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CELL DIVISION
Why do Cells Divide?
• Unicellular organisms do it to make new
individuals
• Multicellular organisms do it to grow, repair,
and maintain SA:V ratios– there are about 10,000,000 mitotic cell divisions/
sec in the human body!
How do they do it?
• PROKARYOTES use binary fission:
– DNA attaches directly to
membrane
• DNA replicates and attaches to
different points on membrane
• Plasma membrane grows between
these two points
• A segment of cell wall begins to
form between the DNA
Eukaryotic Cells
• Use Mitosis and Meiosis
– Nuclear contents (DNA) are copied
& divided
– Cell splits into two by cytokinesis
– Complexity of Eukaryotes:
• they are big and contain many
organelles
• they contain 700x the DNA of Proks
• they contain many chromosomes
(not just one)
The Cell Cycle
• Represents recurring events
that take place from the
beginning of one cell
division to the beginning of
the next.
• Includes:
– cell division
– growth
– DNA replication
– preparation for the next
division
A few Questions?
• Why is the cell cycle called a “cycle”?
– It represents recurring events
• Why do you think that it is important for a cell
to grow in size during its cell cycle?
– If a cell did not grow in size, each cell
division would produce progressively
smaller cells
• What might happen to a cell if all events
leading up to cell division took place as they
should, but the cell did not divide?
– The cell would grow increasingly larger--- to
a point at which the cell could no longer
exchange materials with the environment
efficiently enough to live.
– Animation
THREE PARTS TO THE CELL CYCLE
• Interphase– majority of life cycle,
preparation for cell
reproduction
• M Phase
– Nuclear division
• Cytokinesis– The cytoplasm divides
And the cell splits in 2
Interphase
• G0- Gap0 Phase- where cells spend most
of their time, just acting like cells, until
signals are sent to their nucleus that it’s
time to divide.
– some cells stay here
ex. muscle & nerve cells
• G1- Gap 1 Phase-
– cell grows and carries out normal
metabolism
– organelles duplicate
• S (synthesis) Phase
– all DNA in chromosomes replicates
• G2 – Gap2 Phase
– Cell grows and preps for mitosis
– Animation
• Draw a labeled picture of the Cell Cycle
C & C P. 216
1. How does the importance of cell division differ in unicellular and
multicellular eukaryotes?
Cell division in unicellular organisms results in the production of
new individuals; in multicellular organisms, it results in new tissue
2. What is the major event in M phase? S phase? G0 phase?
M, mitosis; S, DNA synthesis (replication); G0 stopping point,
metabolism
3. What determines whether a cell stays in interphase or divides?
Signals received in G1 or G2
4. What are some factors that might influence the cell cycle?
A trigger protein may help control the cell cycle. This protein may
itself be influenced by hormones, cell size, and position in a tissue
5. Explain why DNA replication is important in cell division
Each offspring/daughter cell needs a complete set of genetic
material.
DNA: The genetic material
A few questions to ponder today:
• What is DNA?
• How did we figure out the structure of
this important biomolecule?
• How is it replicated for the cell cycle?
• Steps to the discovery of DNA:
– 1869- Freidrich Miesher
• purified DNA from pus cells obtained from discarded surgical
bandages
– 1923- Robert Feulgen
• developed a procedure that stained this material in the nucleus
• This enabled him to measure the amount of “stuff”in the nucleus
• He concluded that virtually every cell nucleus in any plant or animal
has the same amount of this “stuff”
The Unknown…Was it DNA or a protein
that carried the hereditary instructions
from generation to generation?
Early scientists saw the structure of the DNA
molecule as too simple to possibly contain the
complex genetic information of an organism
• 1828- Fred Griffith was trying to prepare a vaccine
against a pneumonia-causing bacterium.
•He discovered that harmless bacteria could turn virulent
when mixed with bacteria that caused disease.
•In other words, he discovered what is now called
transformation
a change in phenotype caused when bacteria cells take up
foreign genetic material.
animation
• 1944- Oswald Avery discovered that DNA is responsible for
the transformation that occurred in Griffith’s mice.
• 1952- Alfred Hershey and Martha Chase settled the
controversy.
– DNA is the molecule that store genetic info in living cells
• 1951- Roseland Franklin fired x-rays through DNA
crystals, trying to learn how the atoms of the DNA
molecule were arranged
• 1953- James Watson and Francis Crick were shown
Franklin’s photo and immediately determined the
double helical structure of DNA.
DNA Structure
Nucleotides are the subunits:
– a phosphate group
– a 5-carbon sugar molecule
• Deoxyribose
• 5’ end- where the Phosphate group is
• 3’ end –OH (hydroxyl) end
– a nitrogen –containing base
• Purines- made up of 2 rings of
carbon and nitrogen atoms
– Adenine (A)
– Guanine (G)
• Pyrimadines- made up of a single
ring of carbon and nitrogen atoms
– Thymine (T)
– Cytosine (C)
Chargaff’s Rule- (1947)Erwin Chargaff, a chemist who worked at Columbia
found that:
– for each organism studied, the amount of adenine
always equaled the amount of thymine
– and cytosine always equaled guanine.
– This became known as the Base-Pairing Rules
Why does this complementarity exist?
□ The two chains of nucleotides are held
together by 2 or 3 weak hydrogen bonds
that form between the nitrogenous bases:
– Cytosine and guanine have three
hydrogen bonds
– Thymine and adenine have two
hydrogen bonds
□ Each rung of the DNA ladder needs to
have a total of three rings
• Also, One of the strands of DNA runs in
the 3’ to 5’ direction and the other runs
in the 5’ to 3’ direction.
antiparallel
• This is called _______________
What does 5’ and 3’ mean???
The symbols 5' and 3' (pronounced "five prime" and "three prime") refer to
the two ends of a strand of DNA.
The carbons on the deoxyribose sugar are numbered 1-5. One end of a DNA
strand begins with a phosphate group; this is attached to the 5' carbon
(carbon #5) of a deoxyribose.
A new nucleotide is attached by combination of its phosphate to the 3'
carbon of the sugar. The two strands of a DNA molecule are antiparallel, so
the 5' end of one strand is paired with the 3' end of its complementary
strand.
http://wiki.answers.com/Q/What_do_5'_and_3'_mean_when_talking_abou
t_a_cell_reproducing_itself
DNA cont.
• DNA is the molecular
“blueprint”—
– This “instruction manual”
makes up an organisms genes.
– Encoded in the precise order of
its nucleotides
– The genetic alphabet only
contains 4 letters (ACTG).
– Human DNA contains more
than 3 billion base pairs!
COMPARING PROKARYOTIC AND
EUKARYOTIC DNA:
PROKARYOTES
EUKARYOTES
Configuration of DNA
Circular
linear
Length
1000 micrometers
No
1.8 meters
Yes
Associated proteins
Transient proteins
Histones
# of Chromosomes per
Cell
1
2- 1,262
DNA housed in nucleus?
HOW DO ALL THESE BASE PAIRS FIT IN
THE NUCLEUS?
– a NUCLEOSOME consists of 2 loops of DNA wrapped
around a central cluster of 8 histone molecules.
– This shortens the length of DNA by 1/6.
– This is shortened even further by its coiling into
thicker fibers
DNA REPLICATION
• We know we need to copy a cells DNA before a cell
can divide, but how is DNA copied?
• There were 3 possible models for DNA copies to be
made while obeying the Base Pairing or Chargaff’s
Rules :
– Conservative replication
leaves the original DNA molecule intact and
generates a completely new molecule
• Sort of like a photo copier
•
– Dispersive replication
• produces two DNA molecules with sections of both
old and new DNA interspersed along each strand.
– Semi-conservative replication
• produces molecules with both old and new DNA,
but each molecule would be composed of one old
strand and one new one.
• Journey Into DNA
• Investigating DNA Replication
The Process of DNA Replication
• Simplified DNA Replication Activity
1. The double helix unwinds with the help of enzymes called DNA
HELICASES.
• The enzymes break the hydrogen bonds between nitrogen bases
• Strands are held apart and prevented from twisting back into their
double-helical shape by a SINGLE STRAND BINDING PROTEIN.
• The two areas on either end of the DNA where the DNA helix
separates are called REPLICATION FORKS.
2. RNA PRIMASE- inserts as starter of RNA nucleotides at the initiation
point.
Animation
3. DNA POLYMERASE III moves along each of the DNA strands reading the
nucleotide on the template and joins the complementary nucleotide
onto the end of the new strand by following the Base-Pairing Rules
(as many as 1000 base pairs can be added each second)
– Recall, one of the strands of DNA runs in the 3’ to 5’
direction and the other runs in the 5’-3’ direction
– DNA Polymerase can only move/build in one
direction- new DNA nucelotides can only be added
to the 3’ end (replication proceeds 5’-3’ on each
strand)
– Because DNA is antiparallel, DNA Polymerase
molecules move in opposite directions along the
two strands.
– The two newly synthesized strands grow in opposite
directions- one towards the fork, the other away
from it.
– The strand that is assembled continuously and
towards the replication fork is called the LEADING
STRAND,
– The strand that is assembled discontinuously and
away from the replication fork is called the LAGGING
STRAND. These short fragments are called OKAZAKI
FRAGMENTS.
4. DNA POLYMERASE I- strips away the RNA
PRIMERS and inserts DNA nucleotides
5. LIGASE- seals the gaps in the sugar phosphate
backbone where Okazaki fragments come
together.
Checking for Errors
• Errors sometimes occur.
• An important feature of DNA
replication is DNA polymerases
ability to “proofread”.
– This enzyme is capable of
backtracking, removing
incorrect nucleotides and
replacing it with the correct
one.
– Still one error per one billion
nucleotides typically occurs.
– This type of error is called a
MUTATION.
Multiple Forks
• Replication does not begin at one end
of the DNA molecule and end at the
other.
• Each chromosome contains a single
long strand of DNA.
• The replication of a typical human
chromosome with one pair of
replication forks would take 33 days!
• For this reason, each human
chromosome is replicated in about 100
sections that are 100,000 nucleotides
long (this takes only 8 hours).
C & C p. 222
1. What are 3 different roles of DNA polymerase?
DNA polymerase synthesizes DNA from the four nucleotides, proofreads
its own work, and corrects single base insertion errors
2. Explain how DNA synthesis can proceed in both directions form a
replication origin, even though DNA polymerase can synthesize DNA only
in one direction.
Back synthesis on the lagging strand products short fragments that are
joined by enzymes.
3. How is the number of replication errors in cells kept to a minimum?
Enzymes detect and repair the errors; DNA polymerase has proofreading
and error-correcting functions.
4. How does the cell repair damaged DNA?
By enzyme-mediated mechanisms, including excision repair
5. How are histones involved in gene expression?
Histones bind to DNA and form nucleosomes. This tightly packed
structure turns off DNA by excluding the enzymes involved in gene
expression.