Mitosis and Meiosis
Biology – Mr. T. Miller
Mitosis
• The division of the nucleus that forms
new somatic cells.
Somatic Cells
• Normal body cells (they have the whole
number of chromosomes).
• Normal humans have 46 chromosomes
in their somatic cells.
Chromosomes
• DNA is organized into collections of
genes called chromosomes
• Gene – a piece of DNA that controls a
trait.
• Normally chromosomes look like
unwound pieces of thread.
• When dividing the DNA condenses into
small, compact structures.
The Cell Cycle
The Cell Cycle
• The G1 Phase – (Growth Phase) The
cytoplasm increases and organelles
multiply.
• The S Phase – (Synthesis Phase) The cell
copies the DNA for future division.
• The G2 Phase – (Pre-Mitosis Phase) The
cell continues to grow and make the
materials that will make the spindle.
• The M Phase (Mitotic Phase) – The
nucleus and cell divide.
Interphase
• Cells spend the
most time here (G1,
S and G2 phases)
• Not dividing, but are
getting ready to.
• Chromosomes and
centrioles replicate.
• Chromosomes are
unwound.
Prophase
• Chromosomes
condense
• Nuclear envelope
breaks down
• Spindle begins to
form from centrioles
• Centrioles move
toward poles
Metaphase
• Nuclear envelope is
gone
• Centrioles are at the
poles
• Chromosomes are
on the equator
• Spindles connect
centrioles to
chromosomes
Anaphase
• Cleavage furrow
forms, beginning
cytokinesis
(cytoplasm division)
• Centromeres divide,
chromosomes are
single stranded
• Chromosomes move
toward poles
Telophase
• Chromosomes at
poles
• Cleavage furrow
deepens
• Nuclear envelope
begins to reform
Daughter Cells
• Chromosomes
unwind
• Cells re-enter
interphase
• Only 1 centriole at
first
Apoptosis
• Programmed cell death.
• What happens if there is an error in
apoptosis?
• Mitosis does not happen non-stop.
• What happens if there are errors in the
genes that control mitosis?
• What is it called when there are errors
in both mitosis and apoptosis?
• Cancer
Meiosis
• Division of the nucleus that makes new
gametes.
• Gametes are reproductive or sex cells.
Interphase
Prophase I
Detail of Prophase I Cont’d
• Crossing over happens when
homologous chromosomes “trade”
genes.
• This increases genetic diversity.
Metaphase I
Anaphase I
Telophase I
Daughter Cells I
Prophase II
Metaphase II
Anaphase II
Telophase II
Daughter Cells II
Daughter Cells of Meiosis
• We start with parent cells with the full
number of chromosomes (diploid) and
get daughter cells with half the number
(haploid).
• Four daughter cells are formed,
however, in males you get 4 useable
sperm cells, in females you get one
useable egg and 3 polar bodies that are
re-absorbed by the body (egg gets
most of the cytoplasm).
Errors in Meiosis
• When errors take place in meiosis, the
gametes do not end up with the right
number of chromosomes.
• Usually, this results in cells that can
not be used in reproduction.
• Occasionally error containing cells do
succeed in reproduction, but disorders
such as Down Syndrome result.