3. Sexual life cycle
• Made up of meiosis and fertilization
• Diploid cells
– Somatic cells of adults have 2 sets of
chromosomes
• Haploid cells
– Gametes have only 1 set of chromosomes
• Offspring inherit genetic material from 2
parents
3
5. 5
• Life cycles of sexually reproducing organisms
involve the alternation of haploid and diploid
stages
• Some life cycles include longer diploid phases,
some include longer haploid phases
• In most animals, diploid state dominates
– Zygote first undergoes mitosis to produce diploid cells
– Later in the life cycle, some of these diploid cells
undergo meiosis to produce haploid gametes
7. 7
Features of Meiosis
• Meiosis includes two rounds of division
– Meiosis I and meiosis II
– Each has prophase, metaphase, anaphase, and
telophase stages
• Synapsis
– During early prophase I
– Homologous chromosomes become closely
associated
– Includes formation of synaptonemal complexes
• Formation also called tetrad or bivalents
8. 8
• First meiotic division is termed the “reduction
division”
– Results in daughter cells that contain one homologue
from each chromosome pair
• No DNA replication between meiotic divisions
• Second meiotic division does not further reduce
the number of chromosomes
– Separates the sister chromatids for each homologue
9. The Process of Meiosis
• Meiosis I
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I
• Meiosis II
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II
• Meiotic cells have an
interphase period that
is similar to mitosis
with G1, S, and G2
phases
• After interphase,
germ-line cells enter
meiosis I
9
11. Prophase I
• Chromosomes coil tighter and become visible,
nuclear envelope disappears, spindle forms
• Each chromosome composed of 2 sister
chromatids
• Synapsis
– Homologues become closely associated
– Crossing over occurs between nonsister chromatids
– Remain attached at chiasmata
• Chiasmata move to the end of the chromosome
arm before metaphase I
11
12. 12
Crossing over
• Genetic recombination between nonsister
chromatids
• Allows the homologues to exchange
chromosomal material
• Alleles of genes that were formerly on
separate homologues can now be found
on the same homologue
• Chiasmata – site of crossing over
– Contact maintained until anaphase I
15. 15
Metaphase I
• Terminal chiasmata hold homologues
together following crossing over
• Microtubules from opposite poles attach to
each homologue
– Not each sister chromatid as in mitosis
• Homologues are aligned at the metaphase
plate side-by-side
• Orientation of each pair of homologues on
the spindle is random
18. 18
Anaphase I
• Microtubules of the spindle shorten
– Chiasmata break
• Homologues are separated from each other and
move to opposite poles
– Sister chromatids remain attached to each other at
their centromeres
• Each pole has a complete haploid set of
chromosomes consisting of one member of each
homologous pair
• Independent assortment of maternal and
paternal chromosomes
20. 20
Telophase I
• Nuclear envelope re-forms around each
daughter nucleus
• Sister chromatids are no longer identical
because of crossing over (prophase I)
• Cytokinesis may or may not occur after
telophase I
• Meiosis II occurs after an interval of
variable length
22. 22
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23. 23
Meiosis II
• Resembles a mitotic division
• Prophase II: nuclear envelopes dissolve
and new spindle apparatus forms
• Metaphase II: chromosomes align on
metaphase plate
• Anaphase II: sister chromatids are
separated from each other
• Telophase II: nuclear envelope re-forms
around 4 sets of daughter chromosomes;
cytokinesis follows
28. Final result
• Four cells containing haploid sets of
chromosomes
• In animals, develop directly into gametes
• In plants, fungi, and many protists, divide
mitotically
– Produce greater number of gametes
– Adults with varying numbers of gametes
28
29. 29
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30. Errors in Meiosis
• Nondisjunction – failure of chromosomes
to move to opposite poles during either
meiotic division
• Aneuploid gametes – gametes with
missing or extra chromosomes
• Most common cause of spontaneous
abortion in humans
30
31. 31
Meiosis vs. Mitosis
Meiosis is characterized by 4 features:
1. Synapsis and crossing over
2. Sister chromatids remain joined at their
centromeres throughout meiosis I
3. Kinetochores of sister chromatids attach
to the same pole in meiosis I
4. DNA replication is suppressed between
meiosis I and meiosis II
