Chapter 9Cell Division and Mitosis

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Why do Cells divide?

• Growth

• Reproduction (for single cell organisms)

• Repair

Cycle of Life

• The cycle of life includes fertilization of gametes, cell division, and growth, production of gametes, and death.

• All of life depends of life

Overview of Division mechanisms• Before a cells are able to reproduce, there

must be a division of nucleus and its DNA• Mitosis: used by multicellular organisms

for growth by repeated division of somatic (body cells)– This division helps cells grow, replace dead,

or worn-out cells and repair tissues

• Meiosis: only occurs in germ cells that divide to form gametes (sex cells)

Chromosomes

• Each chromosomes is a molecule of DNA complexed with proteins

• Human DNA is 2 meters long– Prior to cell division, each threadlike

chromosomes is duplicated to form two sister chromatids held together by a centromere

– The centromere is also the region where the duplicated chromosome will attach to the microtubules of the spindle during nuclear division

Continue

• Proteins called histones tightly bind to DNA and cause spooling into a structural unit called nucleosome

Mitosis and Chromosome Number

• Each organisms has a definite chromosome number– Example: Humans have 46 chromosomes

• Chromosomes exists in pairs (one from each parent)– Humans have 23 pairs of chromosomes– Somatic Cells are diploid (pairs)– Germ cells (sperm and egg cells) are haploid

(half the number of total chromosomes)

Cell Cycle

• A recurring sequence of events that extends from the time of a cell’s formation until each division is complete– Three phases of cell cycle: Interphase,

Mitosis , and Cytokinesis

Interphase• Is the portion of the cell cycle is which the

cell prepares for cell division (nuclear and cytoplasmic)

• Three phases of Interphase– G1 phase: Carbohydrates, lipids, and proteins

for cell’s own use and for export assembled– S phase: DNA is copied and proteins are

synthesized used in organizing the condensed chromosomes

– G2 phase: the proteins that will drive mitosis to completion are produced

Mitosis Overview

• Nuclear Division that occurs in four phases: Prophase, Metaphase, Anaphase, and Telophase (PMAT)

• Spindle apparatus (fibers) moves toward chromosomes– Composed of two sets of microtubules– Extend from two poles of cell and overlap at

the cell’s equator

Prophase

• Chromosomes become visible as rodlike units, each consisting of two sister chromatids

• Nuclear envelope begins to break down

• Spindle apparatus move toward the poles

Prometaphase

• Nuclear envelope fragments

• Spindle apparatus attach to the centromere

Metaphase (M&Ms)

• Longest Stage of Mitosis

• Nuclear membrane breaks up completely in the transition between pro- and metaphase

• Chromosome aligns at the cell’s equator, halfway between the poles—also known as the metaphase plate

Anaphase

• Sister chromatids separate and move toward opposite poles

• Spindle apparatus shorten and pull the chromosomes toward the poles

• Once separated each chromatid is now an independent chromosomes

Telophase

• Two daughter chromosomes of each original chromatid pair at opposite pair

• Chromosome return to the threadlike form typical of chromosome

• Each daughter cell has the same number of chromosomes as the parent cell

Cytokinesis • Division of cytoplasm

• Plants– Because of rigid cell wall, the cytoplasm of

plant cells can not simply pinch off, the plant cell forms a cell plant to separate the two nucleus

– Made from vesicles derived from the Golgi apparatus

Continue…• The flexible plasma membrane of animal

cells can be squeezed in the middle to separate the two daughter cells– First sign of cleavage is the appearance of a

cleavage furrow, a shallow groove in the cell surface near the old metaphase plate

Mitosis is amazing!!!

• Mitosis is accomplished with astonishing accuracy

• There are times when mistakes happen (too many chromosomes or chromosomes deleted).– This is known as a mutation (genetic

mistakes)

Binary Fission

• Prokaryotes cell division

• 1- Bacterium have circular chromosomes replicate and move apart

• 2- the point chromosome replication is known as origin of replication

• 3- Each origin of replication will move opposite end of the cell

Continue…

• 4- Cell elongates

• 5- Cytoplasm begins to pinch in

• 6- divides

Cell Cycle Control

• Cell cycle control system: a cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle– Checkpoint: in the cell cycle is a critical

control point where stop and go-ahead signals can regulate the cycle

– Animal cells generally have built in stop signals to halt the cell cycle

– 3 major checkpoints: G1, G2, M phases

Regulatory Proteins

• 1- Kinases: drives the cell cycle– Present at constant concentration throughout

the entire cell cycle throughout the entire cell cycle

• 2- Cyclin: attaches to kinases– Cyclically fluctuating in the cell cycle – At checkpoint times is when you see the

fluctuating – This helps the cell have the correct number of

chromosomes

Growth Factor

• A protein released by certain cells that stimulates other cells to grow

Density-dependent inhibition

• Phenomenon in which crowded cells stop growing

Loss of cells cycle in cancer cells• Cancer cells do not respond to the body’s

control mechanisms– Transformation the process that converts a

normal cell to a cancer cell• Immune system recognize a transformed cells

and destroys it• However, if the cell evades destruction it may

proliferate and form a tumor (a mass of abnormal cells within otherwise normal tissue)

Two types of tumors

• 1- Benign Tumor: abnormal cells remain at the original site

• 2- Malignant tumor: becomes invasive enough to impair the functions of one or more functions (this is when a person have cancer)

Metastasis

• Spread of cancer cells to locations distant from their original site– This is usually done through the blood vessels

and lymph vessels