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Unit Chunk 2A: Mitosis, the Cell Cycle, Gene Expression & Cell Differentiation
Day One: The Cell Cycle - Interphase
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Cells do not continue to grow bigger as organisms do.
Cells continue to produce more cells to foster the growth of an organism.
Cell division is how eukaryotic cells reproduce to make new cells.
Mitotic cell division—MITOSIS—produces two new clone daughter eukaryotic cells
with the same DNA as the original parent cell.
Eukaryotes divide via the Cell Cycle.
• There are two Stages for the Cell Cycle:
1) Interphase – G1, S, and G2
2) Mitosis
EQ: Describe the Interphase Stage of the Cell Cycle.
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First Stage – Interphase – G1, S, & G2
Growth 1 (G1) - the phase in which a cell spends most of its time growing,
synthesizing proteins, and performing its job for the organism
Towards the very end of this phase, the cell gets ready to divide by reproducing its
organelles.
Synthesis (S) – DNA Replication: in this phase the cell makes an exact duplicate
of DNA in its nucleus. The chromatin now appears as a dense mass in the nucleus.
Growth 2 (G2) – a cell checkpoint occurs; if there are no errors in the replicated
DNA and the cell is ready, “GO” proteins signal the cell to proceed into mitosis. If
DNA errors are detected or the cell is not ready, “Stop” proteins signal the cell to
NOT proceed with mitotic division and to make repairs.
G2 – If the cell cannot repair the damaged DNA, proteins will signal the cell to
undergo programmed death – Apoptosis. If the cell does not die, uncontrolled cell
growth may lead to cancer. If a “Stop” or “Go” protein gets damaged, this may also
result in uncontrolled cell division or cancer.
Day Two: The Cell Cycle - Mitosis
EQ: Describe the Second Stage of the Cell Cycle--Mitosis.
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Second Stage – Mitosis – M
Cell’s nucleus divides into 2 new nuclei
Cell divides to form two new cells
Chromatin condenses to form distinct X chromosomes and the nuclear membrane
breaks down
X chromosomes line up in the middle of cell
X chromosomes separate and move to opposite ends of the cell.
New nuclear membrane forms around each of the two new sets of chromosomes.
Chromosomes unwind to become chromatin
Cytoplasm divides to become two new daughter cells with same DNA as the parent
EQ: What three functions does Mitosis perform for a multicellular organism?
Mitosis produces:
New cells for the growth of a multicellular organism
New cells to replace dead cells to maintain a multicellular organism
New cells that can differentiate to create different types of cells that do different jobs
for a multicellular organism
Day Three: Gene Expression & Cell Differentiation
In the development of most multicellular organisms, a single cell--a fertilized egg
called a zygote—reproduces to make identical cells called embryonic stem cells.
Each stem cell has the potential to become a specialized body cell, with a different
structure for a specific function.
A zygote produces a large number of embryonic cells through mitosis.
Mitosis results in two new daughter cells with the same DNA--two clone cells.
So the process of cell division alone only leads to increasing numbers of identical
cells.
* Through gene expression, embryonic cells express, activate, or repress, do not
activate, specific groups of genes. This, in turn, controls what proteins each cell will make.
It also controls what proteins a cell will NOT make.
* This causes cell differentiation, the production of cells that are different in structure and
function. These embryonic cells have made different specialized cells --BUT ALL THESE
CELLS STILL HAVE THE SAME DNA!!!
EQ: HOW does gene expression regulate cell differentiation???
Gene expression regulates cell differentiation by determining what groups of genes are
expressed or repressed in a cell, and this controls what proteins are made or not made for
that cell. This makes cells different and specialized in both their structure and function-cell differentiation.
Day Four: Gene Expression & Cell Differentiation
Cell Development and Differentiation in Humans
1) Sperm and egg--the fusion of two different sets of DNA; 23 chromosomes in each
2) The product: one fertilized cell, a zygote with 46 chromosomes
3) Zygote divides into two clone stem cells, which divide into four identical stem cells,
which divide into eight stem clones and so forth.
4) Cells on the outside, inside, and middle initiate gene expression and begin to
differentiate.
5) Cell reproduction and differentiation continues, and the specialized cells begin to
organize into groups that will form the tissues, organs, and organ systems of the most
complex multicellular organism—a human being.
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Embryonic cells, which have not yet differentiated into various cell types, are called
Embryonic Stem Cells - ESCs.
Stem cells found in adult organisms, for instance in bone marrow, are called Adult
Stem Cells - ASCs.
All these cells--zygote, stem cells, and all the specialized cells--have the
SAME genes with the same DNA!
But the genes in these cells express themselves differently making different
cells with different structures and functions, or cell differentiation.
EQ: Distinguish between a stem cell and a specialized differentiated cell like a
blood cell or neuron.
Stem cells are unspecialized cells that can reproduce to make more clone stem
cells, and, they also have the ability to differentiate into one or more types of
specialized body cells through gene expression.
Specialized body cells that have differentiated, like a blood cell or a neuron, can
only reproduce clone cells like themselves. Once a cell differentiates, the
process cannot be reversed!
The process of cell differentiation is the basis for the hierarchical
organization of multicellular organisms and their cells, tissues, organs, and
organ systems.
EQ: Develop and label a model to illustrate the role of mitosis and cell differentiation
in producing and maintaining complex multicellular organisms.