Download Meiosis

Meiosis
During meiosis, but NOT mitosis, the homologous chromosomes "find" each
other and pair up, lying side by side. During this time, an important process
called Crossing over occurs:
Crossing over: sections of chromosomes from homologues are swapped (or
"synapse")- but only during prophase I of meiosis, when homologues are paired.
In humans, 2-3 cross-over events occur per chromosome pair. The result:
chromosomes after meiosis are genetically unique and have combinations of
DNA derived from both parents.
Prophase I: Homologues are paired up
Metaphase I: Homologues align in pairs
Anaphase I: Homologues separate; sister chromatids remain
attached.
Telophase I (not shown): 2 new cells form. Each has 1 copy of each
homologue, but still has duplicated sister chromatids
Meiosis II: Goal: Separate the sister chromatids. This is
essentially a mitotic division
Prophase II (not shown): Chromosomes [sister chromatids]
condense
Metaphase II (not shown): Chromosomes [sister chromatids] align
at the metaphase plate
Anaphase II (not shown): Chromosomes [sister chromatids]
separate
Telophase II (not shown): Chromosomes [sister chromatids]
separate. There are now 4 haploid (n) cells
Summary: At the end of meiosis, 4 daughter cells are formed = eggs or sperm.
Each contains half as many chromosomes as the parent cell (n); each cell is
genetically different from its parents and from its "siblings". Fertilization of an
egg by a sperm restores the chromosome number to 2n.
Alteration of Generations: All sexually reproducing organisms (animals, plants
and fungi) alternate between haploid (n) and diploid (2n) states.
Genetic variation "reshuffling the genes" comes from several sources:
Ok...people may say that you may have your mom's nose and your dad's
hair, but what does that mean? Why do you look different (but somewhat similar)
from your brothers and sisters, and from your parents, if you all basically have
the same chromosomes? Each meiosis and fertilization is literally like a 'roll of
a dice' - so no two individuals are alike!
1. Crossing Over: The chromosomes you receive from your mom's egg and
your dad's sperm and NOT the same chromosomes that your mom and dad
have. They are new, 'shuffled' versions of their chromosomes - and you only get
ONE of them from each parental unit....
2. Independent assortment: Homologues line up or "shuffle" randomly on the
metaphase plate in Meiosis I. With 23 chromosomes assorting independently,
there are 223, or 8 million, possible assortments of chromosomes inherited for
every cell!!
3. Random fertilization: The ovum has 8 million possible chromosome
combinations, so does the sperm cell. 8 million x 8 million = 64 trillion
possible diploid combinations in EACH AND EVERY zygote! WOW!!
In other words, you're unique...(...just like everyone else.....).
The birds 'n the bees: (we'll be brief here)....
During sexual reproduction, fertilization of the ovum (n, 23 chromosomes) by
the sperm (n, 23 chromosomes) restores the diploid number and creates a
zygote (2n, 46) (awww..) that divides and grows by mitosis to form a multicellular
human.
The former zygote
~ 1 year after fertilization.....
Cute, multicellular, and diploid...
Instructions not included.