The Cell Cycle and Cell Division

The Cell Cycle and Cell Division
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
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, or cell division: a parent cell duplicates its genetic material and then divides into two similar cells. Cell division is important in growth and repair of multicellular organisms and the reproduction of all organisms.

Figure 7.1 The Importance of Cell Division
Figure 7.1 The Importance of Cell Division Cell division is the basis for (A) reproduction, (B) growth, and (C) repair and regeneration of tissues.

Organisms have two basic strategies for reproducing themselves:
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Organisms have two basic strategies for reproducing themselves: Asexual reproduction Sexual reproduction

The offspring are clones—genetically identical to the parent
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Asexual reproduction The offspring are clones—genetically identical to the parent Any genetic variations are due to mutations (changes in DNA sequences due to environmental factors or copying errors)

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction
Single-celled prokaryotes usually reproduce by binary fission Single-celled eukaryotes can reproduce by mitosis and cytokinesis Many multicellular eukaryotes can also reproduce by asexual means

Figure 7.2 Asexual Reproduction on a Large Scale
Figure 7.2 Asexual Reproduction on a Large Scale This forest of aspens in Utah’s Wasatch Mountains arose via asexual reproduction. Genetically, all these trees are virtually identical.

Involves fusion of gametes Results in offspring with genetic variation
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Sexual reproduction Involves fusion of gametes Results in offspring with genetic variation Gametes form by meiosis—a process of cell division that reduces genetic material by half

DNA in eukaryotic cells is organized into chromosomes.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction DNA in eukaryotic cells is organized into chromosomes. Somatic cells: body cells not specialized for reproduction Each somatic cell contains two sets of chromosomes that occur in homologous pairs. One homolog came from the female parent and one from the male parent and have corresponding genetic information.

Gametes have only one set of chromosomes— one homolog from each pair.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Gametes have only one set of chromosomes— one homolog from each pair. They are haploid; number of chromosomes = n Fertilization: two haploid gametes fuse to form a zygote They are diploid; number of chromosome in zygote = 2n

All sexual life cycles involve meiosis:
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction All sexual life cycles involve meiosis: Gametes may develop immediately after meiosis Or each haploid cell may develop into a haploid organism (haploid stage of the life cycle) that eventually produces gametes by mitosis Fertilization results in a zygote and begins the diploid stage of the life cycle.

Figure 7.3 Sexual Life Cycles Involve Fertilization and Meiosis (Part 1)
Figure 7.3 Sexual Life Cycles Involve Fertilization and Meiosis In sexual reproduction, haploid (n) cells or organisms alternate with diploid (2n) cells or organisms.

The essence of sexual reproduction is:
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The essence of sexual reproduction is: Random selection of half the diploid chromosome set to form a haploid gamete Followed by fusion of haploid gametes from separate parents to make a diploid cell This results in shuffling of genetic information in a population, and no two individuals have exactly the same genetic makeup.

Four events in cell division:
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Four events in cell division: Reproductive signals initiate cell division DNA replication 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
Prokaryotes divide by binary fission: results in reproduction of the entire organism. Reproductive signals may be environmental factors such as nutrient availability.

ori—where replication starts ter—where replication ends
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Replication: Most prokaryotes have one circular chromosome with two important regions: ori—where replication starts ter—where replication ends Replication occurs as the DNA is threaded through a “replication complex” of proteins at the center of the cell.

Segregation: 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, using ATP. An actin-like protein provides a filament along which ori and other proteins move.

Figure 7.4 Prokaryotic Cell Division: Binary Fission
Figure 7.4 Prokaryotic Cell Division: Binary Fission The process of cell division in a bacterium involves DNA replication, DNA segregation, and cytokinesis.

Cytokinesis: After chromosome segregation, the cell membrane pinches in by contraction of a ring of protein fibers under the surface. As the membrane pinches in, new cell wall materials are deposited, resulting in separation of the two cells.

Eukaryotic cells divide by mitosis followed by cytokinesis.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Eukaryotic cells divide by mitosis followed by cytokinesis. Reproductive signals are usually related to functions of the entire organism, not the environment of a single cell. Most cells in a multicellular organism are specialized and do not divide.

DNA replication only occurs during a specific stage of the cell cycle.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Replication of each chromosome occurs as they are threaded through replication complexes. DNA replication only occurs during a specific stage of the cell cycle.

In segregation, one copy of each chromosome ends up in each of the two new cells. More complex than in prokaryotes: eukaryotes have a nuclear envelope, and there are multiple chromosomes. Cytokinesis in plant cells (which have cell walls) is different than in animal cells (no cell walls).

In mitosis, one nucleus produces two daughter nuclei, each containing the same number of chromosomes as the parent nucleus. Mitosis is continuous, but it is convenient to subdivide it into phases.

The cell cycle is the period from one cell division to the next, divided into stages in eukaryotes. M phase: Mitosis (segregation of chromosomes into two new nuclei), followed by cytokinesis. Interphase: cell nucleus is visible and cell functions occur, including DNA replication.

Figure 7.5 The Phases of the Eukaryotic Cell Cycle
Figure 7.5 The Phases of the Eukaryotic Cell Cycle The eukaryotic cell cycle has several phases. DNA in the interphase nucleus is diffuse and becomes compacted as mitosis begins.

Interphase has three subphases:
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Interphase has three subphases: G1 (Gap 1)—variable, may last a long time S phase (synthesis)—DNA is replicated G2 (Gap 2)—the cell prepares for mitosis; synthesizes microtubules for segregating chromosomes

Prophase: three structures appear Condensed chromosomes
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Prophase: three structures appear Condensed chromosomes Reoriented centrosomes Spindle

Even during interphase, DNA is packaged by winding around specific proteins, and other proteins coat the DNA coils. In prophase, the chromosomes become much more tightly coiled and condensed.

After replication, each chromosome has two DNA molecules called sister chromatids, joined at a region called the centromere.

Karyotype: the condensed chromosomes for a given organism can be distinguished by their sizes and centromere positions

Karyotype analysis was used to identify and classify organisms, but DNA sequencing is more commonly used today. Karotype analysis is still used to identify chromosome abnormalities.

The centrosome determines orientation of the spindle.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells The centrosome determines orientation of the spindle. Consists of two centrioles—hollow tubes formed by microtubules. The centrosome is duplicated during S phase; centrosomes move towards opposite sides of the nucleus at the G2–M transition. Centrosome position determines the plane of cell division—important in the development of multicellular organisms.

Centrosomes serve as poles toward which the chromosomes move.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Centrosomes serve as poles toward which the chromosomes move. The spindle forms between the poles from microtubules: Polar microtubules overlap in the middle region of the cell and keep the poles apart. Astral microtubules interact with proteins attached to the cell membrane; also help keep the poles apart.

Sister chromatids become daughter chromatids after separation.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Kinetochore microtubules attach to kinetochores on the chromatid centromeres. Sister chromatids attach to kinetochore microtubules from opposite sides so that the two chromatids will move to opposite poles. Sister chromatids become daughter chromatids after separation.

Prometaphase: the nuclear envelope breaks down and chromatids attach to the kinetochore microtubules. Metaphase: the chromosomes line up at the midline of the cell. Anaphase: the chromatids separate, and daughter chromosomes move toward the poles.

Figure 7.6 The Phases of Mitosis (1)
Figure 7.6 The Phases of Mitosis Mitosis results in two new nuclei which are genetically identical to each other and to the nucleus from which they were formed. In the micrographs, the green dye stains microtubules (and thus the spindle); the blue dye stains the chromosomes. The chromosomes in the diagrams are stylized to emphasize the fates of the individual chromatids.

Figure 7.6 The Phases of Mitosis (2)
Figure 7.6 The Phases of Mitosis Mitosis results in two new nuclei which are genetically identical to each other and to the nucleus from which they were formed. In the micrographs, the green dye stains microtubules (and thus the spindle); the blue dye stains the chromosomes. The chromosomes in the diagrams are stylized to emphasize the fates of the individual chromatids.

Two mechanisms move the chromosomes to opposite poles:
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Two mechanisms move the chromosomes to opposite poles: Kinetochores have molecular motor proteins (kinesin and dynein), which move the chromosomes along the microtubules. The kinetochore microtubules shorten from the poles, drawing the chromosomes toward the poles.

Telophase: nuclear envelopes form around each set of chromosomes and nucleoli appear, and the spindle breaks down and chromosomes become less compact.

Cytokinesis: In animal cells, the cell membrane pinches in between the nuclei. A contractile ring of actin and myosin microfilaments forms on the inner surface of the cell membrane; the two proteins produce a contraction to pinch the cell in two.

Figure 7.7 Cytokinesis Differs in Animal and Plant Cells
Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (A) A HeLa cell (a type of human cancer cell) undergoing cytokinesis. In this flurorescence micrograph, nuclei are yellow, mitochondria are red, and actin filaments are green. (B) An electron micrograph of a plant cell in late telophase. Plant cells divide differently than animal cells because they have cell walls.

In plant cells, vesicles from the Golgi apparatus appear along the plane of cell division. The vesicles fuse to form a new cell membrane. Contents of vesicles also contribute to forming the cell plate—the beginning of the new cell wall.

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 there does not appear to be a precise mechanism for distribution of the cytoplasmic contents.

Table 7.1

Concept 7.3 Cell Reproduction Is Under Precise Control
Cell reproduction must be under precise control. If single-celled organisms had no control over reproduction, they would soon overrun the environment and starve to death. In multicellular organisms, cell reproduction must be controlled to maintain body form and function.

Prokaryotic cells divide in response to environmental conditions. In eukaryotes, cell division is related to the needs of the entire organism. Mammals produce growth factors that stimulate cell division and differentiation. Example: platelets in the blood secrete growth factors that stimulate cells to divide to heal wounds.

Progression through the eukaryotic cell cycle 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
Figure 7.8 The Eukaryotic Cell Cycle The cell cycle consists of a mitotic (M) phase, during which mitosis and cytokinesis take place, and a long period of growth known as interphase. Interphase has three subphases (G1, S, and G2) in cells that divide.

Specific substances trigger the transition from one phase to another. The first evidence for these substances came from cell fusion experiments. Fusion of mammalian cells at G1 and S phases showed that a cell in S phase produces a substance that activates DNA replication.

Figure 7.9 Regulation of the Cell Cycle (Part 1)
Figure 7.9 Regulation of the Cell Cycle Nuclei in G1 do not undergo DNA replication, but nuclei in S phase do. To determine if there is some signal in the S cells that stimulates G1 cells to replicate their DNA, cells in G1 and S phases were induced to fuse, creating cells with both G1 and S properties.a [a P. N. Rao and R. T. Johnson Nature 225: 159–164.]

The trigger substances turned out to be protein kinases: cyclin-dependent kinases (CDKs). They catalyze phosphorylation of proteins that regulate the cell cycle and are activated by binding to cyclin, which exposes the active site (allosteric regulation).

CDKs function at cell cycle checkpoints: G1 checkpoint is triggered by DNA damage. S checkpoint is triggered by incomplete replication or DNA damage. G2 checkpoint is triggered by DNA damage. M checkpoint is triggered by a chromosome that fails to attach to the spindle.

Each CDK has a cyclin to activate it, which is made only at the right time. After the CDK acts, the cyclin is broken down by a protease. Synthesis and breakdown of cyclins is important in controlling the cell cycle. Cyclins are synthesized in response to various signals, such as growth factors.

Figure 7.10 Cyclins Are Transient in the Cell Cycle
Figure Cyclins Are Transient in the Cell Cycle Cyclins are made at a particular time and then break down. In this case, the cyclin is present during G1 and activates a CDK at that time.

Example: control of the restriction point (R) G1–S cyclin–CDK catalyzes phosphorylation of retinoblastoma protein (RB). RB normally inhibits the cell cycle at R, but when phosphorylated, it 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 haploid cells produced by meiosis are genetically different from one another and from the parent cell.

Figure 7.11 Mitosis and Meiosis: A Comparison (Part 1)
Figure Mitosis and Meiosis: A Comparison Meiosis involves two cell divisions, the first of which is very different from the single division of mitosis. Meiosis II is similar to mitosis, in that the centromeres separate during anaphase, allowing the chromatids of the two homologous pairs to separate into four daughter chromosomes that are genetically distinct from the parental chromosomes.

Figure 7.11 Mitosis and Meiosis: A Comparison (Part 2)
Figure Mitosis and Meiosis: A Comparison Meiosis involves two cell divisions, the first of which is very different from the single division of mitosis. Meiosis II is similar to mitosis, in that the centromeres separate during anaphase, allowing the chromatids of the two homologous pairs to separate into four daughter chromosomes that are genetically distinct from the parental chromosomes.

The function of meiosis is to:
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity The function of meiosis is to: Reduce the chromosome number from diploid to haploid Ensure that each haploid cell has a complete set of chromosomes Generate diversity among the products

Meiosis I Homologous chromosomes come together and line up along their entire lengths. The homologous chromosome pairs separate, but individual chromosomes made up of two sister chromatids remain together.

Figure 7.12 Meiosis: Generating Haploid Cells (1)
Figure Meiosis: Generating Haploid Cells In meiosis, two sets of chromosomes are divided among four daughter cells, each of which has half as many chromosomes as the original cell. The four haploid cells are the result of two successive nuclear divisions. The micrographs show meiosis in the male reproductive organ of a lily; the diagrams show the corresponding phases in an animal cell. (For instructional purposes, the chromosomes from one parent of the original organism are colored blue and those from the other parent are red.)

Meiosis I is preceded by an S phase during which DNA is replicated. Each chromosome then consists of two sister chromatids. At the end of meiosis I, two nuclei form, each with half the original chromosomes (one member of each homologous pair). The centromeres did not separate, so each chromosome is still two sister chromatids.

Not preceded by DNA replication Sister chromatids separate
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiosis II Not preceded by DNA replication Sister chromatids separate End result: four haploid cells that are not genetically identical

Figure 7.12 Meiosis: Generating Haploid Cells (2)
Figure Meiosis: Generating Haploid Cells In meiosis, two sets of chromosomes are divided among four daughter cells, each of which has half as many chromosomes as the original cell. The four haploid cells are the result of two successive nuclear divisions. The micrographs show meiosis in the male reproductive organ of a lily; the diagrams show the corresponding phases in an animal cell. (For instructional purposes, the chromosomes from one parent of the original organism are colored blue and those from the other parent are red.)

Shuffling of genetic material during meiosis occurs by two processes:
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Shuffling of genetic material during meiosis occurs by two processes: Crossing over In prophase I homologous chromosomes (synapsis) and the four chromatids form a tetrad, or bivalent.

The homologs seem to repel each other at the centromeres but remain attached at chiasmata.

Genetic material is exchanged between nonsister chromatids at the chiasmata. Any of the four chromatids in the tetrad can participate, and a single chromatid can exchange material at more than one point. Crossing over results in recombinant chromatids and increases genetic variability of the products.

Figure 7.13 Crossing Over Forms Genetically Diverse Chromosomes
Figure Crossing Over Forms Genetically Diverse Chromosomes The exchange of genetic material by crossing over results in new combinations of genetic information on the recombinant chromosomes. The two different colors distinguish the chromosomes contributed by the male and female parents of the organism whose cell is undergoing meiosis.

Prophase I may last a long time.
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, meiosis continues up to decades later during the monthly ovarian cycle and is completed only after fertilization.

Independent assortment
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Independent assortment At anaphase I, it is a matter of chance which member of a homologous pair goes to which daughter cell. The greater the number of chromosomes, the greater the potential for genetic diversity. In humans, 223 (8,388,608) different combinations of maternal and paternal chromosomes can be produced.

Meiosis is complex, and errors can occur. Nondisjunction
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiosis is complex, and errors can occur. Nondisjunction Homologous pair fails to separate at anaphase I Sister chromatids fail to separate at anaphase II Both result in aneuploidy—an abnormal number of chromosomes.

Figure 7.14 Nondisjunction Leads to Aneuploidy
Figure Nondisjunction Leads to Aneuploidy Nondisjunction, shown here occurring in meiosis I, results in aneuploidy: one or more chromosomes are either lacking or present in excess. Generally, aneuploidy is lethal to a developing embryo.

Most human embryos from aneuploid zygotes do not survive. Many miscarriages are due to this. The most common human aneuploidy is trisomy 16. Trisomy 21 (Down syndrome) is one of the few aneuploidies that allow survival.

Polyploidy Sometimes, organisms with triploid (3n), tetraploid (4n), and even higher numbers can form. This can occur through an extra round of DNA replication before meiosis, or lack of spindle formation in meiosis II. Polyploidy occurs naturally in some species and can be desirable in plants.

Translocation Crossing over between non-homologous chromosomes in meiosis I Location of genes relative to other DNA sequences is important, and translocations can have profound effects on gene expression.

A translocation that occurs in humans between chromosomes 9 and 22 can result in a form of leukemia.

Cells can die in one of two ways:
Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Cells can die in one of 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.

Apoptosis is genetically programmed cell death. Two possible reasons:
Concept 7.5 Programmed 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 are prone to genetic damage that can lead to cancer—especially true of epithelial cells that die after days or weeks

Cell detaches from its neighbors DNA is cut into small fragments
Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Events of apoptosis: Cell detaches from its neighbors DNA is cut into small fragments Membranous lobes (“blebs”) form and break into fragments Surrounding living cells usually ingest remains of the dead cell by phagocytosis

Figure 7.15 Apoptosis: Programmed Cell Death
Figure Apoptosis: Programmed Cell Death (A) Many cells are programmed to “self-destruct” when they are no longer needed, or when they have lived long enough to accumulate a burden of DNA damage that might harm the organism. (B) Both external and internal signals stimulate caspases (or similar enzymes in plants), which break down specific cell constituents, resulting in apoptosis.

Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms
Plants use apoptosis in the hypersensitive response. They protect themselves from disease by undergoing apoptosis at the site of infection by a fungus or bacterium, preventing spread to other parts of the plant.

Programmed cell death is controlled by signals:
Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Programmed cell death is controlled by signals: Internal signals may be linked to cell age or damaged DNA. Both internal and external signals lead to activation of caspases, which hydrolyze target proteins in a cascade of events. The cell dies as caspases hydrolyze proteins of the nuclear envelope, nucleosomes, and cell membrane.

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 mutated 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 are inactive in cancer cells. Example: RB blocks the cell cycle at R. HPV causes synthesis of E7 protein, which fits into the protein-binding site of RB, thereby inactivating it.

Figure 7.16 Molecular Changes Regulate the Cell Cycle in Cancer Cells
Figure Molecular Changes Regulate the Cell Cycle in Cancer Cells In cancer cells, oncogene proteins become active (A) and tumor suppressor proteins become inactive (B).

Chemotherapy drugs stop cell division by targeting cell cycle events. Some drugs block DNA replication; others damage DNA, stopping cells at G2; and still others prevent normal functioning of the mitotic spindle. Unfortunately, these drugs also act on normal cells and are toxic to rapidly dividing cells in the intestines, skin, and bone marrow.

Research into more specific chemotherapy drugs is ongoing. Example: a drug has been identified that affects the protein produced as a result of the translocation between chromosomes 9 and 22. It has been successful at treating leukemia caused by this translocation.

The Cell Cycle and Cell Division

Similar presentations

Presentation on theme: "The Cell Cycle and Cell Division"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

The Cell Cycle and Cell Division

Similar presentations

Presentation on theme: "The Cell Cycle and Cell Division"— Presentation transcript:

Similar presentations

About project

Feedback