7

The Cell Cycle and Cell Division

Chapter 7 The Cell Cycle and Cell Division

Key Concepts

• 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • 7.3 Cell Reproduction Is Under Precise Control

Chapter 7 The Cell Cycle and Cell Division

Key Concepts

• 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity • 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms

Chapter 7 Opening Question How does infection with HPV result in uncontrolled cell reproduction?

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The lifespan of an organism is linked to

cell reproduction

—

usually called cell division.

Organisms have two basic strategies for reproducing themselves: • Asexual reproduction • Sexual reproduction Cell division is also important in growth and repair of tissues.

Figure 7.1 The Importance of Cell Division (Part 1)

Figure 7.1 The Importance of Cell Division (Part 2)

Figure 7.1 The Importance of Cell Division (Part 3)

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction In

asexual reproduction

the offspring are

clones

—genetically identical to the parent.

Any genetic variations are due to

mutations

.

A unicellular prokaryote may reproduce itself by

binary fission.

Single-cell eukaryotes can reproduce by

mitosis.

Other eukaryotes are also able to reproduce through asexual or sexual means.

Figure 7.2 Asexual Reproduction on a Large Scale

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction

Sexual reproduction

requires

gametes

—two parents each contribute one gamete to an offspring. Gametes form by

meiosis

—a process of cell division.

Gametes —and offspring—differ genetically from each other and from the parents.

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction DNA in eukaryotic cells is organized into

chromosomes

.

A

chromosome

consists of a single molecule of DNA and proteins.

Somatic cells

—body cells not specialized for reproduction Each somatic cell contains

two sets of chromosomes (homologs)

homologous pairs.

that occur in

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Gametes contain only

one

set of chromosomes —one homolog from each pair.

Haploid

cell —Number of chromosomes =

n

Fertilization

—Two haploid gametes (female egg and male sperm) fuse to form a

zygote

.

Chromosome number in zygote = 2

n

and cells are

diploid

.

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction All kinds of sexual life cycles involve meiosis:

Haplontic

life cycle —in protists, fungi, and some algae —zygote is only diploid stage After zygote forms it undergoes meiosis to form haploid

spores

, which germinate to form a new organism.

Organism is haploid, and produces gametes by mitosis —cells fuse to form diploid zygote.

Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 1)

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction

Alternation of generations

—most plants, some protists; meiosis gives rise to haploid spores Spores divide by mitosis to form the haploid generation (

gametophyte

).

Gametophyte forms gametes by mitosis. Gametes then fuse to form diploid zygote (

sporophyte

), which in turn produces haploid spores by meiosis.

Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 2)

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction

Diplontic

life cycle —animals and some plants; gametes are the only haploid stage A mature organism is diploid and produces gametes by meiosis.

Gametes fuse to form diploid zygote; zygote divides by mitosis to form mature organism.

Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 3)

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The essence of sexual reproduction is that it allows the

random selection of half the diploid chromosome set.

This forms a haploid gamete that fuses with another to make a diploid cell.

Thus, no two individuals have exactly the same genetic makeup.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Four events must occur for cell division: •

Reproductive signal

—to initiate cell division •

Replication

of DNA •

Segregation

—distribution of the DNA into the two new cells •

Cytokinesis

—division of the cytoplasm and separation of the two new cells

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells In prokaryotes, cell division results in reproduction of the entire organism.

The cell: • Grows in size • Replicates its DNA • Separates the DNA and cytoplasm into two cells through

binary fission

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Most prokaryotes have one chromosome, a single molecule of DNA —usually

circular

.

Two important regions in reproduction: •

ori

- where replication starts •

ter

- where replication ends

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Replication occurs as the DNA is threaded through a “replication complex” of proteins in the center of the cell.

Replication begins at the

ori

towards the

ter

site.

site and moves

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells As replication proceeds, the

ori

complexes move to opposite ends of the cell.

DNA sequences adjacent to the

ori

region actively bind proteins for the segregation, hydrolyzing ATP for energy.

An actin-like protein provides a filament along which

ori

and other proteins move.

Figure 7.4 Prokaryotic Cell Division

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Cytokinesis begins after chromosome segregation by a pinching in of the plasma membrane —protein fibers form a ring.

As the membrane pinches in, new cell wall materials are synthesized resulting in separation of the two cells.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Eukaryotic cells divide by mitosis followed by cytokinesis.

Replication

of

DNA

occurs as long strands are threaded through replication complexes.

DNA replication only occurs during a specific stage of the cell cycle.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells In

segregation of DNA

after cell division, one copy of each chromosome ends up in each of the two new cells.

In eukaryotes, the chromosomes become highly condensed.

Mitosis

segregates them into two new nuclei — the cytoskeleton is involved in the process.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells

Cytokinesis

follows mitosis.

The process in plant cells (which have cell walls) is different than in animal cells (which do not have cell walls).

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells The

cell cycle

—the period between cell divisions In eukaryotes it is divided into mitosis and cytokinesis —called the M phase—and a long interphase.

During

interphase

, the cell nucleus is visible and cell functions including replication occur Interphase begins after cytokinesis and ends when mitosis starts.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Interphase has three subphases: G1, S, and G2.

G1

(

Gap 1

) —variable, a cell may spend a long time in this phase carrying out its functions

S phase

(

Synthesis

) —DNA is replicated

G2

(

Gap 2

) —the cell prepares for mitosis, synthesizes microtubules for segregating chromosomes

Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 1)

Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 2)

Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 3)

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells In mitosis,

one nucleus produces two daughter nuclei each containing the same number of chromosomes as the parent nucleus

. Mitosis is continuous, but can be can be divided into phases —prophase, prometaphase, metaphase, anaphase, and telophase.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells During interphase, only the nuclear envelope and and the nucleolus are visible.

The

chromatin

(DNA) is not yet condensed.

Three structures appear in

prophase:

• The condensed chromosomes • Centrosome • Spindle

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells

Condensed chromosomes

appear during prophase.

Sister chromatids

—two DNA molecules on each chromosome after replication

Centromere

—region where chromatids are joined

Kinetochores

are protein structures on the centromeres, and are important for chromosome movement.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells The

karyotype

of an organism reflects the number and sizes of its condensed chromosomes.

Karyotype analysis can be used to identify organisms, but DNA sequence is more commonly used.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Segregation is aided by other structures: The

centrosome

determines the orientation of the spindle apparatus.

Each centrosome can consist of two

centrioles

—hollow tubes formed by microtubules.

Centrosome is duplicated during S phase and each moves towards opposite sides of the nucleus.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Centrosomes serve as

mitotic centers

or

poles

; the

spindle

forms between the poles from two types of microtubules: •

Polar microtubules

in the center.

form a spindle and overlap •

Kinetochore microtubules

—attach to kinetochores on the chromatids. Sister chromatids attach to opposite halves of the spindle.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Chromosome separation and movement is highly organized.

During

prometaphase

, the nuclear envelope breaks down.

Chromosomes consisting of two chromatids attach to the kinetochore mictotubules.

Figure 7.6 The Phases of Mitosis (1)

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Durin

metaphase

, chromosomes line up at the midline of the cell.

During

anaphase

, the separation of sister chromatids is controlled by M phase cyclin Cdk; cohesin is hydrolyzed by

separase.

After separation, they move to opposite ends of the spindle and are referred to as

daughter chromosomes

.

Figure 7.6 The Phases of Mitosis (2)

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells A protein at the kinetochores —

cytoplasmic dynein

—hydrolyzes ATP for energy to move chromosomes along the microtubules towards the poles.

Microtubules also shorten, drawing chromosomes toward poles.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells

Telophase

separated: occurs after chromosomes have • Spindle breaks down • Chromosomes uncoil • Nuclear envelope and nucleoli appear • Two daughter nuclei are formed with identical genetic information

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Cytokinesis: Division of the cytoplasm differs in plant and animals • In animal cells, plasma membrane pinches between the nuclei because of a

contractile ring

of microfilaments of actin and myosin.

Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 1)

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Plant cells: Vesicles from the Golgi apparatus appear along the plane of cell division • These fuse to form a new plasma membrane.

• Contents of vesicles form the

cell plate

—the beginning of the new cell wall.

Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 2)

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells After cytokinesis:

Each daughter cell contains all of the components of a complete cell

.

Chromosomes are precisely distributed.

The

orientation

of cell division is important to development, but organelles are not always evenly distributed.

Concept 7.3 Cell Reproduction Is Under Precise Control The reproductive rates of most prokaryotes respond to environmental conditions. In eukaryotes, cell division is related to the needs of the entire organism.

Cells divide in response to extracellular signals, like

growth factors

.

Concept 7.3 Cell Reproduction Is Under Precise Control The eukaryotic cell cycle has four stages: G1, S, G2, and M.

Progression is tightly regulated —the

G1-S

transition is called

R

, the

restriction point

.

Passing this point usually means the cell will proceed with the cell cycle and divide.

Figure 7.8 The Eukaryotic Cell Cycle

Concept 7.3 Cell Reproduction Is Under Precise Control Specific signals trigger the transition from one phase to another.

Evidence for substances as triggers came from

cell fusion

experiments.

Nuclei in cells at different stages, fused by polyethylene glycol, both entered the phase of DNA replication (S).

Figure 7.9 Regulation of the Cell Cycle (Part 1)

Concept 7.3 Cell Reproduction Is Under Precise Control Transitions also depend on activation of

cyclin dependent kinases

(

Cdk’s

). A

protein kinase

is an enzyme that catalyzes phosphorylation from ATP to a protein.

Phosphorylation changes the shape and function of a protein by changing its charges.

Concept 7.3 Cell Reproduction Is Under Precise Control Cdk is activated by binding to

cyclin

(by

allosteric regulation

); this alters its shape and exposes its active site.

The G1-S cyclin-Cdk complex acts as a protein kinase and triggers transition from G1 to S.

Other cyclin Cdk’s act at different stages of the cell cycle, called

cell cycle checkpoints

.

Figure 7.10 Cyclins Are Transient in the Cell Cycle

Concept 7.3 Cell Reproduction Is Under Precise Control Example of G1-S cyclin-Cdk regulation: Progress past the restriction point in G1 depends on

retinoblastoma protein (RB

).

RB normally inhibits the cell cycle, but when phosphorylated by G1-S cyclin-Cdk, RB becomes inactive and no longer blocks the cell cycle.

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity

Meiosis

consists of

two

nuclear divisions but

DNA is replicated only once

. The function of meiosis is to: • Reduce the chromosome number from diploid to haploid • Ensure that each haploid has a complete set of chromosomes • Generate diversity among the products

Figure 7.11 Mitosis and Meiosis: A Comparison

Figure 7.12 Meiosis: Generating Haploid Cells (Part 1)

Figure 7.12 Meiosis: Generating Haploid Cells (Part 2)

Figure 7.12 Meiosis: Generating Haploid Cells (Part 3)

Figure 7.12 Meiosis: Generating Haploid Cells (Part 4)

Figure 7.12 Meiosis: Generating Haploid Cells (Part 5)

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiotic division reduces the chromosome number. Two unique features: • In

meiosis I

,

homologous pairs of chromosomes come together and line up

along their entire lengths.

• After metaphase I,

the homologous chromosome pairs separate

, but individual chromosomes made up of two sister chromatids remain together.

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiosis I is preceded by an S phase during which DNA is replicated.

Each chromosome then consists of two sister chromatids, held together by cohesin proteins.

At the end of meiosis I, two nuclei form, each with half the original chromosomes —still composed of sister chromatids.

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Sister chromatids separate during

meiosis II

, which is

not

proceeded by DNA replication.

The products of meiosis I and II are four cells with a haploid number of chromosomes.

These four cells are not genetically identical

.

Two processes may occur: Crossing over and independent assortment

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity In prophase of meiosis I homologous chromosomes pair by

synapsis

.

The four chromatids of each pair of chromosomes form a

tetrad

,or

bivalent

.

The homologs seem to repel each other but are held together at

chiasmata.

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity

Crossing over

is an

exchange of genetic material

that occurs at the chiasma.

Crossing over results in

recombinant

chromatids and increases genetic variability of the products.

In-Text Art, Ch. 7, p. 138

Figure 7.13 Crossing Over Forms Genetically Diverse Chromosomes

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Prophase I may last a long time.

• Human males: Prophase I lasts about 1 week, and 1 month for entire meiotic cycle • Human females: Prophase I begins before birth, and ends up to decades later during the monthly ovarian cycle

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity

Independent assortment

during anaphase I also allows for chance combinations and genetic diversity.

After homologous pairs of chromosomes line up at metaphase I, it is a matter of chance which member of a pair goes to which daughter cell.

The more chromosomes involved, the more combinations possible.

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiotic errors:

Nondisjunction

—homologous pairs fail to separate at anaphase I —sister chromatids fail to separate, or homologous chromosomes may not remain together Either results in

aneuploidy

—chromosomes lacking or present in excess

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Organisms with triploid (3

n

), tetraploid (4

n

), and even higher levels are called

polyploid

.

This can occur through an extra round of DNA duplication before meiosis, or the lack of spindle formation in meiosis II.

• Polyploidy occurs naturally in some species, and can be desirable in plants.

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity If crossing over happens between

non homologous chromosomes

, the result is a

translocation

.

A piece of chromosome may rejoin another chromosome, and its location can have profound effects on the expression of other genes.

Example: Leukemia

In-Text Art, Ch. 7, p. 140

Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Cell death occurs in two ways: • In

necrosis

, the cell is damaged or starved for oxygen or nutrients. The cell swells and bursts.

Cell contents are released to the extracellular environment and can cause inflammation.

7.5 ProConcept 7.ammed Cell Death Is a Necessary Process in Living Organisms •

Apoptosis

is genetically programmed cell death. Two possible reasons:

The cell is no longer needed

, e.g., the connective tissue between the fingers of a fetus.

Old cells may be prone to genetic damage that can lead to cancer

—blood cells and epithelial cells die after days or weeks.

Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Events of apoptosis: • Cell detaches from its neighbors • Cuts up its chromatin into nucleosome-sized pieces • Forms membranous lobes called “blebs” that break into fragments • Surrounding living cells ingest the remains of the dead cell

Figure 7.14 Apoptosis: Programmed Cell Death (Part 1)

Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Cell death cycle is controlled by signals: • Lack of a mitotic signal (growth factor) • Recognition of damaged DNA External signals cause membrane proteins to change shape and activate enzymes called

caspases

—hydrolyze proteins of membranes.

Figure 7.14 Apoptosis: Programmed Cell Death (Part 2)

Answer to Opening Question Human papilloma virus (HPV) stimulates the cell cycle when it infects the cervix.

Two proteins regulate the cell cycle:

Oncogene

proteins are positive regulators of the cell cycle —in cancer cells they are overactive or present in excess

Tumor suppressors

are negative regulators of the cell cycle, but in cancer cells they are inactive —can be blocked by a virus such as HPV

Figure 7.15 Molecular Changes Regulate the Cell Cycle in Cancer Cells