1. Chapter 13 148 Meiosis and Sexual Life Cycles
Heredity or inheritance is the transmission of traits from one generation to the next generation.
Genes are the units of heredity which are responsible for traits.
Locus: the specific location of a gene in a chromosome.
Chromosome: DNA and proteins.

2. 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

3. 148 Sexual reproduction: Asexual reproduction: Clone:
Life cycle: a sequence of stages from the time
of conception of an organism to
the production of its own
offspring.

4. 148 Karyotype: Homologous chromosomes (homologues):
Somatic chromosomes (autosomes):
Diploid cells:
Haploid cells:
Gametes:
Fertilization or syngamy:
Zygote:
Meiosis:

5. 149 The Variety of Sexual Life Cycles:
In the human life cycle, the gametes are the only haploid cells. The multicellular organism is diploid. In many fungi and some protists, zygote is the only diploid stage. Meiosis occurs immediately after zygote formation. Mitosis produces a multicellular haploid organism. Gametes are produced from the haploid organism by mitosis.

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

7. 149
Plants and some algae undergo a type of life cycle called alternation of generations which have diploid and haploid multicellular stages. The multicellular diploid stage is called the sporophyte, and the multicellular haploid stage is called the gametophyte. Meiosis in sporophyte produces haploid cells called spores. The spore divides by mitosis to form a multicellular haploid individual (gametophyte) which produces gametes by mitosis. Fertilization of the gametes gives rise to a zygote which divides by mitosis to form a multicellular diploid individual.

8. (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
MEIOSIS
FERTILIZATION
Figure 13.6b Three types of sexual life cycles—plants and some algae 2n 2n
Zygote
Diploid
multicellular
organism
(sporophyte)
Mitosis
(b) Plants and some algae

9. 149 Sexual Life Cycle in Animals:
Sexual Life Cycles in Plants and Some Algae:
Sexual Life Cycles of Some Fungi and Some Algae:
Refer to Fig. 13.6, page 252 in text.

10. 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 n
MEIOSIS
FERTILIZATION
Figure 13.6c Three types of sexual life cycles—most fungi and some protists 2n
Zygote
(c) Most fungi and some protists

11. 150 Meiosis: Interphase I:
During interphase each chromosome replicates to form two sister chromatids which are still joined by the centromere. The centriole pairs also replicate.

12. 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

13. 150 First Meiotic Cell Division: Prophase I:
In addition to the events that take place in the prophase of mitosis, synapsis of homologous chromosomes and the formation of tetrads also take place in prophase I. Crossing over of homologous chromosomes also occurs. The sister chromatids are held together by centromeres and nonsister chromatids are linked by chiasmata, the points of crossing over. This phase occupies more than 90% of the time required for meiosis. It may last for several days.

14. 150
Metaphase I:
The homologous chromosomes in tetrad formation line up in the equatorial plane of the cell.
Anaphase I:
The homologous chromosomes separate and move to opposite poles of the cell. There is no splitting of centromeres. Each chromosome has two chromatids, which are still joined by a centromere. Cytokinesis begins during late anaphase.

15. 150
Telophase I:
Two haploid nuclei are formed. Each chromosome contains double amount of nucleic acid because each chromosome has two chromatids, and each chromatid has one DNA.
Cytokinesis:
Division of cytoplasm forms two daughter cells, each contains half the amount of DNA as compared to the parental cell. In some species, after the first meiotic cell division there is a short period called interkinesis, which is similar to an interphase in mitosis except that there is no replication of DNA. Some species immediately undergo the second meiotic cell division without interkinesis.

16. Prophase I Metaphase I Centrosome (with centriole pair) Centromere
Fig. 13-8b
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Centromere
(with kinetochore)
Sister
chromatids
Chiasmata
Spindle
Metaphase
plate
Figure 13.8 The meiotic division of an animal cell
Homologous
chromosomes
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore

17. Telophase I and Cytokinesis
Fig. 13-8c
Telophase I and
Cytokinesis
Anaphase I
Sister chromatids
remain attached
Figure 13.8 The meiotic division of an animal cell
Homologous
chromosomes
separate
Cleavage
furrow

18. 150 Second Meiotic Cell Division: Prophase II:
Chromosomes become thickened and shortened. Centrioles move to opposite poles and send out the spindle apparatus. The nucleolus and nuclear membrane begin to disappear. No synapsis or crossing over occurs.

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

20.
Metaphase II:
The chromosomes line up in the equatorial plane with the kinetochores of the sister chromatids facing the opposite poles.
Anaphase II:
The centromeres of sister chromatids split. The sister chromatids separate to form chromosomes which move to opposite poles. During late anaphase II, cytokinesis begins. The line of cell division is perpendicular to the first division plane.

21. Telephase II and Cytokinesis
Fig. 13-8f
Telephase II and
Cytokinesis
Anaphase II
Sister chromatids
separate
Haploid daughter cells
forming
Figure 13.8 The meiotic division of an animal cell

22. 151
Telophase II:
Two new nuclei are formed. The chromosomes begin to lengthen to form chromatins. Nucleolus and centriole are also formed. Spindle fibers disappear.
Cytokinesis:
Division of cytoplasm of each of the two haploid cells forms a total of four haploid cells.

23. Prophase I Nonsister of meiosis chromatids held together
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Figure The results of crossing over during meiosis

24. Prophase I Nonsister of meiosis chromatids held together
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

25. Prophase I Nonsister of meiosis chromatids held together
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Anaphase I
Figure The results of crossing over during meiosis

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

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

28. 151 Mitosis and Meiosis Compared: Meiosis Mitosis
Synapsis of homologous No
chromosomes and the
formation of tetrads.
Crossing over of homo No
logous (nonsister)
chromatids visible by
the appearance of
chiasmata.

29. 151 Meiosis Mitosis At anaphase I, 3. At anaphase,
centromeres do not centromeres
split and sister split and sister
chromatids do not chromatids
separate. The sister separate to form
chromatids of each chromosomes,
chromosome move which move to
to the same pole the opposite poles.

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
Anaphase
Telophase
Anaphase I
Figure 13.9 A comparison of mitosis and meiosis in diploid cells
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. 151 Meiosis Mitosis No interphase between 4. Interphase
meiosis I and II between each
cell division.
Formation of four Formation of
haploid cells (gametes) two cells which
which have half the have the identical
number of chromosomes number of
as the parental cell chromosomes as
the parental cell.

32. 151-152 Origins of Genetic Variation Among Offspring:
Independent Assortment of Chromosomes:
The number of possible combinations in
human gametes is 2n where n is equal to 23. It amounts to about 8 million possible assortments of chromosomes inherited from that individual’s mother or father.
Crossing Over:
It also contributes to genetic variability. It occurs when portions of nonsister chromatids trade places.

33. 152 Random Fertilization:
A human gamete represents one out of 8.4 million possible genetic combinations. A zygote, which results from the fusion of sperm and egg, has 70 trillion diploid combinations (the exact number is 70,368,744,177,664).

34. 152 Evolutionary Significance of Genetic Variation:
The two sources of genetic variation are sexual reproduction and mutations. Genetic variation provides the raw material for evolution. Natural selection results in the survival of those organisms with the right type of genetic makeup that enables them to adapt in an ever changing environment.