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Chapter 9

Biology 1201A Chapter Notes - Chapter 9: Genetic Recombination, Dna Replication, Gamete


Department
Biology
Course Code
BIOL 1201A
Professor
Jennifer Waugh
Chapter
9

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Chapter 9 - Genetic Recombination
Why It Matters
When mating, it is important that the individuals are genetically different from one
another.
If populations have genetic variability, they have potential to evolve.
Natural Selection can act on this variability such that certain variants leave more
offspring than others.
Over time, the relative proportion of different variants will change - this is
evolution in action.
The ultimate source of genetic diversity is mutation of the DNA sequence, often
resulting from errors during DNA replication.
Since mutations a relatively rare, diversity is amplified through various mechanisms that
shuffle existing mutations into novel combinations.
This process is called genetic recombination - the cutting & pasting of DNA
backbones into new combinations.
Allows "jumping genes" to move, inserts some viruses into the
chromosomes of their hosts, underlies the spread of antibiotic resistance
in archaea and bacteria, & is the heart of meiosis in eukaryotic organisms.
Puts the "sexual" in sexual reproduction - w/o it, reproduction is asexual,
& offspring are simply identical clones of their parent.
most of the recombination discussed in this chapter occurs b/w regions of DNA that are
very similar, but not identical, in the sequence of bases.
These regions are called homologous chromosomes.
Homology allows different DNA molecules to line up & recombine precisely.
Once homologous regions of DNA are paired, enzymes break a covalent bond in
each of the 4 sugar-phosphate backbones.
The free ends of each backbone are then exchanged & reattached to those of
the other DNA molecule.
The result is 2 recombined molecules in which the original DNA is bound to the
other DNA, & vice-versa.
Cutting & pasting 4 DNA backbones results in ONE recombination event.
9.1 - Mechanism of Genetic Recombination.
In its most general sense, genetic recombination requires the following:
Two DNA molecules that differ from one another.
A mechanism for bringing those molecules into close proximity.
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A collection of enzymes to "cut," "exchange," & "paste," the DNA back together.
Figure 9.1 - basic diagram of genetic recombination.
1. Two molecules of DNA w/ similar sequence are brought together in close
proximity.
2. Enzymes nick DNA backbones, swap the ends, & reattach them.
3. Final result is 2 recombined DNA molecules.
9.3 - Genetic Recombination in Eukaryotes: Meiosis.
Sexual Reproduction - the production of offspring through the union of male & female
gametes.
Depends on meiosis - a specialized process of cell division that recombines DNA
sequences & produces cells w/half the number of chromosomes present in the
somatic (body) cells.
At fertilization, nuclei of an egg & sperm of a cell fuse, producing an egg called a zygote,
in which the chromosome number of the typical species is restored.
W/o meiosis, fertilization would double the number of chromosomes in each
subsequent generation.
Both meiosis & Fertilization mix genetic information into new combinations; thus, none
of the offspring of a mating pair are likely to be genetically identical to their parents or
siblings.
9.3a - Meiosis Occurs in Different Places in Different Organismal Life Cycles.
Although the life cycle of nearly all eukaryotes alternates with one basic set of
chromosomes (haploid), & a stage w/ 2 basic sets of chromosomes (diploid,) evolution
has produced a wide variety in the relative timing of mitosis, meiosis, & fertilization
among different species.
It might be assumed that gametes are made by meiosis - this is true only for humans &
other animals.
In the life cycle of houseplants & some fungi living in the soil in the park, the
haploid products of meiosis are spores, not gametes.
These spores divide by mitosis to form multicellular bodies that, in turn, make
gametes by mitosis.
Many organisms make gametes by mitosis.
Animals
Follow the pattern in which the diploid phase dominates the life cycle, the haploid
phase is reduced, & meiosis is followed directly by gamete formation.
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Can be thought of as the "diploid life cycle," since the diploid stage is
multicellular.
In males, each of the 4 nuclei produced by meiosis is enclosed in a separate cell by
cytoplasmic divisions, & each of the 4 cells differentiates into a functional sperm cell.
In females, only 1/4 nuclei becomes functional as an egg nucleus.
Fertilizations restores the diploid phase of the life cycle - thus, animals are haploids only
as sperm or eggs, & no mitotic divisions occur during the haploid phase of the life cycle.
Most Plants & Some Fungi
These organisms alternate b/w haploid & diploid generations in which, depending on
the organism, either generation may dominate the life cycle, & mitotic division occur in
both phases.
Can be thought of as the, "alternating-generations life cycle," since both diploid
& haploid stages can be multicellular.
In these organisms, fertilization produces the diploid generation, in which the
individuals are called sporophytes.
After they grow into maturity by mitotic division, some of their cells undergo
meiosis, producing haploid, genetically different cells called spores.
These spores are not gametes; they germinate & grow directly by mitotic
divisions into a generation of haploid cells called gametophytes.
At maturity, the nuclei of some cells in the gametophytes develop into egg or
sperm nuclei.
All the egg / sperm nuclei produced by a particular gametophyte are
genetically identical b/c they arise through mitosis.
Fusion of a haploid egg & sperm nucleus produces a diploid zygote nucleus that
divides by mitosis to produce the diploid sporophyte generation again.
In all plants, except bryophytes, the diploid sporophyte generation is the most visible
part of the plant.
The gametophyte generation is reduced to an almost microscopic stage that
develops in the reproductive parts of the sporophytes.
EX - flowering plants.
the female gametophyte remains in the flower, while the male
gametophyte is released as pollen.
When pollen comes in contact with a flower of the same species, it
releases a haploid nucleus that fertilizes a haploid egg cell of the female
gametophyte in the flower.
The resulting cell (zygote), reproduces by mitosis to form a sporophyte.
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