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Chapter Introduction

The Life of a Eukaryotic Cell

8.1 Cell Division in Eukaryotes

8.2 The Phases of the Cell Cycle

DNA Replication

8.3 DNA Structure

8.4 DNA Synthesis

8.5 DNA Repair

Mitosis and Cell Division

8.6 The Stages of Cell Division

8.7 Differences in Mitosis

Regulation of the Cell Cycle

8.8 Control of the Cell Cycle

8.9 Checkpoints

Chapter Highlights

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A Compare the processes of cell division in prokaryotes and eukaryotes.

Learning Outcomes

By the end of this chapter you will be able to:

B Describe the four phases of the cell cycle and how they are controlled.

C Summarize the events of DNA replication and evaluate the importance of correcting DNA replication errors.

D Describe the states of mitosis and compare and contrast mitosis in plant and animal cells.

E Describe how the cell cycle is regulated.


How is this event important in the growth and development of multicellular organisms?

Transport Systems What is happening to

these cells?

You can see the root tip cells of an onion plant at various stages of the cell cycle in this light micrograph (x400).


Transport Systems• Eukaryotic cells cycle through a series

of ordered processes that result in duplication of the cell.

• Errors in either DNA replication or mitosis can seriously damage or kill a cell or even a multicellular organism.

You can see the root tip cells of an onion plant at various stages of the cell cycle in this light micrograph (x400).


• Eukaryotic cell division is part of a more complex series of stages called the cell cycle.


8.1 Cell Division in Eukaryotes

• Unicellular eukaryotes, such as yeast and Amoeba, divide to produce two new identical organisms.

A scanning electron micrograph of the cilia-covered Tetrahymena, x6000, is shown here. This single-celled eukaryote is shown in a late stage of cell division.


• Multicellular organisms usually develop from a single fertilized egg cell.

8.1 Cell Division in Eukaryotes (cont.)

• Plants have specialized regions at the tips of their roots and stems where, through repeated cell divisions, they produce the new cells that develop into the mature tissues of growing roots, stems, leaves, and other organs.

• During animal development, cell division produces many different types of cells that form the nerves, skin, and other organs.



• Eukaryotic cell division requires accurate replication and equal division of the genetic information encoded in the cell’s DNA.

8.1 Cell Division in Eukaryotes (cont.)

• Cell division also replaces cells that simply wear out or are damaged during the life of an organism.

• The cell cycle is remarkably similar in all eukaryotes.



• As a single cell completes the cell cycle, it becomes two new daughter cells.

8.2 The Phases of the Cell Cycle

• When a eukaryotic cell divides, its nuclear membrane breaks down, the individual chromosomes separate, and they are distributed to the daughter cells in a process called mitosis.


• The period between divisions is called interphase.


• Cells pass in order through, and are always in one of, the five phases of the cell cycle known as:

8.2 The Phases of the Cell Cycle (cont.)

– G1 (Gap 1 or prereplication)


– S (DNA Synthesis)

– G2 (Gap 2 or premitosis)

– M (Mitosis)

– G0 (Gap 0 or nondividing cells).


• When a cell in G0 or G1 receives a signal to divide, it passes through the restriction point (R).

• Once a cell passes the restriction point, it cannot return to G1 or G0 without completing a full cell cycle.

• Different types of cells vary in their ability to leave G0 and commit to a cell-division cycle.




• During the S phase, the DNA of each chromosome replicates to form a new identical set of chromosomes.

• During G2, the cell prepares for mitosis by synthesizing specific types of RNA and proteins.

• During interphase, the chromosomes spread out and fill up the nucleus.




• Mitosis, sometimes called nuclear division, is a series of events that ensures that each new daughter cell receives one copy of each chromosome.

• The division of the whole cell which occurs after mitosis is called cytokinesis.



• After cytokinesis, each daughter cell enters G1.


• Mitosis provides each daughter cell with a complete set of chromosomes that are the same type and number as those of the parent cell.

DNA Replication

8.3 DNA Structure

• The process of DNA replication depends on the molecular shapes of DNA and its nucleotide bases.

• Base pairing depends on how many hydrogen bonds each nitrogen base can form with its counterpart.


• Adenine (A) pairs only with thymine (T) because these two bases can make two hydrogen bonds.

8.3 DNA Structure (cont.)

DNA Replication

• Guanine (G) pairs only with cytosine (C) because three hydrogen bonds hold them together.

A short section of a DNA molecule, as it would appear if uncoiled and flattened, is depicted here. Sugar-phosphate bonds connect the nucleotides along each strand. Hydrogen bonds between the nitrogen bases connect the two strands.


• DNA strands are parallel but run in opposite directions in an antiparallel arrangement.

8.3 DNA Structure (cont.)

DNA Replication

A short section of a DNA molecule, as it would appear if uncoiled and flattened, is depicted here. Sugar-phosphate bonds connect the nucleotides along each strand. Hydrogen bonds between the nitrogen bases connect the two strands.


• The synthesis of new DNA during the S phase of the cell cycle is a multistep process that can be divided into three major parts:

8.4 DNA Synthesis

DNA Replication

1. binding of enzymes to existing DNA

2. unwinding of the double helix

3. synthesis of a new matching strand for each existing strand


The three major parts of DNA replication


• First, enzymes and other proteins involved in DNA synthesis bind to specific regions of chromosomes called replication origins.

• The proteins include an enzyme that unwinds the double helix, an RNA-synthesizing enzyme, and DNA polymerase, the enzyme that catalyzes the formation of the new DNA strands.

8.4 DNA Synthesis (cont.)

DNA Replication

• The combination of DNA and proteins is called a replisome.


• Prokaryotes have one origin of replication while in eukaryotes there are multiple origins.


• DNA polymerase can add nucleotides only to the end of an existing nucleic-acid strand.

DNA Replication


• Synthesis of the new matching strand occurs continuously on only one of the original strands called the leading strand.


• On the other original strand, called the lagging strand, synthesis occurs in short segments.

DNA Replication



DNA Replication

• This type of replication is known as semiconservative (half conservative) replication because each of the two new double-stranded DNA molecules conserves one strand (half) of the original DNA, but adds one strand of new DNA.


• Proteins are involved in wrapping the DNA into the tightly condensed structure called the chromosome.

DNA Replication

A DNA molecule wraps around histone proteins to form nucleosomes, the basic packing unit of eukaryotic chromosomes. The coiled, beaded chain of DNA with its nucleosomes forms still thicker coils that make up the chromosome.



• Any change in the sequence of a cell’s DNA is known as a mutation.

8.5 DNA Repair

• Mutations can be nonharmful (silent), harmful, or lethal to the cell.

• Mutations in human cells that persist to the next cell division are inherited by the daughter cells and cause many diseases.

DNA Replication


• Cells have processes to detect and correct errors in replication as well as damage to DNA by environmental factors such as mutagenic chemicals.

• The DNA polymerase that produces the DNA itself proofreads its own work and replaces any mismatched neucleotides.

• Most mutations consist of base pairs that cannot form hydrogen bonds and are repaired through an excision repair.

DNA Replication

8.5 DNA Repair (cont.)


Excision repair


• When DNA replication is complete, the cell passes from the S phase to the G2 phase.


8.6 The Stages of Cell Division

• The two copies of each chromosome made during the S phase are called sister chromatids.

This scanning electron micrograph, x87,000, shows a replicated chromosome in metaphase with its pair of sister chromatids joined at their centromere.


8.6 The Stages of Cell Division (cont.)


• As a cell enters the M phase, sister chromatids are still attached by proteins at a narrow point called the centromere.

This drawing illustrates the chromosome’s structures.


• If segregation occurs correctly, each new nucleus receives one copy of each chromosome.

• A mistake at this stage will result in daughter cells with abnormal numbers of chromosomes called aneuploid cells.




• The process of mitosis, once begun, is continuous but is considered to have four distinct steps.

• Individual chromosomes are not visible during interphase (a).



All photos in this sequence are of the interphase and stages of mitosis in the blood lily Haemanthus. Chromosomes are stained blue, and microtubules are stained red.


• Prophase begins when the nuclear membrane breaks down into small vesicles and the chromosomes condense.

• Microtubules begin to form around the nucleus (b) and join to form a mitotic spindle (c).



• The microtubules are anchored to protein structures that surround the centrioles (if present), called the spindle poles.


• Within each centromere is a protein complex called the kinetochore.

• Some of the microtubules in the spindle bind to the kinetochores of each chromatid so that a chain of microtubules connects each chromatid to a spindle pole.

• Sister chromatids move to opposite poles.




• Metaphase is the second step of mitosis.

• By this time, motor proteins in the kinetochores have pulled the chromosomes into a ring between the two poles, forming the metaphase plate (d and e).




• In the third step, anaphase, enzymes break down the protein holding sister chromatids together.

• The sisters separate, and the motor proteins of their kinetochores pull them along the spindle microtubules to opposite spindle poles (f and g).




• In telophase (h and i), the chromosomes begin to expand, and the nuclear envelope re-forms around them, producing two new nuclei.

• Soon after this, cytokinesis divides the cell in two, as the plasma membrane constricts between the nuclei and completes cell division.




• Although the major events and molecular players of cell division are similar in all eukaryotic cells, there are some subtle differences.

8.7 Differences in Mitosis

– Cytokinesis begins during anaphase in most animal cells.

– At cytokinesis in plants, vesicles containing cellulose begin to congregate and fuse between the two nuclei, forming the plasma membranes and completing the cell wall between the two new cells.

– In some fungi, such as yeast, the nuclear envelope forms a bud instead of breaking down, and the spindle poles are embedded in the nuclear membrane.



• The controls that regulate the order and timing of cell-cycle events are of major interest to scientists who study the eukaryotic cell cycle.


8.8 Control of the Cell Cycle

– Something in S-phase and M-phase cells can cause G1 and G2 nuclei to advance to the next phase (S or M).

– A factor or factors in S-phase cells can enter G1 nuclei and initiate DNA replication.

– M-phase cells can move G2 nuclei into mitosis.


• Proteins called cyclins regulate progression through the cell cycle.

8.8 Control of the Cell Cycle (cont.)

• The most important cyclins are the G1 cyclins and the mitotic cyclins.


G1 cyclins peak at S phase, and mitotic cyclins peak at metaphase in M phase.


• Cyclins act by binding to various kinases, which are enzymes that transfer a phosphate group from ATP to other enzymes.

• The phosphate group activates these enzymes.

• The quantity of these kinases in the cell remains steady throughout the cycle but they are active only when bound to the appropriate cyclin.




• As the amount of a particular cyclin rises, it activates more kinases which activate various enzymes needed for progress through the cell cycle.






The abundance of different cyclins varies during the cell cycle. Each cyclin activates specific kinases. The kinases activate some enzymes directly at (a) and signal the cell to synthesize other proteins needed to progress to the next phase of the cycle at (b).


• Eukaryotic cells have an elaborate system called checkpoint control that monitors the condition of the DNA, the chromosomes, and the mitotic spindle.

8.9 Checkpoints

• Checkpoint controls consist of proteins that detect mistakes and damage and put the cell into cell-cycle arrest until the damage is fixed.


• Without checkpoint controls, mitosis could produce daughter cells with damaged or missing chromosomes.


• Checkpoints throughout the cell cycle ensure that problems are corrected before the cycle progresses preventing the production of daughter cells with genetic damage.


8.9 Checkpoints (cont.)


• When the regulators involved in preventing cells from leaving the G0 stage are inactivated, cells may divide at the wrong time.

• Mutations in the genes that encode these proteins can lead to uncontrolled growth known as cancer.


8.9 Checkpoints (cont.)


Summary

• There are five phases of the cell cycle: G0, G1, S, G2, and M.

• Interphase consists of G1, S, and G2.

• DNA synthesis occurs in S phase and begins at replication origins.

• Replisomes catalyze the synthesis of two new strands of DNA that are complements of the two parental strands. The leading strand is synthesized in a continuous process. The lagging strand replicates in short stretches of DNA that are then joined.

• Eukaryotic DNA replication is semiconservative; the resulting chromosomes each contain an old strand of DNA paired with a newly synthesized strand.

• The eukaryotic cell cycle forms two offspring cells from a parent cell.


Summary (cont.)

• The newly replicated sister chromatids are segregated to the daughter nuclei during mitosis.

• In mitosis, arrays of microtubules form a mitotic spindle. Microtubules emanating from the spindle poles link to others that attach to the chromosomes at their centromeres. Motor proteins in the kinetochores then transport the chromosomes toward the metaphase plate.

• At anaphase, the sister chromatids separate and migrate to the spindle poles.

• The nuclear envelope re-forms around the new daughter nuclei in telophase. Cytokinesis usually follows mitosis.

• Errors in replication are repaired by the DNA polymerase itself, while damaged segments may be repaired by the excision-repair system.


Summary (cont.)

• Errors that occur during this process are closely monitored by checkpoint control.

• Failure of the checkpoint-control system is one important step in the development of cancer.

• The timing and sequence of events are linked to the synthesis and disappearance of various cyclins throughout the cell cycle.


Reviewing Key TermsMatch the term on the left with the correct description.

___ mitosis

___ cyclins

___ kinetochore

___ telophase

___ aneuploid

___ G0

a. group of proteins that regulate the progression of the cell cycle

b. a condition of having an abnormal number of chromosomes

c. stage in the cell cycle characterized by new nuclei forming at opposite ends of the cell

d. the phase of the cell cycle when nuclear division occurs

e. links chromosomes to the mitotic spindle

f. a resting stage in the cell cycle

d

a

e

c

b

f


Reviewing Ideas1. What is the basic, most common cause

of cancer?

The basic, most common cause of cancer is a mutation in the genes that encode the checkpoint proteins, or tumor-suppressor genes.


Reviewing Ideas2. What is an aneuploid cell and how do

they form?

Aneuploid cells are daughter cells with abnormal numbers of chromosomes. These cells are formed as a result of a mistake during chromosome segregation.


Using Concepts3. Why do prokaryotes only require one replication

origin while eukaryotes require many?

In prokaryotes, one replication origin is sufficient because bacteria contain only one small chromosome, which can replicate quickly. Eukaryotes have multiple chromosomes, each containing much more DNA than bacteria chromosomes do.


Using Concepts4. Could you study cell division in yeast to help

understand human cell division? Explain.

Scientists have gained much of their knowledge of the cell cycle from studies of yeast. This is possible because the cell cycle is remarkably similar in all eukaryotes. Studies have found that yeast and human cells perform the cell cycle in a comparable fashion using very similar proteins.


Synthesize5. Suppose you spent too much time in the sun and developed a severe

sunburn. Undoubtedly, some cells were damaged by the excessive UV exposure. What prevents this damage from becoming cancerous automatically?

Your cells have the ability to detect and repair mutations caused by environmental factors such as UV radiation. Proteins in the cell can detect DNA damage and put the cell into cell-cycle arrest until the damage is fixed, preventing the mutation from being passed on to daughter cells.


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Stages of the cell cycle

Excision repair

Excision repair

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