Tuesday December 3, 2013
Test on DNA, Cell Cycle Gene Expression, Replication,
Mitosis, & maybe Mutations 12/11-12/12
Agenda:
O Homework Check
O Discussion: Why Cells Divide, Chromosomes & Preparing for Mitosis:
-------------Warm Up-----------------------------------------------1. What is DNA replication and what is its goal?
a
Without looking at your
notes, what is…
a.
b.
c.
d.
e.
f.
g.
h.
e
b
d
f
c
----------HW: -----------O Re-read outlines on CH9: Cell cycle, Chromosomes and Mitosis.
g
g
h
Homework Check
Mitosis Objective
• Students will understand the process of The Cell
cycle.
• The students will also be able to demonstrate
the process of Mitosis and how the purpose
aides life.
The Two Possible
Outcomes
• Upon conception, a sperm fertilizes an egg.
• From this point, the DNA from dad and mom
are joined to give a new organism all the
DNA it needs to live.
• But the size of a fertilized egg is
microscopic.
• How do we, as organisms, get to be the size we are when we
finally finish growing from such simple origins?
• There are two solutions.
• One way is for that one fertilized cell to keep getting larger and
larger.
• But that would result in a gigantic blob and you know cells are
very small.
Amoeba Norris OR Chuck Norris
Unicellular
Multi-celled
Option Two
• The second option, the one that really happens, is
we grow as a result of the number of our cells
growing.
• The number of cells grow from the existing cells that
are already there.
• The existing cells split in half, giving birth to 2
identical versions of itself.
• The growing number of cells each survive
independently but work together to give rise to
complex multicellular organisms, like humans.
• This process of cell division is a vital part of
development, of healing, of life.
DNA Packaging: Objectives
• List 3 reasons why cells
divide.
• Describe three levels of
structure in the DNA
packaging in the nucleus.
• Explain what are daughter
cells and why they are
identical to the parent cells.
• Describe how cells (both
prokaryotic and eukaryotic)
prepare for division.
•
•
•
•
•
•
•
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Vocabulary:
Gene
Chromosome
Chromatin
Histone
Nucleosome
Chromatid
Centromere
Cells don’t last forever
• Remember, cells don’t last
forever… they have a life
span like we do as
organisms.
• You’ve learned in G1 cells
primarily grow, performing
transcription and translation.
• You’ve also learned that in S
phase DNA is duplicated.
• We will now focus on G2,
when cells finish growing and
prepare for division.
What do these things have in common?
• Human Growth
• Growth of plant
• Wound healing
What is Cell Reproduction?
• As the body of a multicellular organism grows
larger, its cells do not just grow large too.
• Instead, the body grows by increasing the
number of the same size cells.
– An adult human can produce up to around 2 trillion
cells per day!
• This is known as cellular reproduction.
• Or more commonly, mitosis.
Why Cells Reproduce
• There are three main reasons cells divide:
• 1. To help tissues and organs grow.
– We continuously add cells as we grow to get bigger.
• 2. To replace cells.
– As old cells die or cells get damaged new cells take
their place in order to maintain health.
– As the cell ages, it continues to grow. When it gets
too big it need to be replaced to limit cell size.
• 3. To repair broken cells.
– When cells get damaged due to trauma they need to
be replaced.
Why Cells Reproduce…
Growth & Development
• Aging is inevitable.
• But a cell only last so long.
– Some last days, some decades
• As one cell dies others replace it.
Why Cells Reproduce…
Replacing Old Cells
• What constitutes an old cell?
• A cell is old when it gets too big.
• As a cell ages it grows by producing more proteins &
more organelle.
• As a cell grows its surface-area-to-volume ratio
decreases.
• Remember, cells are small because it is more efficient to
be small than big.
Think of a Cell as a Protein Factory
Why Cells Reproduce…
Replacing Broken Cells = Healing Wounds
• What constitutes BROKEN CELLS?
• Cells damaged from:
– Trauma
– Burns
– Cancer
Recap
• What are three reasons cells reproduce?
– To allow an organism to grow.
– To replace old cells (that get too big).
– To allow an organism to replace damage.
Why Cells Reproduce, Making New Cells
• This all ties in to one major theme.
• Other than repairing broken cells or for growing…
• Cells divide when they reach a
certain size, because larger cells
are more difficult to maintain.
• Small cells are MUCH more
efficient than large cells.
Facts of Cell Division
• When a cell divides it forms “daughter” cells
– Each newborn “daughter” cell is smaller than the
parent and has a higher surface area–to-volume ratio
than its parent does.
– It inherits half of the parent cell’s organelle necessary
for life (mitochondria, ER, Golgi, Etc.)
– Also, each new daughter cell gets an entire, exact
copy of the parent cell’s DNA. (produced in replication)
– They are, in essence, identical twins & clones of the
parent.
What Must Happen First…DNA
Packaging
• You now know why a cell divides.
– It’s to replace old or damaged cells,…
– To grow,…
– Or when the cell reaches a certain size.
• Now, before you learn about the
process of mitosis, you need to learn
about the structure of chromosomes
and how DNA is prepared for efficient
separation into two daughter cells.
Chromosomes
• You know that DNA carries
Genetic Information.
• The genetic information is
arranged on large
molecules of DNA
organized into hereditary
units called genes.
– Recall: A gene is a unit of
heredity that consists of a
segment of nucleic acid
that codes for a functional
unit of RNA or protein.
Chromosomes
Eukaryotic Chromosomes
• During most of a cell’s life, its
chromosomes exist in the nucleus
like spaghetti noodles in a bowl.
• This form is called chromatin.
• Chromatin = a complex of DNA & proteins called
• In this form, the genetic information is easily accessed to
make proteins.
• But before a cell can divide, the DNA must be condensed,
or wound up into a smaller, more organized unit…into
chromosomes.
• WHY? = so that DNA doesn’t get messed up during
division.
Chromosomes
• Ultimately, before a cell can
divide its genes in DNA it’s
organized and packaged
(condensed) into structures
called chromosomes.
– Depending on the organism,
chromosomes are linear
structures (eukaryote) or a
circular structure
(prokaryote) that is made up
of DNA and proteins.
START
STOP
1st level of Organization.
Eukaryotic Chromosomes
• The first level of packaging is
done by a class of proteins
called histones. A complex of
eight histones come together
to form a disc-shaped core.
– A type eukaryotic protein found
in the chromosome
• The long DNA molecule is
wound around a series of
histone cores in a regular
manner to make what is called
a nucleosome.
– A eukaryotic structural unit of
chromatin of DNA wound around
histones
• Under an electron
microscope, this
level of packaging
resembles beads
on a string.
2nd Level of Organization
• The 2nd Level
• The string of nucleosomes line up in a spiral to
form a cord that is 30 nm in diameter.
3rd Level of Organization
Eukaryotic Chromosomes
• The 30-nm nucleosome cord forms visible chromosomes
around protein scaffolding.
• These looped domains then coil into the final, most
highly condensed form of the chromosome.
• Many dense loops of chromatin form the rod-shaped
structures that can be seen in regular light microscopes.
– After the several condensing steps, the chromatin is dense
enough to be seen by the eye in one of our microscopes
– Each chromatid is made of a single, long
molecule of DNA.
• .
CHROMATID #2
Eukaryotic Chromosomes
• Chromosomes are drawn as “X”s
• It’s actually 2 identical chromosomes
that have been replicated joined in the
center.
• Each of the two thick strands of a fully
condensed, duplicated chromosome
are called a chromatid.
CHROMATID #1
Chromosomes
Chromosomes
Eukaryotic Chromosomes
• Identical pairs of chromatids, called sister
chromatids, are held together at a region called the
centromere.
– The region of the chromosome that holds the two
sister chromatids together during mitosis
• Why do the sister chromatids need to be
identical?
• During cell division, the sister chromatids are
separated at the centromere, and one ends up in
each daughter cell. They need to be identical so
the daughter cells will have the exact same genes
and do the exact same things.
Summary
• All new cells are produced by the division of preexisting
cells.
– This idea is a cornerstone of CELL THEORY
• G1 = (Transcription and Translation) Cells “Live”. They
make proteins & the directions for those proteins are
contained in the genes of DNA.
• S = (Replication) DNA and all the genes are duplicated
so there exists two copies of all the genetic information.
G2 = Organelle is duplicated and DNA is condensed
into chromosomes.
• Mitosis and Cytokinesis = (Next Topic) Division of the
nucleus and cytoplasm.
• The process of cell division involves more than cutting a cell into
two pieces. Each new cell must have all of the equipment
(organelle… mitochondria, etc) needed to stay alive.
CONCEPT CHECK
Where are CHROMATIDS?
PickElectron
a letter(s).
A Chromosome Under
Microscopy
A
E
B
C
D
CONCEPT CHECK
Where is the
A Chromosome Under CENTROMERE?
Electron Microscopy
A
E
B
C
D
The place where 2 sister
chromatids are joined
CONCEPT CHECK
What is CHROMATIN?
A Chromosome Under Electron Microscopy
A
E
B
C
D
This is the stuff
that all DNA in
chromosome form
is made of.
CONCEPT CHECK
What is the
A Chromosome Under CHROMOSOME?
Electron Microscopy
A
E
B
C
D
Chromosomes are the
most condensed and
organized form of DNA
Concept Check:
B
Talk with shoulder partner
and determine which letter
corresponds to the
graphic.
A
C
D
H
A
F
E
C
B
G
Word Bank:
E
Sister chromatid
DNA
Nucleus
30nm-cord
DNA around protein scaffold
Centromere
Chromosome
Nucleosome
D
F
H
G
Summary: Answer these questions.
• Why do Cells Divide?
• What happens to DNA before a cell can divide?
• What helps DNA condense?
• What is special about daughter cell and parent
cell DNA?
Summary of
Chromosome
Packaging
Application
• Tissue Regeneration
• Pinky Nail Regrowth
Prokaryotic Chromosomes
Prokaryotic Chromosome Organization
• Prokaryotes are much simpler than eukaryotes.
• A prokaryotic cell has a single circular molecule of DNA.
– Normally, it’s connected on both ends, in a loop
• This loop of DNA contains thousands of genes.
• A prokaryotic chromosome is condensed through
repeated twisting or winding, like a rubber band twisted
upon itself many times.
• This is called supercoiling.
Preparing for Cell Division
Prokaryotes
• Once supercoiled, the bacterial DNA is protected
• In order for bacteria to divide the DNA must unwind, get
copied and attached to the inner cell membrane
• Once attached the bacteria is ready to divide.
Preparing for Cell Division
Prokaryotes
• Division in Prokaryotes is much more basic.
• The process of Prokaryotic cell division is called binary
fission.
• It happens in three steps.
1. The cytoplasm is divided when a new cell membrane
forms between the two DNA copies.
2. The cell grows until it nearly doubles in size.
3. The cell is then constricted in the middle, like a long
balloon being squeezed near the center.
• Eventually the dividing prokaryote is pinched into two
independent daughter cells, each of which has its own
circular DNA molecule exactly the same as the parent.
Binary Fission
Preparing for Cell Division, continued
Eukaryotes
• The reproduction eukaryotic cells is more complex than that of
prokaryotic cells.
• Reasons:
• Eukaryotic cells have many organelles. In order to form two living
cells, each daughter cell must contain enough of each organelle to
carry out its functions.
• The daughter cells being smaller only need a few organelle at first
but as it grows, makes more of each organelle.
• The DNA within the nucleus must also be copied, sorted, and
separated.
• Eventually, the daughter cells become mature cells, exactly like their
parent cells, equipped with the same amount of DNA & organelle.
Comparing Cell Division in Prokaryotes and Eukaryotes
Because of its complexity, eukaryotic cell division takes roughly 24 hours.
Prokaryotic cell division requires only 20 minutes. Being infected by one cell can
result in our bodies being overrun by millions of bacteria in less than a day.
Summary Reflection
• In your notes, list 3 things you have learned
today.
• List at least one thing you are unsure about and
need to study more.
• Next up: You will use this information to better
understand mitosis!
Summary
• Because larger cells are more difficult to maintain, cells
divide when they grow to a certain size.
• Many proteins help package eukaryotic DNA into highly
condensed chromosome structures.
• All newly-formed cells require DNA, so before a cell
divides, a copy of its DNA is made for each daughter
cell.
The Organization of DNA
• Take a piece of string and stretch it out.
• This is how long all the DNA in just one cell would be if it
were stretched out… 2 meters.
• It takes great organization to get this much DNA to fit
into each of the 100 trillion cells you have.
• Try fitting two “chromosomes” into your “nucleus”.
• Try once just shoving it in then wrap the chromosome
around a paperclip.
• Which way is more organized?
• This organization is called condensation.
• What does the word condensed mean?