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Cell Division
Mitosis and Meiosis
Mitosis
• Asexual
reproduction
• Purpose:
Growth,
maintenance,
and repair
Cell Cycle
1. Interphase
Cells are in interphase
90% of the time.
–
–
–
G1- growth
S - chromosome
replication
G2- Prepare for mitosis
2. Mitosis (nuclear division)
3. Cytokinesis (cytoplasmic
division)
Interphase
(pre game show)
• Chromosomes are copied (# doubles)
• Chromosomes appear as threadlike coils
(chromatin) at the start, but each
chromosome and its copy (sister
chromosome) change to sister chromatids
at end of this phase
Synthesis Phase (S phase)
• S phase during interphase of the cell
cycle
• Nucleus of eukaryotes
S
DNA replication takes
place in the S phase.
phase
G1
interphase
G2
Mitosis
-prophase
-metaphase
-anaphase
-telophase
6
DNA Replication Facts
• DNA has to be copied before a cell
divides in order to make new cells
• New cells will need identical DNA
strands
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How do cells replicate their DNA?
1. The DNA double
helix is
unwound by
an enzyme.
2.
Another enzyme moves down the 2 original
DNA strands and
adds complementary nucleotides.
Base Pairing Rule
3. Two separate
DNA molecules
are formed, each
with
one new
strand and one
old strand.
Practice
DNA Replication
Replication Mistakes
• 6 billion base pairs per cell.
• All are replicated in a few
hours.
• Mistake happens one per
billion nucleotides.
Mistakes = Mutations
• Mutations are changes in the
base sequence of the DNA
molecules.
• This can have serious affects on
genes and the functions of cells.
• Cancer is caused by mutations in
DNA.
Mutagens
• Physical or chemical factor
that increases the frequency of
mutations.
• Examples: UV rays, X-rays,
smoking
Overview of Mitosis
Parent Cell=2n
(diploid)
Daughter cell= 2n
Daughter cell= 2n
(diploid)
(diploid)
PROPHASE
CENTRIOLES APPEAR AND MOVE
TO OPPOSITE ENDS OF THE CELL
Spindle fibers form between centrioles
Centriole
Spindle
Fibers
METAPHASE
• Chromosomes “meet” in the middle.
• Spindle fibers attach to the
centromere of each chromosome.
ANAPHASE
• Sister chromatids separate from
each other and are “pulled” to
opposite ends of the cell.
Telophase
• Cell plate forms and cell membrane begins
pinching in= “cleavage.”
• Chromosomes look like threads
(chromatin) again. Nuclear envelope forms
again.
Cytokinesis
• Division of the cytoplasm
• Cell membrane completely
pinches off.
• Cycle starts again.
Results:
2 diploid
(2n)
daughter
cells,
genetically
identical to
parent cell
Meiosis
Meiosis
Type: Sexual
Reproduction
Purpose: To
create haploid
gametes
Meiosis
• Types of cells that undergo
meiosis: Gametes (sex cells)
– Egg and sperm
Meiosis 1
First division of meiosis
• Prophase 1: Each chromosome dupicates and
remains closely associated. These are called sister
chromatids. Crossing-over can occur during the
latter part of this stage.
• Metaphase 1: Homologous chromosomes align at
the equatorial plate.
• Anaphase 1: Homologous pairs separate with
sister chromatids remaining together.
• Telophase 1: Two daughter cells are formed with
each daughter containing only one chromosome of
the homologous pair.
Meiosis II
Second division of meiosis: Gamete formation
• Prophase 2: DNA does not replicate.
• Metaphase 2: Chromosomes align at the
equatorial plate.
• Anaphase 2: Centromeres divide and sister
chromatids migrate separately to each pole.
• Telophase 2: Cell division is complete.
Four haploid daughter cells are obtained.
Meiosis: Overview
Parent Cell:
diploid= 2n
After Meiosis I
After
Meiosis II
Daughter Cells:
Haploid= n ; and all genetically different from each other and parent cell
???
Why would it be important
for the gametes to
contain half as many
chromosomes as a
somatic cell?
Comparing
Mitosis and Meiosis
Mitosis vs. meiosis
Meiosis – key differences from
mitosis
•
•
•
•
Meiosis reduces the number of chromosomes by half.
Daughter cells differ from parent, and each other.
Meiosis involves two divisions, Mitosis only one.
Meiosis I involves:
– Synapsis – homologous chromosomes pair up.
– Crossing over of non-sister chromatids occur.
– In Metaphase I, homologous pairs line up at metaphase
plate.
– In Anaphase I, sister chromatids do NOT separate.
– Overall, separation of homologous pairs of chromosomes,
rather than sister chromatids of individual chromosome.
Exit Ticket
• Name 3 key differences
between mitosis and meiosis.