Cell Increase and Decrease
• Cell division increases the number of
somatic (body) cells
• Two parts of cell division:
• Mitosis (division of nucleus)
• Cytokinesis (division of cytoplasm)
• Apoptosis (cell death) decreases the
number of cells.
•Cell division occurs when:
•Body growth
•Maintenance and repair
•Fighting infection
•Replacing worn/dead cells
•Apoptosis occurs when:
•Tail of tadpole disappears  frog
•Skin between human fingers and toes dies
during development
•Both cell increase and apoptosis occur during
normal development and growth – example of
homeostasis.
The Cell Cycle
• An orderly sequence of events that occurs
from the time a cell is first formed until it
divides into two new cells.
http://www.cellsalive.com/cell_cycle.htm
• Most of the cell cycle is spent in interphase:
• G1 stage – cell growth, cell doubles its
organelles (cell structures), prepares for
DNA replication
• S stage – DNA replication occurs
• G2 stage – cell makes proteins needed for
cell division
• Amount of time spent in interphase varies –
average for adult mammals is 20 hours
• Nerve cells and muscle cells exit the cell
cycle  G0 phase
• Following interphase is the M stage,
including mitosis and cytokinesis
(definitions slide #1).
• During mitosis, two copies of DNA made
during replication are separated, and
become the nuclei of the two daughter
cells – takes about 4 hours.
• The cell cycle ends when cytokinesis, the
splitting of the cytoplasm, is complete.
Control of the cell cycle
•Three checkpoints:
1.During G1 prior to the S stage – if DNA is
damaged, apoptosis occurs
2.During G2 prior to the M stage – will not
proceed if DNA is damaged or not copied
3.During the M stage prior to the end of
mitosis – if chromosomes are not properly
aligned
Chromosome Structure
•In a non-dividing cell – genetic material
is in the form of chromatin (DNA &
protein)
•In a dividing cell, chromatin undergoes
coiling to form chromosomes
•Proteins called histones package the DNA
so it can fit into the nucleus (2 meters of
DNA fit into nucleus that is 5
micrometers)
•After replication, there are 2 identical
sister chromatids, held together by a
centromere
•Each species has a set number of
chromosomes:
•Humans – 46
•Crayfish – 200
•Corn – 20
•Adder’s tongue fern – 1262
•Chimpanzee - 48
•Sand dollar – 52
•Dog – 78
•Cat - 32
•Body cells contain the diploid (2n)
number of chromosomes – 2
chromosomes of each kind (1 from each
parent)
•Sex cells (eggs and sperm) contain only
1 chromosome of each kind – haploid (n)
number of chromosomes
•Mitosis – occurs in body cells – diploid
cells divide to produce diploid cells –
daughter cells are genetically identical to
parent cells
http://www.cellsalive.com/mitosis.htm
Animation of mitosis
Mitosis overview
1.
2.
3.
4.
5.
6.
7.
8.
Centriole
Chromatin
Nucleolus (in yellow)
Nuclear membrane
Spindle fibers
Chromosome (replicated)
Centromere
Sister Chromatids (each half of replicated
chromosomes)
9. Daughter Chromosomes (once the replicated
chromosome splits)
10.Cell membrane
11.Cleavage furrow
12.Asters
13.Centrosome (=aster + centriole)
Late Interphase
•Centrosomes
(which contain
pair of centrioles
and an aster –
which are short
microtubules)
duplicate
•Chromatin
condenses into
chromosomes
Early Prophase
•Chromosomes
become visible
•Centrosomes
move to opposite
ends of the cell
•Nuclear
membrane and
nucleolus
disappear
Late Prophase
•Spindle fibers
form
•Chromosomes
become attached
to spindle fibers
– centromere
attaches to
spindle fibers
Metaphase
•Chromosomes
line up at
metaphase plate
– equidistant
from poles
Anaphase
•Centromeres
holding sister
chromatids divide
•Sister chromatids
separate, becoming
daughter
chromosomes, and
move toward
opposite ends of cell
Telophase
•Spindle disappears
•Nuclear membrane
reappears
•Chromosomes turn
into chromatin
•Nucleolus reappears
Mitosis in Plant Cells
•Same phases as in animal cells
•Have centrosome and spindle, but no
centrioles or asters
• Cytokinesis, or division of cytoplasm,
accompanies mitosis.
• Cleavage of the cytoplasm begins in
anaphase, but is not completed until just
before the next interphase.
• Newly-formed cells receive a share of
organelles made during interphase.
http://www.copernicusproject.ucr.edu/ssi/HighSchoolBioResources
/Cell Division/Mitosis/Mitosis_Video_Quiz.asf
Cytokinesis in Animal Cells
• A cleavage furrow (indentation of
membrane where cell will divide) begins
at the end of anaphase.
• A band of actin and myosin filaments,
called the contractile ring, slowly forms a
constriction between the two daughter
cells.
• A narrow bridge between the two cells is
apparent during telophase, then the
contractile ring completes the division.
Cytokinesis in animal cells
Cytokinesis in Plant Cells
• The rigid cell wall surrounding plant cells
cannot form a cleavage furrow.
• Instead, a cell plate forms from vesicles
released by the Golgi apparatus (a part of
the cell that processes proteins)
• New plant cell walls form and are later
strengthened by cellulose fibers.
Cytokinesis in plant cells
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Cell Division in Prokaryotes
The process of asexual reproduction in
prokaryotes is called binary fission.
The two daughter cells are identical to the
original parent cell, each with a single
chromosome.
Following DNA replication, the two
resulting chromosomes separate as the
cell elongates.
Cell divides without cell structures seen
in plants & animals
Animation of binary
fission
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•
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1)
2)
Meiosis
Produces sex cells (gametes) – eggs &
sperm
Reduces the chromosome number so that
egg or sperm cells each have only one of
each kind of chromosome (2n  1n).
The process ensures that the next
generation will have:
the diploid number of chromosomes
a combination of traits that differs from
that of either parent.
Overview of meiosis
http://www.cellsalive.com/meiosis.htm
• Meiosis involves two cell divisions and
produces four haploid cells.
• Humans have 23 pairs of homologous
chromosomes (chromosomes with the
same genes), or 46 chromosomes total.
• Prior to meiosis I, DNA replication
occurs.
• In many organisms, haploid daughter
cells mature into gametes (sex cells –
eggs and sperm)
• Fertilization (fusion of egg and sperm) restores the diploid number of
chromosomes
Phases of Meiosis
• The same four phases seen in mitosis –
prophase, metaphase, anaphase, and
telophase – occur during both meiosis I
and meiosis II.
• The period of time between meiosis I and
meiosis II is called interkinesis.
• No replication of DNA occurs during
interkinesis
Animation
Meiosis square dance
Meiosis I
Prophase I
•Nuclear memebrane & nucleolus
disappear
•Spindle forms
•Homologous chromosomes pair during
synapsis
Metaphase I
•Homologous chromosomes line up at
metaphase plate
Anaphase I
•Homologous chromosomes separate &
move to opposite poles
Telophase I
•Nuclear membrane and nucleolus reappear
•Cytokinesis occurs
Interkinesis
•Period of time between meiosis I and II
Meiosis II
Prophase II
•Spindle reappears, nucleolus and
nuclear membrane disappear
•Chromosomes attach to spindle
Metaphase II
•Chromosomes line up at metaphase
plate
Anaphase II
•Sister chromatids separate, becoming
daughter chromosomes
Telophase II
•Spindle disappears, nuclear membrane
and nucleolus reappear
•Cytokinesis divides the cells
Genetic Recombination
• Genetic variation occurs in several
ways:
1) Crossing-over of nonsister chromatids –
occurs during prophase I
2) Independent assortment of homologous
chromosomes – separate in a random
manner – 223 or 8,388,608 possible
combinations of the 23 pairs of
chromosomes
3) Combining of chromosomes of
genetically different gametes during
fertilization
• Between crossing over, combinations of
gametes produced, and random
combining of sperm and egg, variation
is endless in the human population
Meiosis vs. Mitosis
Mitosis
•DNA replication
occurs only once
during interphase.
•One cell division
•Two diploid
daughter cells –
genetically
identical to parent
Meiosis
•DNA replication
occurs only once
during interphase.
•Two cell divisions.
•Four haploid
daughter cells –
genetically different
from parent
Mitosis
•Daughter cells are
identical to each
other
•Occurs in all
somatic cells for
growth and repair
Meiosis
•Daughter cells are
different from each
other
•Occurs only in the
reproductive organs
for the production
of gametes
Meiosis compared to mitosis
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The Human Life Cycle
Requires both mitosis and meiosis.
In males, meiosis occurs as
spermatogenesis and produces 4 haploid
sperm.
In females, meiosis occurs as oogenesis
and produces 1 egg cell.
Mitosis is involved in the growth of a
child and repair of tissues during life.
Life cycle of humans
Spermatogenesis
• Diploid primary spermatocytes undergo
meiosis I to produce haploid secondary
spermatocytes.
• Secondary spermatocytes divide by
meiosis II to produce 4 haploid
spermatids.
• Spermatids mature into sperm with 23
chromosomes.
Spermatogenesis
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Oogenesis
Diploid primary oocyte undergoes meiosis I
to produce one haploid secondary oocyte and
one haploid polar body.
Secondary oocyte begins meiosis II, stops at
metaphase II, and is released from the ovary.
Meiosis II will be completed only if sperm
are present.
Following meiosis II, there is one haploid
egg cell with 23 chromosomes and up to
three polar bodies.
Polar bodies serve as a dumping ground for
extra chromosomes – will disintegrate.
Oogenesis
• In humans, both sperm cells and the egg
cell have 23 chromosomes each (1n)
• Following fertilization of the egg cell by a
single sperm, the zygote has 46
chromosomes, the diploid number (2n)
found in human somatic cells.
• The 46 chromosomes represent 23 pairs of
homologous chromosomes.
• Cell differentiation occurs during
development resulting in a variety of cell
types