Transcript Section 5.1: The Cell Cycle

Chapter 5:
Cell Growth
and
Division
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Section 5.1: The Cell Cycle
• To this point we have learned what a cell is
and what parts make up a cell.
• This chapter will explain how cells duplicate
themselves.
• The duplication of cells is called the Cell Cycle.
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Section 5.1: The Cell Cycle
• The cell cycle is a regular pattern of growth,
DNA replication, and cell division that occurs
in eukaryotic cells.
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Section 5.1: The Cell Cycle
• There are 4 main stages to the cell cycle:
1. Gap 1 Stage (G )- cells grow, carry out normal
functions and start replicating its organelles.
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Most of a cell’s life is spent in (G1)
2. Synthesis- Cell’s start to copy the DNA code.
3. Gap 2 Stage (G2)- cell continues normal growth
patterns, this is also one of the final check points
to see if everything is “ok”.
4. Mitosis (M-Phase)-this is when the nucleus and
the cell is actively dividing.
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We will talk more about this later
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Section 5.1: The Cell Cycle
• Although most all cells go through the cell cycle,
all cells go through at different rates.
• The rate of cell division is dependent upon your
body’s need for certain cells
Cell Type
Approximate Life Span
Skin Cells
2 weeks
Red Blood Cells
4 months
Liver Cells
300-500 Days
Intestine-internal lining
4-5 Days
Intestine- muscle and other tissue
16 years
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Section 5.1: The Cell Cycle
Cell Type
Approximate Life Span
Skin Cells
2 weeks
Red Blood Cells
4 months
Liver Cells
300-500 Days
Intestine-internal lining
4-5 Days
Intestine- muscle and other tissue
16 years
• Remember this is just the life span we are talking
about, not the amount of time spent in the cell
cycle.
– For most human cells (G1, S, G2 &M) takes 12 hours.
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Section 5.1: The Cell Cycle
• Cells that divide very rarely enter a stage that some
scientists call G0 State.
– In G0, cells are unlikely to divide, although they continue
to carry out all the normal functions.
• Some cells, such as neurons, seem to be in stage G0
their entire lifespan and never divide.
– Recently, scientists have discovered that neurons can
divide, but scientists don’t know why they don’t.
– When we find how to make neurons divide, conditions like
paralysis and strokes would be temporary.
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Section 5.1: The Cell Cycle
• Scientists have often wondered why cells are the size
that they are and why do they divide when they do.
• Studying these two ideas have led scientist to some basic
understandings about cells:
•
Cells have upper and lower size limits!
-If cells were too small, then they would not be able to fit all
of the necessary organelles and molecules into the cell.
-For example, a cell with too few mitochondria would not be
able to function.
-If cells get to big, than the ratio of surface area to volume
gets to far out of wack!
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Section 5.1: The Cell Cycle
• When a cell increases in size, the volume of that cell
increases at a much faster rate than the size.
• Because many substances like oxygen, nutrients, and waste
must enter and leave the cell, if the cell gets too big it will
expend more energy getting materials into and out of the
cell than the amount of ATP’s it can make.
• If a cell gets to big, it becomes to inefficient to survive and
will eventually die. Instead, it divides to put the surface
area to volume ration back in balance.
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Section 5.1: The Cell Cycle
• Generally speaking, cells divide for 2 reasons:
1. Keep surface area to volume ratio.
2. Repair and replace old cells.
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Section 5.2: Mitosis & Cytokinesis
• In order for this whole process of the cell cycle to
work, the genetic material must be duplicated.
• DNA is located on chromosomes, which are long
threads of DNA.
• The amount of DNA in just one of your cells is
about 10 feet long.
– So, How does it fit?
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Section 5.2: Mitosis & Cytokinesis
• DNA goes through many phases to get prepared
to divide.
• During interphase, DNA and the chromosome it is
on are loosely organized and it looks like
spaghetti!
• At this point, the genetic material is called
Chromatin.
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Section 5.2: Mitosis & Cytokinesis
• Chromatin is very unorganized and is tangled
together.
• Before mitosis and cell division, the DNA must
organize so it can get duplicated properly.
• Chromatin starts condensing and coiling around
proteins called Histones.
• These proteins organize DNA into specific
chromosomes.
• Remember, each of these chromosomes have
been duplicated in the S-stage, so there are two
of each.
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Section 5.2: Mitosis & Cytokinesis
• It would be impossible for the cell to separate
chromatin equally, so the genetic information
must organize.
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Section 5.2: Mitosis & Cytokinesis
• Remember, the chromosomes Telomere
have already been duplicated so
there are two of them and they
form an “X” shape.
• Each “leg” is called a chromatid.
• Each chromatid is held together
by a centromere.
• Each end of a chromatid is called a
Telomere.
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Section 5.2: Mitosis & Cytokinesis
• Telomeres do not code for genes!
– they simply keep the chromosome from unwrapping
or connecting with other chromosomes.
• Once a duplicated chromosome has organized,
mitosis and cytokinesis can occur.
• Mitosis has 4 phases and each can be indentified
by what the chromosomes are doing.
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Section 5.2: Mitosis & Cytokinesis
1. Prophase: chromatin chromosomes, nuclear
membrane breaks down, spindle fibers start to
appear, centrioles migrate to the poles.
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Section 5.2: Mitosis & Cytokinesis
2. Metaphase: shortest phase… spindle attaches to
centromere as duplicated chromosomes line up
on cell equator.
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Section 5.2: Mitosis & Cytokinesis
3. Anaphase: sister chromatids separate… spindle
shortens…chromosomes move to the poles.
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Section 5.2: Mitosis & Cytokinesis
4. Telophase: The opposite of prophase. Nuclear
membrane reforms, spindle disappears,
chromosomes uncoil.
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Section 5.2: Mitosis & Cytokinesis
• After telophase starts, cytokinesis
usually starts as well.
• Cytokinesis is the splitting of the
cell into two daughter cells.
• Cytokinesis, is an example of what
happens in animal cells.
• In plant cells the cell membrane
can not pinch in because of the
cell wall. Instead, a cell plate
forms between the two nuclei.
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Section 5.3: Cell Cycle Regulation
• Many factors affect the cell cycle. These factors
control the process of cell division.
– External Factors:
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Cell to cell contact:
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When cells come in contact with each other they stop dividing.
Cells release chemicals:
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This signals other cells to divide.
– Internal Factors:
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Enzymes called Cyclin-Dependent Kinases:
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Carry cells through many check points to make sure the cell gets
through the whole cell cycle.
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Section 5.3: Cell Cycle Regulation
•
When cell division is not
controlled, cancer is the end
product.
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Cancer is just uncontrolled
cell growth.
Cells usually grow and divide
until they come in contact
with another cell and then
stop.
If the division doesn’t stop,
tumors are formed.
Tumors are big piles of rapidly
dividing cells.
There are two types of
tumors
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Section 5.3: Cell Cycle Regulation
• There are two types of tumors:
1. Benign: cancer cells remain clustered together.
2. Malignant: cancer cells break away and spread to
other parts of the body (metastasize)
• So what makes cancer cells harmful?
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Section 5.3: Cell Cycle Regulation
• So what makes cancer cells harmful?
– Cancer cells do not carry out the functions that
certain type of cell is supposed to.
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For example, lung cells have to exchange oxygen and
carbon dioxide.
– Cancer cells in the lungs don’t do this, so while these
cancer cells are taking up space, they are not
functioning.
– Eventually the good cells die out and you are left with
all cancer cells.
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Section 5.4: Other Types of Cell Reproduction
• Not all cells reproduce by
mitosis!
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Prokaryotes no not because
they have no nucleus.
Most prokaryotes reproduce
asexually by a process called
binary fission.
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The cell simply duplicates its DNA
and divides into roughly two equal
cells.
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Section 5.4: Other Types of Cell Reproduction
• Some eukaryotes also reproduce asexually.
• Mitotic reproduction is common in simpler
plants and animals.
– Mitotic reproduction can take several forms:
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Budding- a small projection grows on the surface of the
parent organism, forming a separate new individual.
Fragmentation- a parent organism splits into pieces, each
of which can grow into a new organism.
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Section 5.5: Multicellular Life
• In multicellular organisms, cells communicate
and work together in groups that form complex
organisms.
Cells Tissues  Organs  Organ Systems  Organisms
• In multicellular organisms, cells go through the process
of cell differentiation.
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Section 5.5: Multicellular Life
• Cell Differentiation:
– Early cells of organisms are called stem cells.
– Stem Cells:
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These are cells that do not have a function
As the organism grows, the stem cells turn into all of the
cells your body needs.
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