1. Meiosis and Sexual Life Cycles
Chapter 13
Meiosis and Sexual Life Cycles

2. Genetics- the scientific study of heredity and variation
Living organisms are distinguished by their ability to reproduce their own kind
Genetics- the scientific study of heredity and variation
Heredity- the transmission of traits from one generation to the next
Variation- differences in appearance that offspring show from parents and siblings
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3. Inheritance of Genes
Genes- units of heredity, made up of segments of DNA
Gametes (sperm and eggs)
Locus- specific location of a gene on a chromosome
DNA is packaged into chromosomes
One set of chromosomes is inherited from each parent
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4. Asexual VS.Sexual Reproduction
Asexual reproduction= one parent producing genetically identical offspring by mitosis
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. Chromosomes in Human Cells
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
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6. TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere
Fig. 13-3b
TECHNIQUE
5 µm
Pair of homologous
replicated chromosomes
Centromere
Figure 13.3 Preparing a karyotype
Sister
chromatids
Metaphase
chromosome

7. sex chromosomes are called X and Y
females= homologous pair of X chromosomes (XX)
Males= one X and one Y chromosome
The 22 pairs of chromosomes that do not determine sex are called autosomes
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8. homologous chromosomes= one chromosome from each parent
The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father
A diploid cell (2n) has two sets of chromosomes
humans diploid number is 46 (2n = 46)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

9. 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)

10. A gamete= single set of chromosomes, and is haploid (n)
Humans 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
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11. Behavior of Chromosome Sets in the Human Life Cycle
Fertilization- union of gametes (the sperm and the egg)
Fertilized egg= zygote and has one set of chromosomes from each parent
The zygote produces somatic cells by mitosis and develops into an adult
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12. At sexual maturity, the ovaries and testes produce haploid gametes
Gametes are produced by meiosis.
Meiosis results in one set of chromosomes in each gamete
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

13. 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)

14. Meiosis reduces the number of chromosome sets from diploid to haploid
Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II
Results 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
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15. The Stages of Meiosis Meiosis I= homologous chromosomes separate
Meiosis I= produces two haploid daughter cells with replicated chromosomes
Meiosis II= sister chromatids separate
Meiosis II= produces four haploid daughter cells with unreplicated chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

16. 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

17. Division in meiosis I occurs in four phases:
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I and cytokinesis
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18. Prophase I
occupies more than 90% of the time required for meiosis
Chromosomes begin to condense
In synapsis, homologous chromosomes loosely pair up, aligned gene by gene
In crossing over, nonsister chromatids exchange DNA segments
Each pair of chromosomes forms a tetrad, a group of four chromatids

19. tetrads line up at the metaphase plate
Metaphase I
tetrads line up at the metaphase plate
Microtubules from the poles are attached to the kinetochore of the homologous chromosomes
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20. pairs of homologous chromosomes separate
Anaphase I
pairs of homologous chromosomes separate
One chromosome moves toward each pole
Sister chromatids remain attached at the centromere and move as one unit toward the pole
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

21. Telophase I and Cytokinesis
each half of the cell has a haploid set of chromosomes
each chromosome still consists of two sister chromatids
Cytokinesis forms two haploid daughter cells
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

22. 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

23. 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

24. spindle apparatus forms
Prophase II
spindle apparatus forms
chromosomes (each still composed of two chromatids) move toward the metaphase plate
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25. sister chromatids are arranged at the metaphase plate
Metaphase II
sister chromatids are arranged at the metaphase plate
Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

26. sister chromatids separate
Anaphase II
sister chromatids separate
sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

27. Telophase II and Cytokinesis chromosomes arrive at opposite poles
Nuclear envelope form, and the chromosomes begin decondensing
Cytokinesis separates the cytoplasm
At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

28. Mitosis VS. Meiosis
Mitosis= cells that are identical to the parent cell (1 cell → 1 identical 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

29. 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

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