Transcript General Botany Lab

EKU General Botany Lab
Mitosis Review
Mitotic cell division begins
by the DNA molecules coiling and thickening
to begin to form recognizable chromosomes.
This process usually takes a long time.
Here is a nucleus that is not
actively dividing. Nuclei like this
are in interphase of the cell cycle.
Notice that the nucleoli and
the nuclear envelope (membrane)
are clearly visible.
These nuclei are in prophase
of mitosis. The nucleoli are
becoming indistinct and the
chromosomes are coiling and
shortening. This gives the
nucleus a granular
appearance.
Mitotic division continues
When the chromosomes move to the middle of the cell.
This phase of mitosis is called metaphase.
Then, every chromosome divides
lengthwise and the halves move
in opposite directions. This stage
is called anaphase.
Finally, the chromosomes
reach the ends . . .
of groups of microtubules called the spindle. When they have reached that point, we say
that the cell is in telophase of mitosis.
During telophase, if the rest of the cell is going to
divide, a new cell wall forms, and the “daughter”
nuclei gradually
return to interphase.
These (this slide and
the previous two slides)
are the main events of
mitotic cell division.
Stages of the cell cycle
with some details
•
•
•
•
•
interphase
prophase
metaphase
anaphase
telophase
Interphase
Is the part of a cell’s life when it is not dividing.
DURING INTERPHASE,
The cell “hums” with normal
metabolic activity.
DNA molecules are fully
extended, making RNA
molecules
Nucleoli are plainly visible.
At some point during interphase,
the single DNA molecules each
make an entire lengthwise copy
of themselves. Each new DNA
molecule remains attached to
the DNA molecule that
produced it.
These are plant cells. Notice the cell walls.
Prophase
is the stage where the nucleus is preparing to divide. A lot happens,
but it all happens gradually.
DURING PROPHASE,
The DNA molecules and their
associated protein molecules
shorten, thicken, and become
recognizable as double-strand
chromosomes.
The nucleolus (sometimes there’s
more than one) becomes indistinct
and disappears.
The nuclear envelope (double
membrane) disappears.
A new organelle, the spindle,
forms. The spindle is constructed
from microtubules and microfibrils.
The spindle is the structure that will actually divide the nucleus, and make sure each new nucleus
receives exactly the right number of chromosomes.
The chromosomes move to the center of the spindle and attach to spindle microtubules.
When the centromeres attach to the
spindle, the cell is in the stage we call
metaphase.
Animal or plant?
Animal or plant?
Then, the centromeres of each chromosome divide.
When that happens, anaphase, the next stage, has begun.
Anaphase begins when the centromeres
divide
When the centromeres divide, it means each of the
chromatids (= long halves of a chromosome) is now
separate from its duplicate copy. The two chromatids
of each chromosome move in opposite directions
along the spindle.
All of the other chromosomes are doing the same
thing – one chromatid goes in one direction, and
the other chromatid goes in the opposite direction.
Eventually, all of the chromatids reach the ends of
the spindle and stop moving. (The ends of the
spindle are sometimes called the “poles” of the
spindle.)
When the chromosomes reach the end of the
spindle, telophase has begun.
(Notice the spindle fibers between the two groups
of anaphase chromosomes, inside the red circle.)
Telophase
is the final phase of the division of a nucleus
Telophase is basically the reverse of prophase.
The chromosomes become indistinct and gradually
disappear, as the DNA molecules become extended
once more. The nuclear envelope and nucleolus
reappear. Normal cell metabolism begins to occur,
and the “daughter cells” slip into interphase.
Usually, cytokinesis (= division of the cytoplasm)
occurs during telophase.
In animals, microfibrils contract inside the cell
membrane to pinch the cell in half.
In plants, the endomembrane system secretes
non-living material in a plane through the middle
of a cell, dividing the cytoplasm approximately in
half.
During cytokinesis, other cell organelles (other than
the nucleus) are not necessarily divided equally
into the new daughter cells.
Here are some views of cytokinesis.
In animals, the structure that
forms the two new cells is
called a cleavage furrow.
In plants, the structure
that forms the two new
cells is called a phragmoplast,
or cell plate. It looks like a line
across the middle of the spindle.
Summary of mitosis
Mitosis results -2 cells
with same number of chromosomes as parent cell
genetically identical to parent cell
~1/2 the volume of the parent cell
Mitosis significance –
It is not part of the sexual process
It is how multicellular organisms grow
Please note: these single-strand
chromosomes will make copies
of themselves during interphase.