Download Monday, November 17, 2014 Agenda: Cell Organelle Analogy

Monday, November 17, 2014
Agenda:
Cell Organelle Analogy Poster
Learning objective: Students will
develop a model for cells using analogies.
HW: Finish reading/Venn Diagrams
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We will present tomorrow
•  Make sure your presentation includes:
•  The analogy and function for each cell
part
•  Everyone in the group saying
something
•  Make sure you talk about the hydrolytic
enzymes with lysosomes
•  Keep it under 2 minutes
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Monday, November 17, 2014
Agenda:
Meiosis and Genetic Variation
Learning Objective: Students will
understand how meiosis contributes to
genetic variation.
HW: Review Meiosis notes.
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Warm-up
•  Discuss the questions using pictures A
and B.
Picture A
Picture B
Meiosis and Genetic Variation
Why did some of the mice babies
have black fur and some had
white fur?
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Genome
•  A Genome is the complete set of
someone’s DNA.
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Genes
•  Eukaryotic DNA is
organized in chromosomes.
–  Genes have specific places
on chromosomes.
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Heredity
•  Heredity is a way of passing
genetic information on to
offspring.
•  The Chromosome Theory of
Heredity says that
chromosomes carry genes.
•  Gene – “unit of heredity”.
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Reproduction
•  Asexual Reproduction
–  Many single-celled organisms reproduce by
splitting, budding, and parthenogenesis.
–  Some multicellular organisms can reproduce
asexually, which makes clones. (offspring that are
genetically identical to the parent).
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Sexual Reproduction
•  Sexual reproduction
is when two gametes
come together.
•  A fertilized egg is
called a zygote.
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•  Reproducing sexually
allows genes to mix
into new
combinations. This
process is called
genetic
recombination.
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Debate: Do you think it is better to
reproduce sexually or asexually?
What are the benefits of each?
Benefits of
Asexual
Reproduction
Benefits of Sexual
Reproduction
- Faster
- Not dependent
on a mate
- More genetic
diversity
- Less likely to
get a bad gene
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Chromosomes
•  A karyotype is a picture of an
individual’s chromosomes.
•  The light and dark bands are different
genes.
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Karyotyping
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Homologous Chromosomes
•  Two copies of the same chromosome are
called homologues. Together, they are called
a homologous pair.
•  Autosomes (chromosomes 1-22) have
homologues, sex chromosomes do not (X
and Y don’t match).
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In humans …
•  A diploid cell has 2 copies of each
chromosome.
•  A haploid cell only has 1 copy of each
chromosome.
•  The haploid number is the number of
chromosomes in one set (n). In humans, n
= 23.
•  Gametes (sperm/ova) are haploid (n).
•  Somatic cells are diploid (2n).
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Chromosome Numbers
All are even numbers –
diploid (2n) sets of
homologous
chromosomes!
Ploidy = number of
copies of each
chromosome.
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Human cells are
diploid because they
have 2 copies of each
chromosome.
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Meiosis
•  How do we get a haploid cell (gamete)
from a diploid cell (somatic)?
•  Answer: Meiosis!
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How is meiosis different from mitosis?
•  Meiosis reduces the number of chromosomes
by half.
•  In Meiosis the cell divides twice : Meiosis I and
Meiosis 2.
The DNA copies once
(during Prophase I),
but is pulled apart
twice (during
Anaphase I and
Anaphase II).
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In Meiosis I:
Homologous chromosomes
pair up and some DNA
from each crosses over to
the other chromosome.
This process is called
crossing over.
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Meiosis 1
First division of meiosis
•  Prophase 1: Each chromosome dupicates
and remains closely associated. These are
called sister chromatids. Crossing-over can
occur during the latter part of this stage.
•  Metaphase 1: Homologous chromosomes
align at the center.
•  Anaphase 1: Homologous pairs separate
with sister chromatids remaining together.
•  Telophase 1: Two daughter cells are formed
with each daughter containing only one
chromosome of the homologous pair.
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Meiosis II
Second division of meiosis: Gamete formation
•  Prophase 2: DNA does not replicate.
•  Metaphase 2: Chromosomes align at
the equatorial plate.
•  Anaphase 2: Centromeres divide and
sister chromatids migrate separately to
each pole.
•  Telophase 2: Cell division is complete.
Four haploid daughter cells are
obtained.
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Mitosis vs. meiosis
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Meiosis creates genetic variation
•  During normal cell growth, mitosis produces
daughter cells identical to parent cell (2n
to 2n)
•  Meiosis results in genetic variation by
shuffling of maternal and paternal
chromosomes and crossing over.
No daughter cells formed during meiosis
are genetically identical to either mother or
father
During sexual reproduction, fusion of the
unique haploid gametes produces truly
unique offspring.
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Independent assortment
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Independent assortment
Number of combinations: 2n
e.g. 2 chromosomes in haploid
2n = 4; n = 2
2n = 22 = 4 possible combinations
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In humans
e.g. 23 chromosomes in haploid
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!
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Crossing over
Chiasmata – sites of crossing
over, occur in synapsis.
Exchange of genetic material
between non-sister chromatids.
Crossing over produces
recombinant chromosomes.
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Random fertilization
At least 8 million combinations from Mom,
and another 8 million from Dad …
>64 trillion combinations for a diploid
zygote!!!
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Meiosis & sexual life cycles
•  Life cycle = sequence
of stages in organisms
reproductive history;
conception to
reproduction.
•  Somatic cells = any
cell other than
gametes, most of the
cells in the body.
•  Gametes produced by
meiosis.
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Generalized animal life cycle
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Sex is costly!
•  Large amounts of energy required to find a
mate and do the mating: specialized
structures and behavior required
•  Intimate contact provides route for infection
by parasites (AIDS, syphillis, etc.)
•  Genetic costs: in sex, we pass on only half of
genes to offspring.
•  Males are an expensive luxury - in most
species they contribute little to rearing
offspring.
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But …
•  More genetic diversity: more potential for survival
of species when environmental conditions
change.
–  Shuffling of genes in meiosis
–  Crossing-over in meiosis
–  Fertilization: combines genes from 2 separate
individuals
•  DNA back-up and repair.
–  Asexual organisms don't have back-up copies of genes,
sexual organisms have 2 sets of chromosomes and
one can act as a back-up if the other is damaged.
–  Sexual mechanisms, especially recombination, are
used to repair damaged DNA - the undamaged
chromosome acts as a template and eventually both
chromosomes end up with the correct gene.
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Study Questions
•  1. What happens as homologous
chromosomes pair up during prophase I
of meiosis?
•  2. How does metaphase of mitosis
differ from metaphase I of meiosis?
•  3. What is the sole purpose of
meiosis?
•  4. What specific activities, involving
DNA, occur during interphase prior to
both mitosis and meiosis?
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"5. Compare mitosis and meiosis on the
following points:
•  " a. number of daughter cells
produced.
•  " b. the amount of DNA in the
daughter cells in contrast to the original
cell.
•  " c. mechanism for introducing
genetic variation.
•  6. What is a zygote and how is it
formed?
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