Transcript Meiosis - greenebio

Meiosis
 Meiosis occurs in sexual reproduction when a
diploid germ cell produces four haploid
daughter cells that can mature to become
gametes (sperm or egg).
Meiosis
Goal: reduce genetic material by half
Why? n (mom) + n (dad) = 2n (offspring)
from mom
from dad
child
too
much!
meiosis reduces
genetic content
Just
right!
Meiosis: cell division in two parts
Sister
chromatids
separate
Chromatids
separate
Meiosis I
(reduction
division)
Meiosis II
(equational
division)
Diploid 2n
Haploid n
Haploid n
Result: one copy of each chromosome in a gamete.
Meiosis I : the reduction division
Spindle
fibers
Nucleus
Nuclear
envelope
Prophase I
(early)
(diploid)
Prophase I
(late)
(diploid)
Metaphase I
(diploid)
Anaphase I
(diploid)
Telophase I
(diploid)
1st Period begins
 http://highered.mcgrawhill.com/sites/0072495855/student_view0/
chapter28/animation__how_meiosis_work
s.html
Prophase I
Early prophase
Late prophase
Chromatids pair.
Crossing over occurs.
Chromosomes condense.
Spindle forms.
Nuclear envelope fragments.
Metaphase I
Chromatid homologous
pairs align
along the equator of the
cell. This is random and
results in
Genetic variation
Homologous – a
chromosome with the same
gene sequence as another,
one is paternal and the other
is maternal
Metaphase I
Homologous chromosomes
cross over and genes may be
exchanged.
Crossing over results in
genetic variation.
Anaphase I
Chromosomes separate and
move to opposite poles.
Sister chromatids remain
Attached at their centromeres.
Telophase I
Nuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into two.
Meiosis II
Gene X
Sister chromatids carry
identical genetic
information.
Meiosis II produces gametes with
one copy of each chromosome and thus one copy of
each gene.
Meiosis II : the equational division
Prophase II
(haploid)
Metaphase II
(haploid)
Anaphase II
(haploid)
Telophase II
(haploid)
Four
Non-identical
haploid
daughter cells
Prophase II
Nuclear envelope fragments.
Spindle forms.
Metaphase II
Chromosomes align
along equator of cell.
Anaphase II
Sister chromatids separate
and move to opposite poles.
Telophase II
Nuclear envelope
assembles.
Chromosomes unravel.
Spindle disappears.
Cytokinesis divides cell into
two.
Results of meiosis
• Four haploid cells
• One copy of each
chromosome
“Putting It All Together” - Fertilization
What Meiosis is About
Meiosis allows the creation of unique individuals
through sexual reproduction.
http://highered.mcgrawhill.com/sites/0072495855/student_v
iew0/chapter28/animation__unique_
features_of_meiosis.html
Nondisjunction – When chromosomes
do not separate during meiosis.
Chromosomal Mutation Extra or less
chromosomeshttp://glencoe.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::5
50::400::/sites/dl/free/0078695104/383925/
Chapter11_NGS_VisualizingNondisjunction
_10_10_06.swf::Visualizing%20Nondisjunc
tion
1st Period Begins Here
Mitosis
Meiosis
Number of divisions
1
2
Number of daughter cells
2
4
Yes
No
Same as parent
Half of parent
Where
Somatic cells
Sex cells
When
Throughout life
At sexual maturity
Growth and repair
Sexual reproduction
Genetically identical?
Chromosome #
Role
MITOSIS
MEIOSIS
Occurs in somatic (body) cells
Occurs only in gonads (sex organs:
ovary/testes)
Produces cells for repair,
maintenance, growth, asexual
reproduction
Only produces gametes (sex cells:
egg/sperm)
Results in identical diploid (2n) daughter Reduction division results in haploid (n)
cells
cells
 2 Factors
 What causes genetic variations? Why are
no to individuals alike?
 2 Factors
 What causes genetic variations? Why are
no to individuals alike?
Animation: Crossing over, independent assortment.
What causes genetic diversity?
1. CROSSING OVER. –TETRADS
What causes genetic diversity?
1. CROSSING OVER. –TETRADS
2. RANDOM ASSORTMENT OF
CHROMOSOMES. - MEIOSIS
3. GENE MUTATION
4. NONDISJUNCTION - FAILURE OF
CHROMOSOMES TO SEPARATE - - - 2N +1,
2N - 1
5. FERTILIZATION – WHICH EGG AND SPERM
JOIN FORM ZYGOTE?
What causes genetic diversity?
2. RANDOM ASSORTMENT OF
CHROMOSOMES. - MEIOSIS
What causes genetic diversity?
3. GENE MUTATION
What causes genetic diversity?
4. NONDISJUNCTION - FAILURE OF
CHROMOSOMES TO SEPARATE - - - 2N +1,
2N - 1
What causes genetic diversity?
5. FERTILIZATION – WHICH EGG AND SPERM
JOIN FORM ZYGOTE?
 What is the difference between a gene
mutation and a chromosomal
mutation?
 Gene mutations are usually with the
nitrogen base, chromosomal mutations
involve hundred’s of genes.
What is non-disjunction?
 Failure of chromosomes to separate
during meiosis.
Trisomy 21
Down’s Syndrome
When does crossing – over occur?
 Prophase I of Meiosis
 Is it meiosis or mitosis
 Power Point – Game- White Boards – Get
a 2 sheets of paper – Write the word
Mitosis (Write Big) on one and Meiosis on
the other.