Transcript Cell Division - St. Clairsville High School

Cell Division
Why do cells divide?
Cells must divide in order for
the surface area (cell
membrane) to keep up with
the volume of the cell.
Cell Cycle
Stage 1 : Interphase
Stage 2: Mitosis
Stage 3: Cytokinesis
Stage 1: Interphase
The period before cell division occurs.
The cell grows to mature size and
produces copies of organelles.
DNA makes a copy of itself (DNA
replication.)
Cell stores energy and produces
structures for division.
Stage 2: Mitosis
The cell’s nucleus divides into two new
nuclei. One copy of the DNA goes to
each new daughter cell.
There are four phases: prophase,
metaphase, anaphase, and telophase.
Prophase
The chromatin in the cell’s nucleus begins to get thicker and are now
called chromosomes. Each identical rod, or strand, of the chromosome
is called a chromatid. The two strands are held together by a structure
called a centromere.
Metaphase
The chromosomes attach to the spindle fibers and line up
in the middle of the cell.
Anaphase
The chromosomes get pulled by the spindle fibers to
each side of the cell.
Telophase
The two new nuclei of the daughter cells form, and a
nuclear membrane appears around both.
Stage 3: Cytokinesis
During cytokinesis, the cytoplasm divides, distributing the
organelles into each of the two new cells. The animal cell’s
membrane pinches in, and the plant cell forms a cell plate.
Each daughter cell has the same number of chromosomes as
the original parent cell.
DNA Replication- A cell
makes a copy of its DNA
before mitosis begins.
In 1953, two scientists, James
Watson and Francis Crick,
figured out the structure of the
DNA molecule.
The DNA molecule looks like a
twisted ladder called a “double
helix”.
The sides are made of sugar
and phosphate.
The rungs are made of
nitrogen bases: adenine,
thymine, guanine, and
cytosine.
These bases always pair
together:
A-T
C-G
The length of the cell cycle
varies from cell to cell.
Most of the cell’s life is spent in
interphase.
Some cells, such as the human brain
cells, never divide.
DNA Replication
starts when the two sides of the
DNA molecule unwind or “unzip”
at the hydrogen bond holding the
nitrogen bases together. Next
free floating nucleotides (sugar,
phosphate, and a nitrogen base)
attach to their complimentary
bases. A always pairs with T,
while G always pairs with C.
http://www.pbs.org/wgbh/aso/tryit/dna/
DNA replication is needed so
that the DNA and
chromosomes in the
daughter cells are exactly
like the parent cell.