Goals: Be able to…
Sex
Describe the purposes of sex, fertilization, and
meiosis.
Draw a generic human life cycle, including new
terms and chromosome #s.
Explain what happens during different stages of
meiosis, and how meiosis I and II are different.
Describe how meiosis increases genetic diversity.
Differentiate between meiosis and mitosis.
Explain how trisomy 21 occurs.
Explain why interspecies hybrids are often sterile.
ASEXUAL reproduction
ASEXUAL reproduction
Sonoran spotted whiptail lizard
SEXUAL reproduction: 2 parents give
rise to genetically unique offspring.
Sea star
SEX costs, so
why have it?
1
1 pair: Homologous chromosomes
Humans have 23 pairs
Have the same genes
Different versions of those
genes = alleles
NOT sister chromatids!
Fig 5.20
Fig 5.21
Non-homologous chromosomes have different
genes
How are homologous chromosomes like a pair of
shoes?
Rules:
Gene = letter
Allele = Capital or small letter (version)
Fig 5.21
After S-phase (replication)
Homologous chromosomes
What is the difference between homologous
chromosomes and sister chromatids?
Sister chromatids
Sister chromatids
Fig 5.22
2
46
Humans have 23 pairs
46
The human life cycle
92?
Fig 5.20
46
46
46
Meiosis
Meiosis
Meiosis
23
23
23
Sperm
Egg
46
Meiosis
23
Fertilization
Sperm
Zygote
46
Mitosis
46
Meiosis: Divides chromosome # in 1/2
23
Fertilization: Joining gametes to form a
zygote
23
46
46
Parent
cell
Egg
23
23
Gametes
Gametes
Zygote
Restores original chromosome #
3
1 set = haploid: 23
2 sets = diploid: 46
True/False: There can be homologous
chromosomes in a haploid cell
3 sets = triploid: ??
How many chromosomes
would a triploid human
have?
Lloyd
True/False: There can be sister chromatids in a
haploid cell
Where does meiosis occur in humans?
Meiosis
1. Decrease
chromosome #
Fertilization
Meiosis
Zygote
Diploid
Haploid
Purpose of meiosis
2. Increase genetic
variability
Diploid
Fig 5.23
Meiosis reduces chromosome number:
DNA replicates once, but divides twice.
If meiosis occurs, it happens at the same spot in
the cell cycle as mitosis.
Haploid
23
Replication
Diploid
46
Haploid
23
Diploid
46
Haploid
23
Meiosis I
Haploid
23
Haploid
23
Haploid
23
Meiosis II
Fig 5.24
4
Replication occurs like normal
Meiosis I: homologous chromosomes
separate
Anaphase I
Prophase I
1 chromosome
Metaphase I
1 chromosome
with 2 sister
chromatids!
Fig 5.25
Meiosis I: homologous chromosomes
separate
2 chromosomes each
Fig 5.25
4 chromosomes
Meiosis I: separate
homologous chromosomes
Diploid Æ Haploid
Diploid Æ Haploid
Fig 5.25
Meiosis II: separate
sister chromatids
Haploid Æ Haploid
Meiosis II: sister chromatids separate
Metaphase II
Telophase I
1. How many chromosomes are present in each
human cell after telophase I? After telophase II?
Anaphase II
Prophase II
2. How many sister chromatids are present after
telophase I? After telophase II?
Telophase II
Fig 5.25
5
Meiosis I: Increase genetic diversity 2 ways
If NO crossing over during Prophase I:
RL
RL
Crossing over
rl
Random alignment
rl
2 different gametes
Fig 5.25
1. Crossing over during Prophase I
Fig 5.26
1. Crossing over during Prophase I
How many different gametes? What are they?
Homologous chromosomes swap pieces
Fig 5.26
Fig 5.26
1. Crossing over during Prophase I
RL
Rl
3. Why does crossing over have to happen in
prophase I, not prophase II?
rL
rl
4 different gamete types
Fig 5.26
6
Random alignment at Metaphase I
Homologous chromosomes line up differently
B+H+
B+H+
What would happen if non-homologous
chromosomes crossed over?
B- HB- HFig 5.27
Random alignment at Metaphase I
Homologous chromosomes line up differently
B+H-
Random alignment at Metaphase I
Homologous chromosomes line up differently
B+H-
B- H+
B- H+
Fig 5.27
Fig 5.27
Random alignment of shoes
Meiosis in a nutshell:
Interphase: DNA replicates
Meiosis I: Homologous
chromosomes separate
Meiosis II: Sister
chromatids separate
7
Meiosis: Summary
Begin
Interphase
Diploid (46) cell
Mitosis
Diploid (46) cell
DNA replicates Each chromosome has
2 sister chromatids
Meiosis I
Homologous
chromosomes
separate
Meiosis II
Sister
chromatids
separate
2 haploid (23) cells
Each chromosome has
2 sister chromatids
ALL chromosomes
line up along middle
4 haploid (23) cells
Sister chromatids
pulled apart
Meiosis video
Genetically identical
Fig 5.28
Meiosis vs. Mitosis
Mitosis
Meiosis
Homologs line
up
Meiosis II
½ genes
Meiosis
Mitosis
Purpose:
Cut chrom # in ½ ,
Introduce genetic
variability
Make exact
copies
Mechanism:
2 divisions
1 division
Homo. chrom Yes
line up at
metaphase?
No
Fig 5.28
Sex Gone Wrong!
A fly has 6 chromosomes (3 homologous pairs).
Draw a fly cell at metaphase of mitosis and a
different fly cell at metaphase I of meiosis.
Errors during meiosis and their consequences
• Trisomy 21
8
Why do chances of Trisomy 21 increase with
Mom’s age?
Trisomy 21: Down Syndrome
Fig 7.14
Meiosis I nondisjunction:
Homologous chromosomes fail to separate
Meiosis II nondisjunction:
Sister chromatids fail to separate
Nondisjunction
during Meiosis I
Amniocentesis to detect Down
Syndrome in at-risk women
During fertilization…
=
+
aBB
Nondisjunction
during Meiosis II
aB
aaBBB
9
Sex Gone Wrong!
Errors during meiosis and their consequences
1. In your own words, explain why Down
syndrome occurs
• Trisomy 21
• Hybrids
Interspecies hybrid sterility
Horse
Why are
mules sterile?
Diploid = 64,
Haploid = 32
x
Donkey
Diploid = 62,
Haploid = 31
=
Mule
Diploid = 63,
Haploid = ?
No homologous chromosomes
10
Polyploidy: multiple SETS of chromosomes
Bananas from the store are triploid (3 sets) and
cannot produce proper gametes.
2. Explain in your own words why a triploid
organism cannot produce balanced gametes
11