Cell Biology Lec.5 Dr:Buthaina Al-Sabawi Date: Cell Biology Lec.5 Dr:Buthaina Al-Sabawi Date: The Cell Cycle The cell cycle, is the series of events that leads to duplication and division of a cell. The cell cycle, is the series of events that leads to duplication and division of a cell.

Division cycle consists of 4 coordinated processes:(1) cell growth,(2) DNA replication, (3)distribution of the duplicated chromosomes,(4) cell division Division cycle consists of 4 coordinated processes:(1) cell growth,(2) DNA replication, (3)distribution of the duplicated chromosomes,(4) cell division The alternation between mitosis and interphase called cell cycle Cell cycle divided into 2 stages. Mitosis(4 stages) (1)Prophase (2) Metaphase (3) Anaphase (4) Telophase Interphase, is itself divided into three phases

(1) G1 (Presynthesis) (2) S (DNA synthesis) (3) G2 (Post DNA duplication) The cell cycle divided into four periods, G1, S, G2 and mitosis (M) The cell cycle divided into four periods, G1, S, G2 and mitosis (M) G1(Gap1) the cell grow in size and the cellular organelles increase in number, the cell is metabolically active & continuously grows but does not replicate its DNA. (S) is the time when the cell duplicates its genetic material by synthesizing DNA. During duplication, each chromosome doubles to produce identical sister chromatids, and also centrioles replicate take place.

G2 (Gap2) is the internal between chromosome duplication and the beginning of mitosis, during which cell growth continues and proteins are synthesized in preparation for mitosis is restored to its normal size. Cell cycle activities may be temporarily or permanently suspended in a G0 phase. Some cells, such as skin cells, divide continuously through out the life of the organism. Other cells, such as skeletal muscle cells and nerve cells, are arrested in the G1 stage. Cardiac muscle cells are arrested in the G2-stage. Some cells, such as skin cells, divide continuously through out the life of the organism. Other cells, such as skeletal muscle cells and nerve cells, are arrested in the G1 stage. Cardiac muscle cells are arrested in the G2-stage.

The division cycle is regulated by extracellular signals from the environment (nutrient, size of cell, growth factors. Growth factors: are molecules that attach to the plasma membrane receptors and thereby bring about cell growth Availability of G.F controls the animal cell cycle at a point in the late G1 called the restriction point. If growth factors are not available during G1, the cells enter in a rest stage of the cycle (G0)..

The coordination between different phases of cell cycle is dependent on a system of checkpoints and feedback controls that prevent entry into the next phase of the cell cycle until the events of the preceding phase have been completed. Several cell cycle checkpoints function to ensure that incomplete or damaged chromosome are not replicated and passed on to the daughter cells.

the cell cycle is highly regulated, and checkpoints control transitions between cell- cycle stages. Checkpoints are biochemical circuit that, that detect external or internal problems and send inhibitory signals to the cell-cycle system. the cell cycle is highly regulated, and checkpoints control transitions between cell- cycle stages. Checkpoints are biochemical circuit that, that detect external or internal problems and send inhibitory signals to the cell-cycle system. There are four major types of checkpoints.  The restriction point.  DNA damage checkpoints.  DNA replication checkpoints.  Spindle assembly checkpoints ( also called metaphase checkpoints).

Kinases are enzymes that activate proteins by transferring a phosphate group from ATP to the protein being activated. An activated protein is needed for the cell cycle to proceed from G1 to S. Similarly, another activated protein is needed to move the cycle from G2 to mitosis. Kinases are enzymes that activate proteins by transferring a phosphate group from ATP to the protein being activated. An activated protein is needed for the cell cycle to proceed from G1 to S. Similarly, another activated protein is needed to move the cycle from G2 to mitosis. Kinases activate these proteins and thus stimulate the cell cycle to continue. Kinases are normally inactive and must be activated before they can activate other proteins. Cyclin-dependent kinases become activated by combining with a protein called cyclin

Under normal conditions, cyclin combines with kinase only when growth factors are present. For example, damaged tissue releases growth factors to stimulate cell division needed to repair the tissue. Under normal conditions, cyclin combines with kinase only when growth factors are present. For example, damaged tissue releases growth factors to stimulate cell division needed to repair the tissue. S-Kinase combines with S-cyclin and the resulting active complex stimulates DNA replication

Some growth factors are being used in medicine, one example is erythropoietin, which stimulates proliferation, differentiation, and survival red blood cell (erythrocytes) precursors in the bone marrow. The organism has elaborate regulatory systems that control cell reproduction& differentiation are controlled by a group of genes called Proto- oncogenes, alterating the structure of expression of these genes promotes the production of tumors.

Several factors that inhibit cell reproduction are collectively called Chalones. Several factors (e.g., chemical substances, certain types of radiation, viral infections) can be induce abnormal cell proliferation that bypasses normal regulation mechanisms for controlled growth and result in the formation of tumors. The term tumor, initially used to denote any localized swelling in the body caused by inflammation or abnormal cell proliferation. A tumor which is caused by abnormal proliferation of cell may either Benign Malignant

Benign: slow growth & not invade other tissues nor spreading to distant body sites. Malignant: rapid growth & invade other tissues, spreading throughout the body (metastasis). Only malignant tumors are properly referred to as cancer. Cancer, can result from abnormal proliferation of any of the different kinds of cell in the body. So there are more than a hundred distinct types of cancer, which can vary substantially in their behavior and response to treatment.