Why Do Cells Divide Instead of Just
Growing Bigger and Bigger?
The cell's DNA would no longer be able to serve the
increasing needs of the growing cell.
2. The cell has more trouble moving enough nutrients
and wastes across the cell membrane.
1.

Larger cells have a smaller ratio of surface area to
volume.
Prokaryotic vs. Eukaryotic
 Prokaryotic Cell Division:
 Simple matter of separating the contents of the cell into
two parts.
 Eukaryotic Cell Division:
Cell Cycle
Prokaryotic DNA
 located in the cytoplasm
 Have a single circular
DNA molecule (usually
referred to as the cell's
chromosome)
Eukaryotic DNA
• Located in the nucleus in
the form of chromatin/
chromosomes
Eukaryotic Cell Cycle
 Cell Cycle - Series of
events that cells go
through as they grow and
replicate.
 Interphase - Period of the
cell cycle between cell
divisions (consists of S,
G1, and G2 phase)
 M Phase –Mitosis
(division of the cell
nucleus) Cytokinesis
(division of the
cytoplasm)
Cell Cycle- Interphase
 G1 Phase – Period of intense
growth before DNA
replication
 S Phase – DNA
(chromosome) replication
takes place, still little cell
growth
 G2 Phase – Period of more
growth and final
preparations before cell
starts dividing.
 There are 3 “checkpoints” that allow the cells to continue
moving through the cell cycle.
What do all of these
have in common?
1) growth from an infant to an
adult
2) repairing or replacing damaged
issues/organs
3) cancer
Is Mitosis Asexual
or Sexual?!??
Asexual Reproduction = Mitosis
 Asexual Reproduction - Reproduction of genetically
identical cells to the parent cell without the fusion of
gametes; not necessary to have two parents
 Examples include:
1.
2.
Single-celled organisms
Source of new cells when a multicellular organism grows
and develops (skin cell, liver cell)
Four Phases of Mitosis
Prophase - Chromosomes visible/centrioles; nuclear envelope disappears
2. Metaphase – Centriole go to opposite ends of the cell; spindle fibers
1.
emerge from the centrioles; chromosomes line up in middle of cell
3. Anaphase - Sister chromatids are pulled to opposite ends of cell
4. Telophase - Chromosomes condense back to chromatin and nuclear
envelope reappears
(The four phases are followed by Cytokinesis)
Four Phases of Mitosis
Cytokinesis
 Usually occurs at the same time as telophase.
 Cytokinesis - The division of the cytoplasm itself.
Cytokinesis: Plant vs. Animal Cells
 Animal Cells:
 The cell membrane is pinched into
two nearly equal parts = cleavage
furrow.
 Plant Cells:
 A structure known as the
cell plate forms midway
between the divided
nuclei and gradually
develops into a separating
membrane.
 A cell wall then begins to
appear in the cell plate.
What Phase Of the Cell Cycle Is It?
A
D
B
C
E
F
What Phase Of the Cell Cycle Is It?
A
B
C
E
F
Anaphase
D
What Phase Of the Cell Cycle Is It?
A
B
Anaphase
D
C
Telophase
E
F
What Phase Of the Cell Cycle Is It?
A
Anaphase
D
B
C
Telophase
Cytokinesis
E
F
What Phase Of the Cell Cycle Is It?
A
Anaphase
D
Interphase
B
C
Telophase
Cytokinesis
E
F
What Phase Of the Cell Cycle Is It?
A
Anaphase
D
Interphase
B
C
Telophase
Cytokinesis
E
F
Metaphase
What Phase Of the Cell Cycle Is It?
A
Anaphase
D
Interphase
B
C
Telophase
Cytokinesis
E
F
Metaphase
Prophase
Lets take a deeper look at what DNA
is before we learn how it replicates...
 Deoxyribonucleic Acid
(DNA) - The material
inside the nucleus that
carries all the genetic
information
 Responsible for
determining all organism’s
traits

Eye color, body structure,
and cellular enzyme
production
The Components of DNA

DNA is a long molecule made up of
repeating individual units called
nucleotides

Nucleotides are made up of three parts:
1. Sugar
2. Phosphate Group
3. Nitrogenous Base
Nucleotides Put Together = DNA
5’
3’
3’
5’
Nitrogenous Bases

DNA contains four nitrogenous bases:
1.
2.
3.
4.
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Complimentary Bases
 Chargaff discovered that there were
always equal amounts of A & T. And
equal amounts of C & G in cells.
 Adenine pairs with Thymine, and
Guanine pairs with Cytosine
 The complimentary bases are held
together by weak hydrogen bonds.
 (No matter what organism it is!!!!)
 [A] = [T]
 [G] = [C]
Purines and Pyrimidines
 Purines have two rings
in their structures.

Adenine and guanine
belong to this group
 Pyrimidines have one
ring in their structure.

Cytosine and thymine
belong to this group
Structure of DNA
 Phosphate Group &
Deoxyribose Sugar
 Form the backbone or sides of
the ladder.
 Nitrogenous Bases
 Form the “rungs” of the ladder
or middle of the molecule.
The Shape of DNA
 DNA looks like a “twisted ladder” called a Double
Helix.
Discovering DNA
1. Rosalind Franklin Used X-ray diffraction;
Showed the X-shaped
pattern of DNA (helix
shape)
2. Watson and Crick –
Two scientists that
discovered the double
helix shape in 1953
Chromosome
 Long, continuous single
piece of DNA, contains
many genes
Genes
 Sequence of DNA that
codes for a protein and
thus determines a trait
Chromosomes
 In eukaryotic cells, the genetic
information that is passed on
from one generation of cells to
the next is carried by
chromosomes.
 The cells of every organism
have a specific number of
chromosomes
 The cells of fruit flies have 8
chromosomes.
 Human cells have 46
chromosomes.
 Carrot cells have 18
chromosomes.
Chromosomes
 Well before cell division, each chromosome is replicated,
or copied.
Chromosomes
 Each chromosome consists of two
identical “sister” chromatids.
 When the cell divides, the “sister”
chromatids separate from each other.
 One chromatid goes to each of the two
new cells.
Chromosomes
• Each pair of chromatids is attached at an area called
the centromere.
Chromosome
Structure
 DNA molecules are surprisingly long
• The chromosome of the prokaryote E. coli, which can
live in the human colon, contains 4,639,221 base pairs!
• Analogy: Imagine trying to pack a 300-meter length of
rope into your school backpack!
 DNA must be dramatically folded and tightly packed
to fit within the cell
Cell Cycle- Interphase
 G1 Phase – Period of
intense growth before
DNA replication
 S Phase – DNA
(chromosome)
replication takes place,
still little cell growth
 G2 Phase – Period of
more growth ;
organelles replicate;
final preparations
before cell starts
dividing.
Cell Cycle – M Phase (cell division)
 Cell Division - The process by
which a cell divides into two
new “daughter” cells.
 M Phase –Mitosis (division
of the cell nucleus)
Cytokinesis (division of the
cytoplasm)
 Before cell division occurs,
the cell replicates, or copies,
all of its DNA.
 Each daughter cell gets one
complete set of genetic
information.
CHROMATIN
What does the
DNA look like?
Present in what
stage of the cell
cycle?
CHROMOSOMES
CHROMATIN
CHROMOSOMES
What does the
DNA look like?
Present in what
stage of the cell
cycle?
Cell
Divison
Interphase
(M Phase)
DNA Replication
 Before a cell divides, it duplicates its DNA via a process
called DNA Replication
 During the S phase of cell division
 Ensures that each resulting cell will have a complete set
of DNA molecules
 Result: Two identical DNA molecules (each DNA
molecule has one original strand and one new strand)
How
DNA
Replication
Occurs
Step 1:

Helicase, an enzyme,
“unzips” the DNA molecule


Hydrogen bonds between
the base pairs are broken
Result = 2 separated strands
of DNA
How DNA Replication Occurs
Step 2: A replication fork is formed.
How DNA Replication Occurs
Step 3: Free floating nucleotides pair up
with complimentary bases
• Each strand of DNA serves as a template,
or model, for the new strand
• DNA Polymerase – Enzyme that joins
individual nucleotides to produce a DNA
molecule and also “proofreads” each new
DNA strand
You Try It!
 What is the complimentary DNA strand if the original
DNA strand is AGTCTA?
You Try It!
 What is the complimentary DNA strand if the original
DNA strand is AGTCTA?
Original Strand: AGTCTA
Complimentary Strand: TCAGAT
 If a meerkat’s parent skin cell contained 34
chromosomes; after cell division how many
chromosomes would each of the meerkat’s
daughter skin cells contain?
 17 chromosomes
 34 chromosomes
 68 chromosomes
 None because skin cells don’t go through cell
division
 If a meerkat’s parent skin cell contained 34
chromosomes; after cell division how many
chromosomes would each of the meerkat’s
daughter skin cells contain?
 17 chromosomes
 34 chromosomes
 68 chromosomes
 None because skin cells don’t go through cell division
 Asexual reproduction produces cells that are
genetically identical to their parents for
reasons of growth, repair and replacement.
Regulating the Cell Cycle
 Not all cells move through the cell cycle at the same
rate.
 Cell growth and cell division are controlled.
 Cyclins are proteins that regulate the timing of the cell
cycle in eukaryotic cells.
Regulating the Cell Cycle
 If cells are placed in a petri dish containing food:
 Cells will grow until they come in contact with other
cells then they stop growing.
 If cells are removed from the center of the dish:
 The cells bordering the open space will begin dividing
until they have filled the empty space.
Regulating the Cell Cycle
 This occurs in an
injury as well:
 If a cut in the
skin or a break
in a bone
occurs, cells at
the edges of the
injury are
stimulated to
divide rapidly.
 When the
healing process
nears
completion, the
rate of cell
division slows
down.
Cancer
 Cancer - Disorder in which some of the body's own
cells lose the ability to control growth.
 Cancer cell divide uncontrollably and form masses of
cells (tumors)
 Cancer cells may spread throughout the body:

Disrupts normal activities and causes serious medical
problems or even death
There are many reasons that cells can divide and grow out of
control:
* exposure to environmental causes (such as
carcinogens or ultraviolet (UV) radiation from the sun)
*nutritional deficiencies
* genetic causes
*some infections
(All of these will
result in DNA
mutations)