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Chapter #6
Cell
Reproduction
Section 6-1:
Chromosomes
A. Chromosome
Structure
1)DNA (deoxyribonucleic acid) contains the
information needed to direct a cell’s activities.
2)A gene is a segment of DNA that codes for a
specific piece of information.
3)A chromosome is a rod-shaped structure that
forms when a single DNA molecule & its
associated proteins coil tightly before cell
division.
A. Chromosome
Structure
4)Chromosomes make a copy of themselves just
before cell division.
a) The two copies are called chromatids.
b) The chromatids are attached by a protein disk at a
point called the centromere.
A. Chromosome
Structure
5)Each body cell contains two sets of each
chromosome.
a) The two sets are called homologues, or homologous
chromosomes. You received one homologue from
each parent.
b) When a cell contains two homologues of each
chromosome, it is termed diploid. This is
represented as 2n. Humans have 46 chromosomes,
so 2n = 46.
A. Chromosome
Structure
6)Each sex cell contains just one set of
chromosomes.
a) Sex cells (eggs & sperm) are also called gametes.
b) Gametes are haploid, meaning they only have one
homologue of each chromosome. This is represented as
n. Humans have 23 chromosomes in their gametes, so
n = 23.
7)When an egg cell is fertilized by a sperm cell,
a zygote is formed. The zygote contains two
homologues of each chromosomes, so it is
considered diploid (2n).
B. Chromosomes Affect
Development
1) Each of the 46 chromosomes play an important
part in determining how the body develops and
functions. A person must have the correct
number of chromosomes in his or her cells.
2) In most cases, humans who are missing even
one chromosome do not survive the early stages
of embryonic development.
a) Monosomy = A diploid cell is missing a chromosome.
b) Trisomy = A diploid cell has an extra chromosome.
• Down syndrome is an example of trisomy in
chromosome #21.
B. Chromosomes Affect
Development
3) A karyotype can be made to examine a
person’s chromosome.
4) Trisomy & monosomy occur because of
nondisjunction during the formation of the
gametes.
a) Nondisjunction occurs when the chromosomes fail
to separate during cell division.
b) As a result, one gamete will receive both
homologues of a chromosome, while the other
gamete won’t receive one at all.
B. Chromosomes Affect
Development
5) Prenatal testing can be performed if parents
are at risk for genetic diseases or
nondisjunction disorders.
a) Amniocentesis: A small amount of the amniotic
fluid is removed. The fluid contains cells from the
fetus that can be analyzed as a karyotype.
b) Chorionic Villi Sampling: A sample of the chorionic
villi is removed from the placenta. The cells can be
grown in a lab and analyzed as a karyotype.
B. Chromosomes Affect
Development
6) Alterations in chromosome structure
a) Mutations: changes in an organism’s genetic
material.
b) Deletion: a fragment of a chromosome breaks off or
is lost
c) Duplication: a segment of a chromosome attaches
to another chromosome
d) Inversion: a fragment of a chromosome reattaches
in the reverse orientation
e) Translocation: a fragment joins a non-homologous
chromosome
C. Chromosomes
Determine Sex
1) 22 of the 23 pairs of human chromosomes
are the same in males & females. They are
called autosomes.
2) The chromosomes that are different in
males & females are the sex chromosomes.
They carry the genes that determine sex.
We will later learn that the sex
chromosomes also carry some traits not
associated with sex.
C. Chromosomes
Determine Sex
3) There are two sex chromosomes:
a) The X chromosome is larger & looks like the letter X
b) The Y chromosome is smaller and shorter
4) In humans, the genes that cause a fertilized egg
to develop into a male are located on the Y
chromosome.
a) Any individual with a Y chromosome is male
b) Any individual without a Y chromosome is female
C. Chromosomes
Determine Sex
5) The X chromosome contains traits necessary
for life, so males have one X and one Y
chromosome and females have two X
chromosomes, though there are some
exceptions:
a) XXY = Klinefelter’s syndrome
b) XYY = Increased antisocial behavior
c) XO = Turner’s syndrome
d) XXX = Triple-X syndrome
C. Chromosomes
Determine Sex
6) In some insects, such as grasshoppers, there
is no Y chromosome. In such cases, females
are XX and males are XO.
7) In birds, moths, & butterflies, the male is XX
and the female is XO.
Section 6-2:
Mitosis & Cell
Division
A. Bacteria
1) Cell division in single-celled bacteria takes
place in two stages:
a) The DNA is copied
b) The cell splits into two equal halves by a process
called binary fission
2) Binary fission is a form of asexual
reproduction that produces identical offspring.
a) A new plasma membrane is added at a point on the
membrane between the two DNA copies.
b) The growing plasma membrane pushes inward and
the cell is constricted in two.
c) A new cell wall forms around the new membrane
and eventually the dividing bacterium is pinched
into two independent cells.
B. Eukaryotic cells undergo nuclear division.
The life of a eukaryotic cell is traditionally
diagramed as a cell cycle that consists of 5
phases shown in Figure 6-8 on page #127.
The Cell Cycle
1) G1 Phase --> This is the growth phase of
the cell. It is where the cell grows rapidly
and carries out its routine functions. For
most organisms, this phase occupies the
major portion of the cell’s life between
divisions.
2) S Phase --> This is when the DNA is copied.
At the end of this phase, and individual
chromosome consists of two chromatids
attached at the centromere.
The Cell Cycle
3) G2 Phase --> Preparations for nuclear
division are made during this phase.
Mitochondria and other organelles
replicate. Microtubules are reassembled
which will be used to form the spindle fibers
during mitosis.
4) M Phase --> This is the phase in which
mitosis occurs. Mitosis is the process by
which the nucleus of a cell is divided into
two nuclei, each with the same number and
kinds of chromosomes.
5) C Phase --> This is when the cytoplasm
divides during a process called cytokinesis.
C. What happens
during mitosis and cell
division?
1) A eukaryotic cell spends most of its life in the
G1, S, and G2 phases, which are collectively
called interphase.
2) As interphase ends & mitosis begins, the
chromosomes begin to condense. Enzymes
break down the nuclear envelope.
3) Centrioles (in animal cells) start to separate
and move to opposite ends of the cell.
a) As the centrioles move apart, a network of protein
cables, called the spindle, forms between them.
b) Each cable is called a spindle fiber.
4) A second group of microtubules extends out
from a region of the centromere of each
chromosome called the kinetochore. It is a
disk of protein that serves as a platform for
assembling microtubules.
a) The two sets of microtubules extend out toward
opposite poles of the cell.
b) Each set continues to grow until it makes contact
with the pole of the spindle, with one chromatid
attached to each pole.
5) Once the microtubules are attached to the
centromeres, the centromeres split, freeing the
chromatids from each other.
6) Mitosis is now simply a matter of reeling in
the chromatids, now considered
chromosomes, to the poles. When they arrive,
each pole has one complete set of
chromosomes.
7) In the final step of cell division, a new nuclear
envelope forms around each pole, forming two
nuclei. Then cytokinesis takes place, and the
cytoplasm divides in half. Two new cells have
now formed.
D. Stages of Mitosis
1) Prophase  The chromosomes begin
condensing and become visible. The nuclear
envelope begins to break down, and the
network of spindle fibers becomes visible.
2) Metaphase The chromosomes move to the
center of the cell and line up along the
“equator”. Once in place at the equator,
each chromosome is held in place by the
microtubules attached to the kinetochore.
D. Stages of Mitosis
3) Anaphase The two chromatids physically
separate when the centromere divides. The
chromatids, which may now be called a
chromosome, move toward opposite poles of
the cell as the fibers attached to them
shorten.
4) Telophase The chromosomes uncoil and a
new nuclear envelope forms. The spindle
fibers break down and disappear. Mitosis is
complete.
E.During cytokinesis, the cytoplasm is cleaved in
half, and the cell membrane grows to enclose
both cells.
1) Animal cells and other cells that lack cell walls are
pinched in half by a belt of protein threads.
2) Plant cells have a rigid cell wall and a different strategy
of cell division.
a) Vesicles formed by the Golgi bodies fuse at the
equator of the cell and form the cell plate, which
is a membrane across the middle of the cell.
b) A new cell wall then forms on both sides of the
cell plate.
Section 6-3:
Meiosis
A. What Happens
During Meiosis?
1) Meiosis is the mechanism that halves the
number of chromosomes in cells. The diploid
number is reduced to the haploid number.
2) Before meiosis, the DNA is copied, like in
mitosis.
3) Meiosis consists of two divisions:
a) Meiosis I Homologous chromosomes separate into
two cells
b) Meiosis II The two chromatids of each
chromosome separate into two haploid cells
4) One diploid cell that undergoes meiosis
produces four haploid cells.
a) In animals, meiosis often results in haploid gametes
(see figure 6-11)
b) In plants, meiosis often leads to spores, haploid cells
that later lead to the production of gametes.
5) During meiosis, two unique events occur:
a) In the beginning of meiosis I, homologous
chromosomes pair up next to each other. While paired,
the arms of the chromosomes exchange reciprocal
segments of DNA in a process called crossing-over.
b) Because there is only one replication of DNA but two
divisions, meiosis halves the number of chromosomes,
called skipping replication.
B. Meiosis I
1) Prophase I  The chromosomes condense,
the nuclear envelope breaks down, the
homologous chromosomes pair up &
crossing-over occurs.
2) Metaphase I  The pairs of homologous
chromosomes are moved by spindle fibers to
the equator of the cell.
B. Meiosis I
3) Anaphase I  The homologues separate.
The chromatids do not separate, but
remained joined at the centromere. This is
the stage in which nondisjunction can occur.
4) Telophase I  The individual chromosomes
gather at each of the two poles. The
cytoplasm divides, forming two new cells
with half the number of chromosomes of the
original cell. This is why meiosis I is often
called “reduction division”.
C. Meiosis II
1) Prophase II  The centrioles in the newly
formed cells split again and move to
opposite poles. The spindle reforms.
2) Metaphase II  The chromosomes line up in
the middle of the cell. The fibers attach to
the centromeres.
C. Meiosis II
3) Anaphase II  The centromeres divide and
the chromatids are pulled apart. Each one
is now called a chromosome.
4) Telophase II  The chromosomes are at
opposite poles of the cell. Each new cell has
the haploid number of chromosomes.
5) Cytokinesis follows telophase II.
D. Crossing-Over
1) Crossing-over is an efficient way to produce
genetic recombination.
2) As a result, the two chromatids of a
chromosome no longer contain identical
genetic material.
3) Crossing-over provides a source of genetic
variation, which impacts the speed at which
evolution occurs. (See Fig. 6-13)