Cell Division
When a cell reaches its maximum size, the nucleus gives the
signal for cell division.
I. Cell Division
Needed for 3 major life processes:
1. growth: increased number of cells
2. repair: mending skin, blood vessels, etc.
3. reproduction
There are 2 types of cell division
1. mitosis: used for growth and repair, mitosis
produces cells that are identical to the parent cell;
parent and offspring cells have identical DNA
(genetic material)
a. used for asexual reproduction: process by
which an organism simply duplicates its genetic
material and splits into 2 separate, identical
organisms
2. meiosis: special form of cell division; produces cells
that are NOT identical to the parent cell
a. used in sexual reproduction, which produces
an offspring that has a combination of genetic
material from 2 parent organisms
b. the products of meiosis have only HALF the
amount of DNA (genetic material) found in
each parent cell
II. Mitosis
 process of cell division that gives 2 identical cells from
one parent cell
A. Cell Cycle
1. sequence of phases in the life cycle of a cell
2. covers the period of time from the beginning of one
cell division to the beginning of the next cell division
3. the cell cycle has two parts:
a. interphase (growth and preparation)
b. cell division (mitosis)
B. Interphase:
1. occurs between divisions
2. during interphase, the cell produces all of the
material necessary for cell growth and for cell
division
3. Interphase – longest part of the cell cycle (90%)
4. Includes replication (copying) of the genetic
material before cell division
a. DNA is in the form of chromatin (thin, fibrous
strands) while it is being copied
C. Stages of Mitosis: Prophase, Metaphase, Anaphase
and Telophase
1. Prophase: chromatin condenses to form
chromosomes (which are made of DNA)
a. each species of organism’s chromosomes are
unique – in size, shape, number, etc.
b. chromosomes usually occur in pairs
examples:
all humans have 23 pairs of chromosomes
fruit flies have 4 pairs
some plants have hundreds of pairs
some molds have thousands of pairs
c. each chromosome has been copied (during
interphase) and the IDENTICAL chromosomes
are called sister chromatids
d. the sister chromatids are joined together at a
point called the centromere
sister chromatid
(each half of
the “X” is one)
chromatin
(interphase)
chromosome
(prophase)
centromere
e. also during prophase: nuclear membrane
(around nucleus) and the nucleolus disappear;
mitotic spindle forms (fibrous scaffolding inside
the cell)
2. Metaphase:
a. chromosomes are pulled to the center of the cell
and begin to line up in the middle
(if the cell was a globe, then chromosomes would
line up on the equator)
3. Anaphase
a. centromeres divide
b. spindle fibers start to pull the sister chromatids
apart
1. one of each pair goes to the poles of the cell
2. complete set of chromosomes at each end
Think of chromosomes being pulled from the
equator to the North and South Poles
4. Telophase
a. 2 nuclei are formed
b. nuclear membranes form around each set of
chromosomes
c. chromosomes uncoil  chromatin
d. mitotic spindle disassembles
D. Cytokinesis – split cytoplasm
1. animal cells: cell membrane of the parent cell starts
to fold inward to the center until 2 cells are formed
from pinching the parent cell in half
2. plant cells: a cell plate forms in the middle of the
cell
a. cell plate consists of a double membrane
b. a new cell wall forms between the double
membrane of the cell plate
c. the double membrane remains and forms part of
the cell membrane of the 2 new cells
____________________________________________________
Vocabulary for Meiosis
Gene: section of DNA that controls a particular trait or
characteristic (ex. eye color)
Chromosome: lots of DNA condensed; each chromosome
contains lots of genes
Homologous chromosomes: same length, size, and shape; code
for the same traits but might have different versions of the
genes for these traits
Example: 2 homologous chromosomes carry the code for
eye color
 one chromosome might have the gene for blue
eyes while the other chromosome has the gene
for brown eyes
 same traits (eye color), different versions (blue, brown)
Our 23 pairs of chromosomes are 23 homologous pairs (46
chromosomes).
Diploid: di=2 ; genes are in pairs and therefore, chromosomes
are in pairs (2 sets of chromosomes)
Human cells are normally diploid: chromosomes are in
pairs (23 pairs).
Haploid: half of the normal number of chromosomes
Humans: diploid
 23 pairs  46 chromosomes
haploid  not pairs  23 chromosomes
Symbols:
n = number of sets of chromosomes
2n = diploid, 2 sets of chromosomes
1n = 1 set of chromosomes
somatic cells: normal body cells of an organism; usually
diploid; made through mitosis
gametes: sex cells of an organism; haploid; made through
meiosis
Meiosis
1. Called reduction division because it reduces the number of
chromosomes by half
2n
(diploid)
meiosis
1n
4n
(haploid)
(tetraploid)
6n
meiosis
meiosis
2n
(haploid)
3n
2. Meiosis is used in sexual reproduction.
3. Results in 4 new haploid cells, called gametes.
4. Used for gametogenesis (process of making gametes)
Two types of gametogenesis:
1. spermatogenesis : process of making sperm
2. oogenesis: process of making eggs
Homologous Chromosomes
Further Explanation
• Homologous chromosomes are the same length, size, shape, and code for
the same TRAITS.
• The location of these traits is the same. On the diagram below, locations
are indicated with a solid black line on the chromosome.
• However, the specific GENES for these traits can be different or it can be
the same.
• Homologous chromosomes are NOT IDENTICAL. Sister chromatids are
identical.
• In the diagram below, notice that the TRAITS are written in green, while
the GENES for those traits are written in blue.
TRAIT TONGUE ROLLING
GENE  ROLLING
TRAIT  THUMB SHAPE
GENE  HITCHHIKER’S
TRAIT  EYE PIGMENT
GENE  NO PIGMENT
TRAIT  TONGUE
ROLLING
GENE  NONROLLING
TRAIT  THUMB SHAPE
GENE  HITCHHIKER’S
TRAIT  EYE PIGMENT
GENE  NO PIGMENT