Download The Cell Cycle

2/25/2016
The Cell Cycle
The Cell Cycle
• The cell cycle is the series of events that take
place in a eukaryotic cell from the time of its
formation and the moment it replicates itself.
▫ What’s a eukaryotic cell?
 Has a nucleus enclosed by a membrane and
membrane organelles. Animals, plants, fungi
(mushrooms). NOT bacteria.
How do prokaryotic cells divide?
• Asexual
Reproduction
▫ A single-celled
organism can divide to
become two living
organisms (asexual
reproduction)
▫ In prokaryotes, this is
called binary
fission.
▫ No chromosomes are
formed in binary
fission.
▫ The cell replicates its
DNA, then splits in
half.
Why do cells need to divide?
• Why don’t they just get bigger and
bigger?
▫ As a cell grows larger, it become
more ineffective at transporting
materials in and out of a cell.
Why do cells need to divide?
• Sexual Reproduction
▫ A multicellular organism can grow from a
single cell created by fertilization (zygote).
• Growth
▫ Divisions of a single cell can result in
numerous cells.
• Repair
▫ Damaged cells can repair themselves as they
spend time in interphase.
▫ New cells can be created to replace damaged
or dead cells.
The Cell Cycle
• Includes two stages:
1. Interphase = growth and
replication of DNA. The cell spends
most of its life in this phase.
2. Cell Division or M phase=
nuclear division (mitosis) and
splitting of the cell (cytokinesis)
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Interphase
• The period of time before cell division.
• Interphase is divided into 3 phases: G1, S, and G2.
• At any given time, about 90% of cells are in
interphase.
Interphase
• The G1 phase represents the primary growth of
the cell (most of the time is spent in this phase).
A great amount of protein synthesis occurs
during this stage.
• During the S (Synthesis) phase, DNA synthesis
and replication occurs.
• During the G2 phase, the cell is growing rapidly
to prepare for mitosis.
• http://www.cellsalive.com/cell_cycle.htm
• At any given time, about 90% of cells are
in interphase.
Mitosis and Cytokinesis
• Mitosis- the process by which the nucleus of the
cell divides into two nuclei, each with the same
number and kinds of chromosomes as the parent
cell.
• Cytokinesis- the process by which the cytoplasm
divides, forming two cells.
G0 Phase
• Sometimes cells exit the cell cycle (usually from
G1) and enter the G0 phase.
• In the G0 phase, cells are alive and metabolically
active, but do not divide. Cells do not copy their
DNA and do not prepare for cell division.
• If these cells are damaged, they cannot be
replaced.
• Heart, muscle, eyes and brain.
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• The cell cycle has a regular system of checks and
balances that prevents damaged or mutated cells
from proceeding to the next phase. One way an
organisms deals with the problems is to kill the
damaged cell before it passes on the problem to
its daughter cells. This is a normal process
called apoptosis. (Some normal cells also go
through this process.)
Chromosomes
What are chromosomes made of?
Chromosomes
• During cell division, DNA (genetic
material) packs into chromosomes
• Eukaryotic cells have chromosomes made
up with distinct lengths of DNA.
• Human cells have 46 chromosomes.
• The DNA of a chromosome is about 5 cm long
• The DNA in a human is about 2-3 meters long
• How does it get so small???
• Chromatid- chromosomes have two identical sides,
each is called a chromatid. CHROMATIDS FORM
AS THE DNA MAKES A COPY OF ITSELF BEFORE
IT DIVIDES.
• Centromere- attaches the chromatids in the center
and holds them together until they separate during
cell division.
What are chromosomes made of?
• Chromatin- uncoiled DNA and
protein.
• Histones- proteins that the DNA are
wrapped around
• DNA in a human chromosome is
10,000 times the length of the
chromosome!
1. Chromatid
2.Centromere
3.Short arm
4.Long arm
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How do chromosomes form?
• Chromosomes come in homologous pairs
• https://www.dnalc.org/resources/3d/07-howdna-is-packaged-basic.html
▫ Duplicate chromosomes are called homologous pairs
• Homologous pair= 1 of each was given by each
parent for a total of 2
• Homologous pairs are the same size, shape, and
have the same genes on them.
• If one has a gene for hair color, so would the
other.
Karyotype
• Shows all the
chromosomes as a
picture where they are
paired with their
duplicates
(homologous pairs)
• Sex chromosomeschromosomes that
determine the sex of
an organism
▫ X and Y in humans
• Autosomes- all other
chromosome besides
the sex chromosomes
QOD 2/9/16
• Some forms of cancers are treated with
radiation, similar to ultraviolet light. Why might
it be beneficial to irradiate cancer cells?
• The cancer cells will be damaged so that they
will no longer function as normal cells, i.e. they
will not divide or may be induced to undergo
apoptosis.
QOD, cont.
• Chemotherapy (treatment for cancer) utilizes
chemicals that disrupt various parts of the cell
cycle, targeting rapidly growing cells. Taxol is
one such drug that prevents the mitosis phase
from taking place. Explain how this drug is
useful as a cancer treatment.
• The cancer cells are rapidly dividing. When the
drug inhibits cell division, the tumor will stop
growing.
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• How might targeting rapidly growing cells
explain common chemotherapy side effects such
as hair loss and nausea?
Mitosis
How do living things grow and
repair themselves?
These dividing bone marrow cells are undergoing
mitosis to give rise to new blood cells
• Living things must grow and develop. At times
they suffer injuries or damage, or cells simply
wear out. New cells must be formed for the
organism to survive. What process must occur
to make a new, properly functioning cell?
MITOSIS!
This amoeba is undergoing cell division to create a
genetically identical daughter cell.
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How old are you?
▫ Look at your arm: Skin cells, about 2 weeks old.
▫ Paper cut? The blood from the cut is about four
months old.
▫ Stomach lining: cells last about five days
▫ Liver: a year and a half
▫ The billion+ cells that make you up regenerate
about every seven years on average.
IPMAT – in the order of
the letters
Prophase
Prophase
• Centriole pairs move
toward opposite
poles
• Spindle fibers begin
to extend from
centrioles. Form
asters
• Duplicated
chromosomes form
as long threads
• Nuclear membrane
breaks down
• Nucleolus disappears
• Chromosomes clearly
visible and begin to
move to equator of cell
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Metaphase
•Spindle fibers align
the chromosomes
along the middle of
the cell nucleus.
Anaphase
•The chromosomes separate (into
chromatids) and move to opposite sides of
the cell.
•Chromatids move to poles
Telophase
•Chromatids arrive at opposite poles of cell
• New nuclear membranes form around the
chromatids .
• Centrioles and spindle
fibers disappear
• Chromosomes stretch out
• Nuclear membrane forms
around each mass of
chromosomes
• Cleavage furrow/cell plate
begins to form
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Cytokinesis- splitting of cell membrane
Cytokinesis
• Cytokinesis in animal cells begins with the
formation of a cleavage furrow
– A fiber ring composed of a protein around
the center of the cell contracts pinching the
cell into two daughter cells, each with one
nucleus.
– The cleavage furrow appears during telophase
• Cytokinesis in plant cells
▫ During telophase, a new cell wall begins
to form at the metaphase plate
 This is called a cell plate
▫ The plant cell continues to grow this cell
wall until it divides the cell into 2
Results of Mitosis
•Mitosis is the division of a NUCLEUS
•Mitosis produces two new nuclei that have the
SAME number of chromosomes as the original
nucleus.
•So the parent and daughter cells are
genetically identical
1.Prophase
2.Metaphase
3.Anaphase
4.Telophase
•MITOSIS IS ASEXUAL reproduction
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Uncontrolled cell growth
• Cancer- a disorder in which cells have
lost the ability to control their own
rate of growth.
• Tumor- mass of abnormal cells (A)
• A series of enzymes control the cell cycle at each
checkpoint.
Meiosis
Necessary for SEXUAL reproduction
Meiosis produces gametes (sex cells)
• Cells reproduce through mitosis to make exact
copies of the original cell. This is done for
growth and repair. Sexually-reproducing
organisms have a second form of cell division
that produces reproductive cells with half the
number of chromosomes. This process is called
meiosis, and without it, humans, oak trees,
beetles, and all other sexually reproducing
organisms would be vastly different than they
are today.
• Meiosis produces gametes (sex cells).
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Types of Cell Division
• Somatic cells undergo mitosis for cellular division
Diploid VS Haploid
▫ Somatic cells are BODY CELLS and are
considered diploid or 2n
▫ Diploid cells have 2 of each chromosome
▫ Human cells contain a total of 46
chromosomes (23 pairs)
 23 from biological mother; 23 from
biological father
▫ Mitosis results in 2 identical diploid cells
from 1 diploid cell.
Types of Cell Division
• Gametes (egg and sperm) are made from cells
undergoing meiosis
▫ Gametes are only produced in the gonads
(ovaries and testes)
▫ Each of the four gametes produced from a
parent cell is considered haploid or n
 Each of the four daughter cells has ½ the
number of chromosomes (only 1 copy of
each chromosome as the parent
 Each of the four daughter cells is
• If an organism’s somatic cells have 100
chromosomes, how many would the organism’s
gametes have?
genetically different
Human cells have
46 chromosomes
EXCEPT
sex cells (gametes)
have only 23 (half)
• Cells preparing to divide by meiosis undergo the
G1, S and G2 stages of interphase. During
interphase, the cell will copy its DNA during the
S phase, so cells begin meiosis with a duplicate
set of chromosomes, just as cells beginning
mitosis do. Meiosis I ends with two cells.
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Model 5 –
Meiosis I
• Alleles are alternative forms of the same gene.
For example, gene A may contain the
information for fur color. One allele “A” may
result in white fur, while the alternative allele “a”
may result in black fur. Homologous
chromosomes are chromosomes that contain
the same genes, although each chromosome in
the homologous pair may have different alleles.
Meiosis I
Meiosis I (starts with 2N)
Prophase I
▫ Crossing over can occur; exchange of DNA
between homologs and/or 2 sister chromatids
Metaphase I
Metaphase I
• Tetrads move to the equator;
homologous pairs stay together
(so they are side by side)
Anaphase I
• Homologous pairs separate
and move to each pole
(One chromosome of each
pair is pulled to each pole)
• Chromatids still joined at
centromere (unlike in
Mitosis)
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Telophase I
• Nucleus reforms, spindles
disappear (similar to
mitosis)
• Cytoplasm divides forming
2 daughter cells
• Cytokinesis and a short
interphase without DNA
replication also occur
Model 7 - Meiosis II
Crossing Over
• The exchange of DNA
between homologs
and/or sister
chromatids
• Causes genetic
recombination because
new mixtures of genetic
material are created.
• Resulting chromatids
will be genetically
different from the
parent chromatids
Meiosis II
(chromosomes do not double; ends with 4 (1nhaploid) cells
Meiosis II
Prophase II
• New spindle fiber forms
• Nuclear membrane and nucleolus break apart
• Chromosomes form
Metaphase II
• chromosomes move to the equator
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Meiosis II
Anaphase II
• Centromere divides; sister chromatids (1/2 of
each chromosome) moves to opposite poles
Telophase II
• Nuclear membrane reforms; Spindle disappears
• Cytokinesis occurs
• 4 daughter cells from one parent
Meiosis II Review
• Each chromosome
contains 2 chromatids
• They line up at the center
of each cell
• Paired chromatids
separate
• 4 daughter cells are
produced (haploid)
Reproduction and Meiosis
• Meiosis in males ♂
• 1. Part of
spermatogenesis
• 2. Occurs in the
testes
• 3. Always results in
four sperm cells
• Meiosis in females ♀
•Part of oogenesis
•Occurs in the ovaries
•One cell receives most of
the cytoplasm; the other
3 cells are polar bodies
(will die)
•Results in mature egg
• ( with 23
chromosomes)
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• Before replication, one chromosome is composed of
one DNA molecule. Following replication, each
chromosome is composed of two DNA molecules; in
other words, DNA replication itself increases the
amount of DNA but does not increase the number of
chromosomes. The two identical copies—each
forming one half of the replicated chromosome—are
called chromatids. During the later stages of cell
division these chromatids separate longitudinally to
become individual chromosomes.
How does the math work out? (MITOSIS)
• In case the difference between chromosomes
and chromatids is a little unclear in your book
and my notes so far… TECHNICALLY…
▫ People have 46 chromatids before DNA is
replicated and 92 chromatids after (or 46
chromosomes)
▫ After mitosis, the 46 chromosomes (or 92
chromatids) split up
▫ So, each daughter cell has 46 chromatids (just
like the parent originally did
 Which will then go on to replicate and
become 46 chromosomes
How does the math work out?(MEIOSIS)
▫ During meiosis I, 46 chromosomes are made
again, then they are split into 2 daughter cells,
which each get 23 chromosomes (NOT
chromatids)
▫ Then, during meiosis II, those 23
chromosomes (46 chromatids) are split into
their individual chromatids
 So the 2 daughter cells from meiosis I
each create 2 more daughter cells that will
have 23 chromatids each.
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