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The Cell Cycle

Chapter 12

• Objectives• Describe the structural organization of a prokaryotic and

eukaryotic genome.

• Describe the major events of cell division that enable the genome of one cell to be passed on to two daughter cells.

• List the phases of the cell cycle and describe the sequence of events that occurs during each phase.

• List the phases of mitosis and describe the events characteristic of each phase.

• Compare cytokinesis in animals and plants.

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• Explain how the abnormal cell division of cancerous cells escapes normal cell cycle controls.

• Distinguish between benign, malignant, and metastatic tumors.

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The Key Roles of Cell Division

• The continuity of life is based upon the reproduction of cells, or cell division– unicellular organisms reproduce by cell division– multicellular organisms depend on cell division for

• development from a fertilized cell• growth

• repair

– the cell division process is an integral part of the cell cycle

• cell division results in genetically identical daughter cells

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Underlying Steps of Cell Division

• Three steps occur in every cell division process, independent of the the type of cell involved– The DNA of the cell is copied prior to the

beginning of division– During division, the two copies of the cellular DNA

are separated– The cell then divides into two daughter cells

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Cellular Organization of the Genetic Material

• Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA

• A cell’s endowment of DNA, its genetic information, is called its genome– the DNA molecules in a cell are packaged into

chromosomes

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

• In animals somatic cells have two sets of chromosomes and gametes have one set of chromosomes– human somatic cells have 46 chromosomes and

gametes have 23 chromosomes

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Distribution of Chromosomes During Cell Division

• In preparation for cell division DNA is replicated and the chromosomes condense

• Each duplicated chromosome has two sister chromatids, which separate during cell division

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Eukaryotic Cell Division

• Eukaryotic cell division consists of two parts– mitosis, the division of the nucleus– cytokinesis, the division of the cytoplasm

• In meiosis sex cells are produced after a reduction in chromosome number

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Phases of the Cell Cycle

• The cell cycle consists of two phases – the mitotic phase– interphase

• The mitotic phase alternates with interphase in the cell cycle

• Interphase can be divided into subphases– G1 phase– S phase– G2 phase

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Mitosis

• The mitotic phase is made up of mitosis and cytokinesis

• While continuum, there are several established dividing points for mitosis resulting in five distinct phases– Interphase: duplication of genetic material, ends

with visible chromosomes– Prophase: chromatin coils and condenses into

chromosomes; mitotic spindle forms from MTOC’s; nucleoli disappear

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– Prometaphase: nuclear membrane dissolves; spindle microtubules attach to the chromosomes at the kinetochores

– Metaphase: the spindle is completely formed; chromosomes align in single file with centromeres on metaphase plate

– Anaphase: the chromatids separate and migrate to spindle poles

– Telophase: reverse of prophase– Cytokinesis: division of cytoplasm

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The Mitotic Spindle: A Closer Look

• The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis

• The spindle arises from the centrosomes and includes spindle microtubules and asters

• Some spindle microtubules attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate

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• In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell– movement of the chromosomes is driven by

addition or subtraction of protein subunits to the kinetochore end of spindle microtubules

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• Nonkinetechore microtubules from opposite poles overlap and push against each other, elongating the cell

• In telophase genetically identical daughter nuclei form at opposite ends of the cell

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Cytokinesis: A Closer Look

• Cytokinesis differs in plants and animals– in animals, a ring of microfilaments contracts

around periphery of cell• this forms a cleavage furrow that eventually divides the

cytoplasm

– in plants, vesicles containing cell wall material collect on spindle equator

• the vesicles fuse from the inside out forming cell plate which gradually develops into a new cell wall between new cells

• membranes surrounding the vesicles fuse to form new parts of the plasma membranes

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Binary Fission

• Prokaryotes (bacteria) reproduce by a type of cell division called binary fission

• In binary fission the bacterial chromosome replicates– the two daughter chromosomes actively move

apart

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Factors Affecting Cell Division

• The cell cycle is regulated by a molecular control system

• The frequency of cell division varies with the type of cell

• These cell cycle differences result from regulation at the molecular level

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Evidence for Cytoplasmic Signals

• Molecules present in the cytoplasm regulate progress through the cell cycle

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The Cell Cycle Control System

• The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock

• There are three major check points in cell cycle– G1 of interphase– G2 of interphase– M phase

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• The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received– release of growth factor at each of these

checkpoints allows cell cycle to continue

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The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases

• 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

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Internal and External Signals at the Checkpoints

• Both internal and external signals control the cell cycle checkpoints– Growth factors stimulate other cells to divide

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• In density-dependent inhibition crowded cells stop dividing

• Most animal cells exhibit anchorage dependence in which they must be attached to a substratum to divide

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Loss of Cell Cycle Controls in Cancer Cells

– Cancer cells do not respond normally to the body’s control mechanisms

– cancer cells do not exhibit density-dependent inhibition

– some do not show anchorage dependence

• Uncontrolled cell division forms tumors– benign-no metastasis – malignant tumor-metastasize

• named for organ or tissue of origin

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• A benign tumor becomes malignant when cancerous cells from the tumor mass spread to new sites and continue to proliferate– movement of the cancer cells is mediated by

either blood or lymph systems

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Development of cancer involves multiple genetic changes in cells

• Human chromosomes contain tumor suppressing genes– these genes must be mutated for a cell to become

cancerous

• Oncogenes are genes which normally function in cell signaling and regulation of cell division– when these genes are mutated, normal regulation

of cell division is lost and the cell divides continuously

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Common treatments for cancer

• Radiation– disrupts normal processes of cell division

• cancer cells are more susceptible

• Chemotherapy– disrupts cell division

• Recent advances in understanding the cell cycle and cell cycle signaling have led to advances in cancer treatment

• Coupled with the ability to sequence the DNA of cells in a particular tumor, treatments are becoming more “personalized”

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