Transcript Meiosis and Reproduction10

Meiosis and Reproduction
California Science Standards #1c,
2a, 2b, 2c, 2d, 2e, 2f, 2g
1
Meiosis
• Only certain cells in a multicellular
organism undergo meiosis.
– Special diploid cells (spermatogonia) in the
testis of the male
• Produce the haploid sperm
– Special diploid cells (oogonia) in the female
ovary
• Produce the haploid eggs
Meiosis
• One cell (oogonia or spermatogonia)
undergoes meiosis and is divided into four
daughter cells
Spermatogenesis
Movie: “repro_sperm.mov,” © Glencoe, 2000
Oogenesis
Mitosis vs. Meiosis
Mitosis vs. Meiosis
• Mitosis makes exact copies of cells
• Meiosis makes gametes (egg, sperm)
• Mitosis makes daughter cells with 100% of
the parent cell’s DNA
• Meiosis makes daughter cells with 50% of
the parent cell’s DNA
Egg (50%) + sperm (50%) = zygote
(100% of DNA)
Movie: “repro_fertilization.mov,” © Glencoe, 2000
Man or Woman?
• Only 1 chromosome makes us different!
Meiosis
• 8 phases:
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Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II
Movie: “Meiosis.mov,” © Wadsworth, 1997
Prophase I
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DNA is already duplicated (from interphase)
DNA condenses into chromosomes
Spindle fibers appear (“fishing line”)
Nucleus breaks down
Homologous chromosomes are paired
(synapsis)
• Crossing-over
– Trade pieces of DNA
Prophase I: Crossing Over
• Between a sister chromatid from one
chromosome and a sister chromatid from
the other homologous chromosome
• Results in genetic recombination
– “Shuffle the DNA deck”
Metaphase I
• Tetrads (homologous chromosome pairs)
line up randomly
• Spindle fibers attach
to centromeres
Anaphase I / Telophase I
• A1- Independent assortment
– Each homologous chromosome migrates (“gets
reeled in”) to an opposite pole (but which pole?)
• T1- Cytokinesis begins (pinch off)
– Each of the cells are haploid- ½ # chromosomes
– But, they are in the duplicated state
A Closer Look…
2nd “Shuffle”
• Results in genetic
recombination
– “Shuffle the DNA
deck” again
Prophase II / Metaphase II
• P2- Spindle fibers form
• M2- Chromosomes move to the midline
(equator)
Movie Review: “Meiosis_g.mov,” © Glencoe, 2000
Anaphase II / Telophase II
• A2- Chromatids separate
and migrate toward
opposite poles
• T2– A nucleus re-forms around
chromosomes in each of
the FOUR new cells
– Cytokinesis
• Each of the cells are haploid½ # chromosomes
• Now they are back to the
unduplicated state
Review Q’s
• Track the amount of DNA
from interphase, through
Meiosis I and II, to interphase
again (use the terms haploid,
diploid, duplicated, and
unduplicated).
• Speculate: Why is it
advantageous for a species
to use genetic recombination
(shuffle the DNA)?
Gestation
From One to a Trillion
Fertilization
• Gestation=the length of a normal
pregnancy
– Begins after fertilization
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Sperm and egg fuse to form a zygote
1n + 1n = 2n
Haploid + haploid = diploid
½ chromosomes (mom’s 23) + ½
chromosomes (dad’s 23) = 1 whole set of
chromosomes (baby’s 46)
• 50% DNA + 50% DNA = 100% DNA
Cleavage & Implantation
• Zygote divides by mitosis into a ball of
cells called a morula (cleavage)
• Morula divides into a blastocyst, which is a
ball of cells filled with a fluid cavity
• Blastocyst burrows and embeds itself into
the lining of the uterus (implantation)
Development of Embryo
• 1st trimester: most dramatic (from one
cell into 3 types of cells: endoderm,
ectoderm, mesoderm); heartbeat, brain
activity
Development of Embryo
• 2nd trimester: heartbeat heard, skeleton
formed, can suck thumb, and kick
Development of Embryo
• 3rd trimester: rapid growth, viable
outside mother, responds to music,
sound, and light
Development of Embryo
• Ready for birth (~270 days after fertilization)
• Now trillions of cells big