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Transcript
€ The
cell cycle is defined as the series of
changes that a cell undergoes from time
it forms until it divides.
€-
Interphase used to be considered a
time of rest; but it is a very active period.
€ During this time, cell grows, and
maintains its routine functions.
€ Interphase is divided into phases based
on the sequence of activities:
• S phase- DNA replicates
• G1 and G2 phase –cell growth and DNA is
packed.
€ Mitosis
is the cell division of somatic cells
and produces two identical daughter
cells.
€ These new cells are identical, each of
them have 46 chromosomes.
€ Two main parts in mitosis
• 1- Karyokinesis- Nucleus divides
• 2-Cytokinesis-Cytoplasm divides
€ Mitosis
Stages:
• Prophase- Condensation of chromatin fibers
forming chromosomes.
x Each chromosome consist of two identical structures
called chromatids that are attached together at the
center by the centromere
€ Spindle
fibers attach to the centromeres
of the chomosomes so that a fiber
accompanying on chromatid attaches to
one centromere and the fiber
accompanying to the otherchromatid
attaches to its centromere.
€ The
chromosomes move along the
spindle fibers and align in the middle.
Metaphase plate
€ The
centromeres of the chromatids
separate and these identical chromatids
are now considered individual
chromosomes.
€ The separated chromosomes move in
opposite directions.
€ The spindle fibers shorten and pull their
attached chromosomes toward the
centrioles at opposite sides of cell.
€ The
final stage of mitosis begins when
the chromosomes complete their
migration toward the centiroles.
€ A nuclear envelope forms around each
chromosome set, and nucleioi becomes
visible.
€ It
is the cytoplasmic division
€ The
cytoplasmic division starts during the
anaphase. The cell membrane starts to
constrict around the middle.
€ The ring of spindles pinches and separtates
the two newly formed nuclei and half of
organelles go into different cells.
€ The newly formed cells may have different
size and number of organelles but the same
number of chromosomes.
How often a cell divides depends on the type of cell.
Skin cells, liver cells, and blood forming cells divide
constantly.
€ A physical basis for mitotic clock is the DNA at the tip
of chromosome.
€ Other external factors that control mitosis are the
kinases ( proteins) and cyclins , hormones and
growth factors.
€ Internal factors that control the mitosis are the
surface area ratio between cell and membrane and
the cell size.
€ Other external factors are availability
€ Too frequency mitosis crates a disorganized mass
called TUMOR
€
€
€
€
€
€
Two types of tumors:
1. Bening: It remains in place like a
lump,eventually interfering with the functions of
the tissue.
2. Malignant: or cancerous tumor may resemble a
crab. Cancer cells if not stopped, they
metastatize to other sites.
Two major types of genes cause cancer.
• Oncogenes: activates other genes that cause cell division
• Tumor suppresor genes: hold mitosis
€ Cells
that retain the ability to divide
themselves allow for continual growth
and renewal.
€ Stem Cell- divides mitotically to yield
either two daughter cells as themselves
or one daughter cell that is a stem cell
and another partially specialized.
€ The
partially specialized cell is termed
progenitor cell.
€ Stem
cells and progenitor cells are
described in terms of their
potential,according to the fates of their
daughter cells.
• 1- totipotent- can give rise to every cell type.
• 2- pluripotent- their daughter cells can follow
different pathways.