BIO NOTES - Genetic Variation
● We have seen that meiosis provides a way to keep the chromosome number
constant generation after generation.
○ Without meiosis, the chromosome number of the next generation would
continually increase.
○ The events of meiosis also help ensure that genetic varia.tion occurs with
each generation.
○ Genetic variation is essential for a species to be able to evolve and adapt
in a changing environment.
○ Asexually repro.ducing organisms, such as the prokaryotes, depend
primarily on mutations to generate variation among offspring.
○ This is suf.ficient for their survival, because they produce great numbers of
offspring very quickly.
○ Although mutations also occur among sexually reproducing organisms,
the reshuffling of genetic mate.rial during sexual reproduction ensures that
offspring will have a different combination of genes than their parents.
○ Meiosis brings about genetic variation in two key ways: crossing-over and
inde.pendent assortment of homologous chromosomes.
● Genetic Recombination
○ Crossing-over is an exchange of genetic material between non-sister
chromatids of a bivalent during meiosis I.
○ In humans, it is estimated that an average of two to three crossovers
occur between the nonsister chromatids during meiosis.
○ At synapsis, homologues line up side by side, and a nucleoprotein lattice
appears between them
■ This lattice holds the bivalent chiasmata of nonsister chromatids 1
and 3
■ Crossing-over during meiosis I.
● a. The homologous chromosomes pair up, and a
nucleoprotein lattice develops between them.
, ○ This is an electron micrograph of the lattice.
○ It “zippers” the members of the bivalent together, so
that corresponding genes on paired chromosomes
are in alignment.
● b. This visual representation shows only two places where
nonsister chromatids 1 and 3 have come in contact.
● c. Chiasmata indicate where crossing-over has occurred.
The exchange of color represents the nonsister chromatids
during meiosis I.
● d. Following meiosis II, daughter chromosomes have a new
combination of genetic material due to crossing-over, which
occurred between Daughter chromosomes together in such
a way that the DNA of the duplicated chro.mosomes of each
homologue pair is aligned.
○ This ensures that the genes contained on the
nonsister chromatids are directly aligned.
○ Now crossing-over may occur.
○ As the lattice breaks down, homologues are
temporarily held together by chiasmata (sing.,
chiasma), regions where the nonsister chromatids are
attached due to DNA strand exchange and
crossing-over.
○ After exchange of genetic information between the
nonsister chro.matids, the homologues separate and
are distributed to different daughter cells.
■ To appreciate the significance of crossing-over, keep in mind that
the members of a homologous pair can carry slightly different
instructions, or alleles, for the same genetic traits.
■ In the end, due to a swapping of genetic material during
crossing-over, the chromatids held together by a centromere are no
longer identical.
● We have seen that meiosis provides a way to keep the chromosome number
constant generation after generation.
○ Without meiosis, the chromosome number of the next generation would
continually increase.
○ The events of meiosis also help ensure that genetic varia.tion occurs with
each generation.
○ Genetic variation is essential for a species to be able to evolve and adapt
in a changing environment.
○ Asexually repro.ducing organisms, such as the prokaryotes, depend
primarily on mutations to generate variation among offspring.
○ This is suf.ficient for their survival, because they produce great numbers of
offspring very quickly.
○ Although mutations also occur among sexually reproducing organisms,
the reshuffling of genetic mate.rial during sexual reproduction ensures that
offspring will have a different combination of genes than their parents.
○ Meiosis brings about genetic variation in two key ways: crossing-over and
inde.pendent assortment of homologous chromosomes.
● Genetic Recombination
○ Crossing-over is an exchange of genetic material between non-sister
chromatids of a bivalent during meiosis I.
○ In humans, it is estimated that an average of two to three crossovers
occur between the nonsister chromatids during meiosis.
○ At synapsis, homologues line up side by side, and a nucleoprotein lattice
appears between them
■ This lattice holds the bivalent chiasmata of nonsister chromatids 1
and 3
■ Crossing-over during meiosis I.
● a. The homologous chromosomes pair up, and a
nucleoprotein lattice develops between them.
, ○ This is an electron micrograph of the lattice.
○ It “zippers” the members of the bivalent together, so
that corresponding genes on paired chromosomes
are in alignment.
● b. This visual representation shows only two places where
nonsister chromatids 1 and 3 have come in contact.
● c. Chiasmata indicate where crossing-over has occurred.
The exchange of color represents the nonsister chromatids
during meiosis I.
● d. Following meiosis II, daughter chromosomes have a new
combination of genetic material due to crossing-over, which
occurred between Daughter chromosomes together in such
a way that the DNA of the duplicated chro.mosomes of each
homologue pair is aligned.
○ This ensures that the genes contained on the
nonsister chromatids are directly aligned.
○ Now crossing-over may occur.
○ As the lattice breaks down, homologues are
temporarily held together by chiasmata (sing.,
chiasma), regions where the nonsister chromatids are
attached due to DNA strand exchange and
crossing-over.
○ After exchange of genetic information between the
nonsister chro.matids, the homologues separate and
are distributed to different daughter cells.
■ To appreciate the significance of crossing-over, keep in mind that
the members of a homologous pair can carry slightly different
instructions, or alleles, for the same genetic traits.
■ In the end, due to a swapping of genetic material during
crossing-over, the chromatids held together by a centromere are no
longer identical.
