Chapter 8
Section 2: Cell Division
• About 2 trillion cells are produced in by an
adult human body each day by cell division
• Cell division happens differently in
prokaryotes and eukaryotes because
prokaryotes have cell walls and lack nuclei and
other membrane-bound organelles
• Prokaryotes also only have one circular
chromosome attached to the plasma
membrane
Cell Division in Prokaryotes
• Binary fission is the division of a prokaryotic cell
into two offspring cells
• DNA is copied, which results in two genetically
identical chromosomes
• The cell then grows until it reaches twice the size
and the cell membrane begins to push inward,
constricting the cell in the middle
• A new cell wall then forms around the new cell
membrane until the two cells split completely
into two new cells
http://www.youtube.com/watch?v=3cD3U2pgb5w
Cell Division in Eukaryotes
• In eukaryotes, BOTH the cytoplasm AND the
nucleus have to divide
• MITOSIS is cell division that results in two new
genetically identical cells
• This occurs in organisms that are undergoing
growth, development, repair or asexual
reproduction
• MEIOSIS is cell division that results in haploid
cells that are genetically different; this creates
gametes
The Cell Cycle
• The cell cycle is just a repeating set of events
in the life of a cell
• Cell division is only one phase of the cell cycle
• The time in between each cell division is called
interphase
• Interphase is divided into three phases: G1, S
and G2
• Cell division is divided into two phases:
mitosis and cytokinesis
Interphase
• Cells spend the majority of their time in
interphase
• Following cell division, the new cells are about
half the size of the original cell, so in G1 phase,
the new cells grow
• After cells reach their mature size, they have to
copy their DNA: this happens in the S phase (for
synthesis)
• Once the DNA is copied, cells prepare for cell
division in the G2 phase
MITOSIS
• Mitosis happens in 4 stages:
– Prophase: DNA tightens into chromosomes
– Metaphase: Chromosomes line up in the middle
of the cell and fibers attach to centromeres
– Anaphase: Chromatids of each chromosome pair
separate at the centromere and move to opposite
ends of the cell
– Telophase: Chromosomes reach each end of the
cell, fibers dissemble, and chromosomes loosen to
chromatin
Control of Cell Division
• The cell spends most of its life in Interphase, which has
a couple of “check points”
• These “check points” allow the cell cycle to stop or go
depending on whether the cell passes some “tests”
– G1 checkpoint: If the cell is healthy and the right size,
proteins at this checkpoint will allow it to go to the S phase
– G2 checkpoint: DNA repair enzymes check DNA replication
and make sure that there is no damage; proteins will allow
it to go to mitosis
– Mitosis checkpoint: Proteins will signal the end of mitosis if
everything has passed and the cell will enter G1 phase
What Happens if Control is Lost?
• The proteins that make sure that cells grow
and divide properly are coded for in genes
• Sometimes cell growth and division is
disrupted if the proteins do not work properly
• If this happens, it could lead to cancer, which
is the uncontrolled growth of cells
• Cells could also overproduce growthpromoting molecules, which can also lead to
an increase in cell division
Cancerous Cells
Think About It
• What would happen if cytokinesis occurred
before mitosis?
• What would happen if chromosomes did NOT
replicated during interphase?
http://www.youtube.com/watch?v=DD3IQknCEdc