Cell Biology Cell Biology Lec.5 Dr:Buthaina Al-SabawiLec.5 Dr:Buthaina Al-Sabawi Date:21-12-2011 Date:21-12-2011

The Cell CycleThe Cell Cycle The cell cycle, is the series of events that The cell cycle, is the series of events that leads to duplication and division of a cell.leads to duplication and division of a cell.

Division cycle consists of 4 coordinated processes:(1) Division cycle consists of 4 coordinated processes:(1) cell growth,(2) DNA replication, (3)distribution of the cell growth,(2) DNA replication, (3)distribution of the duplicated chromosomes ,(4) cell division duplicated chromosomes ,(4) cell division

The alternation between mitosis and interphase called cell The alternation between mitosis and interphase called cell cyclecycle

Cell cycle divided into 2 stagesCell cycle divided into 2 stages..Mitosis(4 stages)Mitosis(4 stages)(1)Prophase (1)Prophase (2) Metaphase(2) Metaphase(3) Anaphase(3) Anaphase(4) Telophase(4) TelophaseInterphaseInterphase, is itself divided into three phases , is itself divided into three phases

(1) G1 (Presynthesis)(1) G1 (Presynthesis)(2) S (DNA synthesis)(2) S (DNA synthesis)(3) G2 (Post DNA duplication) (3) G2 (Post DNA duplication) The cell cycle divided into four periods, The cell cycle divided into four periods, G1, S, G2G1, S, G2 and and

mitosismitosis (M) (M)G1G1(Gap1) the cell grow in size and the cellular (Gap1) the cell grow in size and the cellular

organelles increase in number, the cell is organelles increase in number, the cell is metabolically active & continuously grows but does metabolically active & continuously grows but does not replicate its DNA.not replicate its DNA.

(S)(S) is the time when the cell duplicates its genetic is the time when the cell duplicates its genetic material by synthesizing DNA. During duplication, material by synthesizing DNA. During duplication, each chromosome doubles to produce identical sister each chromosome doubles to produce identical sister chromatids, and also centrioles replicate take place. chromatids, and also centrioles replicate take place.

G2G2 (Gap2) is the internal between chromosome (Gap2) is the internal between chromosome duplication and the beginning of mitosis, during duplication and the beginning of mitosis, during which cell growth continues and proteins are which cell growth continues and proteins are synthesized in preparation for mitosis is restored synthesized in preparation for mitosis is restored to its normal size. Cell cycle activities may be to its normal size. Cell cycle activities may be temporarily or permanently suspended in a G0 temporarily or permanently suspended in a G0 phase.phase.

Some cells, such as skin cells, divide continuously Some cells, such as skin cells, divide continuously through out the life of the organism. Other cells, through out the life of the organism. Other cells, such as skeletal muscle cells and nerve cells, such as skeletal muscle cells and nerve cells, are arrested in the G1 stage. Cardiac muscle are arrested in the G1 stage. Cardiac muscle cells are arrested in the G2-stage.cells are arrested in the G2-stage.

The division cycle is regulated by extracellular The division cycle is regulated by extracellular signals from the environment (nutrient, size of signals from the environment (nutrient, size of cell, growth factors. cell, growth factors.

Growth factorsGrowth factors:: are molecules that attach to are molecules that attach to the plasma membrane receptors and thereby the plasma membrane receptors and thereby bring about cell growthbring about cell growth

Availability of G.F controls the animal cell cycle Availability of G.F controls the animal cell cycle at a point in the late G1 called the restriction at a point in the late G1 called the restriction point. If growth factors are not available point. If growth factors are not available during G1, the cells enter in a rest stage of during G1, the cells enter in a rest stage of the cycle (G0).the cycle (G0)...

The coordination between different phases The coordination between different phases of cell cycle is dependent on a system of of cell cycle is dependent on a system of checkpoints and feedback controls that checkpoints and feedback controls that prevent entry into the next phase of the prevent entry into the next phase of the cell cycle until the events of the preceding cell cycle until the events of the preceding phase have been completed.phase have been completed.

Several cell cycle checkpoints function to Several cell cycle checkpoints function to ensure that incomplete or damaged ensure that incomplete or damaged chromosome are not replicated and chromosome are not replicated and passed on to the daughter cells.passed on to the daughter cells.

the cell cycle is highly regulated, and the cell cycle is highly regulated, and checkpoints control transitions between cell- checkpoints control transitions between cell- cycle stages. cycle stages. Checkpoints are biochemical circuit Checkpoints are biochemical circuit that, that detect external or internal problems and that, that detect external or internal problems and send inhibitory signals to the cell-cycle system. send inhibitory signals to the cell-cycle system.

There are four major types of checkpoints.There are four major types of checkpoints. The restriction point.The restriction point. DNA damage checkpoints.DNA damage checkpoints. DNA replication checkpoints.DNA replication checkpoints. Spindle assembly checkpoints ( also called Spindle assembly checkpoints ( also called

metaphase checkpoints).metaphase checkpoints).

KinasesKinases are enzymes that activate proteins by are enzymes that activate proteins by transferring a phosphate group from ATP to the transferring a phosphate group from ATP to the protein being activated. An activated protein is protein being activated. An activated protein is needed for the cell cycle to proceed from G1 to needed for the cell cycle to proceed from G1 to S. Similarly, another activated protein is needed S. Similarly, another activated protein is needed to move the cycle from G2 to mitosis. to move the cycle from G2 to mitosis. 

KinasesKinases activate these proteins and thus stimulate activate these proteins and thus stimulate the cell cycle to continue.the cell cycle to continue.

Kinases are normally inactive and must be Kinases are normally inactive and must be activated before they can activate other proteins. activated before they can activate other proteins. Cyclin-dependent kinases become activated by Cyclin-dependent kinases become activated by combining with a protein called combining with a protein called cyclincyclin

Under normal conditions, cyclin combines Under normal conditions, cyclin combines with kinase only when growth factors are with kinase only when growth factors are present. For example, damaged tissue present. For example, damaged tissue releases growth factors to stimulate cell releases growth factors to stimulate cell division needed to repair the tissue. division needed to repair the tissue. 

S-Kinase combines with S-cyclin and the S-Kinase combines with S-cyclin and the resulting active complex stimulates DNA resulting active complex stimulates DNA replicationreplication

Some growth factors are being used in medicine, Some growth factors are being used in medicine, one example is erythropoietin, which stimulates one example is erythropoietin, which stimulates proliferation, differentiation, and survival red proliferation, differentiation, and survival red blood cell (erythrocytes) precursors in the bone blood cell (erythrocytes) precursors in the bone marrow.marrow.

The organism has elaborate regulatory systems The organism has elaborate regulatory systems that control cell reproduction& differentiation are that control cell reproduction& differentiation are controlled by a group of genes called Proto-controlled by a group of genes called Proto-oncogenes, alterating the structure of oncogenes, alterating the structure of expression of these genes promotes the expression of these genes promotes the production of tumors.production of tumors.

Several factors that inhibit cell reproduction are Several factors that inhibit cell reproduction are collectively called Chalones. Several factors (e.g., collectively called Chalones. Several factors (e.g., chemical substances, certain types of radiation, viral chemical substances, certain types of radiation, viral infections) can be induce abnormal cell proliferation infections) can be induce abnormal cell proliferation that bypasses normal regulation mechanisms for that bypasses normal regulation mechanisms for controlled growth and result in the formation of tumors.controlled growth and result in the formation of tumors.

The term tumor, initially used to denote any localized The term tumor, initially used to denote any localized swelling in the body caused by inflammation or swelling in the body caused by inflammation or abnormal cell proliferation.abnormal cell proliferation.

A tumor which is caused by abnormal proliferation of cell A tumor which is caused by abnormal proliferation of cell may eithermay either

Benign MalignantBenign Malignant

Benign: slow growth & not invade other tissues Benign: slow growth & not invade other tissues nor spreading to distant body sites.nor spreading to distant body sites.

Malignant: rapid growth & invade other tissues, Malignant: rapid growth & invade other tissues, spreading throughout the body (metastasis). spreading throughout the body (metastasis). Only malignant tumors are properly referred to Only malignant tumors are properly referred to as cancer.as cancer.

Cancer, can result from abnormal proliferation of Cancer, can result from abnormal proliferation of any of the different kinds of cell in the body. So any of the different kinds of cell in the body. So there are more than a hundred distinct types of there are more than a hundred distinct types of cancer, which can vary substantially in their cancer, which can vary substantially in their behavior and response to treatment.behavior and response to treatment.