Transcript Meiosis - TeacherWeb



Requires two parent
cells from
◦ two separate
organisms OR
◦ two sexually
different parts of a
single organism
Produces offspring
that are genetically
different from either
parent
 To
make sex cells (gametes)
 Two
haploid (1n) gametes are
brought together through
fertilization to form a diploid
(2n) zygote (fertilized egg)
2n = 6
1n =3
 Is
a zygote an
example of a
haploid or
diploid cell?
◦ Diploid

Before Meiosis
occurs (similar to
mitosis), DNA must
Occurs in
be replicated
Interphase

Occurs prior to
division
 When
does the
replication of
the
chromosomes
take place?
◦ During
Interphase (sphase)
Gene X
Homologous
chromosomes
(a pair of
chromosomes that
carry alleles for
the same traits)
Sister
Chromatids
Diploid (2n): cells that have both
sets of homologous chromosomes
 What
are
homologous
chromosomes?
- a pair of
chromosomes that
carry alleles for
the same traits
from mom
from dad
child
too
much!
meiosis reduces
genetic content
The right
number!


Meiosis reduces the chromosome number by half
Fertilization then restores the 2n (diploid) number
Homologs
separate
Meiosis
I
Sister
chromatids
separate
Meiosis
II
Diploid
Haploid
Spindle
fibers
Nucleus
Early
Prophase I
(diploid)
Late
Prophase
I
Nuclear
envelope
Metaphase
Anaphase Telophase I
I
I
Early prophase I Late prophase I
•Homologous
chromosomes pair
forming tetrads.
•Crossing over occurs.
•Chromosomes condense.
•Spindle forms.
•Nuclear envelope breaks
down.
Homologous chromosomes
(each with sister
chromatids)
Join to form a
TETRAD
Called Synapsis
 How
many
chromatids
are in a
tetrad?
-
4



Homologous
chromosomes in a
tetrad cross over
each other
Pieces of
chromosomes or
genes (alleles) are
exchanged
Produces genetic
diversity in the
offspring by
creating new
combinations of
alleles.
Crossing-over produces new
combinations of alleles through
independent assortment
 When
does
Crossing over
happen?
During
Meiosis I
-
Homologous pairs
of chromosomes
align along the
equator of the
cell
Homologous chromosomes
separate and move to
opposite poles.
Sister chromatids remain
attached at their
centromeres.
The nuclear envelopes
reassemble.
The spindle disappears.
Cytokinesis divides cell
into two.
 Meiosis
I is
also called
________
_________?
Reduction
Division
-
Gene X
Only one homolog of each
chromosome is present
in the cell.
Sister chromatids carry
identical genetic
information.
Meiosis II produces gametes with
one copy of each chromosome and
thus one copy of each gene.
Prophase Metaphase
II
II
Anaphase
Telophase
II
4 genetically
II
different
haploid cells
The nuclear
envelope breaks
down.
The spindle forms.
Chromosomes align
along the equator of
the cell.
Equator
Pole
Sister chromatids
separate and
move to opposite
poles.
The nuclear envelope
assembles.
The chromosomes
decondense.
The spindle
disappears.
Cytokinesis divides
cell into two.
Gametes (egg & sperm)
form
Four haploid cells with
one copy of each
chromosome
One allele (variation) of
each gene
Compare mitosis with
meiosis- what do
you notice is
similar?
The stages have the
same names
Oogenesis
or
Spermatogenesis




Occurs in the testes
Two divisions produce
4 spermatids
Spermatids mature
into sperm
Men produce about
250,000,000 sperm
per day
Primary Spermatocyte (diploid)
Secondary
spermatocyte (haploid)
Spermatids
(haploid)





Occurs in the ovaries
Two divisions produce 3 polar bodies that
die, and 1 egg
Polar bodies die because of unequal division
of cytoplasm
An immature egg is called an oocyte
Starting at puberty, one oocyte matures into
an ovum (egg) approximately every 28 days
Primary Oocyte (diploid)
Primary Polar Body
(haploid)
Secondary
Oocyte
(haploid)
ootid
Secondary
oocyte
Are the cells
that result
from Meiosis
II diploid or
haploid?
They are haploid
Mitosis
Meiosis
2
Number of
divisions
1
Number of
daughter cells
2
4
Yes
No
Chromosome #
Same as parent
Half of parent
Where
Somatic (body)
cells
Sex (gamete) cells
When
Throughout life
At sexual maturity
Growth and
repair
Sexual reproduction
Genetically
identical?
Role
http://www.bbc.co.uk/schools/gcsebitesi
ze/science/add_edexcel/cells/mitosisac
t.shtml
http://www.youtube.com/watch?v=iCL6d
0OwKt8