Download 11.3 Notes

Ch.11.4 - Meiosis
Formation of
Gametes
(egg & sperm)
Our Chromosomes
• 46 Chromosomes (23 pairs)
• #1-22 Pairs are Autosomes (body cell chromosomes):
• Determine all traits except gender
• #23 pair are Sex chromosomes
referred to as X & Y
• male (XY)
• female (XX)
X - Chromosome
Y-Chromosome
Chromosome Numbers
Vary in organisms
A closer look at Chromosome
Pairs
Karyotype
Picture of chromosomes
arranged by size
Pairs 1-22 = autosomes
Pair 23 = sex chromo
Detects abnormalities &
XX female or XY male
Homologous Chromosomes (Buddy-Buddy)
• Matching pair = homologous chromosomes
• Homologous chromosomes: 2 chromosomes (one from
mom and one from dad) that are alike in:
• size,
• location of centromere,
• dark/light banding pattern of genes
Remember: only non matching pair are sex
chromosomes
Homologous Chromosome Pairs
Mom’s Blue
eye gene
Dad’s Brown
eye gene
*Allele – different
versions of the
same gene (traits)*
• If a cell has all 46 chromo (23 pairs), it’s called
a diploid cell
• Shorthand: 2N
Homologous Pair 
Somatic Cell
• Body Cell
• Skin cell, etc.
Gamete
• Sex Cell
• Egg/sperm
• Mitosis
• Meiosis
• 2 sets of DNA
• 46 total chromo
• 1 set from each parent
• 1 set of DNA
• 23 total chromo
• ½ set from each parent
• DIPLOID (2N)
• HAPLOID (N)
Important Vocab
• Somatic Cell vs. Gamete
• Body cell vs. Sex cell
• Diploid (2N) vs. Haploid (1N)
• 2 sets of DNA vs. 1 set of DNA
• Homologous Chromosomes
• Matching pairs of chromo in 2N cell
• Alleles
• Different version of the same trait
• Fertilization
• When sperm meets egg and combines DNA
• Zygote
• Cell in growth state following fertilization
• Tetrad
• Structure containing 4 chromatids
Meiosis Info…
Similar but different from Mitosis:
1. Sex cell division only
2. Involves 2 cell divisions
3. Results in 4 cells with half the normal
genetic info
• Produces gametes (egg/sperm)
• Male Testes (spermatogenesis)
• Female Ovaries (oogenesis)
Why Do we Need Meiosis?
Basis of sexual reproduction
• Accounts for individual genetic diversity
• You are unique! You look a little bit like your mom and a
little like your dad!
Two haploid (1n) gametes are brought
together through fertilization to form a
diploid (2n) zygote (fertilized egg)
1N
Fertilization
1N
2N
Here’s the key to your
“uniqueness”
Gene X
Homologous Pair
same genes,
different alleles
Sister
Chromatids
(same genes,
same alleles)
Homologous pairs separate in meiosis and therefore
different alleles (versions of traits) separate. So
many combos of traits are possible!
Meiosis Forms Haploid Gametes
Meiosis must reduce the chromosome # by half
Fertilization then restores the 2n number
23 chromo from egg + 23 chromo from sperm = you
46!
from mom
from dad
child
too
much!
EGG
SPERM
YOU
meiosis reduces
genetic content
The right
number!
EGG
SPERM
YOU
Meiosis: 2-Part Cell Division
Interphase
46
Homologous
Pair
separate
Meiosis
I
Diploid
(2x46 = 92)
1 replication of chromosomes is
followed by 2 cell divisions
(aka Interphase only happens once!)
Sister
chromatids
separate
Diploid (46)
Meiosis
II
Haploid
(23)
Meiosis: Reduction Division
• 2 part cell division
• Meiosis I
• Interphase I
• PMAT I
Interphase
• Meiosis II
• PMAT II
Meiosis I
• End result:
4 genetically different
haploid cells
• 4 sperm or 1 egg
Meiosis II
Meiosis I: Reduction Division
Nucleus
Early
Prophase I
(Chromosome
number
doubled)
Chromo Spindle
pair up fibers
Late
Prophase
I
Nuclear
envelope
Metaphase
Anaphase Telophase I
I
I
(diploid)
Meiosis II: Reducing Chromo #
Prophase
II
Metaphase
Telophase
II
Anaphase
4 genetically
II
II
different
haploid cells
Interphase I
• SAME as MITOSIS
• Chromosomes will double
• G1, S, G2
Prophase I
•Homologs pair up and
form tetrad (a pair of
homologous chromosomes
Chromosomes condense.
Spindle forms.
Nuclear envelope
disappears.
Crossing over occurs
Tetrads Form in Prophase I
Homologous chromosomes
Pair up
(each with sister
chromatids)
Join to form a
TETRAD
Called Synapsis
Crossing-Over occurs in Prophase I
Tetrad Forms
Definition: Pieces of
chromosomes or genes
are exchanged
Advantage of sexual
reproduction =
genetic variation!
Crossing-Over
It’s hard to predict what traits you’ll get from mom and dad
because there is so many possible combinations!
**THIS IS ONE SOURCE OF GENETIC VARIABILITY!**
Genetic Variability is due to…
1. Crossing over
2. Independent Assortment
3. Random Segregation
4. Random Fertilization
Meaning: You are unique for these 4 reasons!
• Independent
Assortment
• Random
Segregation
• The way a pair of
chromosomes lines up
during metaphase is not
dependent on other pairs.
• Random chromatids
separate into the newly
forming eggs/sperm
• Aka Mom’s chromosomes
don’t necessarily all line
up on one side
• Aka if you inherit mom’s
hair color, you might get
her brown hair trait OR
her blonde hair trait
Independent
Assortment
Random Segregation
Metaphase I and Anaphase I
Metaphase I
Homologous pairs of
chromosomes align
along the equator
Anaphase I
-Homologs separate and
move to opposite poles.
-Sister chromatids remain
attached at their
centromeres.
Telophase I and Cytokinesis
Nuclear envelopes
reassemble.
Spindle disappears.
Cytokinesis divides cell
into two new diploid cells.
Meiosis II-occurs in 2 cells
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/gene.
Meiosis II: Reducing Chromo #
Prophase
II
Metaphase
Telophase
II
Anaphase
4 genetically
II
II
different
haploid cells
• Prophase II
• Nuclear envelope disappears
• Spindle fibers form
• No Crossing Over

Metaphase II

Chromosomes align along
the equator
Anaphase II
Equator
Pole
Sister chromatids
separate and
move to opposite
poles.
Telophase II
Nuclear envelope
reforms.
Chromosomes loosen
into chromatin.
Spindle breakdown.
Cytokinesis breaks
the cells into 2 new
daughter cells
Results of Meiosis
Gametes (egg & sperm) form
Four haploid cells with one
copy of each chromosome
One allele of each gene
Different combinations of
alleles for different genes
along the chromosome
Prophase II
Prophase I
Metaphase II
Metaphase I
Anaphase II
Anaphase I
Telophase II
Telophase I
Cytokinesis
Cytokinesis
Meiosis Animation
Overview of Meiosis
Oogenesis & Spermatogenesis
THE PROCESS OF MAKING
EGG AND SPERM
Spermatogenesis
“Creation of sperm”
Testes
2 divisions produce
4 viable haploid spermatids
Spermatids mature into
sperm
Men produce about
250,000,000 sperm per day
Spermatogenesis in the Testes
Spermatid
Spermatogenesis
Oogenesis
“Creation of Eggs”
Ovary
2 divisions produce 3 polar bodies that die
+ 1 viable egg
Polar bodies die because of unequal
division of cytoplasm
Starting at puberty, if unfertilized, one
immature oocyte matures into an ovum
(egg) every 28 days  MENSTRUAL CYCLE
Oogenesis in the Ovaries
**Egg cells are special…only one egg is made every
time meiosis occurs; the other 3 cells (polar
bodies) that are made are much smaller & are
discarded (Remember: the egg cell is the largest
cell you’ll come across)
Oogenesis
1st polar body
may divide
(haploid)
a
Mitosis
Oogonium
(diploid)
A
X
X
a
X
a
a
Polar
bodies
die
X
Meiosis I
Meiosis II
X
Primary
oocyte
(diploid)
A
X
A
X
Secondary
oocyte
(haploid)
Oocyte
A
X
polar body
(dies)
Mature
Egg (ovum)
Meiosis is IMPORTANT… Genetic Variability
1. Independent assortment
(late pro/early meta I&II)
-chromosomes line up randomly
2. Law of Segregation
(Late meta/ana I&II)
-spindle fibers attach randomly to chromosomes and
separate.
3. Crossing-over
(Pro I)
pieces of sister chromatids are switched
4. Random fertilization
Random sperm + random egg are combined
What are the possibilities?
***Total possible chromosome combinations due to
independent assortment = 2n [for humans = 223 =
8,388,608]
***Total possible chromosomally different zygotes
due to fertilization = (223)2 =
70,368,744,000,000
***Possible genetically different zygotes per
couple if crossing-over occurs only once = (423)2
= 4,951,760,200,000,000,000,000,000,000
--advantageous b/c variability needed for evolution
Comparison of Divisions
Mitosis
Meiosis
2
# of divisions
1
Number of
daughter cells
2
4
Yes
No
Same as parent
Half of parent
Where
Somatic cells
Gamete cells
When
Throughout life
At sexual maturity
Genetically
identical?
Chromosome #
Role Growth and repair
Sexual reproduction
What’s the difference between Mitosis &
Meiosis ?
46 Single
92
Chromosomes
(23 Pairs)
Single
Chromosomes
(46 Pairs)
46 Single
Chromosomes
(23 Pairs)
46 Single
Chromosomes
(23 Pairs)
46
Single
Chromosomes
(23 Pairs)
92
Single
Chromosomes
(46 Pairs)
46
Single
Chromosome
s
(23 Pairs)
23 singles
23 singles
46
Single
Chromosom
es
(23 Pairs)
23 singles
23 singles