In eukaryotes, heritable information is passed to the

next generation via processes that include the cell cycle and

mitosis or meiosis plus fertilization

Cell Cycle and Mitosis 8.1 to 8.11

Genetic information is stored and transmitted through DNA

All the DNA in a cell constitutes the cell’s genome

A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells)

DNA molecules in a cell are packaged into chromosomes

Fig. 12-3

20 µm

Genetic Information is stored and transmitted through DNA

Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus

Somatic cells (nonreproductive cells) have two sets of chromosomes - DIPLOID

Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells - HAPLOID

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

Fig. 12-4 0.5 µm Chromosomes

Chromosomeduplication(including DNAsynthesis)

Chromo-some arm

Centromere

Sisterchromatids

DNA molecules

Separation ofsister chromatids

Centromere

Sister chromatids

The cell cycle is a complex set of stages that is highly regulated with checkpoints, which determine the ultimate fate of the cell

Interphase

Majority of the cell cycle

Time when a cell’s metabolic activity is very high and the cell performs various functions

3 stages◦G1 – cell growth◦S – DNA replication (Synthesis of DNA)◦G2 – prepare for mitosis

M Phase (mitotic phase)

About 10% of the cell cycle

2 stages◦Mitosis – nuclear division◦Cytokinesis – cytoplasm division

Results in two genetically identical cells

The cell cycle is directed by internal controls or checkpoints.

Cell Cycle Checkpoints

• 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

Cell Cycle Checkpoints

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

MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase

Fig. 12-17M G1 S G2 M G1 S G2 M G1

MPF activityCyclinconcentration

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

Degradedcyclin

CdkG 1

S

G 2

M

CdkG2checkpointCyclin is

degradedCyclinMPF

(b) Molecular mechanisms that help regulate the cell cycle

Cyclin accumulation

Internal and External signals provide stop-and-go signs 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

Fig. 12-19

Anchorage dependence

Density-dependent inhibition

Density-dependent inhibition

(a) Normal mammalian cells (b) Cancer cells25 µm25 µm

Mitosis passes a complete genome from the parent cell to the daughter cell

Mitosis is conventionally divided into five phases:◦Prophase◦Prometaphase◦Metaphase◦Anaphase◦Telophase

Cytokinesis is well underway by late telophase

Fig. 12-6b

PrometaphaseProphaseG2 of InterphaseNonkinetochore

microtubulesFragmentsof nuclearenvelope

Aster CentromereEarly mitoticspindle

Chromatin(duplicated)

Centrosomes(with centriolepairs)

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Kinetochore Kinetochoremicrotubule

Fig. 12-7

Microtubules Chromosomes

Sisterchromatids

Aster

Metaphaseplate

Centrosome

Kineto-chores

Kinetochoremicrotubules

Overlappingnonkinetochoremicrotubules

Centrosome 1 µm

0.5 µm

Fig. 12-6d

Metaphase Anaphase Telophase and CytokinesisCleavagefurrow

Nucleolusforming

Metaphaseplate

Centrosome atone spindle pole

SpindleDaughterchromosomes

Nuclearenvelopeforming

Fig. 12-8b

Kinetochore

MicrotubuleTubulinSubunits

Chromosome

Chromosomemovement

Motorprotein

Fig. 12-9

Cleavage furrow100 µm

Contractile ring ofmicrofilaments

Daughter cells

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

Vesiclesformingcell plate

Wall ofparent cell

Cell plate

Daughter cells

New cell wall

1 µm

Summary of Mitosis

What must occur before mitosis?◦DNA replication◦Pass G2 checkpoint

What is “checked” at the ‘M’ checkpoint?◦Chromosome attachment to microtubule

What follows mitosis?◦Cytokinesis

What are the products of the mitosis?◦2 genetically identical cells

Why do we need mitosis?◦Growth, repair, asexual reproduction