1. Meiosis and Sexual Life Cycles10
Meiosis and Sexual Life Cycles

2. Overview: Variations on a Theme
Living organisms are distinguished by their ability to reproduce their own kind
Heredity: the transmission of traits from one generation to the next
Variation: the differences in appearance that offspring show from parents and siblings
Genetics: the scientific study of heredity and variation
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3. Comparison of Asexual and Sexual Reproduction
Asexual reproduction:, a single individual passes genes to its offspring without the fusion of gametes
Results in a clone
A group of genetically identical individuals from the same parent
Asexual reproduction is less expensive than sexual reproduction
Sexual reproduction: 2 parents give rise to offspring that have unique combinations of genes inherited from the 2 parents
Sexual reproduction is nearly universal among animals
Overall, genetic variation is evolutionarily advantageous
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4. MEIOSIS Key Haploid gametes (n = 23) Haploid (n) Egg (n) Diploid (2n)
Figure 10.5
Key
Haploid gametes (n = 23)
Haploid (n)
Egg (n)
Diploid (2n)
Sperm (n)
MEIOSIS
FERTILIZATION
Testis
Ovary
Figure 10.5 The human life cycle
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
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5. TERMINOLOGY Somatic cell: a body cell; excludes sperm or egg cells
Diploid (2n); 2 sets of chromosomes
Total 46 chromosomes (2n) in every somatic cell of humans (every species has its own diploid number)
Gametes: sex cells; sperm or egg
Haploid (n); have only 1 set of chromosomes
23 chromosomes in each human gamete (half of total)
Fertilization: union of 2 gametes to form zygote
Zygote: fertilized egg; diploid (2n) cell
As you develop from zygote into mature adult, genetic information is passed to all somatic cells by mitosis

6. Terminology (Con’d)
Gene: unit of inheritance; packaged as chromosomes; contains DNA and histone proteins
Locus: the exact position a gene is found on a chromosome
Karyotype: an ordered display of the pairs of chromosomes from a cell
Homologous Chromosomes or Homologues: the two chromosomes in each pair of your 23 pairs; one homologue is from Mom; the other homologue is from Dad
Sex chromosomes: are called X and Y
Human females have a homologous pair of X chromosomes (XX)
Human males have one X and one Y chromosome
The remaining 22 pairs of chromosomes in human males are called autosomes
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7. Various Sexual Reproduction Among Organisms
Figure 10.6
Various Sexual Reproduction Among Organisms
Haploid (n)
Haploid multi-
cellular organism
(gametophyte)
Haploid unicellular or
multicellular organism
Diploid (2n) n
Gametes n n
Mitosis n
Mitosis
Mitosis n
Mitosis n n n n n
MEIOSIS
FERTILIZATION
Spores n n
Gametes
Gametes n
MEIOSIS
FERTILIZATION
Zygote
MEIOSIS
FERTILIZATION 2n 2n 2n 2n
Diploid
multicellular
organism
Zygote
Diploid
multicellular
organism
(sporophyte) 2n
Figure 10.6 Three types of sexual life cycles
Mitosis
Mitosis
Zygote
(a) Animals
(b) Plants and some algae
(c) Most fungi and some protists
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8. Animals Haploid (n) Diploid (2n) Gametes n n n MEIOSIS FERTILIZATION
Figure
Animals
Haploid (n)
Diploid (2n)
Gametes n n n
MEIOSIS
FERTILIZATION
Zygote
Figure Three types of sexual life cycles (part 1: animal) 2n 2n
Diploid
multicellular
organism
Mitosis
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9. KARYOTYPE A photomicrograph of an individual’s
somatic cell metaphase chromosomes
arranged in sequence
Human karyotypes often made with
lymphocytes (one of your 5 WBC)
Used to screen for chromosomal
abnormalities

10. HOMOLOGOUS CHROMOSOMES (HOMOLOGUES)
One homologue is inherited from each parent
Mom Homol
Sister chromatid
Dad homol
Sister chromatid
Tetrad
A pair of chromosomes with the same size, centromere position and staining pattern; 22 pair; the 23rd pair are the sex chromosomes (XX = female, and XY = male)

11. Meiotic Cell Division
Meiotic cell division is essential for sexual reproduction
The combination of genetic material from 2 parents
Sexual Reproduction: involves meiosis (synthesis of gametes) & fertilization (union of 2 gametes to form zygote/fertilized egg)
Results in four daughter cells.
Each daughter cell contains half the number of chromosomes as the parent.
Each daughter cell (gamete) is genetically unique
Offspring have a unique combination of genes inherited from both parents
Results in great genetic variability
Offspring vary from each parent & from each other.
Sex cells, or gametes, are produced by meiotic cell division. This type of cell division includes two rounds of nuclear division.

12. duplicated chromosomes Meiosis II
Figure 10.7
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Pair of duplicated
homologous
chromosomes
Chromosomes
duplicate
Sister
chromatids
Diploid cell with
duplicated
chromosomes
Meiosis I
Homologous
chromosomes
separate
Figure 10.7 Overview of meiosis: how meiosis reduces chromosome number
Haploid cells with
duplicated chromosomes
Meiosis II
Sister chromatids
separate
Haploid cells with unduplicated chromosomes
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13. GENERAL CHARACTERISTICS OF MEIOSIS
Meiosis: special type of cell division that reduces the number of chromosomes by half
Produces haploid sperm cells in testes & haploid egg cells in ovaries
Fertilization….the union of haploid gametes….produces diploid offspring
Meiosis has 2 divisions:
Start meiosis with a germ cell = stem cell that will undergo halving of chromosomes to become sperm/egg
DNA replication in S phase of Interphase
Meiosis I: chromosome number is halved by separating the 2 chromosomes of each homologue; P1, M1, A1, T1/C
No Interphase between Meiosis I and Meiosis II
Meiosis II: chromosome number is still halved, but sister chromatids are now separated: P2, M2, A2, T2/C

14. duplicated chromosomes Meiosis II
Figure
Meiosis I
Homologous
chromosomes
separate
Haploid cells with
duplicated chromosomes
Meiosis II
Sister chromatids
separate
Figure Overview of meiosis: how meiosis reduces chromosome number (part 2: meiosis I and II)
Haploid cells with unduplicated chromosomes
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15. Details of Meiosis I
Interphase: chromosomes duplicate in S phase; 23 prs from Mom and 23 prs from Dad; still can’t see 46 prs of sisters in G2
Prophase I: can now see 46 prs sister chromatids
Homologues pair up/synapsis
Form a tetrad. Have 23 tetrads/23 prs homologues
Do crossing over/chiasmata
Metaphase I: tetrads (homologues & sisters) line up
Mendel’s Independent assortment
Anaphase I: tetrads split up (homologues w/sisters separate
Mendel’s Principle of Segregation
Telophase1/C: 2 haploid cells form
Chromosomes still double as sister chromatids
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16. G1 S G2 Interphase Pair of homologous chromosomes in
Figure
Interphase G1
Pair of homologous
chromosomes in
diploid parent cell
Pair of duplicated
homologous
chromosomes
Chromosomes
duplicate S
Figure Overview of meiosis: how meiosis reduces chromosome number (part 1: interphase)
Sister
chromatids
Diploid cell with
duplicated
chromosomes G2
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17. Prophase I Tetrad after Synapsis Key Maternal set of
Figure 10.4
Prophase I
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Sister chromatids
of one duplicated
chromosome
Centromere
Figure 10.4 Describing chromosomes
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Tetrad after Synapsis
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18. Separates homologous chromosomes
Figure a
MEIOSIS I:
Separates homologous chromosomes
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Kinetochore
(at centromere)
Sister
chromatids
Chiasmata
Kinetochore
microtubules
Spindle
microtubules
Metaphase
plate
Figure a Exploring meiosis in an animal cell (part 1a: prophase I and metaphase I)
Fragments
of nuclear
envelope
Pair of
homologous
chromosomes
Centromere
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19. Chiasmata Tetrad Region of crossing over; exchange of genetic material
Through the process of crossing over, meiosis allows homologous chromosomes of maternal origin and paternal origin to undergo an exchange of DNA segments.
This process is random, resulting in completely unique chromosomes when meiosis is complete. No nucleotides are gained or lost in this process.
Crossing over also helps hold together the bivalents for metaphase I.
Region of crossing over; exchange of genetic material 19

20. Synaptonemal complex Crossover Crossover Chiasmata Figure 10.9-2
Figure Crossing over and synapsis in prophase I
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21. Separates homologous chromosomes
Figure b
MEIOSIS I:
Separates homologous chromosomes
Telophase I
and Cytokinesis
Anaphase I
Sister chromatids
remain attached
Figure b Exploring meiosis in an animal cell (part 1b: anaphase I, telophase I and cytokinesis)
Cleavage
furrow
Homologous
chromosomes
separate
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22. MEIOSIS I: Separates homologous chromosomes
Figure
MEIOSIS I: Separates homologous chromosomes
Telophase I
and Cytokinesis
Prophase I
Metaphase I
Anaphase I
Sister chromatids
remain attached
Centrosome
(with centriole pair)
Kinetochore
(at centromere)
Sister
chromatids
Chiasmata
Spindle
microtubules
Kinetochore
microtubules
Metaphase
plate
Figure Exploring meiosis in an animal cell (part 1: meiosis I)
Cleavage
furrow
Fragments
of nuclear
envelope
Homologous
chromosomes
separate
Pair of
homologous
chromosomes
Centromere
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23. To separate homologues Now pre-gametes are haploid
Why Do We Need Meiosis I?
To separate homologues
Now pre-gametes are haploid

24. Now haploid, but with sisters still attached
SUMMARY OF MEIOSIS I
Have 2 pre-sperm or pre-egg cells of 23 pairs of sister chromatids each
Now haploid, but with sisters still attached

25. Details of Meiosis II No Interphase between Meiosis 1 and 2
Meiosis II: separates the sister chromatids & 4 haploid daughter cells with result, each containing single chromosomes
Division in meiosis II also occurs in 4 phases
Prophase II
Metaphase II
Anaphase II
Telophase II and cytokinesis
Similar to mitosis
Suggests that meiosis evolved from mitosis
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26. MEIOSIS II: Separates sister chromatids
Figure
MEIOSIS II: Separates sister chromatids
Telophase II
and Cytokinesis
Prophase II
Metaphase II
Anaphase II
Sister chromatids
separate
Figure Exploring meiosis in an animal cell (part 2: meiosis II)
Haploid daughter
cells forming
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27. WHY DO WE NEED MEIOSIS II?
To separate the sister chromatids
Identical to mitosis (except for chiasmata and independent assortment)

28. Now have 4 sperm cells or 4 egg cells with 23 chromosomes each
SUMMARY OF MEIOSIS II
Now have 4 sperm cells or 4 egg cells with 23 chromosomes each

29. Separates homologous chromosomes Separates sister chromatids
Figure 10.8
MEIOSIS I:
MEIOSIS II:
Separates homologous chromosomes
Separates sister chromatids
Telophase I
and Cytokinesis
Telophase II
and Cytokinesis
Prophase I
Metaphase I
Anaphase I
Prophase II
Metaphase II
Anaphase II
Figure 10.8 Exploring meiosis in an animal cell
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30. Cytoplasmic Division
In meiosis, division of the cytoplasm differs between males & females:
Male meiosis:
Produces 4 functional sperm cells
Female meiosis:
Produces 1 functional egg cell/oocyte & 3 nonfunctional polar bodies
In meiotic cell division, The division of the cytoplasm differs between males and females.
In females, the cytoplasm is divided very unequally in both meiotic divisions. Most of the cytoplasm is retained in one meiotic product called the oocyte, which can develop into a functional egg cell, and the other products receive only small amounts of cytoplasm. These smaller cells are called polar bodies.
In males, the cytoplasm divides fairly equally, resulting in products that all form functional sperm. During sperm development, most of the cytoplasm is eliminated and what remains is the nucleus in the sperm head and a flagellum to propel it toward the egg.
The products of meiosis are gametes. Each gamete is haploid, containing a single set of chromosomes.
During fertilization, gametes fuse to form a single cell called a zygote. This process restores the original chromosome number.

31. 3 Unique Events to Meiosis
All three occur in meiosis l
Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
Alignment of homologous pairs at the metaphase I plate: Homologous pairs of chromosomes are positioned there in metaphase I
Separation of homologs during anaphase I
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32. GENETIC VARIATION IN MEIOSIS
Crossing over
During Prophase 1
Have 2-3 cross-overs/homologous pair (remember have 23 homologous pairs)
Independent Assortment
During Metaphase 1
Each gamete has 1 in 8 million possible haploid combinations from Mom and Dad
Random Fertilization
An egg of 1 in 8 million possibilities will be fertilized by a sperm with 1 in 8 million possibilities
Results in a zygote that can have 1 in 64 trillion possible diploid combinations

33. COMPARISON OF MITOSIS & MEIOSIS
Reduction division
Genetic variation
In testes/ovaries to make gametes
Pro1: synapsis, tetrads, chiasmata
Meta 1: tetrads line up, indep assort
Ana 1: tetrads separate
Preceded by single replication of chromosomes (to make sister chromatids in S phase)
2 cell divisions: Meiosis 1 and 2
Produces 4 daughter cells
Each having 23 chromosomes
Mitosis
Reproducing Division
No genetic variation
Makes somatic cells
Not applicable
Sister line up; no indep assort
Sisters separate
Same
1 cell division
Produces 2 daughter cells
Each identical having 46 chromosomes (same as parent)

34. Figure a
Figure a A comparison of mitosis and meiosis (part 2a: mitosis table)
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35. Figure b
Figure b A comparison of mitosis and meiosis (part 2b: meiosis table)
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36. Possibility 1 Possibility 2 Two equally probable arrangements of
Figure s3
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Figure s3 The independent assortment of homologous chromosomes in meiosis (step 3)
Daughter
cells
Combination 2
Combination 1
Combination 3
Combination 4
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37. proteins holding sister chromatid arms together. Centromere
Figure s5
Nonsister chromatids
held together
during synapsis
Prophase I
of meiosis
Pair of
homologs
Synapsis and
crossing over
Chiasma
site
Breakdown of
proteins holding sister
chromatid arms together.
Centromere
TEM
Anaphase I
Figure s5 The results of crossing over during meiosis (step 5)
Anaphase II
Daughter
cells
Recombinant chromosomes
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38. Concept 10.4: Genetic variation produced in sexual life cycles contributes to evolution
Mutations: changes in an organism’s DNA
Are the original source of genetic diversity
Mutations create different versions of genes called alleles
Reshuffling of alleles during sexual reproduction produces genetic variation
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