Transcript Diploid zygote is very transient in lower eukaryotes

Chapter 13
–Is Sex Necessary?
Well, is it??
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The Benefits of Sex
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A novel assortment of genes
•Starting with meiosis, cell fusion, the
diploid state, and meiosis again in the
new individual, old combinations of
genes are broken up and new
combinations are created.
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Diploid Phase (well, I guess this isn’t
really a benefit but it is worth
remembering).
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A competitive advantage in a variable
environment.
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And whatever the benefits of sex really
are, all complex, present-day organisms
have evolved through sexual rather than
asexual means.
In Vitro Human Fertilization
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Diploid zygote is very transient in
lower eukaryotes as opposed to the
higher euks which have a dominant
diploid state.
Primitive plants such as mosses and
ferns, euks, have a dominant haploid
state.
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That’s right!!! It’s a clam
egg with hopeful sperm.
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• Summary: ___________ _______________ has been favored by
evolution probably because the _________________ recombination of
genetic information improves the chances of producing at least some
offspring that will ________________ in an unpredictably variable
environment. The sexual reproductive cycle involves the alternation of
______________ and _____________ states; diploid cells divide by
_____________ to form haploid cells, and the haploid cells from two
individuals fuse in pairs at fertilization to form new _________ cells.
In the process, genomes are mixed and recombined to produce
individuals that inherit novel assortments of genes. Most of the life
cycle of higher plants and animals is spent in the _________ phase;
only a small proportion of the diploid cells undergo _________ to
produce haploid cells (the gametes) and the haploid phase is very
_________.
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Chromosome Theory of Heredity
and Meiosis
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Compare and Contrast
• one more nuclear
division is required in
meiosis for a total of 2 to
produce the haploid
gametes.
• each diploid cell undergoing meiosis produces 4
genetically different
haploid cells; mitosis
produces 2 genetically
identical diploid cells.
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Things to note:

sister chromatids in each pair are tightly connected along their length
by the cohesins.

sister chromatids are connected at their centromeres

so it is the chiasma plus the connection of the sister chromatids that
keep these two duplicated chromosomes together.
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Bivalent with 3 chiasmata
•
What to note:
 crossing over does not occur
between sister chromatids of the
same chromosome.
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Importance of crossing over:
mutants that have no
chiasmata fail to separate
normally and so resulting
gametes contain too many or
too few chromosomes.
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• Well, I can see how similarly shaped chromosomes can
pair up but. . . What about those X and Y chromosomes?
Do they pair up also and form chiasmata?
 X chromosomes of females: well, they already are of similar size
so yes, they pair and segregate normally.
 X and Y
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The Crossover Event
requires a “meiotic
endonuclease.”

Crossover Interference
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• Summary: The formation of both eggs and sperm begins in a similar way,
with the process called ______________. In this process ______ (a number)
successive cell divisions following one round of DNA replication give rise to
______ (a number) haploid cells from a single diploid cell. Meiosis is
dominated by prophase of meiotic division I, which can occupy 90% or more
of the total meiotic period. As it enters this prophase, each chromosome
consists of two tightly joined _________ _____________. The two replicated
homologs present in each diploid nucleus then pair to form a bivalent,
consisting of ______ ___________________. Chromosomal crossover events
occur during this time. Each results in the formation of a ______________,
which helps hold each pair of homologs together during ______________ I.
Crossing-over has an important role in _____________ genes during gamete
formation, and it allows geneticists to map the relative positions of genes on
chromosomes. The pairing of homologs culminates in the formation of a
structure called a synaptonemal complex, which somehow serves to spread out
the crossover events along the chromosomes. At anaphase of the first meiotic
cell division, the _____ of the sister chromatids suddenly become unglued,
causing one member of each chromosome pair, still composed of a pair of
sister chromatids linked at their _________________, to be distributed to each
daughter nucleus. A _____________ cell division cycle, without DNA
replication, then rapidly ensues; in anaphase II, each sister chromatid separates
from its sister and is segregated into a separate haploid nucleus.
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Sex Determination in Mammals
• Primordial Germ Cells Migrate into the Developing Gonad
• Most animals, the unfertilized egg is asymmetrical. Upon
fertilization and development of the embryo, specific molecules in
the egg’s cytoplasm become primordial germ cells.
• In mammals . . .
• Totipotent
• Genital Ridge of the gonad
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• So It’s Not Temperature After All?
• The Sry gene on the Y chromosome can redirect a female embryo
to become a male.
• Individuals with a Y chromosome will as males no matter how
many X chromosomes they have. Individuals without a Y
chromosome develop as females, even if they have only one X
chromosome.
• The Y chromosome induces the cells of the genital ridge to
develop into a testis instead of an ovary.
• Sry gene or Sex-determining region of Y
• When this gene is introduced into the genome of an XX
mouse zygote, the transgenic embryo produced a male; but
no sperm production.
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