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Lecture3
Reproduction is when an entity undergoes division that
results in the production of two entities of the same
1. Reproduction of eukaryotic
somatic cells
BIOSCI101Essential
.
In biology this process is based on the reproduction of
cells and the cells genetic material.
Cell division plays several important roles in the life of
iology
. .
an entire individual, in multicellular organisms allows
growth and repair.
Mitotic cell division results in the production two
genetically identical daughter cells.
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The cell cycle
BIOSCI101Essential
iology
Cell cycle = an ordered sequence of events in the life of a
cell. Mitotic (M) phase - division & interphase - non dividing
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a) Interphase
~ 90% of cycle
Phases of the cell cycle
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intense biochemical activity
3 phases
G1 - 1st gap phase
S - DNA synthesis
G2 - 2nd gap phase
iology
b) M phase
mitosis = division of nucleus
cytokinesis = division of cytoplasm
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c) DNA in cells during cell division
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iology
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Overview of mitosis
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iology
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Mitosis - division of an animal cell
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2. Comparison of asexual & sexualreproduction
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sexua exua
One parent Two parents
Single parent
contributes all the
genes
Each parent
contributes half their
genes
iology Clones (copies) Vary genetically
The asexual reproduction of the hydra chip off the old
block or a clone
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3. Ferti lization and meiosis alternate in
the sexual l ife cycles
Life cycle = sequence of stages in an organisms
reproductive history conception to production of aBIOSCI101Essential
new o spr ng.
Fertilization = the fusion of two haploid gamete nuclei to
form diploid zygote nucleus.
Haploid = a single set of chromosomes (1n).
iology
Diploid = the condition in which each autosome is
represented twice (2n).
Meiosis = the two successive nuclear divisions in which
a single diploid (2n) cell forms four haploid (1n) nuclei.
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The human life cycle
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iology
The only cells that are are not produced by mitosis
are the gametes.
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Overview of meiosis
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iology
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4. Meiosis & independent assortment
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iology
A cell in G2 of interphase, following chromosome
replication
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Independent assortment
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iology
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Independent assortment of chromosomes
generates variation among offspring
At metaphase I, homologous pair, each consisting ofBIOSCI101Essential
one ma erna an one pa erna c romosome, a gn n a
random orientation.
Therefore each homologue assorts independently of
other chromosomes.
iology
The number of possible combinations for n = 2 is 4 -(2n), however when n = 23 as in humans, 8 million
combinations of chromosomes inherited from your
mother and father (223) are possible.
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Crossing over occurs during prophase I.
5. Crossing over during meiosis &
random fertilisation
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,
individual chromosomes carrying genes derived from
two different parents.
Crossing over is the exchange of genetic material
between non-sister chromatids.
iology
In humans 1-3 crossover events occur per chromosome
pair.
Shuffles the genes within the chromosomes.
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The random nature of fertilization adds to the genetic
Random fertilization
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.
Even without crossing over in humans each male and
female gamete represents 8 million possible chromosome
combinations.
The fusion of a sin le male amete with a sin le femaleiology
gamete will produce a zygote with any of about 64 trillion
(8 million x 8 million) diploid combinations.
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Mechanism Description of mechanism
Origin of genetic variation in sexually
reproducing organisms
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MutationMutation is the original source of all variation and
is very rare
Crossing overShuffles genes within a chromosome, individuals
carrying genes derived from two different parents.
iology
assortment
independently during gamete formation.
Random
fertilization
At fertilization any male gamete can fuse with any
other female gamete and produce a zygote with
any of a possible ~ 64 trillion diploid combinations
of parental chromosomes.