Download CHAPTER 12 THE CELL CYCLE The Key Roles of Cell Division

CHAPTER 12
THE CELL CYCLE
The Key Roles of Cell Division
Cell division results in genetically identical
daughter cells
The Mitotic phase alternates with interphase
in the cell cycle
Introduction
•
The continuity of life from one cell to another is
based on the reproduction of cells via cell division.
•
This division process occurs as part of the cell cycle
(the life of a cell from its origin in the division of a
parent cell until its own division into two).
two).
Fig. 12.1, Page 216
•
The division of a unicellular ‫ وﺣﯿﺪ اﻟﺨﻠﯿﺔ‬organism (e.g.
(e.g.
Amoeba)) reproduces an entire organism, increasing
Amoeba
the population.
•
Cell division is also central to the development of a
multicellular ‫ ﻋﺪﯾﺪ اﻟﺨﻠﯿﺔ‬organism that begins as a
fertilized egg or zygote.
Figs. 12
12..1, Page 216
2
Multicellular organisms also use cell division
to repair and renew cells that die normally or
by accidents (blood
(
cells from bone marrow).
).
Cell division distributes the genetic
material (DNA) to two daughter cells.
Figs. 12
12..1, Page 216
Division is differ among cells:.
- Skin cells divide frequently.
- Liver cells divide when needed (damage repair).
- Nerve cells and muscle cells do not divide at all.
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Cell division results in genetically
identical daughter cells
 A cell’s genetic information (genome ‫ )اﻟﺒﻨﻚ اﻟـﭽﯾﻨﻰ‬is packaged as DNA.
 In prokaryotes, the genome is often a single long DNA molecule.
 In eukaryotes, the genome consists of several DNA molecules.
 A human cell must duplicate about 3 m of DNA and separate the two copies
such that each daughter cell ends up with a complete genome.
 DNA molecules are packaged into chromosomes.
 Every eukaryotic species has a characteristic
chromosomes in the nucleus.
 Human somatic cells (body
(body cells)
cells) have
chromosomes.
 Human gametes ‫( أﻣﺸﺎج‬sperm or eggs) have
chromosomes, half the number in
a somatic cell ‫اﻟﺨﻠﯿﺔ اﻟﺠﺴﺪﯾﺔ‬..
 Each eukaryotic chromosome consists
linear DNA molecule.
number of
46
23
of a long,
Fig. 12.2, Page 216
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 Each chromosome has hundreds or thousands of genes, the units that specify an
organism’s inherited traits.
 Associated with DNA are proteins that maintain its structure and help control
gene activity.
 This DNA-protein complex, chromatin, is organized into a long thin fiber.
 After the DNA duplication in preparation for cell division, chromatin condenses,
coiling and folding to make a smaller package.
 Each duplicated chromosome consists of two sister chromatids which contain
identical copies of the chromosome’s DNA.
 Asthey
condense,
the
region where the strands
connect
shrinks
to
a
narrow
area,
is
the
centromere.
Fig. 12.3
 Later, the sister chromatids are pulled apart and repackaged into
two new nuclei at opposite ends of the parent cell during cell
division.
 The process of the formation of the two daughter nuclei called
(mitosis) and is usually followed by division of the cytoplasm
(cytokinesis ‫) اﻹﻧﺸﻄﺎر اﻟﺨﻠﻮى‬. It occurs in somatic cells ‫اﻟﺨﻼﯾﺎ اﻟﺠﺴﺪﯾﺔ‬
Chromatid
Chromatin +
DNA
Sister chromatid
Centromere
Homologous
Chromosome
Chromosome
‫اﻟﺼﺒﻎ‬
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In the gonads ‫اﻟﻤﻨﺎﺳﻞ‬
‫اﻟﻤﻨﺎﺳﻞ‬,, cells undergo a meiosis division, which yields four daughter
cells, each with half the chromosomes number of the parent cell.
In humans, meiosis reduces the number of chromosomes from 46 to 23
23..
Each of us inherited 23 chromosomes from each parent: one set in an egg and
one set in a sperm during meiosis.
gametes ‫اﻷﻣﺸﺎج‬
‫((اﻷﻣﺸﺎج‬eggs or sperm) are produced only in gonads ‫( اﻟﻤﻨﺎﺳﻞ‬ovaries or
testes).
The fertilized egg undergoes trillions of cycles of mitosis and cytokinesis to
produce a fully developed multicellular human.
These processes continue every day to replace dead and damaged cell.
Fertilization fuses two gametes together and doubles the number of
chromosomes to 46 again.
Definitions
Genes: Small segments of DNA that specify an organism’s inherited characters.
Genes:
characters.
Chromatin:: A DNAChromatin
DNA-protein complex which is organized into a long thin fiber
Chromosome::In the nucleus of each cell, the DNA molecule is packaged into
Chromosome
thread-like structures called chromosomes. Each chromosome is made up of
DNA tightly coiled many times around proteins called histones that support its
structure.
Chromatids:: Two sister arms (chromatids) formed in each duplicated
Chromatids
chromosome. They contain identical copies of the chromosome’s DNA
Centromere:: The narrow region at which the chromatids are attached to each
Centromere
other
Mitosis::Mitosis is a process of cell division which results in the production of
Mitosis
two identical daughter cells from a single parent cell.
Cyt okinesis
okinesis:: ‫ اﻹﻧﺸﻄﺎر اﻟﺨﻠﻮى‬Is the division of the cytoplasm which
usually follows mitosis.
Meiosis:: Cell division that occurs In the gonads ‫اﻟﻤﻨﺎﺳﻞ‬
Meiosis
‫اﻟﻤﻨﺎﺳﻞ‬,, and yields
four daughter cells, each with half the number of chromosomes of the
parent cell.
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The mitotic phase alternates with interphase
in the cell cycle: an overview
Phases of the Cell Cycle
• The mitotic (M) phase of the cell cycle alternates
with the much longer interphase.
– The M phase includes mitosis and cytokinesis.
– Interphase accounts
for 90% of the cell
cycle.
Fig. 12.5
A. Interphase has three subphases:
A. the G1 phase (“first gap”) centered on growth,
– the S phase (“synthesis”) when the chromosomes are
copied,
– the G2 phase (“second gap”) where the cell completes
preparations for cell division,
– and divides (M).
• The daughter cells may then repeat the cycle.
B. Mitosis: is usually include five subsub-phases ‫ﻣﺮاﺣﻞ ﻓﺮﻋﯿﺔ‬:
A.
B.
C.
D.
Prophase, ‫اﻟﺘﻤﮭﯿﺪﯾﺔ‬
Prometaphase,, ‫ﻗﺒﻞ اﻹﺳﺘﻮاﺋﯿﺔ‬
Prometaphase
Metaphase, ‫اﻹﺳﺘﻮاﺋﯿﺔ‬
Anaphase, ‫اﻹﻧﻔﺼﺎﻟﯿﺔ‬
E.
Telophase. ‫اﻹﻧﺘﮭﺎﺋﯿﺔ‬
By late interphase (G
(G2
2), the
chromosomes have been
duplicated ‫ ﺗﻀﺎﻋﻔﺖ‬but are
loosely packed.
The centrosomes have
been duplicated and begin
to organize microtubules
into an aster (“star”).
Fig. 12
12..5a, 218
11
1)
Prophase, ‫ اﻟﺘﻤﮭﯿﺪﯾﺔ‬the chromosomes are
tightly coiled, with sister chromatids
joined together, The nucleoli disappear
disappear..
The mitotic spindle begins to form and
appears to push the centrosomes away
from each other towards opposite ends
(poles) of the cell
cell..
2)
Prometaphase, ‫ ﻗﺒﻞ اﻹﺳﺘﻮاﺋﯿﺔ‬the nuclear
envelope fragments and microtubules
from one pole attach to one of two
kinetochores (special regions of the
centromere)) while microtubules from
centromere
the other pole attach to the other
kinetochore..
kinetochore
3)
Metaphase, ‫ اﻹﺳﺘﻮاﺋﯿﺔ‬the spindle fibers
push the sister chromatids until they are
all arranged at the imaginary plane
equidistant between the poles, defining
Fig. 12
12..6a, b,c,d
Anaphase, ‫ اﻹﻧﻔﺼﺎﻟﯿﺔ‬the centromeres
Anaphase,
divide, result in separating the
sister chromatids
chromatids.. Each is then
pulled toward the pole to which it
is attached by spindle fibers
fibers.. By the
end, the two poles have equivalent
collections of chromosomes
chromosomes..
Telophase, ‫ اﻹﻧﺘﮭﺎﺋﯿﺔ‬the cell continues to
Telophase,
elongate as free spindle fibers from
each centrosome push off each
other..
other
1)
2)
Two nuclei begin to form,
surrounded by the fragments of the
parent’s nuclear envelope
envelope..
Chromatin becomes less tightly
coiled..
coiled
Cytokinesis, begins as the
Cytokinesis,
division of the cytoplasm
occurs.
Fig. 12
12..6 e and f
Fig. 12.6
Fig. 12.6
B. The cytokinesis
cytokinesis:: ‫ اﻹﻧﺸﻄﺎر اﻟﺨﻠﻮى‬divides the cytoplasm:
• Cytokinesis (division of the
cytoplasm)) typically follows
cytoplasm
mitosis.
• Contraction ‫ إﻧﻘﺒﺎض‬of the cell
pinches the cell into two
new cells
Fig. 12.8, Page 222
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Cell Cycle
Interphase
Division process
Cytokinesis
Mitosis
G1
S
Prophase
‫اﻟﺘﻤﮭﯿﺪﯾﺔ‬
G2
Prometaphase
‫ﻗﺒﻞ اﻹﺳﺘﻮاﺋﯿﺔ‬
Metaphase
‫اﻹﺳﺘﻮاﺋﯿﺔ‬
Anaphase
‫اﻹﻧﻔﺼﺎﻟﯿﺔ‬
Telophase
‫اﻹﻧﺘﮭﺎﺋﯿﺔ‬
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