12 cellcycle text

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

Cell Replication

• The Cell Cycle, Mitosis, Meiosis, and Basic Inheritance

Figure 12.1


• Unicellular organisms

– Reproduce by cell 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 division for

a. Development from a fertilized cellb. Growthc. 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


• Cell division results in genetically identical daughter cells

• The DNA molecules in a cell

– Are packaged into chromosomes

• A cell’s endowment of DNA, its genetic information

– Is called its genome


• Eukaryotic chromosomes

– Consist of chromatin, a complex of DNA and protein that condenses during cell division

• In animals

– Somatic cells have two sets of chromosomes (diploid)

– Gametes have one set of chromosomes (haploid)


• In preparation for cell division

– DNA is replicated and the chromosomes condense

• Each duplicated chromosome

– Has two sister chromatids, which separate during cell division0.5 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 of

– Mitosis, the division of the nucleus

– Cytokinesis, the division of the cytoplasm

• In meiosis

– Sex cells are produced after a reduction in chromosome number


Phases of the Cell Cycle

• The cell cycle consists of

– The mitotic phase

– InterphaseINTERPHASE

G1

S(DNA synthesis)

G2Cyto

kines

is

Mito

sis

MITOTIC(M) PHASE

Figure 12.5


• The mitotic phase

– Is made up of mitosis and cytokinesis

• Interphase can be divided into subphases

– G1 phase

– S phase

– G2 phase


• Mitosis consists of five distinct phases

– Prophase

– Prometaphase

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


– Metaphase

– Anaphase

– Telophase

Centrosome at one spindle pole

Daughter chromosomes

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate Nucleolus

forming

Cleavagefurrow

Nuclear envelopeforming

Figure 12.6


The Mitotic Spindle: A Closer Look

• The mitotic spindle

– Is an apparatus of microtubules that controls chromosome movement during mitosis

• The spindle arises from the centrosomes

– And includes spindle microtubules and asters

CentrosomeAster

Sisterchromatids

MetaphasePlate

Kinetochores

Overlappingnonkinetochoremicrotubules

Kinetochores microtubules

Centrosome

ChromosomesMicrotubules0.5 µm

1 µm

Figure 12.7


• In anaphase, sister chromatids separate

– And move along the kinetochore microtubules 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


• Nonkinetechore microtubules from opposite poles

– Overlap and push against each other, elongating the cell

• In telophase

– Genetically identical daughter nuclei form at opposite ends of the cell


Cytokinesis: A Closer Look

• In animal cells

– Cytokinesis occurs by a process known as cleavage, forming a cleavage 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 cytokinesis

– A cell plate 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 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

Figure 12.10


Binary Fission

• Prokaryotes (bacteria)

– Reproduce by a type of cell division called binary fission Origin of

replication

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


• The 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


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

Figure 12.14

Control system

G2 checkpoint

M checkpoint

G1 checkpoint

G1

S

G2M


• The clock has specific checkpoints

– Where the cell cycle stops until a go-ahead signal is received

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 12.15 A, B


The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases

• Two types of regulatory proteins are involved in cell cycle control

• Cyclins and cyclin-dependent kinases (Cdks)


• The activity of cyclins and Cdks– Fluctuates during the cell cycle

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

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

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 MMPF activity

Cyclin

Time

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

(b) Molecular mechanisms that help regulate the cell cycle

MPF promotes mitosis by phosphorylating various proteins. MPF‘s activity peaks during metaphase.

3

Figure 12.16 A, B

M


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

• Both internal and external signals

– Control the cell cycle checkpoints


• Cancer cells

– Exhibit neither density-dependent inhibition nor anchorage dependence

25 µm

Cancer cells do not exhibitanchorage dependence or density-dependent inhibition.

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

(b)

Figure 12.18 B


Loss of Cell Cycle Controls in Cancer Cells

• Cancer cells

– Do not respond normally to the body’s control mechanisms

– Form tumors


• Malignant tumors invade surrounding tissues and can metastasize

– Exporting cancer cells to other parts of the body where they may form secondary tumors

Cancer cells invade neighboring tissue.

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

4Cancer cells spread through lymph and blood vessels to other parts of the body.

3A tumor grows from a single cancer cell.

1

Tumor

Glandulartissue

Cancer cell

Bloodvessel

Lymphvessel

MetastaticTumor

Figure 12.19


Meiosis

• Overview: Hereditary Similarity and Variation

• Living organisms

– Are distinguished by their ability to reproduce their own kind


• Heredity

– Is the transmission of traits from one generation to the next

• Variation

– Shows that offspring differ somewhat in appearance from parents and siblings

• Genetics

– Is the scientific study of heredity and hereditary variation

Figure 13.1


• Offspring acquire genes from parents by inheriting chromosomes

• Genes

– Are the units of heredity

– Are segments of DNA

• Each gene in an organism’s DNA

– Has a specific locus on a certain chromosome

• We inherit

– One set of chromosomes from our mother and one set from our father


Comparison of Asexual and Sexual Reproduction

• In asexual reproduction

– One parent produces genetically identical offspring by mitosis

Figure 13.2

Parent

Bud

0.5 mm


• In sexual reproduction

– Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

• Fertilization and meiosis alternate in sexual life cycles

• A life cycle

– Is the generation-to-generation sequence of stages in the reproductive history of an organism


Sets of Chromosomes in Human Cells

• In humans

– Each somatic cell has 46 chromosomes, made up of two sets (diploid 2n = 46)

– One set of chromosomes comes from each parent (haploid)


5 µmPair of homologous

chromosomes

Centromere

Sisterchromatids

Figure 13.3

• A karyotype

– Is an ordered, visual representation of the chromosomes in a cell


• Homologous chromosomes

– Are the two chromosomes composing a pair

– Have the same characteristics

– May also be called autosomes

• Sex chromosomes

– Are distinct from each other in their characteristics

– Are represented as X and Y

– Determine the sex of the individual, XX being female, XY being male


• In a cell in which DNA synthesis has occurred

– All the chromosomes are duplicated and thus each consists of two identical sister chromatids

Figure 13.4

Key

Maternal set ofchromosomes (n = 3)

Paternal set ofchromosomes (n = 3)

2n = 6

Two sister chromatidsof one replicated

chromosome

Two nonsisterchromatids in

a homologous pair

Pair of homologouschromosomes

(one from each set)

Centromere


• Unlike somatic cells

– Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes

• At sexual maturity

– The ovaries and testes produce haploid gametes by meiosis

• During fertilization

– These gametes, sperm and ovum, fuse, forming a diploid zygote

• The zygote

– Develops into an adult organism


Figure 13.5

Key

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

SpermCell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploidzygote

(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

• The human life cycle


• In animals

– Meiosis occurs during gamete formation

– Gametes are the only haploid cells

Gametes

Figure 13.6 A

Diploidmulticellular

organism

Key


n

n

n

2n2nZygote

Haploid

Diploid

Mitosis

(a) Animals



nn

n

nn

2n2n

Haploid multicellularorganism (gametophyte)

Mitosis Mitosis

SporesGametes

Mitosis

Zygote

Diploidmulticellular

organism(sporophyte)

(b) Plants and some algaeFigure 13.6 B

• Plants and some algae

– Exhibit an alternation of generations

– The life cycle includes both diploid and haploid multicellular stages



nn

n

n

n

2n

Haploid multicellularorganism

Mitosis Mitosis

Gametes

Zygote(c) Most fungi and some protistsFigure 13.6 C

• In most fungi and some protists

– Meiosis produces haploid cells that give rise to a haploid multicellular adult organism

– The haploid adult carries out mitosis, producing cells that will become gametes


• Meiosis reduces the number of chromosome sets from diploid to haploid

• Meiosis

– Takes place in two sets of divisions, meiosis I and meiosis II


The Stages of Meiosis

• An overview of meiosis

Figure 13.7

Interphase

Homologous pairof chromosomes

in diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicatedchromosomes

1

2

Homologous chromosomes

separate

Haploid cells withreplicated chromosomes

Sister chromatids separate

Haploid cells with unreplicated chromosomes

Meiosis I

Meiosis II


• Meiosis I

– Reduces the number of chromosomes from diploid to haploid

• Meiosis II

– Produces four haploid daughter cells


Centrosomes(with centriole pairs)

Sisterchromatids

Chiasmata

Spindle

Tetrad

Nuclearenvelope

Chromatin

Centromere(with kinetochore)

Microtubuleattached tokinetochore

Tertads line up

Metaphaseplate

Homologouschromosomes

separate

Sister chromatidsremain attached

Pairs of homologouschromosomes split up

Chromosomes duplicateHomologous chromosomes

(red and blue) pair and exchangesegments; 2n = 6 in this example

INTERPHASE MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

• Interphase and meiosis I

Figure 13.8


TELOPHASE I ANDCYTOKINESIS

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS

MEIOSIS II: Separates sister chromatids

Cleavagefurrow Sister chromatids

separate

Haploid daughter cellsforming

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Two haploid cellsform; chromosomes

are still doubleFigure 13.8

• Telophase I, cytokinesis, and meiosis II


A Comparison of Mitosis and Meiosis

• Meiosis and mitosis can be distinguished from mitosis

– By three events in Meiosis l

a. Synapsis and crossing over Homologous chromosomes physically connect

and exchange genetic information

b. Tetrads on the metaphase plate

At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the

metaphase plates

c. Separation of homologues

At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell

In anaphase II of meiosis, the sister chromatids separate


Figure 13.9

MITOSIS MEIOSIS

Prophase

Duplicated chromosome(two sister chromatids)

Chromosomereplication

Chromosomereplication

Parent cell(before chromosome replication)

Chiasma (site ofcrossing over)

MEIOSIS I

Prophase I

Tetrad formed bysynapsis of homologous

chromosomes

Metaphase

Chromosomespositioned at themetaphase plate

Tetradspositioned at themetaphase plate

Metaphase I

Anaphase ITelophase I

Haploidn = 3

MEIOSIS II

Daughtercells of

meiosis I

Homologuesseparate

duringanaphase I;

sisterchromatids

remain together

Daughter cells of meiosis II

n n n n

Sister chromatids separate during anaphase II

AnaphaseTelophase

Sister chromatidsseparate during

anaphase

2n 2nDaughter cells

of mitosis

2n = 6

• A comparison of mitosis and meiosis


• Genetic variation produced in sexual life cycles contributes to greater variation

• Reshuffling of genetic material in meiosis

– Produces genetic variation

• In species that produce sexually

– The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation


Independent Assortment of Chromosomes

• Homologous pairs of chromosomes

– Orient randomly at metaphase I of meiosis


• In independent assortment

– Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs

Figure 13.10

Key

Maternal set ofchromosomesPaternal set ofchromosomes

Possibility 1

Two equally probable arrangements ofchromosomes at

metaphase I

Possibility 2

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4


Crossing Over

• Crossing over

– Produces recombinant chromosomes that carry genes derived from two different parents

Figure 13.11

Prophase Iof meiosis

Nonsisterchromatids

Tetrad

Chiasma,site of

crossingover

Metaphase I

Metaphase II

Daughtercells

Recombinantchromosomes

Variation


Random Fertilization

• The fusion of gametes

– Will produce a zygote with any of about 64 trillion diploid combinations

Number of

children from one couple without

two exactly

the same: 102017


• Mutations

– Are the original source of genetic variation

• Sexual reproduction

– Produces new combinations of variant genes, adding more genetic diversity










Wild dogs 1Aardwolf

African wild dog

Arctic fox

Argentine gray fox

Black-backed jackal

Blanford’s fox

Bat-eared fox

Bush dog

Wild dogs


Wild dogs 2

Wild dogs

Arctic wolf

Cape fox

Corsac fox

Coyote

Crab-eating fox

Culpeo fox

DholeFennec fox


Wild dogs

Arctic fox

Dingo

Dingo

Ethiopian wolf

Falkland Island’s fox

Golden jackal

Tibetan sand fox

Gray wolf


Wild dogs 4

Wild dogs

Gray fox

Hoary zorroKit fox

Maned wolfMexican gray wolf

Raccoon dog

Sand fox

Small-eared dog


Wild dogs 5

Wild dogs

Pale fox

Pampas fox

Red fox

Red wolfSechuan zorro

Timber wolf

Iberian wolf


Q 910 Savolainen, et.alThe origin of the domestic dog from wolves has been established … we examined the mitochondrial DNA

(mtDNA) sequence variation among 654 domestic dogs representing all major dog populations worldwide … suggesting a common origin from a

single gene pool for all dog populations.

Q 910

Savolainen, et.al., ‘Genetic Evidence for an East Asian origin of Domestic Dogs,’ Science, Vol 298:5598, 22 Nov 2002,

pp 1610-1613.