Meiosis
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Transcript of Meiosis
Meiosis
Heredity Heredity
Passing of genetic traits from parent to offspring
Chromosome theory of heredity Chromosomes
carry genes Gene = unit of
heredity
What Meiosis is all About Meiosis allows the creation of unique
individuals through sexual reproduction.
In The Beginning Two
Reproduction in which there is a re-mixing of the genetic material is called sexual reproduction
Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops
Meiosis is the process of producing sperm and eggs (gametes) – the number of chromosomes are halved
Gametes Are Haploid Gametes must have half the
genetic material of a normal cell If the genetic material in the
gametes was not halved, when they combined the zygote would have more genetic material than the parents!
Gametes have exactly one set of chromosomes, this state is called haploid (1n)
Regular cells have two sets of chromosomes, this state is called diploid (2n)
Fertilization Results in a Diploid Zygote
Sperm1nHaploid
nucleus
Egg1n Haploid
nucleus
Fertilization Results in a Diploid Zygote
Sperm1n
Egg1n
Haploidnucleus
Haploidnucleus
Fertilization Results in a Diploid Zygote
Sperm1n
Egg1n Haploid
nucleus
Haploidnucleus
Fertilization Results in a Diploid Zygote
Sperm1n
Egg1n Haploid
nucleus
Haploidnucleus
From Zygote to Embryo
Diploid Zygote2n
From Zygote to EmbryoCleavage
From Zygote to Embryo
Cleavage
From Zygote to Embryo
Cleavage
From Zygote to Embryo
Cleavage
From Zygote to Embryo
Morula
Meiosis 1Prior to division, amount of DNA
doubles
Stages of Meiosis
http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
Independent Assortment during Prophase I
The key difference between Mitosis and Meiosis is the way chromosomes uniquely pair and align in Meiosis
Mitosis The first division of Meiosis
Synapsing UNLIKE in mitosis,
homologous chromosomes line up next to each other during prophase
This process is called synapsing
Lined up homologues are called tetrads
Anaphase 1 During anaphase 1,
each homologous chromosome is pulled to opposite sides of the cell. Unlike mitosis, THE CENTROMERES DO NOT BREAK.
Nuclei may or may not reform following division.
Cytokenesis may or may not occur.
Check out the Tetrads
Meiosis I
In Telophase 1, two daughter cells areformed. They are NOT identical! (Why?)
Meiosis II
Chromosomes are NOT
duplicated again between
Meiosis 1 and Meiosis 2Why not?
Meiosis II Prophase 2: spindle reforms and
chromosomes move toward the metaphase plate
Metaphase 2: sister chromatids lined up on the metaphase plate
Anaphase 2: sister chromatids are separated and pulled toward opposite poles of the cell
Telophase 2 and Cytokinesis: nuclei form at either pole and each cell is finally divided into two identical daughter cells
All Together Now
Independent Assortment
Independent Assortment
Example:2 chromosomes in haploidn = 22n = 22 = 4 possible combinations
In Humans
Example: 23 chromosomes in haploidn = 23
2n = 223 = ~ 8 million possible combinations!
Prophase 1 – Crossing Over Homologous
chromosomes come together
Areas of homologous chromosomes connect at areas called chiasmata
Crossing Over Segments of
homologous chromosomes break and reform at similar locations.
Results in new genetic combinations of offspring.
This is the main advantage of sexual reproduction
Crossing-Over
Random fertilization At least 8 million combinations from Mum
and another 8 million from Dad … >64 trillion combinations for a diploid
zygote!!!
Boy or Girl? The Y Chromosome Decides
X chromosome
Y chromosome
Oogenesis The female gamete is
called an ovum At birth each female
carries a lifetime supply of developing oocytes, each of which is in Prophase I.
A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.
Oogenesis Only one ovum is
produced during meiosis Oogenesis places most of
the cytoplasm into the large egg. The other cells, the polar bodies, do not develop.
All the cytoplasm and organelles go into one egg for nourishment for the young organism that will develop after fertilization.
Spermatogenesis The male gamete is
called a sperm Four spermatocytes
are formed during meiosis
Men are busy - meiosis produces roughly 250,000 sperm a day.
Any Questions?
ReferencesSlides, information and images were taken from
the following presentations:
cchs.churchill.k12.nv.us/marshk/Notes/meiosis.ppt bioweb.wku.edu/Faculty/Bowker/120/mitosis.html edweb.sdsu.edu/ltca/Mitosis_Meiosis_files/slide0001.htm waukesha.k12.wi.us/South/Bio1/MEIOSIS.htm my-ecoach.com/coaching/meiosis_files/meiosis.ppt www.grisda.org/tstandish/teachers/presentations/High%20S
chool/Meiosis.ppt www.biology.usu.edu/courses/biol1010-podgorski/
PPpage.htm