The Cell Cycle and Cell Division

The Cell Cycle and Cell Division

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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)




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


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.


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) that occur in homologous pairs.


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 = 2n and cells are diploid.


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)


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.



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.



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.


What life cycle is this?

You are studying two, tiny plant-like organisms that you have discovered in your yard. One is a small, green fuzzy structure about half an inch high. It is multicellular, and each cell has 6 chromosomes. You notice that it sometimes produces single cells that can move away, independently, in rainwater or dew. Each of these cells also has 6 chromosomes. You observe two of these independent cells fusing together.

The second organism is an upright, brown spike about an inch tall. Each of its cells has 12 chromosomes. You observe that cells in its upper end are undergoing cell division. Each of the parent cells produces four daughter cells that each have 6 chromosomes. The daughter cells are blown away by the wind.

You perform DNA analysis on both organisms. To your surprise, they turn out to share the same genome! They are two different life stages of the same species.


What life cycle is this? (continued)

Working in small groups, develop a hypothesis about this organism’s life cycle.

• How are the green fuzzy stage and the brown spike stage related to each other? How could you test your ideas?

• Next, diagram your organism’s life cycle, using the figures on p.126 of your textbook as a guide. Decide whether this life cycle is most likely haplontic, diplontic, or alternation of generations. On your diagram, label the following structures: sporophyte, gametophyte, spore, and gamete. Specify whether each life stage is haploid (1n) or diploid (2n). Finally, label where meiosis and fertilization occur.


Recall that the small, green fuzzy organism consists of many cells that each have 6 chromosomes, while the brown spike consists of many cells that each have 12 chromosomes.

The small, green fuzzy organism is

a. diploid.

b. haploid.

c. I don’t know.


The small, green fuzzy organism consists of many cells that each have 6 chromosomes. It sometimes produces independent cells that also have 6 chromosomes. Two of these cells can fuse together.

The small, green fuzzy organism is producing

a. diploid spores.

b. haploid spores.

c. diploid gametes.

d. haploid gametes.

e. I don’t know.


The small, green fuzzy organism is

a. a sporophyte.

b. a gametophyte.

c. I don’t know.


The brown spike stage consists of many cells that each have 12 chromosomes. Cells at the top are undergoing cell division; each of these parent cells produces four daughter cells that each have 6 chromosomes. The daughter cells are blown away by the wind.

The brown spike organism is producing

a. diploid spores.

b. haploid spores.

c. diploid gametes.

d. haploid gametes.

e. I don’t know.


The green fuzzy stage is a _______; the brown spike stage is a _______.

a. diploid gametophyte; haploid sporophyte

b. haploid gametophyte; diploid sporophyte

c. diploid sporophyte; haploid gametophyte

d. haploid sporophyte; diploid gametophyte

e. I don’t know.


The life cycle of this green fuzzy / brown spike species is best described as

a. haplontic.

b. diplontic.

c. alternation of generations.

d. I don’t know.

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


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


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


Replication occurs as the DNA is threaded through a “replication complex” of proteins in the center of the cell.

Replication begins at the ori site and moves towards the ter site.


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


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.


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.


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.


Cytokinesis follows mitosis.

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


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.


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)




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.


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


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.


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.


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.


Centrosomes serve as mitotic centers or poles; the spindle forms between the poles from two types of microtubules:

• Polar microtubules form a spindle and overlap in the center.

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


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)


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)


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.


Telophase occurs after chromosomes have separated:

• Spindle breaks down

• Chromosomes uncoil

• Nuclear envelope and nucleoli appear

• Two daughter nuclei are formed with identical genetic information


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)


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)


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 7.2 Both Binary Fission And Mitosis Produce Genetically Identical Cells

Reviewing the stages of mitosis

Working in pairs and not looking at your notes, briefly review the stages of mitosis. First, put the following five stages in order:

• Metaphase

• Telophase

• Propase

• Anaphase

• Prometaphase

Then take turns naming a feature of each of the five stages. (Person A names one feature of the first stage; the person B names another feature of the first stage. Then proceed to the second stage and name two more features, etc. Any feature counts.)

Concept 7.2 Both Binary Fission And Mitosis Produce Genetically Identical Cells

During which of these stages is DNA replicated?

a. Telophase

b. Prometaphase

c. Prophase

d. Anaphase

e. None of the above

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.


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


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)


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.


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


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.


The cell cycle game

The entire classroom is a cell in the G1 phase. Will we enter the S phase and copy our DNA?

You will be assigned one of the following roles:

• DNA copiers: You are the cell’s DNA replication machinery. There is a DNA sequence on the board. Your job is to copy the sequence, exactly, onto a piece of paper. If interrupted or if you make a mistake, you must start over. Call out “DNA copied!” when you finally succeed.

• Retinoblastoma protein: Your job is to prevent the DNA from being copied; that is, you are keeping the cell in the G1 phase. You may try any techniques as long as you don’t touch the DNA-copying people directly. You can block their line of sight, cover their eyes with jackets, block the board with your own body, hide their pencils, etc.—anything that will keep them from copying the DNA. When the game starts, you are not phosphorylated, and that means you are active. Once you are phosphorylated (by being given a phosphate group), you’ll move out of the way and become inactive.


The cell cycle game (continued)

• Cyclin-dependent kinase (Cdk): [Note: You are only active if you are holding a cyclin molecule.] Once you have a cyclin molecule, you will become active and you will phosphorylate all the retinoblastoma proteins you can find. You will do this by giving them phosphate groups.

• Cyclin maker: You are an enzyme that will make many copies of the molecule cyclin (by writing “CYCLIN” on a piece of paper) and handing them to Cdk people. You are not active yet; you’ll only become active when a growth factor arrives and taps you on the head.

• Growth factor: When the game starts, you have not arrived at the cell yet. When cued, make a dramatic entrance, and announce your arrival clearly by saying “The growth factor has arrived!” Then tap all the cyclin makers on the head to activate them.


Which of the following is a true statement about Cdk’s and cyclins?

a. Both are always present in the cell.

b. Cyclins are always present; Cdk’s are only made at certain times.

c. Cdk’s are always present; cyclins are only made at certain times.

d. Cdk’s are active when not bound to a cyclin.

e. I don’t know.

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)






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.


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.


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


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.


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


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


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.


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


Organisms with triploid (3n), tetraploid (4n), 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.


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

Meiosis footrace

Divide the class into two halves. Each half will race the other to produce a complete diagram of the stages of meiosis. Start with two volunteer “runners,” one from each half of the classroom, lined up at the back of the room. When the clock is started, these two runners will run up to the board from the back of the room and draw a single, diploid, eukaryotic cell that has 2 pairs of chromosomes (a long pair and a short pair) that is in Prophase I.

Once a runner’s diagram is completed, he or she will run back to the back of the room and tag the next runner, who will run to the board and draw the next stage of meiosis.

Concept 7.4 Meiosis Halves The Nuclear Chromosome Content And Generates Diversity

Meiosis footrace (continued)

Every diagram should include:

• the name of the phase

• all the chromosomes (drawn clearly enough to see if chromatids are present)

• spindle (if present)

• nuclear membrane (if present)

Non-runners can coach runners from the sidelines. Each team should also be compiling a list of differences between mitosis and meiosis. (Designate one person per team to write down the list.)

At the end, check all diagrams for any errors. Each team gets 2 points per correct diagram, 1 point for every correct item on their list of mitosis/meiosis differences, and a 5 point bonus for finishing first!



Which of the following is not a difference between mitosis and meiosis?

a. Meiosis produces four daughter cells from one parent cell; mitosis produces two.

b. Tetrads form during meiosis but not during mitosis.

c. Meiosis halves the 2n number; mitosis does not.

d. Crossing over occurs only during mitosis.

e. Mitosis has only 1 cell division; meiosis has 2.

Apply the concept

Meiosis halves the nuclear chromosome content and generates diversity

In the anther (male sex organ) of the lily plant, cells undergo mitosis in synchrony. These cells can be removed and studied in the laboratory. An antibody was developed that would bind specifically to a chromosomal protein called B1. A procedure involving antibody binding and staining of the antibody was used to detect the protein in anther cells. The protein was detected at the centromeres of mitotic chromosomes, and its presence or absence was monitored at different stages of the cell cycle. The results were G1, absent; early S, absent; late S through metaphase, present; anaphase, absent; telophase, absent.

1. Propose a role for this protein in sister chromatid function.

2. What rsults would you expect if meiotic cells were examined for this protein?

3. At what stages of meiosis would the protein be present or absent?

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.


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)


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

Concept 7.5 Programmed Cell Death Is A Necessary Process In Living Organisms

Cancer and apoptosis

Working in pairs and not looking at your notes, first review what apoptosis is. See if you can name two major reasons for apoptosis (i.e., two reasons that a multicellular body might need a certain cell to perform apoptosis). Then discuss the following statement:

Cancer represents a failure of apoptosis.

Do you agree with this statement? Why or why not? Be prepared to explain your answer to the class.

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The Cell Cycle and Cell Division