Download Meiosis

Honors Biology
Unit 5
2013-2014
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Genome: Complete complement of an
organism’s DNA.
◦ Includes genes (control traits) and non-coding
DNA organized in chromosomes.
Genome > Chromosome > DNA > Genes
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Eukaryotic DNA is organized
in chromosomes.
◦ Genes have specific places on
chromosomes.
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Heredity – way of
transferring genetic
information to offspring
Chromosome theory of
heredity: chromosomes
carry genes.
Gene – “unit of heredity”.
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Asexual: self-reproducing, produce clones
(offspring genetically identical to parent)
◦ Many single-celled organisms reproduce by
splitting, budding, parthenogenesis.
◦ Some multicellular organisms can reproduce
asexually.
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Fusion of two gametes to produce a
single zygote.
With exception of self-fertilizing
organisms (e.g. some plants), zygote has
gametes from two different parents.
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Total = 46 chromosomes (23 pairs)
44 autosomes + 2 sex chromosomes
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Autosomes:
◦ Code for most genes in your body (not sex
determining chromosomes)
◦ In humans chromosome #1-22
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Sex Chromosomes:
◦ chromosomes that
determine what gender
you will be.
◦ #23 in humans
◦ Females are: XX
◦ Males are XY
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Gametes: sperm/egg
◦ haploid (n)
◦ Haploid= Contains a single set of chromosomes (23)
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Zygote: fertilized egg
◦ now diploid (2n)
◦ Diploid= Contains a two sets of chromosomes (23x2=46)
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Somatic cell: any cell other than gametes,
most of the cells in the body. – All are diploid
(2n)
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Mammals use a chromosomal method of
determining sex: XX is female and XY is
male.
Birds use a ZW system: ZZ is male and ZW is
female.
◦ the evolutionary origin of mammalian and bird sex
chromosomes is different
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Some reptiles use developmental
temperature to determine sex: depends on
the species, but hot is male and cold is
female in some.
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Chromosomes exist in homologous pairs in
diploid cells.
homologous pairs: chromosomes that code for
the same genes but possibly have different
alleles (get one from each parent)
Exception: Sex
chromosomes (X, Y).
All autosomes are in
homologous pairs.
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• Now each homologous
chromosome has a sister
chromatid
• Still considered diploid
(2n) cell because still
just two copies of each
chromosome (even
though they now have a
twin)
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All are even numbers –
diploid (2n) sets of
homologous
chromosomes!
Ploidy = number of
copies of each
chromosome.
Diploidy
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Meiosis: cell division process by which
the number of chromosomes per cell
is cut in half
Why does it occur: Meiosis is used to
produce the haploid(n) gametes
(sperm and eggs)
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Meiosis reduces the number of
chromosomes by half.
Daughter cells differ from parent,
and each other.
Meiosis involves two divisions,
Mitosis only one.
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First division of meiosis
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Prophase 1: Duplicated homologous
chromosomes match up forming tetrads.
Crossing-over occurs at the chiasmata.
Metaphase 1: Tetrads align at the equator of
the cell.
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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• produces recombinant chromosomes, mixing the
genes of the mother and father, recombining them.
• Occurs during Prophase 1
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Second division of meiosis: Gamete formation
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Prophase 2: DNA does not replicate.
Metaphase 2: Chromosomes line up at the
equatorial.
Anaphase 2: Sister chromatids move separately
to opposite ends of the cell.
Telophase 2: Cell division is complete. Four
haploid(n) daughter cells are produced.
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During normal cell growth, mitosis produces daughter
cells identical to parent cell (2n to 2n)
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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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Number of combinations: 2n
e.g. if there are 2 chromosomes in haploid
2n = 4; n = 2
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2n = 22 = 4 possible combinations
e.g. 23 chromosomes in haploid
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!
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At least 8 million combinations from Mom,
and another 8 million from Dad …
>64 trillion combinations for a diploid
zygote!!!
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Male gamete formation - Spermatogenesis
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In males, all 4 products of meiosis
develop into sperm cells. They
lose most of their cytoplasm,
remodel their cell shape, and
grow a long flagellum (tail).
Female gamete formation - Oogenesis
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In females, most of
the cytoplasm goes
into 1 of the 4 meiotic
products, which
becomes the egg.
The other 3 meiotic
cells are small “polar
bodies”, which
degenerate.
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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
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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. (some organisms have more)
◦ 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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1. How does metaphase of mitosis differ from metaphase I of meiosis?
2. What happens as homologous chromosomes pair up during
prophase 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?
5. Compare mitosis and meiosis on the following points:
a. number of daughter cells produced.
b. the amount of DNA in the daughter v. parent cell
c. mechanism for introducing genetic variation.
6. What is a zygote and how is it formed?
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1.
Summarize the process of meiosis in 5
sentences or less (include the
following in your summary)
1. What is the sole purpose of Meiosis?
2. What does it start with and what does it end
with (include males and females)
3. Use and underline vocab words gamete,
diploid, haploid, crossing over, and
homologous pairs, and sister chromatids
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