The Cell Cycle

First stage

Biology
Lec 3
28/12/2015

د. بثينة

The Cell Cycle
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.
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





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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)
G1(Gap1): The first gap in the normal cell cycle is called G1 and is the period when the necessary proteins for DNA replication are synthesized. 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 referred as synthesis phase when DNA synthesis. 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.
The duration of the cell cycle phases varies considerably in different kinds of cells.
Cell cycle activities may be temporarily or permanently suspended in a G0 phase.
Some cells, such as skin cells, divide continuously throughout 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.

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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 Growth factors 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.

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The cell cycle is highly regulated, and checkpoints control transitions

between cell- cycle stages. Checkpoints are biochemical circuits, 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.

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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 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.
S-Kinase combines with S-cyclin and the resulting active complex stimulates DNA replication.
The p53 protein plays role in apoptosis by forcing unfunctional or bad cells to commit suicide. p53 is a protein that functions to block the cell cycle if the DNA is damaged is severe this protein can cause apoptosis (cell death). P53 levels are increased in damaged cells. This allows time
to repair DNA by blocking the cell cycle.





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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 or Malignant.
Benign: Not malignant. A benign tumor is one that does not invade
surrounding tissue or spread to other parts of the body; it is not a cancer.
Malignant: rapid growth & invade other tissues, spreading throughout the body (metastasis). Only malignant tumors are properly referred to as cancer. Malignant tumors are formed from abnormal cells that are highly unstable and travel via the blood stream, circulatory system and lymphatic system.
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

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