Transcript Meiosis

MEIOSIS
(making sperm and egg cells…)
DNA Passes from Parent to
Offspring
•
Two types of reproduction
(making more of you):
1. Asexual reproduction →
a single parent reproduces
by itself
http://artfiles.art.com/images/-/Oxford-Scientific/Green-Hydra-Budding-Photographic-PrintC12940880.jpeg
– Parent and offspring (child)
are genetically identical
– e.g., bacteria, many plants
and fungi
http://lifeboat.com/images/bacteria.jpg
DNA Passes from Parent to
Offspring
2. Sexual reproduction →
2 cells (different parents)
unite to produce the first
cell of the new offspring
– parents and offspring are
genetically unique
Review of Chromosomes
• Chromosome → condensed DNA,
containing all genes
• Normal cells = diploid (2n) → 2 copies
of each chromosome (good idea!
backup copy!)
• In diploid cells, 1 copy came from
each parent (people: 1 from mom, 1
from dad)
Offspring have the same amount of DNA
as their parents…so how can parents only
pass on HALF their genetic material?
What is meiosis?
• Meiosis → special cell division for sexual
reproduction
• Produces haploid (1n) cells → 1 copy of each
chromosome
• Haploid cells = gametes (sperm/eggs)
• Human gametes:
– In ♂, meiosis produces (in testes) 4 sperm cells
– In ♀, meiosis produces (in ovaries) 1 egg cell and
3 polar bodies (that later disintegrate)
Mitosis: cells divide once
Meiosis: cells divide twice
Why do ovary and testes
cells go through meiosis?
Meiosis is “reduction
division.”
reduces the
Why doItmeiosis?
chromosome number in
the gametes by one half.
From diploid (2n) to
haploid (n).
Chromosome numbers…
In humans, 2n = 46
A normal body cell has 46 chromosomes
→ 23 came from mom, 23 from dad; each
set of 23 from mom is similar to the set of
23 from dad
In humans, n = 23
A normal gamete has half the normal
chromosome number (n) → just one set
Chromosome number questions
1. If a horse retina cell has 36 chromosomes,
how many chromosomes does a horse
sperm cell have?
2. If n= 4 for fruit flies, how many
chromosomes does a wing cell have?
3. If the 2n number of a pine tree is 86, how
many chromosomes would be found in a
pine ovum?
4. If n = 16 for goldfish, how many
chromosomes are in a fin cell?
Homologous Pairs
• Homologous pairs → Pairs of chromosomes
with the same sequence of genes
• One is from mom, one from dad
• Genes are in the same order, but
not identical!
– e.g., blood type
Phases of Meiosis
• Remember mitosis? (I)PMAT…
• In meiosis, each phase occurs twice
– Meiosis I: Prophase I, Metaphase I, Anaphase I,
Telophase I, Cytokinesis I
– Meiosis II: Prophase II, Metaphase II, Anaphase II,
and Telophase II, Cytokinesis II
Don’t freak out – I changed the
order of the next slides…
Meiosis I
• In Prophase I, homologous
chromosomes pair up in a
tetrad
– same gene sequences, same
banding pattern, same
position of centromere
• Crossing-over takes place
A Sketch of Chromosomes
Crossing Over (Prophase I)
• During
crossing-over,
homologous
chromosomes
exchange pieces
of DNA
• Allows for a
reshuffling of
genetic material
http://images.google.com/imgres?imgurl=http://www.phschool.com/science/biology_place/labbench/lab3/images/crossovr.gif&imgrefurl=http://www.phschool.com/science/biology_place/labbench/lab3/crossovr.html&h=320&w=240&sz=3&hl=en&start=11&tbnid=GqFKVxw1ZWYopM:&tbnh=118&tbnw=89&prev=/images%3
Fq%3Dcrossing%2Bover%2Bchromosomes%26gbv%3D2%26svnum%3D10%26hl%3Den%26safe%3Dactive%26sa%3DG
Meiosis I
• Meiosis I results in 2 haploid daughter cells
Meiosis II
Meiosis
• Each of the haploid daughter cells from meiosis
I divide again to make 4 haploid cells.
Comparing Mitosis and Meiosis
• The processes of mitosis and meiosis are very
similar, but with some important differences:
Mitosis
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Includes only 1 cycle of cell
division
Occurs in body cells (somatic
cells)
No crossing over of
chromosomes
Results in two genetically
identical diploid cells
Metaphase: 1 duplicated
chromosome per spindle fiber
Meiosis
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Includes 2 cycles of cell
division (meiosis I and meiosis
II)
Occurs only in sex cells
(gametes)
Chromosomes cross over in
Prophase I
Results in four genetically
different haploid cells
Metaphase I: 2 duplicated
chromosomes per spindle fiber
Now, let’s look at our
flipbooks and compare!!
Interphase
• DNA replication
occurs
• DNA is in chromatin
form (unwound, like
spaghetti)
Prophase I
• DNA condenses to
form chromosomes
• Homologous
chromosomes pair up
to form tetrads
• Crossing over takes
place
Metaphase I
• Homologous pairs
line up along center
of cell
• 1 homologous
chromosome and its
duplicate are
attached to each
spindle fiber
Anaphase I
• Homologous pairs
separate, move to
opposite ends of the
cell
Telophase I
• Nuclear membrane
reappears
• Spindle fibers
disappear
Cytokinesis
• Cytoplasm of the two
cells separate
• Meiosis I results in 2
haploid cells that are
genetically different
Prophase II
• The 2 haploid cells
from Meiosis I will
divide again
Metaphase II
• Chromosomes (set of
sister chromatids)
line up along the
center of the cell
• 1 sister chromatid
attaches to each
spindle fiber
Anaphase II
• Centromeres split,
sister chromatids are
pulled apart
Telophase II and Cytokinesis
• Cytoplasm divides,
resulting in 4
genetically different
haploid cells