1. Meiosis and Sexual Life Cycles
Chapter 13
Meiosis and Sexual Life Cycles
生物及解剖學科
藍心婕 10/11/2017

2. It is genes that are actually inherited
Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes
In a literal sense, children do not inherit particular physical traits from their parents
It is genes that are actually inherited
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

3. Genes are the units of heredity, and are made up of segments of DNA
Inheritance of Genes
Genes are the units of heredity, and are made up of segments of DNA
Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs)
Each gene has a specific location called a locus on a certain chromosome
Most DNA is packaged into chromosomes
One set of chromosomes is inherited from each parent
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

4. Comparison of Asexual and Sexual Reproduction
In asexual reproduction, one parent produces genetically identical offspring by mitosis
A clone is a group of genetically identical individuals from the same parent
In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

5. 0.5 mm Parent Bud (a) Hydra (b) Redwoods Fig. 13-2
Figure 13.2 Asexual reproduction in two multicellular organisms
Parent
Bud
(a) Hydra
(b) Redwoods

6. In sexual reproduction
Fertilization of sperm and egg produces offspring (gene recombination)
In asexual reproduction
Offspring are produced by a single parent, without the participation of sperm and egg

7. Concept 13.2: Fertilization and meiosis alternate in sexual life cycles
A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

8. Sets of Chromosomes in Human Cells
Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes
A karyotype is an ordered display of the pairs of chromosomes from a cell
The two chromosomes in each pair are called homologous chromosomes, or homologs
Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

9. Preparation of a karyotype from a blood sample
Blood culture
Fluid
Centrifuge
Packed red and white blood cells
Hypotonic solution
Fixative
White blood cells
Stain
Centromere
Pair of homologous chromosomes
Sister chromosomes
2,600X
A blood culture is centrifuged to separate the blood cells from the culture fluid. 1
The fluid is discarded, and a hypotonic solution is mixed with the cells. This makes the red blood cells burst. The white blood cells swell but do not burst, and their chromosomes spread out. 2
Another centrifugation step separates the swollen white blood cells. The fluid containing the remnants of the red blood cells is poured off. A fixative (preservative) is mixed with the white blood cells. A drop of the cell suspension is spread on a microscope slide, dried, and stained. 3

10. The 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 22 pairs of chromosomes that do not determine sex are called autosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

11. Gametes have a single set of chromosomes
Cells with two sets of chromosomes
Are said to be diploid
Gametes, eggs and sperm, are haploid
With a single set of chromosomes

12. Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of
Fig. 13-4
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Figure 13.4 Describing chromosomes
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

13. For humans, the haploid number is 23 (n = 23)
A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n)
For humans, the haploid number is 23 (n = 23)
Each set of 23 consists of 22 autosomes and a single sex chromosome
In an unfertilized egg (ovum), the sex chromosome is X
In a sperm cell, the sex chromosome may be either X or Y
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

14. Behavior of Chromosome Sets in the Human Life Cycle
Fertilization is the union of gametes (the sperm and the egg)
The fertilized egg is called a zygote and has one set of chromosomes from each parent
The zygote produces somatic cells by mitosis and develops into an adult
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

15. Multicellular diploid adults (2n = 46)
Fig. 13-5
Key
Haploid gametes (n = 23)
Haploid (n)
Egg (n)
Diploid (2n)
Sperm (n)
MEIOSIS
FERTILIZATION
Figure 13.5 The human life cycle
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)

16. The Variety of Sexual Life Cycles
The alternation of meiosis and fertilization is common to all organisms that reproduce sexually
The three main types of sexual life cycles differ in the timing of meiosis and fertilization
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

17. Key Haploid (n) Diploid (2n) Gametes n n n MEIOSIS FERTILIZATION
Fig. 13-6a
Key
Haploid (n)
Diploid (2n)
Gametes n n n
MEIOSIS
FERTILIZATION
Figure 13.6a Three types of sexual life cycles—animals
Zygote 2n 2n
Diploid
multicellular
organism
Mitosis
(a) Animals

18. Plants and some algae exhibit an alternation of generations
This life cycle includes both a diploid and haploid multicellular stage
The diploid organism, called the sporophyte, makes haploid spores by meiosis
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19. A gametophyte makes haploid gametes by mitosis
Each spore grows by mitosis into a haploid organism called a gametophyte
A gametophyte makes haploid gametes by mitosis
Fertilization of gametes results in a diploid sporophyte
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

20. (b) Plants and some algae
Fig. 13-6b
Key
Haploid (n)
Haploid multi-
cellular organism
(gametophyte)
Diploid (2n)
Mitosis
Mitosis n n n n n
Spores
Gametes
Figure 13.6b Three types of sexual life cycles—plants and some algae
MEIOSIS
FERTILIZATION 2n 2n
Zygote
Diploid
multicellular
organism
(sporophyte)
Mitosis
(b) Plants and some algae

21. The zygote produces haploid cells by meiosis
In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage
The zygote produces haploid cells by meiosis
Each haploid cell grows by mitosis into a haploid multicellular organism
The haploid adult produces gametes by mitosis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

22. Haploid unicellular or multicellular organism Diploid (2n)
Fig. 13-6c
Key
Haploid (n)
Haploid unicellular or
multicellular organism
Diploid (2n)
Mitosis
Mitosis n n n n
Gametes
Figure 13.6c Three types of sexual life cycles—most fungi and some protists n
MEIOSIS
FERTILIZATION 2n
Zygote
(c) Most fungi and some protists

23. Like mitosis, meiosis is preceded by the replication of chromosomes
Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid
Like mitosis, meiosis is preceded by the replication of chromosomes
Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II
The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis
Each daughter cell has only half as many chromosomes as the parent cell
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

24. Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number
Interphase
Homologous pair of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number
Meiosis I 1
Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II 2
Sister chromatids
separate
Haploid cells with unreplicated chromosomes

25. Division in meiosis I occurs in four phases:
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I and cytokinesis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

26. Figure 13.8 The meiotic division of an animal cell
Prophase I
Metaphase I
Anaphase I
Telophase I and
Cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
Centrosome
(with centriole pair)
Sister chromatids
remain attached
Centromere
(with kinetochore)
Sister
chromatids
Chiasmata
Spindle
Metaphase
plate
Figure 13.8 The meiotic division of an animal cell
Sister chromatids
separate
Haploid daughter cells
forming
Homologous
chromosomes
Homologous
chromosomes
separate
Cleavage
furrow
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore

27. Division in meiosis II also occurs in four phases:
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II and cytokinesis
Meiosis II is very similar to mitosis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

28. Telophase II and Cytokinesis
Fig. 13-8d
Telophase II and
Cytokinesis
Prophase II
Metaphase II
Anaphase II
Figure 13.8 The meiotic division of an animal cell
Sister chromatids
separate
Haploid daughter cells
forming

29. A Comparison of Mitosis and Meiosis
Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell
Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell
The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

30. Replicated chromosome
Fig. 13-9a
MITOSIS
MEIOSIS
MEIOSIS I
Parent cell
Chiasma
Chromosome
replication
Chromosome
replication
Prophase
Prophase I
Homologous
chromosome
pair
2n = 6
Replicated chromosome
Metaphase
Metaphase I
Figure 13.9 A comparison of mitosis and meiosis in diploid cells
Anaphase
Telophase
Anaphase I
Telophase I
Haploid
n = 3
Daughter
cells of
meiosis I 2n 2n
MEIOSIS II
Daughter cells
of mitosis n n n n
Daughter cells of meiosis II

31. Figure 13.9 A comparison of mitosis and meiosis in diploid cells
Fig. 13-9b
SUMMARY
Meiosis
Mitosis
Property
DNA
replication
Number of
divisions
Occurs during interphase before
mitosis begins
One, including prophase, metaphase,
anaphase, and telophase
Synapsis of
homologous
chromosomes
Does not occur
daughter cells
and genetic
composition
Two, each diploid (2n) and genetically
identical to the parent cell
Role in the
animal body
Enables multicellular adult to arise from
zygote; produces cells for growth, repair,
and, in some species, asexual reproduction
Occurs during interphase before meiosis I begins
Two, each including prophase, metaphase, anaphase, and
telophase
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Four, each haploid (n), containing half as many chromosomes
as the parent cell; genetically different from the parent
cell and from each other
Produces gametes; reduces number of chromosomes by half
and introduces genetic variability among the gametes
Figure 13.9 A comparison of mitosis and meiosis in diploid cells

32. Three events are unique to meiosis, and all three occur in meiosis l:
– Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
– At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes
– At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

33. Mutations create different versions of genes called alleles
Concept 13.4: 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

34. Homologous chromosomes carry different versions of genes
The differences between homologous chromosomes
Are based on the fact that they can bear different versions of a gene at corresponding loci
Tetrad in parent cell (homologous pair of duplicated chromosomes) e c E C
White
Pink
Meiosis
Black
Brown
Chromosomes of
the four gametes
Eye-color genes
Coat-color genes
Brown coat (C); black eyes (E)
White coat (C); pink eyes (e)
Figure 8.17B

35. Origins of Genetic Variation Among Offspring
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation
Three mechanisms contribute to genetic variation:
Independent assortment of chromosomes
Crossing over
Random fertilization
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

36. Independent Assortment of Chromosomes
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs
The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number
For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

37. Possibility 1 Possibility 2 Two equally probable arrangements of
Fig
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Figure The independent assortment of homologous chromosomes in meiosis
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4

38. Crossing Over
Crossing over produces recombinant chromosomes, which combine genes inherited from each parent
Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene
In crossing over, homologous portions of two nonsister chromatids trade places
Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

39. Recombinant chromosomes
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Figure The results of crossing over during meiosis
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes

40. Crossing over adds even more variation
Random Fertilization
Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)
The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations
Crossing over adds even more variation
Each zygote has a unique genetic identity
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

41. The Evolutionary Significance of Genetic Variation Within Populations
Natural selection results in the accumulation of genetic variations favored by the environment
Sexual reproduction contributes to the genetic variation in a population, which originates from mutations
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

42. Nondisjunction in meiosis I
Normal meiosis II
Gametes
n + 1
n 1
Number of chromosomes
Nondisjunction in meiosis II
Normal meiosis I
n -1 n
Number of chromosomes
Figure 8.21B
Figure 8.21A

43. Down syndrome is caused by trisomy 21 An extra copy of chromosome 21
5,000
Figure 8.20A
Figure 8.20B

44. Infants with Down syndrome (per 1,000 births)
The chance of having a Down syndrome child
Goes up with maternal age
Age of mother 45 50 35 30 25 40 20 90 10 60 70 80
Infants with Down syndrome (per 1,000 births)
Figure 8.20C

45. EVOLUTION CONNECTION
New species can arise from errors in cell division
Polyploid organism

46. Fig. 13-UN4