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Meiosis
Remember these key points…
 How many sets of genes are in each
organism?
2 sets - one from the mom, one
from the dad
A cell with 2 sets of genes is said to be
DIPLOID
If the fertilized egg has 2
copies of the genes, how many
copies are in 1 sperm cell or
one egg cell?
 One copy of the genes in the egg cell.
One copy of the genes in the sperm
cell.
 These cells are called HAPLOID
 They have only one set of genes
How does an organism create
haploid cells?
 Glad you asked!
 This happens through a process called
MEIOSIS
 Meiosis - the process of cell division in
which the number of chromosomes per
cell is cut in half
The Phases of Meiosis
 Before Meiosis I begins the chromosomes are
replicated
 In Prophase I the nuclear envelope breaks
down and HOMOLOGOUS chromosomes
(chromosomes that look the same) pair up to
form TETRADS.
The Phases of Meiosis
 Prophase I continued –
 Portions of the tetrads get
exchanged in a process called
CROSSING OVER. This makes
new combinations of alleles so
the new cells will be genetically
different from the original cell.
The Phases of Meiosis
 Metaphase I –
 Microtubules grow from the
centrioles and connect to the
centromeres
 Tetrads line up along the center
of the cell
The Phases of Meiosis
 Anaphase I –
 The chromosome pairs are split
apart (the sister chromatids are
still together)
 Each cell now has the haploid
number of replicated
chromosomes (2N)
The Phases of Meiosis
 Telophase I –
 The chromosomes decondense
in some species
 Nuclear membrane reforms
The Phases of Meiosis
 Cytokinesis I –
 Two new cells are formed
 Each cell has only one
replicated copy of each gene
and each is genetically different
from the mother cell
The Phases of Meiosis
 Prophase II –
 Nuclear membrane dissolves
 Centrioles replicate
The Phases of Meiosis
 Metaphase II –
 Sister chromatids line up in the
center of the cell
 Spindle fibers attach to the
centromeres
The Phases of Meiosis
 Anaphase II –
 Sister chromatids are pulled
apart
The Phases of Meiosis
 Telophase II and cytokinesis II –
 Four new haploid daughter cells
are formed
Meiosis
 Why do it?
 creates cells that are haploid (N) which
allows reproduction to create a baby with
the correct number of genes
 Creates cells that are genetically unique
which gives the baby a new set of traits,
that might make it better able to survive.
The four cells produced by
Meiosis II are called GAMETES
 In a male, the gametes produced are
called sperm
 In a female, only one of the four cells
created becomes the egg. The other
three cells are called polar bodies.
Let’s think…
 What are some ways meiosis is different
from mitosis? (Other than their names!)
Comparing Mitosis and Meiosis
 Place each of the following words or
phrases in one section of your Venn
Diagram:
Makes 2 cells
Makes 4 cells
Genetically different cells
Genetically identical cells
Makes haploid cells
Makes diploid cells
A process of cell division
Produces gametes
Produces normal body cells
Cells produced are used for growth
Cells produced are used for reproduction