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CHAPTER 8
CELL DIVISION
CHROMOSOMES
RECALL: WHAT IS DNA?
- a molecule that stores genetic
information
- stored in the NUCLEUS
During cell division, DNA is coiled into
CHROMOSOMES- rod-shaped
structures made of DNA and
protein
2 types of proteins are associated with
DNA:
HISTONES- help maintain the shape
of the chromosome; aid in packing of
DNA
NONHISTONE- involved in
controlling the activity of specific
regions of the DNA
STRUCTURE OF A
CHROMOSOME
A chromosome consists of 2 identical
halves
- each half is called a CHROMATID
- when a cell divides, each new cell will
receive one chromatid from each
chromosome
A structure called the CENTROMERE
holds the 2 chromatids together
- between cell divisions, DNA is not
coiled as tightly as it is in
chromosomes  CHROMATIN
Chromosomes are simpler in
prokaryotic cells (bacteria)
- only 1 chromosome, not enclosed in
a nucleus
CHROMOSOME
NUMBERS
Each species has a SPECIFIC number
of chromosomes:
Carrot = 8
Cat = 32
Dog = 78
Fruit fly = 8
Human = 46
There are 2 types of chromosomes
found in humans and other animals:
AUTOSOMES & SEX
CHROMOSOMES
SEX CHROMOSOMES- determine the
sex of an organism; may carry genes
for other characteristics
- in humans, either X or Y
Male = XY
Female = XX
AUTOSOMES- all other chromosomes
in an organism
- since 2 of the 46 chromosomes in
humans are sex chromosomes, there
must be 44 autosomes
Every cell of an organism has 2 copies
of each autosome
- the organism gets 1 copy from each
parent
- the 2 copies of each autosome are
called HOMOLOGOUS
CHROMOSOMES
- same size and shape; carry genes
for same traits
KARYOTYPE- picture of all of the
chromosomes in a cell
- the 46 chromosomes exist as 22
homologous pairs of autosomes and
2 sex chromosomes
karyotype
Cells that have 2 sets of chromosomes
are said to be DIPLOID
- all normal human cells, except sperm
and egg cells, are diploid
- commonly abbreviated as 2n
Sperm and egg cells are HAPLOIDcontain only 1 set of chromosomes
Sperm cell = 1n
Egg cell = 1n
- when these combine to create the
first cell of a new organism, the new
cell will be diploid (2n)
CELL DIVISION
CELL DIVISION IN BACTERIA
REMEMBER:
ALL CELLS COME FROM OTHER CELLS
- Cell division is different in prokaryotic cells
and eukaryotic cells
- Cell division in prokaryotes (bacteria)
is called BINARY FISSION
STEP OF BINARY FISSION:
1. Single chromosome copies itself
2. The cell grows until it is twice the cell’s
original size
3. A new cell wall forms between the 2
chromosomes, and the cell splits into 2
new cells
- each cell is an exact copy of the
original cell
- DRAW THE STEPS OF BINARY
FISSION
CELL DIVISION IN
EUKARYOTES
2 TYPES OF DIVISION:
MITOSIS
MEIOSIS
MITOSIS- results in new cells with genetic
material that is IDENTICAL to the original
cell
MEIOSIS- reduces the number of
chromosomes by half in new cells
CELL CYCLE
CELL CYCLE- repeating set of events that
make up the life of a cell
- cell division is only 1 phase of the cell
cycle
- time between divisions is called
INTERPHASE
Cells spend most of their lifetime in
interphase
DIVISIONS OF INTERPHASE
G1 phase- cell grows to mature size
S phase- DNA of cell is copied
G2 phase – growth and preparation for
cell division occurs
** some cells can enter the G0 phase in
which they do NOT copy their DNA and
prepare for division
- many cells in the human body stop
dividing at a certain time, and never divide
again   BRAIN CELLS
MITOSIS
The M phase of the cell cycle is MITOSIS
- Mitosis is the division of the nucleus
- is a continuous process divided into 4
phases:
Prophase
Metaphase
Anaphase
Telophase
PROPHASE
- begins with the
shortening and coiling
of DNA into
chromosomes
- the chromatids stay
connected by the
centromere
- the nucleolus and
nuclear membrane
break down and
disappear
http://www.abibayern.de/jessika12_1/bilder12_1/prophase.gif
PROPHASE cont.
- 2 pairs of dark spots called
CENTROSOMES appear next to the
disappearing nucleus
- each centrosome contains a small pair of
cylinders called CENTRIOLES (only animals)
- the centrosomes move toward opposite poles
of the cell and SPINDLE FIBERS (made of
microtubules) radiate from the centrosomes
PROPHASE cont.
- these spindle fibers are called the MITOTIC
SPINDLE, and serves to divide the
chromatids equally between the 2 new
cells
METAPHASE
- second stage of
mitosis
- chromosomes are
easiest to identify
in this stage
- chromosomes move to
the center of the
dividing cell and are
held in place there
http://www.abibayern.de/jessika12_1/bilder12_1/prophase.gif
ANAPHASE
- chromatids of each
chromosome separate
at the centromere
and slowly move
toward opposite poles
of the cell
- the chromatids are
considered to be
individual
chromosomes
http://www.abibayern.de/jessika12_1/bilder12_1/prophase.gif
TELOPHASE
- after chromosomes reach
opposite ends of the cell,
spindle fibers come apart
and chromosomes return
to chromatin state
- nuclear envelope
forms around EACH
set of chromosomes
- nucleolus forms in each
new cell
http://www.abi-bayern.de/jessika12_1/bilder12_1/prophase.gif
CYTOKINESIS
CYTOKINESIS- division of the cytoplasm
- in animal cells, the cell membrane pinches
inward, eventually splitting the cell in 2
- the area that pinches inward is called
the CLEAVAGE FURROW
- in plant cells, the Golgi apparatus functions
in forming a CELL PLATE which separates
the cell in 2
IN BOTH PLANTS AND ANIMALS, EACH
OFFSPRING CELL RECEIVES AN EXACT
COPY OF THE ORIGINAL CELL’S
CHROMOSOMES
- ALSO ABOUT ½ OF THE ORIGINAL
CELL’S CYTOPLASM AND
ORGANELLES
MEIOSIS
MEIOSIS- process of nuclear division that
reduces the chromosome number in new
cells to HALF the number in the original
cell
- produces GAMETES- haploid
reproductive cells
- human gametes are sperm and egg
cells, each having 23 chromosomes
- when these combine, a ZYGOTE
results having 46 (2n) chromosomes
STAGES OF MEIOSIS
Cells that are preparing to go through
meiosis go through the G1, S, and G2
phases of interphase
- the cell begins meiosis with a
duplicate set of chromosomes, just
like in mitosis
- cells going through meiosis divide
TWICE, so this results in 4 haploid cells
(1n) rather than 2 diploid cells (2n)
There are 2 stages of meiosis:
MEIOSIS I
MEIOSIS II
MEIOSIS I
- these stages are comparable to mitosis
PROPHASE I
- DNA coils tightly into chromosomes
- spindle fibers appear
- nucleus and nucleolus come apart
- homologous chromosomes pair up 
called SYNAPSIS (does NOT occur in
mitosis)
- each pair of homologous
chromosomes is called a TETRAD
- in a tetrad, chromatids of the homologous
chromosomes line up so that genes on
one chromosome are beside
corresponding genes on the other
chromosome
- during synapsis, chromatids within a
homologous pair twist around one
another
- sometimes, portions of the chromatid
break off and attach to the adjacent
chromatid in the homologous pair
- this is called CROSSING-OVER
- genetic material can be exchanged
between maternal and paternal
chromosomes
- crossing-over results in GENETIC
RECOMBINATION
METAPHASE I
- tetrads line up randomly along the
middle of the dividing cell
- spindle fibers from one pole attach to the
centromere of one homologous
chromosome
- spindle fibers from the opposite pole will
then attach to the other homologous
chromosome
ANAPHASE I
- each homologous chromosome (2
chromatids attached by a centromere)
moves to opposite poles of the cell
- the random separation of the
homologous chromosomes is called
INDEPENDENT ASSORTMENT
- this results in random separation of
maternal and paternal chromosomes 
genetic recombination
TELOPHASE I
- chromosomes reach opposite poles of the
cell and cytokinesis begins
- the new cells contain a haploid number
of chromosomes (1n)
MEIOSIS II
- occurs in EACH cell formed during meiosis I,
and DNA is NOT copied before
PROPHASE II
- spindle fibers form and begin to move the
chromosomes toward the midline of the dividing
cell
METAPHASE II
- chromosomes move to the midline of the cell
ANAPHASE II
- chromatids separate and move toward
opposite poles of the cell
TELOPHASE II
- nuclear membrane forms around the
chromosomes in each of the 4 NEW
CELLS
- cytokinesis II occurs resulting in 4 NEW
CELLS, each containing half of the
original cell’s number of chromosomes
FORMATION OF GAMETES
In animals, meiosis produces haploid
reproductive cells called gametes
- meiosis occurs only in the reproductive
organs of animals  testes and ovaries
SPERMATOGENESIS (males)
- in the development of sperm cells, a
DIPLOID reproductive cell divides by
meiosis
- 4 haploid cells called SPERMATIDS are
formed
- each spermatid develops into a mature
sperm cell
- DRAW SPERMATOGENESIS
OOGENESIS (females)
- a DIPLOID reproductive cell divides by
meiosis to produce 1 mature egg cell
(ovum)
- during cytokinesis I and II the cytoplasm
of the original cell is divided
UNEQUALLY between new cells
- one cell receives most of the
cytoplasm of the original cell
- one egg is produced, and the other 3
products called POLAR BODIES
degenerate
- DRAW OOGENESIS
ASEXUAL AND SEXUAL
REPRODUCTION
ASEXUAL REPRODUCTION- production
of offspring from ONE parent
- does not involve meiosis
- in unicellular organisms (bacteria), new
organisms are created by asexual
reproduction
- offspring are genetically identical to
parent
SEXUAL REPRODUCTION- production of
offspring through meiosis and the union
of a sperm and egg
- offspring are genetically different from
the parents because genes are
combined in new ways
- except in identical twins, offspring contain
unique combinations of their parents’
genes
- evolutionary advantage is that sexual
reproduction enables species to adapt
rapidly to new conditions