Cell Cycle & Cell Division

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THE CELL CYCLE

• During G1 phase cell growth, manufacture

of tRNA, rRNA, cell components and

enzymes are made.

• DNA synthesis occurs during the S stage.

• During G2 phase the cell prepares to go

through mitosis.

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The nuclear membrane is intact and the

individual chromosomes are not visible

At interphase, cells are engaged in their

normal metabolic activities such as

photosynthesis, muscle cell contractions,

and glandular-cell secretion.

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THE CELL CYCLE - Gap 0 (G0) Phase

• There are times when a cell will leave the cycle and quit dividing.

• The cells are not in the cell division cycle, they are differentiated or specialized in their function.

• Some cells remain in the G0

phase (e.g., nerve cells) and others get back to the cell cycle (G1 , S, and G2 , e.g., bone cells).

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DIFFERENTIATION

• Even though all cells in the body originate from a single fertilized egg, are genetically identical but they are not the same and have different functions, e.g., nerve cells, muscle cells, red blood cells . . . each with a specific function.

• The process of creating these specialized

cells is called cell differentiation.

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STEM CELLS

• Stem cells are cells that can become any cell type.

• Stem cells found in the bone marrow can differentiate and become any cell e.g., red blood cells, nerve cells . . .

• Bone marrow transplants are used to cure certain diseases is because the stem cells can differentiate to obtain the desired cells.

What Controls Cell Growth & Differentiation

What controls cell size & division?

Mitogens

• A chemical substance that encourages a cell to commence cell division, triggering mitosis.

• Egs. Pokeweed, phytohaemagglutinin, lipopolysaccharides, concanavalin A

• Removal of Mitogens –

•Differentiated cells withdraw from cell cycle with unduplicated DNA

•With reduced macromolecule synthesis cell enters G0 phase

•Other cells show transient arrest & progress to Apoptosis

Cell Cycle Controls

• How does the cell ensure –

–Conversion of G1 to S Phase

–Successful Replication of DNA

–Preparation to initiate Division process

–Proper segregation of chromatids/ chromosomes to opposite poles

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Cell Cycle Check Points

Features of a Control System

• A timer/ clock that turns on at specific time• Mechanism for initiating events in correct

order• Mechanism to ensure each event occurs only

once per cycle• Binary switches that trigger events in

irreversible fashion• Robustness: backup mechanism• Adaptability: with environment

Factors that control cell cycle

• Cdk Inhibitors (CKIs)

• Proteolysis

Factors that control cell cycle

Video

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ABNORMAL CELL DIVISION

• Sometimes cells cannot stop dividing and go through uncontrolled cell division becoming cancerous cells or tumors.

• Uncontrolled cell division in white blood cells

causes leukemia.

Skin cancer

What Is Cancer?

• Cancer is a disease of the cell cycle.

• Cancer cells

–do not respond normally to the cell cycle control system,

–divide excessively, and

–may invade other tissues of the body.

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What Is Cancer?

• The abnormal behavior of cancer cells begins when

– a single cell undergoes genetic changes (mutations) in one or more genes that encode for proteins in the cell cycle control system and

– these changes cause the cell to grow abnormally.

© 2016 Pearson Education, Inc.

Several Mutations cause cancer

Types of Cancer

• Cancer cells can form tumors, abnormally growing masses of body cells.

– If the abnormal cells remain at the original site, the lump is called a benign tumor.

–Malignant tumors can spread into neighboring tissues and other parts of the body, forming new tumors, and can interrupt organ function.

© 2016 Pearson Education, Inc.

Cancer develops in stages

Retinoblastoma

• Mutation in both the alleles

Virus infection can lead to Cancer

Cancer Treatment• There are three main types -

1. Surgery to remove a tumor,

2. In radiation therapy, parts of the body that have cancerous tumors are exposed to concentrated beams of high-energy radiation, which often harm cancer cells more than normal cells. Radiation therapy is often effective against malignant tumors that have not yet spread.

3. Chemotherapy, the use of drugs to disrupt cell division, is used to treat metastatic tumors.

© 2016 Pearson Education, Inc.

Cancer Prevention and Survival

• Although cancer can strike anyone, there are certain lifestyle changes you can make to reduce your chances of developing cancer or increase your chances of surviving it. These include– not smoking,

– exercising adequately,

– avoiding exposure to the sun,

– eating a high-fiber, low-fat diet,

– performing self-exams, and

– regularly visiting a doctor to identify tumors early.