Meiosis

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Chapter 7 Meiosis
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7-1 Meiosis
Meiosis is a form of cell division that Halves
the number of chromosomes When forming
specialized Reproductive cells.
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tudent_view0/exercise14/meiosis_movie.html
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Meiosis involves 2 divisions of the Nucleus -
meiosis I and meiosis II - And each division is
subdivided Into prophase, metaphase, Anaphase
and telophase stages.
Before meiosis begins, the DNA in the Original
cell is replicated.
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There are 8 stages to Meiosis
Step 1 - Prophase I Step 2 - Metaphase I Step 3 -
Anaphase I Step 4 - Telophase I Step 5 - Prophase
II Step 6 - Metaphase II Step 7 - Anaphase
II Step 8 - Telophase II
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In prophase I, the chromosomes Condense, and the
nuclear envelope Breaks down. Homologous
chromosomes Pair all along their length and then
Cross-over. (crossing over occurs when Portions
of a chromatid on 1 Chromosome are broken and
exchanged With the corresponding portions On one
of the chromotids of the Other chromosome.
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In metaphase I, the pairs of Homologous
chromosomes are moved By the spindle to the
equator Of the cell.
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In anaphase I, the homologous Chromosomes
separate. (just like mitosis)
But the chromotids do not separate At their
centromere! Each chromosome Is still composed to
2 chromatids.
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In telophase I, individual chromosomes Gather at
each of the poles. Then Cytokinesis occurs
forming 2 cells.
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Now we get into mitosis II. The Chromosomes do
not replicate Between mitosis I and II!!
In prophase II, a new spindle forms Around the
chromosomes.
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In metaphase II, the chromosomes Line up along
the equator and are Attached at their
centromeres To the spindle fibers.
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In anaphase II, the centromeres Divide, and the
chromatids (now called chromosomes) move
to Opposite poles of the cell.
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And lastly in telophase II, a nuclear Envelope
forms around each set of Chromosomes. The spindle
breaks Down, and the cell undergoes
Cytokinesis.
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The result of meiosis if 4 Haploid cells.
The 4 resulting cells are the Reproductive cells.
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The chromosome pairs separate Independently.
The chromosomes that an offspring Receives from
each pair is A matter of chance.
The random distribution of homologous Chromosomes
during meiosis is called Independent assortment.
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Meiosis and the joining of gametes Are essential
to evolution.
The DNA exchange that occurs at Crossing over
adds even more Recombination to the independent
Assortment of chromosomes that Occurs later in
meiosis.
Thus the number of genetic Combination that can
occur among Gametes it practically unlimited.
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The process by which sperm are Produced in male
animals is Called spermatogenesis.
The process by which gametes are Produced in
female animals is Called oogenesis.
Meiosis for males creates 4 cells, But only 1
cell in females.
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7-2 Sexual Reproduction
Some organisms look exactly like Their parents
and siblings.
These traits are passed down In 2 ways, asexual
and sexual Reproduction.
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In asexual reproduction, a single Parent passes
copies of all of Its genes to each
offspring, Basically creating a clone.
In sexual reproduction, 2 parents Each form
haploid cells, which join To form offspring.
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Asexual reproduction is the simplest And most
primitive Method of reproduction.
But it allows organisms to produce Many offspring
in a short period Of time.
But the DNA does not vary much Between
individuals.
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The entire span in the life of an Organism from
one generation To the next is called a life
cycle.
Eukaryotes that undergo sexual Reproduction can
have one of three Types of sexual life
cycles Haploid Diploid Alternation of
Generations
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The haploid life cycle is the simplest.
Haploid cells occupy the major Portion of the
life cycle, the zygote Is the only diploid cell
and it Undergoes meiosis immediately After
creation.
This type is usually found in protists, Like
fungus.
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The diploid life cycle is the one That we live
in.
Most of the life cycle is diploid, With haploid
cells only for Reproduction.
The haploid gametes join in a Process called
fertilization, which Results in a diploid zygote.
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Plants, algae and some protists Have a life
cycle that regularly Alternate between haploid
And diploid.
The diploid phase of a plant is Called a
sporophyte, while a spore Is haploid.
The gametophyte is the haploid Phase of a plant.
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LET'S SUM IT UP!
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Steps of Meiosis I
Reductional Division !
In humans, n23, so 2n46.
n23 in humans
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Steps of Meiosis II
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THE END