Transcript Meiosis

MEIOSIS
IN 123 & 125
Vocabulary
Important Info
Headings
Mitosis:
•formation of somatic cells (body cells)
•At the end of mitosis each of the 2
daughter cells has 46 chromosomes
•These are your body cells; cells that make
up tissues, organs, etc.
Meiosis:
•Formation of sex cells called gametes.
•At the end of meiosis each of the 4
daughter cells has 23 chromosomes.
•They contain 1/2 the # of chromosomes
Meiosis
Gametes are special
cells used in sexual
reproduction.
 These cells contain
1/2 the normal # of
chromosomes than
the regular body
cells.
 When the sperm and
egg unite the zygote
will contain the
normal # of
chromosomes for
that organism

CELL
DIVISION
CELL TYPE
# OF
DAUGHTER
CELLS
# OF
CHROMOSOMES
Mitosis
Somatic
(body)
2
46
Diploid
Meiosis
Gamete
(sex)
4
23
Haploid
•Meiosis reduces the number of chromosomes to ½ the
# in body cells.
•This is called the haploid number (46  23)
•Reductive division is necessary so that when
reproduction occurs =
•Zygote contains the necessary 46 chromosomes.
Egg(23 chrom) + sperm(23 chrom) = zygote(46 chrom)
2 Divisions of Meiosis
There are 2
divisions of
meiosis:
•Meiosis I and
Meiosis II.
•Must be in order
to end up with:
•Each sex cells
containing:
= 23 chromosomes
Meiosis I
During the first meiotic division,
chromosomes are replicated.
1. Prophase I
• chromatin makes
a copy of itself
• begins to coil up
2. Metaphase I
•homologous pairs of
chromosomes line up
together at the
middle of the cell.
•This forms a tetrad
(meaning 4).
*Homologous chromosomes are
paired chromosomes.
*Each contains genes for the same
trait.
3. Anaphase I•Homologous chromosomes that form each
tetrad separate and move to opposite ends.
•The centromeres
(hold chromatids together) do not split.
4. Telophase I•Spindle fibers
disappears
•Cell divides by:
• Cytokinesis
(a cleavage forms)
Meiosis II
During the second meiotic division
NOTHING is replicated.
1. Prophase II• centriole
doubles
• spindle forms
2. Metaphase II•chromosomes line up at the middle
3. Anaphase II•Individual chromatids move to
opposite ends
4. Telophase II•2 daughter cells
divide into 4 new
daughter cells
•Each new daughter cell
has 23 chromosomes.
•This is ½ the # of the original parent
cell.
Where Does Meiosis Occur?

Testes of the male:
– Called
Spermatogenesis.
– 4 sperm are
produced.

Ovaries of the
female:
– Called Oogenesis.
– 4 cells are produced
• Only 1 becomes an
egg used for
reproduction!!!
• Other 3 dissolve
Meiosis I in Motion
I- chromatin  chromosome & nuclear membrane disappears
P1- centrioles move to opp. Sides of cell
M1- 4 chromosomes line up in middle (tetrad)
A1- chromosomes (intact) pulled to opp. Sides by spindle fibers
T1- cell spits forming 2 new cells
Meiosis II in Motion
P2- centrioles move to opp. Sides of each cell
M2- 2 chromosomes line up in middle
A2- chromatids pulled apart by spindle fibers
T2- cell spits forming 4 daughter cells that contain haploid set of
chromosomes