johnson - the living world: 3rd ed. - all rights reserved - mcgraw hill companies how cells divide...

How Cells Divide

Chapter 6

Outline

• Simple Cell Cycle• Complex Cell Cycle• Chromosomes• Mitosis

Cancer• Meiosis I• Meiosis II• Evolutionary Consequences of Sex

Simple Cell Cycle

• Cell division in bacteria takes place in two stages (Simple Cell Cycle). DNA is copied Cell Splits (Binary Fission)

- Forms two daughter cells• Heredity information in bacteria encoded in

single circle of DNA.

Complex Cell Cycle

• Eukaryotic DNA is contained in linear chromosomes. Long DNA molecules packaged with

proteins.• Mitosis - Mechanism of cell division occurring

in non-reproductive (somatic) cells.• Meiosis - Mechanism of cell division

occurring in reproductive (germ) cells.

Complex Cell Cycle

• G1 phase - Primary growth phase

• S phase - DNA replicates• G2 phase - Microtubule synthesis

• M phase - Chromosomes pull apart• C phase - Cytokinesis

Complex Cell Cycle

Chromosomes

• Human cells each have 23 nearly identical pairs of chromosomes, for a total of 46. Each chromosome contains thousands of

genes that play important roles in body development and function.

Chromosome Structure

• DNA helix is wrapped around proteins with positive charges (histones) and negative charges counteracting each other.

Chromosome Structure

• Homologues - Nearly identical copies of the same chromosomes.

• Diploid Cells - Two copies of chromosomes. Before cell division, each homologue

replicates, resulting in two identical copies (sister chromatids).

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Cell Division

• Interphase - Chromosomes replicate and begin to wind up tightly.

• Mitosis Prophase - Nuclear envelope breaks

down, DNA further condenses, and spindle fibers form.

Metaphase - Chromosomes align at center of cell and kinetochore fibers attach to centromeres.

Mitosis

Anaphase - Centromeres replicate and sister chromatids separate and move towards opposite poles.

Telophase - Nuclear envelope reappears and chromosomes decondense.

Cytokinesis

• Cytokinesis - Division of cytoplasm. Animal cells - Cell pinches in two with

contracting belt of microtubules, forming a cleavage furrow.

Plant cells - Membrane assembled at right angle to mitotic spindle. Cell plate grows outward until it reaches the interior surface of plasma membrane.

Controlling Cell Cycle

• At critical points, further cell progress depends on a central set of switches regulated by cell feedback. G1 – Cell growth assessed G2 DNA replication assessed M mitosis assessed

Cancer

• Cancer - Unrestrained cell growth and division. Tumor - Cluster of cells.

- Benign - Encapsulated and noninvasive.- Malignant - Not encapsulated, invasive,

and shed cells. Metastases - Process of cells

shedding from a malignant tumor and spreading to distant parts of the body.

Cell Cycle Control

• Gene p53 plays a key role in G1 checkpoint of cell division. Gene’s product monitors integrity of DNA,

checking for successful replication.- If protein detects damaged DNA, it halts

cell division and stimulates repair enzymes.

Nonfunctional p53 genes allow cancer cells to repeatedly divide.

Curing Cancer

• Preventing cancer from starting by focusing on decision-making process to divide. Receiving the signal to divide Relay switch Amplifying the signal Releasing the brake Checking that everything is ready Stepping on the gas

Curing Cancer

• Preventing the Spread of Cancer Tumor growth

- Angiogenesis inhibition Metastasis

Molecular Cancer Therapies

Meiosis

• Gamete formation must involve some mechanism to halve the number of chromosomes. Two sets of chromosomes are present in

somatic cells of adults (diploid), but only one set is present in gametes (haploid).

- Sexual Reproduction involves the alteration of meiosis and fertilization.

Alteration of Generations

Stages of Meiosis

• Meiosis I Prophase I - Chromosomes pair up and

exchange segments (Crossing Over). Metaphase I - Chromosomes align at cell’s

center (Independent Assortment). Anaphase I - Homologous pairs pulled

apart. Telophase I - Individual chromosomes

gather at each of the poles.

Crossing Over

Meiosis II

• Mitotic division involving products of Meiosis I. Prophase II - Nuclear envelope breaks

down and spindle fibers form. Metaphase II - Spindle fibers bind to both

sides of centromeres. Anaphase II - Spindle fibers contract,

moving sister chromatids to opposite poles. Telophase II - Nuclear envelope reforms

around four sets of daughter cells.

Evolutionary Consequences of Sex

• Genetic Diversity Independent Assortment Crossing Over Random Fertilization

Review

• Simple Cell Cycle• Complex Cell Cycle• Chromosomes• Mitosis

Cancer• Meiosis I• Meiosis II• Evolutionary Consequences of Sex

johnson - the living world: 3rd ed. - all rights reserved - mcgraw hill companies how cells divide...

living world

center of cell

stages simple cell cycle

copies of chromosomes

c phase cytokinesis

linear chromosomes

metaphase chromosomes

identical pairs of chromosomes

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