Transcript Chapter 9 Cell Division-Proliferation and Reproduction

Chapter 9
Cell Division-Proliferation and
Reproduction
Cell Division
Cell Division
• Cell Division is where one cell becomes two cells.
– Asexual Reproduction is where one parent cell divides
into two identical cells.
– Sexual Reproduction is where genetic information
from two parent cells to create a unique individual
cell.
– Three Types of Cell Division
• Binary Fission
– Asexual Reproduction
• Mitosis
– Asexual Reproduction
• Meiosis
– Sexual Reproduction
Binary Fission
• Binary Fission is form of asexual reproduction
and cell division used by all prokaryotic and
some eukaryotic organisms.
1. The parent cell DNA duplicates, and the cell elongates
2. Cell wall and plasma membrane begin to divide
3. Cross-wall forms completely around divided DNA
4. Cells separate, there are now two daughter cells that are
identical
Binary Fission
Human Genome
• Humans have 23 pairs of chromosomes. This makes
a total of 46 chromosomes.
• A karotype is a description of the number, size, and
shape of chromosomes.
Somatic and Sex Cells
• In humans, somatic (body) cells have two sets
of chromosomes, a total of 46.
• These cells are called diploid or 2N.
• In humans, gametes (sperm or eggs) have one
set of chromosomes, a total of 23. These are
also known as sex or germ cells.
• Gametes have exactly one copy of each
chromosome.
• These are known as haploid or N.
Chromosomes
• What is a chromosome?
– A chromosome is double-stranded DNA molecules
coiled into a short, compact unit containing
genetic material.
• Chromatin is DNA wrapped around histone proteins.
This is to keep them from getting tangled.
Chromosomes
• Uncoiled, each of your chromosomes is about
two inches long.
• So, imagine all 46 of your chromosomes into a
cell’s nucleus that is 5 microns in diameter.
• This is the equivalent of fitting 46 strings, each the
length of a football field into a baseball.
Chromsomes
• A chromatid is one
of two parallel parts
of a chromosome.
• Sister chromadtids
are identical copies
of a chromosome,
attached by a
centromere.
Cell Cycle
• The cell cycle consists of all of the stages of
growth and division for eukaryotic cells.
– It is divided into Interphase and Mitosis.
Interphase
• Interphase is where cells will continue in normal
metabolic activities and begins to prepare for cell
division. Interphase is the longest stage of the cell
cycle.
– G1 Stage
– S Stage (Synthesis)
– G2 Stage
G1 Stage
• In the G1 stage of interphase, the cell will
gather nutrients and other resources from its
environment.
– G1 is the “first growth” phase during the cell’s
primary growth phase
– If a cell will remain in the G1 phase for an
extended period of time, it may be also called the
G0 stage. This is because it is not moving forward
in the cell cycle.
S & G2 Stage
• In the S Stage of Interphase, DNA replication
occurs in the cell.
– This leaves a cell with two identical copies of DNA
for later on in the cell cycle.
• The final part of interphase is the G2 Stage.
• Final preparations are made before going into
mitosis.
• This includes making the proteins used for moving the
chromosomes.
Mitosis
• After interphase, mitosis is the process of the cell cycle
where one parent cell will divide into two daughter
cells. During mitosis, all cellular activity ceases.
• Mitosis is divided into four parts.
• Prophase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis (division of
cytoplasm) typically follows
telophase.
Prophase
• The first stage of mitosis is prophase. There
are three important parts of prophase.
• Chromosomes condense
• Spindle and spindle fibers form
• Nuclear membrane disassembles.
Prophase
• In early prophase, two sets of microtubules,
known as centrioles, begin to separate and
move to opposite poles of the cell.
• Attached to the centrioles, spindle fibers will help
move the chromosomes later in mitosis.
Prophase
• In later prophase, chromosomes begin to appear as 2
chromatids connected at the centromere.
• The nucleolus and nuclear membrane have
disintegrated.
• The spindle fibers have moved farther apart.
• Chromosomes attach to the spindle fibers.
Metaphase
• Metaphase, the second
stage of mitosis, is
where chromosomes
align at the equatorial
plane of the cell.
• At the end of
metaphase, the
centromere holding the
sister chromatids
together divides.
• Each chromatid is now
known as a daughter
chromatid.
Anaphase
• Anaphase, the third stage of mitosis, the sister
chromatids move toward opposite ends of the cell.
• The kineochore is a
protein attached to
each chromatid at the
centromere.
Telophase
• In telophase, the cell finishes mitosis
• Spindle fibers disassembles.
• Nuclear membrane re-forms
• Chromosomes uncoil
• Nucleolus reforms
Cytokinesis
• At the end of telophase, cytoskinesis occurs.
Cytokinesis is when the cytoplasm separate to
form two separate cells.
• For cytokinesis in animal cells, a cleavage furrow is
formed. A cleavage furrow is an indention of the
plasma membrane that pinches to the center of
the cell. This splits the cell in two.
Cytokinesis
• Yet, in plant cells, cytoplasm division occurs
with a cell plate. The cell plate begins to form
at the center of the cell and grows outward to
the cell membrane.
• After cytokinesis,
you have two
daughter cells. Two
exact copies.
Controlling Mitosis
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During cell division, there are checkpoints
which use proteins to evaluate the health of
a cell.
Proto-oncogenes will code for proteins that
provide signals to the cell that will
encourage cell division.
Tumor-supressor genes will code for
proteins to signal stopping cell division
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p53 Protein
The p53 gene will code for the p53 protein, which is
a type of tumor-supressor gene.
If it detects damaged DNA at the end of the G1
phase, it will send out enzymes to fix the problem.
If the cell is deemed healthy afterwards, it is able to
undergo cell division.
p53 Protein
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Yet, if the damage is too far beyond repair,
the p53 protein will cause the cell to digest
itself from the inside out, known as
apoptosis.
The other healthy cells will undergo cell
division to replace the lost cell.
p53 Protein
•
What is the p53 gene is mutated?
–
•
It would code for a p53 protein that might
not be able to monitor for damaged DNA.
Mutations to the p53 gene appear in 40% of all
cancers.
Cancer
•
Cancer is a disease caused by the failure to
control cell division.
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–
Mutagens are agents that mutate or
chemically damage, DNA.
Carcinogens are mutagens that cause cancer.
•
Tar in cigarette smoke is categorized as both a
mutagen and a carcinogen.
Cancer
•
There are several agents that gave been associated with
high risks of cancer:
•
Radiation
– X-Rays
– UV-A from tanning lamps, UV-B
•
Chemicals
– Arsenic
– Benzene
– Asbestos
– Food containing nitrates
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Some have a prediposition to cancer due to their genetic
backgrounds. Mutated DNA may be passed from parent to
offspring
Cancer
•
When uncontrollable mitotic division
occurs, a group of cells form called a tumor.
–
Tumor is a mass of cells not normally found
in a certain portion of the body.
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Benign tumor is a cell mass that does not
fragment and spread beyond its original area
of growth.
Malignant tumors can spread through other
parts of the body.
–
Cells of malignant tumors metastasize, or
move from the original site and begin to grow
new tumors in other regions of the body.
Terminology
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Haploid is a cell with ONE set of chromsomes (N)
Diploid is a cell with TWO sets of chromosomes.
(2N)
Ultimately, one set comes from the haploid cells
provided by each parent (N + N = 2N)
Seed shape
lor
Seed shape
Leaf color
Terminology
•
Homologous chromosomes have the same
appearance and genes.
–
Diploid cells will have many sets of
homologous chromosomes
Flower color
Flower color
Plant height
Plant height
Terminology
•
Non-homologous chromosomes have
different genes on their DNA.
Terminology
During the preparations for meiosis, the
genetic information in a cell will be
copied into sister chromatids.
At this point, sister chromatids are
identical.
The purple sister chromatids on the right
are homologous chromosomes to the
green sister chromatids on the left
because:
1. Same Genes
2. Similar Shapes
3. The centromeres holding sister
chromatids are in the same location
along the chromosomes.
Fig. 9.20,
pg. 175
Sexual Reproduction
• Meiosis transforms diploid cells (2N) into haploid
daughter cells (N)
– Meiosis occurs only in diploid cells, not in haploid cells.
• Fertilization, the fusion of two haploid cells, causes
the transition from haploid (N) to diploid (2N).
– Haploid cells that fuse are called gametes
– This first diploid cell is called a zygote.
• Depending on the organism, mitosis (asexual
reproduction) may occur in haploid cells only, diploid
cells only, or in both haploid and diploid cells.
Alternation of Generations-Human
•In humans, the only
haploid stages are
gametes.
•In males, spermatozoa
begins at puberty and
continues daily until at
least age 70.
•In females, oogenesis
begins as a fetus.
•This egg completes
meiosis after fertilization
by a sperm.
Alternation of Generations-Plant
•In flowering plants and
conifers, the dominant phase
of the life cycle is also diploid.
•Haploid cells are produced in
anthers and in ovaries.
•These haploid cells undergo a
few cycles of mitosis to
produce multicellular haploid
stages.
•Fusion of sperm (N) with egg
(N) produces a zygote (2N)
Meiosis
•
In meiosis, chromosome copy once, divide twice.
– Like mitosis, meiosis is preceded by interphase.
•
This is followed by two rounds of separating
chromosomes to end up with four haploid cells.
Meiosis
• Meiosis
• Meiosis I
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Prophase I
Metaphase I
Anaphase I
Telophase I
• Meiosis II
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Prophase II
Metaphase II
Anaphase II
Telophase II
Interphase
• During interphase, the chromosomes are
copied and the copies (sister chromatids)
remain attached at the centromere.
• The machinery necessary to move
chromosomes (centrioles, etc.) is copied too.
Meiosis I
•Prophase 1 begins as the
chromosomes condense for
packing.
•Spindle fibers begin to form.
•Homologous chromosomes
pair up, these are known as
tetrads.
•Homologous chromosomes
typically exchange some
sections of DNA in a process
called crossing over
Meiosis I
•In metaphase I, all the tetrads
are lined up midway between
the two poles.
•Anaphase I begins when the
homologous chromatids in the
tetrads release each other.
•Spindle fibers pulls the
homologous chromatids to
opposite poles of the cell.
•The homologous
chromosomes are
separated from each other
= segregated.
Meiosis I
•In telophase I, the
chromosomes begin to
uncoil, the new nuclei
form, and the cytoplasm
splits.
•The end of meiosis I
produces two daughter
cells, each with only half
the unique genetic
information of the parent
Meiosis II
•The second cycle of
division in meiosis is just
like mitosis, except the
chromosomes are not
copied during interphase.
•Spindle fibers attach to
the centromeres of sister
chromatids during
prophase II.
Meiosis II
•By metaphase II, all the
sister chromatids are lined
up midway between the
two poles.
•Anaphase II begins when
the centromeres holding
the sister chromatids split
and the sisters become
daughters.
Meiosis II
•In telophase II, the new
nuclei begin to form,
chromosomes unwind,
and the cell begins to
divide.
•During spermatogenesis,
cytokinesis is equal.
•During oogenesis,
cytokinesis is unequal.
Meiosis
•Meiosis is the key to
sexual reproduction,
which generates
populations of genetically
diverse individuals.
•Among these individuals
will be those whose
characteristics allow them
to thrive in a changing
environment.
Genetic Diversity: Mutations
• Five processes are responsible for producing offspring
which differ genetically from their parents.
• 1) Mutations: from point mutations to chromosomal
mutations
• Cystic fibrosis is a common lethal genetic disorder in the
U.S.
• It is caused by a mutation to a gene which produces
proteins that regulate mucus production.
Genetic Diversity: Segregation
• As a result of meiosis, each haploid cell will inherit
only one of the two alleles present in one parent.
• Because homologous chromosomes end up in
different cells during Meiosis I, the two parental
alleles are segregated (separated).
• Gametes will
have either the
allele for type A
blood or type O
blood
Genetic Diversity: Crossing Over
•During prophase of Meiosis I,
homologous chromosomes
exchange sections of DNA crossing over.
•This moves some alleles from one
homologous chromosome to
another
•This creates new combinations of
alleles.
•After crossing over, one
•Without crossing over, the purple purple chromatid has alleles
homologous chromosome has
for type O blood, attached
alleles for type O blood, attached earlobes, and sickle-cell
earlobes, and normal hemoglobin. hemoglobin
Genetic Diversity: Crossing Over
• Without crossing over, meiosis produces two
unique haploid cells.
• After a single cross-over, there are now four
unique haploid cells.
Genetic Diversity: Crossing Over
• Crossing over can happen at multiple places between
homologous chromosomes during Prophase I.
• This additional crossing over events creates more
unique combinations of alleles.
• During meiosis in humans, 2-3 crossing over events
occur between each pair of homologous
chromosomes.
Genetic Diversity: Independent Assortment
• We know that each haploid cell will end up
with one of two homologous chromosomes
from segregation.
Genetic Diversity: Independent Assortment
•But the arrangement of
one pair of homologous
chromosomes has no
impact on the
arrangement of nonhomologous
chromosomes are
inherited independently
•Occurs in Metaphase.
•Independent Assortment
Genetic Diversity: Independent Assortment
• In general, the number of possible
combination of non-homologous
chromosomes due to independent assortment
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2N, where N = # of haploid chromosomes
If N = 1, then there are 21 = 2 unique gametes.
If N = 2, then there are 22 = 4 unique gametes.
If N = 3, then there are 23 = 8 unique gametes.
What about humans?
• If we add in mutations and crossing over, each
gamete is truly unique.
Genetic Diversity: Random Fertilization
• During fertilization, one unique sperm combined
with one unique egg to create a zygote, the initial
diploid stage.
• In humans, that means one of the 8.3 x 106 unique
sperm will combine with one of the 8.3 x 106 unique
eggs.
• Each zygote is the product of these unique
combinations.
• Leads to 70 trillion unique compositions of diploid
cells.
Nondisjunction
•In meiosis, the number of
chromosomes in diploid
cells is reduced to
haploid. However, there
are cases, where
homologous
chromosomes do not
segregate properly.
•Nondisjunction occurs
when homologous
chromosomes do not
separate during cell
division.
Nondisjunction
•If one of these abnormal
gametes unites with a
normal gamete, the
offspring will have an
abnormal number of
chromosomes.
•In monosomy, a cell has
just one pair of homologous
chromosomes.
•In trisomy, a chromosome
is present in three copies.
•Down syndrome is a
result of trisomy-21.