A study in creating sex cells
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Genome
 Genome: Complete complement of an
organism’s DNA.
 Includes genes (control traits) and
non-coding DNA organized in
chromosomes.
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Genes
Eukaryotic DNA is
organized in
chromosomes.
Genes have specific
places on
chromosomes.
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Heredity
 Heredity – way of
transferring genetic
information to
offspring
 Chromosome theory of
heredity:
chromosomes carry
genes.
 Gene – “unit of
heredity”.
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Reproduction
 Asexual
 Many single-celled organisms reproduce by splitting,
budding, parthenogenesis.
 Some multicellular organisms can reproduce
asexually, produce clones (offspring genetically
identical to parent).
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Reproductions: some vocabulary
 Gamete: the name given to sex cells
(either a sperm or an egg)
 Fertilization: the process when the
sperm cell and the egg cell fuse
together
 Zygote: The cell created when the egg
and sperm cell fuse together
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Sexual reproduction
 Fusion of two gametes to produce a single zygote.
 Gametes have half the number of chromosomes
In humans: 23 chromosomes (1n)
 The zygote is formed when the gametes fuse
 In humans, this forms 46 chromosomes (2n)
 Introduces greater genetic variation, allows genetic
recombination.
 With exception of self-fertilizing organisms (e.g.
some plants), zygote has gametes from two
different parents.
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
Importance of Sex Cells
 You need to reduce the number of chromosomes
in sex cells to half (1n)
 Otherwise when they fused, you would keep
doubling the number of chromosomes
 Example:
 In humans, generation 1 would have 46
 Generation2 would have 92
 Generation3 would have 184, etc.
 These organisms would not survive
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Importance of Sex Cells
Sexual repro starts with sex cell
& ends with fertilization
Zygote is formed--in human
now is diploid or 2n with 46
chromosomes
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Why do we need to create sex cells?
 During normal cell growth, mitosis produces
daughter cells identical to parent cell (2n to
2n)
 Meiosis results in genetic variation by
shuffling of the DNA.
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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In humans
If you have 1 chromosome, you have 2 possibilities
If you have 2 chromosome, you have 4 possibilities
With 23 chromosomes in in each gamete
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations
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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!!!
That’s more than all humans that have ever lived.
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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 and disease (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.
 QUESTION: Why is genetic diversity so
important?
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But …
 More genetic diversity = more potential for survival of
species when environmental conditions change.
 Shuffling of genes
 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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In humans …
 23 chromosomes donated by each parent (total
= 46 or 23 pairs).
 Gametes (sperm/ova):
 Contain 22 autosomes and 1 sex chromosome.
 Are haploid (haploid number “n” = 23 in humans).
 Fertilization/syngamy results in zygote with 2
haploid sets of chromosomes - now diploid.
 Diploid cell; 2n = 46. (n=23 in humans)
 Most cells in the body produced by mitosis.
 Only gametes are produced by meiosis.
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MEIOSIS I
 Unlike mitosis, meiosis occurs in two
parts
 Meiosis I is similar to mitosis where
you are creating a 2n number of
chromosomes
 Meiosis II is the process where you
create the 1n number of
chromosomes in the sex cells
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Meiosis I --Sex Cell Formation
 In meiosis, there are 2 divisions of the nucleus: meiosis
I & meiosis II
 Prophase I: double stranded chromosomes and
spindle fibers appear; nuclear membrane and
nucleolus fade (CROSSING OVER occurs here)
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Meiosis I --Sex Cell Formation
 Metaphase I: chromosome pairs (chromatids) line up
 spindle fibers attach to centromeres and centrioles
 Anaphase I: chromotids separate from matching pair
(independent assortment occurs here)
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Meiosis I --Sex Cell Formation
 Telophase I: cytoplasm divides and 2 cells form
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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 equatorial plate.
 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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Summary of Meiosis I
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Meiosis II
At the end of meiosis I, you have
created 2 cells with a 2n number of
chromosomes
The purpose of meiosis II is to take
those 2 cells and create a total of 4
cells, each with n number of
chromosomes
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Meiosis II --Sex Cell Formation
 Prophase II: chromatids and spindle fibers reappear
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Meiosis II --Sex Cell Formation
 Metaphase II: chromatids line up in the center of the cell
 spindle fibers attach to centromere & centriole
 Anaphase II: centromere divides
 chromosomes split and move to opposite poles
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Meiosis II --Sex Cell Formation
 Telophase II: spindle fibers disappear
 nuclear membrane forms around chromosomes at each end of cell
 each nucleus has half the # of chromosomes as the original
(haploid)
 now there are 4 sex cells (daughter cells)
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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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Summary of Meiosis II
Meiosis I and Meiosis II
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Animation
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Meiosis creates genetic variation
 There are a couple of methods to increase genetic
variability in sex cells
 Crossing-over
 Occurs mainly during prophase I
 When adjacent chromatids break of a portion of their
structure and switch places
 Independent assortment
 Occurs mainly during anaphase I
 When homologous chromosomes separate randomly
and move to opposite poles of a dividing cell
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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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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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Chromosomes
 Karyotype:
 ordered display of an individual’s chromosomes.
 Collection of chromosomes from mitotic cells.
 Staining can reveal visible band patterns, gross
anomalies.
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Karyotyping
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Homologues
 Chromosomes exist in homologous pairs in diploid
cells.
Exception: Sex chromosomes (X, Y).
Other chromosomes are known as autosomes,
they have homologues.
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Meiosis – key differences from
mitosis
 Meiosis reduces the number of chromosomes by
half.
 Daughter cells differ from parent, and each other.
 Meiosis involves two divisions, Mitosis only one.
 Meiosis I involves:
 Synapsis – homologous chromosomes pair up.
Chiasmata form (crossing over of non-sister
chromatids).
 In Metaphase I, homologous pairs line up at
metaphase plate.
 In Anaphase I, independent assortment of
chromosomes.
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Mitosis vs. meiosis
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