Chapter 12The Cell Cycle

Cells

• The ability of organisms to produce more of their own kind best distinguishes living things from nonliving matter

• Cell Theory: Living things are made of cells and cells reproduce from other cells– The continuity of life is based on the reproduction

of cells, or cell division

Genome

• All the DNA in a cell constitutes the cell’s genome

• A genome can consist of – Single DNA molecule • common in prokaryotic cells

– Multiple DNA molecules • common in eukaryotic cells

• DNA molecules in a cell are packaged into chromosomes

Chromosomes• Eukaryotic chromosomes consist of chromatin, a

complex of DNA and protein that condenses during cell division

• Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus

• Somatic cells (nonreproductive cells) have two sets of chromosomes – Diploid cells (2n)

• Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells– Haploid cells (n)

Cellular Reproduction• Asexual Reproduction:

Cells divide creating identical cells– In unicellular

organisms, division of one cell reproduces the entire organism

– Multicellular organisms depend on cell division for

– Development from a fertilized cell

– Growth– Repair

Binary Fission in Bacteria

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

• In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart

• The plasma membrane pinches inward, dividing the cell into two

Mitotic Division ChromosomesChromosomal

DNA molecules

Centromere

Chromosomearm

Chromosome duplication(including DNA replication)and condensation

Sisterchromatids

Separation of sisterchromatids intotwo chromosomes

Most eukaryotes reproduce by mitosis• Since prokaryotes

evolved before eukaryotes, mitosis probably evolved from binary fission

• Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis

Cellular Reproduction

• Sexual Reproduction: Gametes fuse to create zygote – Gametes formed during meiosis• a special type of division that can produce sperm and

egg cells• Meiosis creates haploid cells

– After fertilization, cell has unique combination of chromosomes

Cell Cycle and Mitosis

• Eukaryotic cells• Cycle of growth,

DNA synthesis, and division

• Cellular division= one cell divides into 2 cells– identical to

original cell

Interphase and Mitosis

1. G1 Phase= Gap 1 Phase-Cell Growth

2. S Phase= DNA Synthesis-Replication

3. G2 Phase= Gap 2 Phase-Cell Growth

4. M Phase= Mitotic Phase-Cellular Division

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Cell Cycle: G1

• Gap 1 Phase• Growth of cell– Increase in mass

• Increase Cytoplasm

• Increase Proteins• Produce

Organelles

• Metabolic activity is high

Cell Cycle: S phase• DNA synthesis– During S phase, DNA is replicated– Occurs in nucleus

• Eukaryotic cells normally have 2 copies of each chromosome- 1 set from mom, 1 set from dad– Diploid= 2 n– Haploid= n– n= number of different types of chromosomes

Human Chromosomes

Haploid (n)=23 Diploid (2n)=46

Homologous Chromosomes

= pair of chromosomes

Cell Cycle: S Phase

• Prepares DNA for M phase– Duplicates entire genetic code

• Creates 2 new strands of DNA from existing strands

• DNA synthesis uses a semiconservative model– Each DNA molecule has 1 old strand and 1 new

strand

• Diploid Cell: 2n = 6– Haploid= n= 3

• Following DNA synthesis, each of the six chromosomes consists of 2 sister chromatids, connected at the centromere

Sister Chromatids

Centromere

HomologousChromosome

Blue: Paternal ChromosomesPink: Maternal Chromosomes

Chromosomes

• Homologous chromosomes are paired chromosomes which possess genes for the same characteristics. One chromosome comes from mom & one from dad

Sister Chromatids

Centromere

HomologousChromosome

Blue: Paternal ChromosomesPink: Maternal Chromosomes

Chromosomes

Cell Cycle: G2 • Gap 2 Phase• Growth of cell• Increase in mass

• Makes proteins needed for mitosis • Centrosome: the microtubule

organizing center• Each centrosome has pair of

centrioles• The centrosome replicates during

interphase

Cell Cycle: M Phase

• Mitotic Phase1. Mitosis= Nuclear

Division2. Followed by

Cytokinesis• Division of

cytoplasm

Mitosis: 5 Stages

Mitosis is divided into five phases:1. Prophase2. Prometaphase3. Metaphase4. Anaphase5. Telophase

• Cytokinesis overlaps the latter stages of mitosis

Mitosis: 5 Stages1. Prophase• Chromosomes condense• The mitotic spindle is a structure

made of microtubules that controls chromosome movement during mitosis– The spindle includes the

centrosomes, the spindle microtubules, and the asters

• Aster – a radial array of short microtubules– extends from each centrosome

Mitosis: 5 Stages2. Prometaphase• Nuclear envelope breaks up• Two centrosomes migrate to

opposite ends of the cell during prophase and prometaphase

• Some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes – Kinetochores are protein

complexes associated with centromeres

Mitosis: 5 Stages3. Metaphase• Longest phase of mitosis– 20 minutes

• Chromosomes line up at the metaphase plate– an imaginary structure at the midway

point between the spindle’s two poles

• For each chromosome, sister chromatids are attached at kinetochore to spindle fibers from opposite spindle poles

Mitosis: 5 Stages4. Anaphase• Shortest stage of mitosis • Cohesion proteins between sister

chromatids are cleaved– Each chromatid is now an individual

chromosome • Individual chromosomes are

pulled to opposite ends of cells as microtubules shorten

• Cell elongates as microtubules not attached at kinetochores lengthen

Mitosis: 5 Stages5. Telophase• 2 nuclei form – Nucleolus form– Nuclear envelope forms from

fragments – Chromosomes become less

condensed• Cytokinesis– Division of the cytoplasm– Animal cells: cleavage furrow forms

and pinches cell in 2– Plant cells: Formation of cell plate

Cytokinesis

Cell Cycle: M Phase

• Before Mitosis:1 Cell= Diploid (2n)

• After Mitosis: 2 Cells= Diploid (2n)

*End product of mitosis is 2 identical cells

Mitosis

• Cells vary in rate of mitosis– Age matters: children vs. adult– Varies from continuously (skin cells) to only when

necessary to repair damage (internal organ cells)• These differences result from regulation at the

molecular level

Cell Cycle

• The sequential events of the cell cycle are directed by a distinct cell cycle control system– Regulated by both internal and external controls– Example of Internal Control: if kinetochore not attached

to spindle microtubules, sends a molecular signal that delays anaphase

– Example of External Control: growth factors• Proteins released by certain cells that stimulate other cells to

divide

• The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

Controls on Cell Cycle• Multiple checkpoints in cell cycle

– Prevent uncontrolled growth– Repair mistakes

• For many cells, the G1 checkpoint seems to be the most important – If a cell receives a go-ahead signal

at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide

• If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

Controls on Cell Cycle

• Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)

• Cdks activity fluctuates during the cell cycle because it is controlled by cyclins, so named because their concentrations vary with the cell cycle

• MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase

Controls on Cell Cycle• An example of external

signals is density-dependent inhibition, in which crowded cells stop dividing

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

Controls on Cell Cycle• Cancer is a disease of

the cell cycle– Normal cell becomes

cancer cell through transformation• Often results from

mutations in DNA

• Uncontrolled cell growth can lead to tumors– Masses of abnormal

cells

Controls on Cell Cycle• Benign Tumor= abnormal cells

remain at the original site• Malignant tumors= invade

surrounding tissues – Metastasize= export cancer cells to

other parts of the body, where they may form additional tumors

• Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence

• Cancer cells have abnormal cell cycle control system– may lack protein checkpoints,

produce own growth factors or convey growth signal without growth factor