Download Cell Cycle and Mitosis

3.02: Cell Cycle, Mitosis, &
Meiosis
3.02: Cell Cycle
Cell division, also called cell reproduction, occurs in
all organisms. This is how we grow and how we
repair our cells.
 Why do cells divide instead of just growing bigger
and bigger?
 The larger a cell becomes, the more demands it
places on its DNA.
 The larger a cell becomes, the more trouble it
has moving nutrients and wastes across the cell
membrane.
Cell Cycle
 Each cell has only one set of genetic information
stored in its DNA. Before cell division occurs, the
cell must replicate (copy) all of its DNA.
 In prokaryotes, cell division is simple because they
have no nucleus or organelles to organize, they just
split each cell in half in a process called binary
fission. Eukaryotic cell division is much more
complex and happens in stages.
 The first 3 phases of the cycle are known as
interphase. This part makes up most of a cell’s life.
Cell Cycle
1. G1, First growth phase: During G1 a cell grows rapidly
2.
3.
4.
5.
and carries out routine functions. This phase occupies
the major portion of a cell’s life.
S, Synthesis phase: A cell’s DNA is replicated during this
phase.
G2, Second growth phase: Preparations are made for
the nucleus to divide.
M, Mitosis: the division phase during which the nucleus
of a cell is divided into 2 nuclei. Each nucleus has the
same DNA as the parent cell they came from.
Cytokinesis: the process during cell division in which the
cytoplasm divides. This results in two daughter cells,
each with the exact DNA of the parent cell.
Cell Cycle
Cell
growth
Division of
cytoplasm
Division of
nucleus
S
DNA
synthesis
G1
Cytokinesis
Mitosis
G2
Growth
and
preparation
for mitosis
Cell Cycle
 Cells know when to divide because of checkpoints at
different stages of the cell cycle. If one of these
checkpoints fails to do its job because of mistakes in
the DNA called mutations, then the regulation of cell
growth and division is disrupted. Cancer, the
uncontrolled growth of cells, may result from these
mutations.
Chromosomes
 Chromosomes are DNA and proteins tightly coiled
together. Humans have 46 chromosomes in the
nucleus of each cell of our bodies.
 During interphase, the DNA is not visible (this is
called chromatin). As a eukaryotic cell prepares to
divide, these chromosomes become visible and the
DNA is replicated.
 The two exact copies of the DNA that make up each
chromosome are called sister chromatids. The two
chromatids are attached a point called the
centromere.
Chromosomes
 The chromatids separate
during cell division and
each one is placed into a
new daughter cell so that
each new cell has the
same genetic information
as the original cell.
3.02: Mitosis
During mitosis, the nucleus divides to form two
nuclei, each containing a complete set of the cell’s
chromosomes.
 Mitosis involves 1 cell dividing 1 time to produce 2
identical daughter cells.
 Mitosis is called asexual reproduction, because the
new daughter cells are genetically identical to the
parent cell. This means that the number of
chromosomes in each daughter cell is the same as
the number in the parent cell.

Steps of Mitosis: PMAT
1. Prophase:
chromosomes coil
up and become
visible, the nuclear
envelope
surrounding the
nucleus dissolves,
and a spindle forms.
Steps of Mitosis: PMAT
2. Metaphase: the
chromosomes move
to the middle of the
cell and line up
along the equator,
spindle fibers link the
chromatids of each
chromosome to the
opposite poles.
Steps of Mitosis: PMAT
3. Anaphase:
centromeres divide,
the two chromatids
(now called
chromosomes) move
toward opposite
poles as the spindle
fibers attached to
them shorten.
Steps of Mitosis: PMAT
4. Telophase: a nuclear
envelope forms
around the
chromosomes at
each pole, the
chromosomes uncoil
and the spindle
dissolves.
Steps of Mitosis: PMAT
 Cytokinesis follows
mitosis. This is where
the cytoplasm divides
to produce two
identical daughter
cells.
Please add this to your notes:
 In Telophase of plant cells, instead of pinching in
half the cell forms a cell plate.
3.02: Cell Types and Chromosome Number

In an organism, there are somatic cells and there
are sex cells.
o Somatic cells are all of the body’s cells that are not
sex cells.
o Sex cells are sperm and egg.

Somatic cells have the diploid number of
chromosomes, which means one set from the
mother and one set from the father.
o Diploid = full set (2n)
Cell Types and Chromosome Number
 Humans have 46 chromosomes in their somatic
cells, 23 from Mom and 23 from Dad, that are paired
together. These paired chromosomes are called
homologous, meaning they are similar in size,
shape, and genetic content.
Cell Types and Chromosome Number


Gametes (sex cells, egg or sperm) contain only one
set of chromosomes. Since a gamete contains only
one set of chromosomes, it is said to be haploid.
o Haploid = half set (n)
The fusion of two haploid gametes is called
fertilization and results in a zygote. A zygote is a
fertilized egg cell, the first cell of a new individual.
3.02: Meiosis

Meiosis is a form of cell division that halves the
number of chromosomes when forming specialized
reproductive cells, such as gametes or spores.

Meiosis involves 2 divisions of the nucleus –
Meiosis I and Meiosis II. Before meiosis begins,
the DNA in the original cell is replicated (just like
before mitosis).
Meiosis
 Meiosis I (kind of like mitosis, but with
pairs of chromosomes instead of
singles):
1. Prophase I : the chromosomes become
visible, the nuclear envelope dissolves,
and homologous chromosomes pair up
forming a tetrad—2 chromosomes
joined together. Crossing-over occurs
when portions of a chromatid on one
chromosome are broken and
exchanged with the same chromatid
portions on the homologous
chromosome.
Meiosis
2. Metaphase I : pairs of homologous chromosomes
are moved by the spindle to line up at the middle of
the cell.
3. Anaphase I : The homologous chromosomes
separate and each is pulled to the opposite poles of
the cell by the spindle fibers. The chromosomes do
not separate at the centromere as in mitosis, each
chromosome is still composed of 2 chromatids.
4. Telophase I : Individual chromosomes gather at
each of the poles of the cell, then the cytoplasm
divides (cytokinesis) forming 2 new cells.
Meiosis
 Meiosis II (looks just like mitosis, but the chromosomes
5.
6.
7.
8.
are genetically different than the parent cell because of
crossing over):
Prophase II : a new spindle forms around the
chromosomes.
Metaphase II : chromosomes line up along the middle
and are attached at their centromeres to spindle fibers.
Anaphase II : centromeres divide, chromatids move to
opposite poles of the cell.
Telophase II: nuclear envelope forms around each set
of chromosomes, the cell undergoes cytokinesis.
Meiosis I
Meiosis II
Meiosis
 The end result of meiosis is 4 haploid cells from 1
diploid cell. Each new cell has half the genetic
material as the original cell.
3.02: Gametogenesis

In males, these 4 haploid cells are called sperm.
This process by which sperm are produced in male
animals is called spermatogenesis.

In females, the cytoplasm of these 4 haploid cells
divides unevenly so that there is one larger cell with
nearly all the cytoplasm and 3 smaller cells. The
larger cell becomes and egg cell, or ovum, while
the smaller cells are called polar bodies. The
process by which gametes are produced in female
animals is called oogenesis.
Nondisjunction

Sometimes mistakes are made
when gametes are formed in the
body. During meiosis, if the
homologous chromosomes do not
separate properly then the
gametes will have the wrong
number of chromosomes. This is
called nondisjunction.

Then this mutated gamete is used
to make a zygote. That
developing organism will have too
many or too few chromosomes in
each of its cells.
Nondisjunction

Often, this mistake is corrected by nature: the
mother will have a miscarriage.

Babies born with nondisjunction can display certain
serious diseases, such as Down’s Syndrome,
Turner’s Syndrome, or Klinefelter’s Syndrome.
Down’s
Syndrome
3 copies of
chromosome
21; leads to
learning
disabilities,
developmental
disabilities and
a specific facial
appearance.
Turner’s
Syndrome
a female has
only one X
chromosome
instead of the
normal two;
these girls are
short, have a
low hairline, a
webbed neck,
and are sterile.
Klinefelter’s
Syndrome
a male has two
X
chromosomes
and a Y instead
of the normal X
and Y; these
boys have
small testicles,
reduced fertility,
and physical
and behavioral
problems.
Sexual vs. Asexual Reproduction
 In asexual reproduction, a single parent cell passes
on copies of all its DNA to its offspring
 An individual produced by asexual reproduction is a
clone – an organism that is genetically identical to its
parent.
Sexual vs. Asexual Reproduction
 Sexual reproduction involves the formation of
reproductive cells called gametes that have half the
chromosome number as the original cell.
 A diploid mother has haploid eggs, a diploid father
has haploid sperm. In the way the egg and sperm
combine during fertilization to make a to make a
zygote.

haploid (n) + haploid (n) = diploid (2n)
 Since both parents contribute genetic material, the
offspring have traits of both parents but are not
exactly like either parent. Sexual reproduction
causes genetic variation.
Sexual vs. Asexual Reproduction
 Asexual reproduction is simple and efficient. It
allows organisms to reproduce without forming
gametes or finding a mate. However, in an
environment that is constantly changing, some
organisms need variation in order to survive. Sexual
reproduction provides a way to quickly make
different combinations of genes among individuals
and is how organisms evolve over time.
Sexual vs. Asexual Reproduction
 Asexual reproduction = genetically identical
offspring (clones) = mitosis
 1 cell divides 1 time to produce 2 identical cells
 Sexual reproduction = genetic variation = meiosis
1
cell divides 2 times to produce 4 cells with half
the chromosomes
Mitosis
Meiosis
Sexual or
Asexual?
Number of
divisions
Asexual
Sexual
1
2
Number of cells
formed
Type of cell
2
4
Identical to or
different from
parent?
Identical
Different
Crossing over?
No
Yes
Genetic
variation?
No
Yes
Somatic (body cell) Gamete (sex cell)