egg to

multi-celled organism multi-celled organism For repair & renewalFor repair & renewal

– replace cells that die replace cells that die from normal wear & tear from normal wear & tear or from injuryor from injury

Why do cells divide?Why do cells divide?

amoeba

Unicellular organisms reproduce by cell Unicellular organisms reproduce by cell division.division.

100 µm

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

Figure 12.2 A

Multicellular organisms depend on cell Multicellular organisms depend on cell division for:division for:– Development from a fertilized cell.Development from a fertilized cell.– Growth.Growth.– Repair.Repair.

20 µm200 µm

(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).

Figure 12.2 B, C

Importance of Cell DivisionImportance of Cell Division1. Growth and Development1. Growth and Development

2. Asexual Reproduction 3. Tissue Renewal2. Asexual Reproduction 3. Tissue Renewal

Zygote Embryo Fetus Adult1 Cell 100 cells millions cells 100 trillion cells

DNA organization in ProkaryotesDNA organization in Prokaryotes

Nucleoid regionNucleoid region Bacterial Chromosome Bacterial Chromosome

– Single (1) circular DNASingle (1) circular DNA– Small Small

(e.g. (e.g. E. coli E. coli is 4.6X10is 4.6X1066 bp, ~1/100 bp, ~1/100thth human human chromosome)chromosome)

Plasmids Plasmids – extra chromosomal DNA– extra chromosomal DNA

Bacterial FissionBacterial Fission

The cell division process is an integral part The cell division process is an integral part of the cell cycle.of the cell cycle.

The Cell CycleThe Cell Cycle

InterphaseInterphase (90% of cycle)(90% of cycle) • • G1 phase~ growth G1 phase~ growth • • S phase~ synthesis of DNA S phase~ synthesis of DNA • • G2 phase~ preparation for G2 phase~ preparation for cell divisioncell division

Mitotic phase (M-phase)Mitotic phase (M-phase)• • Mitosis~ nuclear division Mitosis~ nuclear division • • Cytokinesis~ cytoplasm Cytokinesis~ cytoplasm

divisiondivision

Some Vocabulary before we get Some Vocabulary before we get into it all…into it all…

GenomeGenome: cell’s genetic : cell’s genetic informationinformation

SomaticSomatic (body cells) cells (body cells) cells GametesGametes (reproductive cells): (reproductive cells):

sperm and egg cellssperm and egg cells ChromosomesChromosomes: condensed DNA : condensed DNA

moleculesmolecules Diploid (2n):Diploid (2n): 2 sets of 2 sets of

chromosomes chromosomes Haploid (1n):Haploid (1n): 1 set of 1 set of

chromosomeschromosomes ChromatinChromatin: DNA-protein : DNA-protein

complexcomplex ChromatidsChromatids: replicated strands : replicated strands

of a chromosomeof a chromosome CentromereCentromere: narrowing “waist” : narrowing “waist”

of sister chromatidsof sister chromatids MitosisMitosis: nuclear division: nuclear division CytokinesisCytokinesis: cytoplasm division: cytoplasm division MeiosisMeiosis: gamete cell division: gamete cell division

1414

Chromosome OrganizationChromosome Organization

When cells divide, daughter cells must each When cells divide, daughter cells must each receive complete copy of DNAreceive complete copy of DNA

Each cell has about 2 meters of DNA in the Each cell has about 2 meters of DNA in the nucleus; thin threads callednucleus; thin threads called chromatin chromatin

Before division, condenses to form Before division, condenses to form chromosomeschromosomes

DNA also replicates before cell division to DNA also replicates before cell division to produce paired produce paired chromatidschromatids

1515

Normal Karyotype (Fig 18.1)Normal Karyotype (Fig 18.1)

Facts About Cell DivisionFacts About Cell Division

Cell division results in genetically identical Cell division results in genetically identical daughter cells.daughter cells.

Cells duplicate their genetic material before Cells duplicate their genetic material before they divide, ensuring that each daughter cell they divide, ensuring that each daughter cell receives an exact copy of the genetic receives an exact copy of the genetic material, material, DNADNA..

Cellular Organization of the Cellular Organization of the Genetic MaterialGenetic Material

A cell’s endowment of DNA, its genetic A cell’s endowment of DNA, its genetic information is called its information is called its genomegenome..

The DNA molecules The DNA molecules in a cell are in a cell are packaged into packaged into chromosomeschromosomes..

50 µm

Figure 12.3

Eukaryotic chromosomes consist of Eukaryotic chromosomes consist of chromatinchromatin, a complex of DNA and protein , a complex of DNA and protein that condenses during cell division.that condenses during cell division.

In animals:In animals:– SomaticSomatic cells have two sets of chromosomes cells have two sets of chromosomes– GametesGametes have one set of chromosomes have one set of chromosomes

Distribution of Chromosomes Distribution of Chromosomes During Cell DivisionDuring Cell Division

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

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

0.5 µm

Chromosomeduplication(including DNA synthesis)

Centromere

Separation of sister

chromatids

Sisterchromatids

Centromeres 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 12.4

Eukaryotic cell division consists ofEukaryotic cell division consists of– MitosisMitosis, the division of the nucleus., the division of the nucleus.– CytokinesisCytokinesis, the division of the cytoplasm., the division of the cytoplasm.

In meiosisIn meiosis– Sex cells are produced after a reduction in Sex cells are produced after a reduction in

chromosome number. This reduction is chromosome number. This reduction is accomplished by a second division.accomplished by a second division.

The mitotic phase alternates with The mitotic phase alternates with interphaseinterphase in the cell cycle. in the cell cycle.

A labeled probe can reveal patterns of gene A labeled probe can reveal patterns of gene expression in different kinds of cells.expression in different kinds of cells.

Phases of the Cell CyclePhases of the Cell Cycle The cell cycle consists ofThe cell cycle consists of

– The mitotic phaseThe mitotic phase– InterphaseInterphase

INTERPHASE

G1

S(DNA synthesis)

G2Cyto

kines

is

Mito

sis

MITOTIC(M) PHASE

Figure 12.5

Interphase can be divided into subphasesInterphase can be divided into subphases1.1. GG11 phase phase

2.2. S phaseS phase

3.3. GG22 phase phase

The mitotic phase is made up of mitosis and The mitotic phase is made up of mitosis and cytokinesis.cytokinesis.

5 Phases of Mitosis5 Phases of Mitosis

5 Phases:5 Phases:

1.1. ProphaseProphase

2.2. PrometaphasePrometaphase

3.3. MetaphaseMetaphase

4.4. AnaphaseAnaphase

5.5. TelophaseTelophase

CELL DIVISION (In Humans)CELL DIVISION (In Humans)

ProphaseProphase: :

1. The nuclear membrane disappears. 1. The nuclear membrane disappears.

2. The 2. The centriolescentrioles (only in animals) move to the opposite ends/poles of (only in animals) move to the opposite ends/poles of the cell.the cell.

3. 3. Spindle FibersSpindle Fibers begin to form. begin to form.

The genetic material recoils into the chromosome state (92 The genetic material recoils into the chromosome state (92 chromatids).chromatids).

Nuclear Membrane

Spindle Fibers

Centriole

Cell Membrane

PrometaphasePrometaphase

– spindle fibersspindle fibers attach to attach to centromerescentromeres creating creating kinetochoreskinetochores

– microtubules attach at microtubules attach at kinetochores kinetochores connect connect centromerescentromeres to to

centriolescentrioles

– chromosomes begin chromosomes begin movingmoving

CELL DIVISION (In Humans)CELL DIVISION (In Humans)

Metaphase:Metaphase: 1. The chromosomes have lined up at the “equator” of the nucleus1. The chromosomes have lined up at the “equator” of the nucleus

2. Spindle fibers coming from the centrioles have attached to the 2. Spindle fibers coming from the centrioles have attached to the chromosomes at the centromeres. chromosomes at the centromeres.

The genetic material is in the chromosome state (92) chromatids).The genetic material is in the chromosome state (92) chromatids).

CELL DIVISION (In Humans) - 4 Phases:CELL DIVISION (In Humans) - 4 Phases:

Anaphase:Anaphase: 1. The chromatids are pulled apart by the spindle fibers. 46 chromatids 1. The chromatids are pulled apart by the spindle fibers. 46 chromatids

going toward each end of the cell (two ends = 92 chromatids).going toward each end of the cell (two ends = 92 chromatids).

2. Cytokinesis begins.2. Cytokinesis begins.

CELL DIVISION (In Humans) - 4 Phases:CELL DIVISION (In Humans) - 4 Phases:

Telophase:Telophase: “Opposite of prophase.” “Opposite of prophase.”

1. Two nuclear membranes reappear1. Two nuclear membranes reappear

2. The centrioles get out of the way2. The centrioles get out of the way

3. Cytokinesis finishes. 3. Cytokinesis finishes.

There are now two identical daughter cells entering G1 Phase, each with 46 chromatids which will There are now two identical daughter cells entering G1 Phase, each with 46 chromatids which will quickly turn into a chromatin state and double during the S phase of interphase to give each body cell quickly turn into a chromatin state and double during the S phase of interphase to give each body cell 23 pairs or 46 individual chromosomes.23 pairs or 46 individual chromosomes.

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 Figure 12.6

Nonkinetochoremicrotubules

5 Phases5 Phases

Centrosome at one spindle pole

Daughter chromosomes

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate

Nucleolusforming

Cleavagefurrow

Nuclear envelopeforming

Figure 12.6

5 Phases5 Phases

The Mitotic Spindle: The Mitotic Spindle: A Closer A Closer LookLook

The The mitotic spindlemitotic spindle Is an apparatus of Is an apparatus of microtubules that controls chromosome microtubules that controls chromosome movement during mitosis.movement during mitosis.

The spindle arises from the The spindle arises from the centrosomes centrosomes and includes spindle microtubules and and includes spindle microtubules and asters (made of centrioles).asters (made of centrioles).

Some spindle Some spindle microtubulesmicrotubules– Attach to the kinetochores of Attach to the kinetochores of

chromosomes and move the chromosomes and move the chromosomes to the chromosomes to the metaphase platemetaphase plate

CentrosomeAster

Sisterchromatids

MetaphasePlate

Kinetochores

Overlappingnonkinetochoremicrotubules

Kinetochores microtubules

Centrosome

ChromosomesMicrotubules0.5 µm

1 µm

Figure 12.7

In anaphase, sister chromatids separateIn anaphase, sister chromatids separate a and nd move along the move along the kinetochorekinetochore microtubules microtubules toward opposite ends of the cell.toward opposite ends of the cell.

EXPERIMENT

1 The microtubules of a cell in early anaphase were labeled with a fluorescent dye that glows in the microscope (yellow).

Spindlepole

Kinetochore

Figure 12.8

Non-kinetechore microtubules from opposite Non-kinetechore microtubules from opposite poles overlap and push against each other, poles overlap and push against each other, elongating the cell.elongating the cell.

In telophase, genetically identical daughter In telophase, genetically identical daughter nuclei form at opposite ends of the cell.nuclei form at opposite ends of the cell.

Cytokinesis: Cytokinesis: A Closer LookA Closer Look

In animal cellsIn animal cells– Cytokinesis occurs Cytokinesis occurs

by a process known by a process known as cleavage, forming as cleavage, forming a a cleavage furrowcleavage furrow.. Cleavage furrow

Contractile ring of microfilaments

Daughter cells

100 µm

(a) Cleavage of an animal cell (SEM)Figure 12.9 A

In plant cells, during In plant cells, during cytokinesiscytokinesis– A A cell platecell plate forms. forms.

Daughter cells

1 µmVesiclesforming cell plate

Wall of patent cell Cell plateNew cell wall

(b) Cell plate formation in a plant cell (SEM)Figure 12.9 B

Mitosis in a plant cellMitosis 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

ChromosomeChromatincondensing

Figure 12.10

Binary FissionBinary Fission

Prokaryotes (bacteria) reproduce by a type Prokaryotes (bacteria) reproduce by a type of cell division called of cell division called binary fissionbinary fission..

Binary FissionBinary Fission

The bacterial The bacterial chromosome chromosome replicates.replicates.

The two The two daughter daughter chromosomes chromosomes actively move actively move apart.apart.

Origin ofreplication

E. coli cell BacterialChromosome

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.4Figure 12.11

The Evolution of MitosisThe Evolution of Mitosis Since prokaryotes preceded eukaryotes by Since prokaryotes preceded eukaryotes by

billions of years it is likely that mitosis billions of years it is likely that mitosis evolved from bacterial cell division.evolved from bacterial cell division.

Certain protists exhibit types of cell division Certain protists exhibit types of cell division that seem intermediate between binary that seem intermediate between binary fission and mitosis carried out by most fission and mitosis carried out by most eukaryotic cells.eukaryotic cells.

A hypothetical sequence for the evolution of mitosisA hypothetical sequence for the evolution of mitosis

Most eukaryotes. In most other eukaryotes, including plants and animals, the spindle forms outside the nucleus, and the nuclear envelope breaks down during mitosis. Microtubules separate the chromosomes, and the nuclear envelope then re-forms.

Dinoflagellates. In unicellular protists called dinoflagellates, the nuclear envelope remains intact during cell division, and the chromosomes attach to the nuclear envelope. Microtubules pass through the nucleus inside cytoplasmic tunnels, reinforcing the spatial orientation of the nucleus, which then divides in a fission process reminiscent of bacterial division.

Diatoms. In another group of unicellular protists, the diatoms, the nuclear envelope also remains intact during cell division. But in these organisms, the microtubules form a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into two daughter nuclei.

Prokaryotes. During binary fission, the origins of the daughter chromosomes move to opposite ends of the cell. The mechanism is not fully understood, but proteins may anchor the daughter chromosomes to specific sites on the plasma membrane.

(a)

(b)

(c)

(d)

Bacterialchromosome

Microtubules

Intact nuclear envelope

Chromosomes

Kinetochore microtubules

Intact nuclearenvelope

Kinetochore microtubules

Fragments ofnuclear envelope

Centrosome

Figure 12.12 A-D