Chapter 9 Cell Division and Mitosis http://www.phschool.com/science/ biology_place/biocoach/mitosisisg /intro.html

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

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• 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 – G 1 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 – G 2 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

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• 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

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• 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: G 1 , G 2 , 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