Download 1. Define the cell cycle? It`s the period of time from the formation of

1. Define the cell cycle?
It’s the period of time from the formation of cell by division to the point when that
cell itself divides
2. Cells are classified into three types according to their capacity to grow and divide
1) Cells with high structural specialization such as nerve cells , muscle cells,
have lost their ability to divide
2) Cells that normally don’t divide , but can be stimulated to begin DNA
synthesis and division when faced with certain stimulus , liver cells for
example can be induced to undergo division by surgical removal of part of
the liver . lymphocytes also stimulated to proliferated by interaction with
antigens
3) Cells that normaly have some level of mitotic activity . for example some
tissues of the human body are subjected to continuous destruction and
must be replaced such as skin cells , epithelial cells , spermatozoa
4) Its important to note , that in each of these cells , there is large population
of reserve cells (stem cells) responsible for the replacement of cells
destroyed under normal physiological conditions
3. What are the most important features of stem cells?
A. They appear morphologically unspecialized
B. Self-renewal after each cell cycle
C. Plasticity ; able to divide and form any cell type according to body needs
4. What are the stages of the cell cycle
1) Interphase
2) Mitosis (M) includes the four phases of cell division (prophase, metaphase,
anaphase, telophase)
3) Cytokinesis where separation of the two daughter cells
5. What are the most important features of interphase ?
A. It’s the period of time during which the metabolic activity of the nucleus is
high
B. It represents the longest duration of cell cycle in which protein and other
biochemical molecules are build up
C. It consists of three mini phases symbolized by G1 , G2 gaps where there is
enlargement of cell size and high cell growth and S phase during which DNA
replication happens
D. The most important biochemical activity of the cells in preparation for cell
division is DNA replication . if DNA synthesis occur , then the cell will divide
, if not , then the cell is said to be in quiescent ‫ ساكن‬phase (G0) to distinguish
it from G1 phase and the cell will not divide
E. the decision whether a cell will continue to divide or not is made in G1
phase and it's about whether or not to start DNA synthesis
F. evidences suggest that a cell in G1 phase must receive a positive signal in
order to proceed into S phase , and that signal come from the increase in
the size of the cell that occur during G1 phase
6. what are the factors that control or regulate cell growth
1) adequate water ; in the absence of water , the cell will go into dormant ‫سبات‬
resting stage such as those of cysts or spores , perhaps remaining dormant
for many years
2) adequate nutrients
3) temperature ; division is greatly inhibited in E.Coli at 15 C , at which a cell
may only give rise to about 128 cells / 24 hours , whereas at 37 C would be
about 2 x 109
4) availability of enzymes affect cell growth directly
5) poison ; these are agents that inhibit cell function and growth such as
cyanide , tetracycline , rifampcin , erythromycin , chloramphenicol
1. write few notes about mitosis and meiosis
A. in mitosis , the nuclear materials divide so that the resulting daughter cells
are genetically identical
B. mitosis occurs in all cells except gametes
C. meiosis on the other hand occurs only in the formation of gametes
D. meiosis is responsible for the variation among human species through
exchange of genetic materials (crossing – over ) that occurs during the
formation of gametes
E. during mitosis , the cell has two pairs of homologus chromosomes , in which
the genetic material (2 x DNA) was duplicated already to (4x DNA) during S
phase of interphase , and each chromosome has divided into two starands
held together by centrome
2. what are the stages of mitosis ?
3.
4.
5.
6.
7.
1) prophase ‫االبتدائي‬
2) metaphase ‫الوسيط‬
3) anaphase
4) telophase
What happens during prophase?
1) the chromosomes shorten and thicken and can be seen clearly under
microscope
2) the mitotic spindles start to form by fibers radiating out from opposite
poles of the cells
what happens during metaphase (second stage of mitosis )
1) all the chromosomes move to the equatorial plane of the cell
What happens during anaphase?
1) the centromes divide
2) centromeres move towords opposite poles of the cells pulling their
chromosome strands with them
what happens during telophase ?
1) the separation of chromatids is completed
2) each one chromatid is known now as daughter chromosome
what happens during cytokinesis
1) the division of the cytoplasm resulting in the formation of two daughter cells