Sexual Reproduction
and Meiosis
Chapter 11
Sexual reproduction includes the fusion of
cells from two organisms to produce the
next generation.
Creates more diversity by mixing information
from two individuals.
Creates a problems in terms of the amount
of information per cell.
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If each cell has X units of information, how
many units will the next generation have?
__2X_____
And the one after this?
___4X____
Ad naseum
How to control the amount of genetic
information per cell?
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Overview of Meiosis
Meiosis is a form of cell division that leads to
the production of gametes.
gametes: egg cells and sperm cells
-contain half the number of chromosomes of an
adult body cell
Adult body cells (somatic cells) are diploid,
containing 2 sets of chromosomes.
Gametes are haploid, containing only 1 set of
chromosomes.
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Sexual reproduction includes the fusion of
gametes (fertilization) to produce a
diploid zygote.
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.
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Fig. 11.2.c
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Features of Meiosis
Meiosis includes two rounds of division –
meiosis I and meiosis II.
During meiosis I, homologous chromosomes
(homologues) become closely associated
with each other. This is synapsis.
Proteins between the homologues hold them
in a synaptonemal complex.
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Features of Meiosis
Crossing over: genetic recombination
between non-sister chromatids
-physical exchange of regions of the chromatids
chiasmata: sites of crossing over
The homologues are separated from each
other in anaphase I.
The kinetochores do NOT split
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Features of Meiosis
Meiosis involves two successive cell
divisions with no replication of genetic
material between them.
This results in a reduction of the
chromosome number from diploid to
haploid.
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The Process of Meiosis
Prophase I:
-chromosomes coil tighter
-nuclear envelope dissolves
-homologues become closely associated in
synapsis
-crossing over occurs between non-sister
chromatids
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The Process of Meiosis
Metaphase I:
-terminal chiasmata hold homologues
together following crossing over
-microtubules from opposite poles attach to
each homologue, not each sister chromatid
-homologues are aligned at the metaphase
plate side-by-side
-the orientation of each pair of homologues on
the spindle is random
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The Process of Meiosis
Anaphase I:
-microtubules of the spindle shorten
-homologues are separated from each other
-sister chromatids remain attached to each
other at their centromeres
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The Process of Meiosis
Telophase I:
-nuclear envelopes form around each set of
chromosomes
-each new nucleus is now haploid
-sister chromatids are no longer identical
because of crossing over
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The Process of Meiosis
Meiosis II resembles a mitotic division:
-prophase II: nuclear envelopes dissolve
and 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;
cytokinesis follows
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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.
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Meiosis vs. Mitosis
Meiosis produces haploid cells that are not
identical to each other.
Genetic differences in these cells arise from:
-crossing over
-random alignment of homologues in
metaphase I (independent assortment)
Mitosis produces 2 cells identical to each
other.
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Fig. 11.9(Part-I)
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Fig. 11.9(Part-II)
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Tab. 13.1
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Sea Turtles
Eggs laid in nests dug in the sand and then covered
Temperature during development
Above a certain temperature – one sex
Below that temperature – the other
Temperature within a nest can vary enough so that
get males & females from the same nest
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Plants
•Perfect - male & female in same flower
•Imperfect - separate male & female flowers
•Monecious: same plant
•Diecious: different plants
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Hermaphroditism - both male &
female sex organs in same organism
Simultaneous Hermaphroditism
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Sequential Hermaphroditism
Some fishes
Some oyster
Some Slipper Shells (Crepidula
fornicata)