Meiosis Notes

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Meiosis Notes

• Cell division to form the gametes, sperm (male
  gamete) and egg (female gamete).
• Normal cells are diploid 2 copies of every gene.
• Gametes are haploid 1 copy of every gene

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Number of Chromosomes
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Overview of Meiosis

• 2 cell divisions.
• Starts with 2 copies of each chromosome
  (homologous), each with 2 chromatids (copies of
  DNA).
• In meiosis I, homologous chromosomes are
  separated into 2 cells.
• In Meiosis II the chromatids are separated into 4
  cells.

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Meiosis I (PMAT I)

• PROPHASE I - The homologous chromosomes pair
  together (Sometimes crossing over occurs).
• METAPHASE I - The pairs of homologous chromosomes
  line up along the middle
• ANAPHASE I - Homologous chromosomes are pulled
  apart.
• TELOPHASE I - One cell becomes two cells with one
  chromosome of the pair (haploid)

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Meiosis II (PMAT II)

• PROPHASE II Prepare to divide
• METAPHASE II Chromosomes line up in the middle
• ANAPHASE II Chromatids (copies of DNA) pull
  apart
• TELOPHASE II The end result is four cells with
  one copy of each gene.

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Stages Of Meiosis Meiosis I
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Stages Of Meiosis Meiosis II
The products of meiosis are 4 haploid cells each
with a unique set of chromosomes.
Telophase I
Prophase II
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Segregation

• In humans meiosis starts with one cell containing
  46 chromosomes (23 pairs) and results in four
  cells containing 23 chromosomes.
• The copies of DNA are separated when gametes are
  formed.

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Independent Assortment

• Homologous chromosomes are positioned randomly so
  any copy can be passed to the gametes with any
  combination of other chromosomes
• There are 2n combinations possible during meiosis
  with n the haploid number of chromosomes for the
  organism

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How many combinations are possible in human
meiosis?

• Possible combinations 2n
• n23 in humans
• 223about 8,300,000 combinations

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Crossing Over

• During Prophase I, the exchange of genetic
  material between homologous chromosomes

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Crossing Over
Because of crossing over, every gamete receives a
unique set of genetic information.
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Fertilization

• The combination of a sperm and an egg which forms
  a zygote.
• 1 sperm (1 of 8 million possible chromosome
  combinations) x 1 ovum (1 of 8 million different
  possibilities) 64 trillion diploid combinations!

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Fertilization Results In A Diploid Zygote
Sperm
Haploid nucleus
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Fertilization Results In A Diploid Zygote
Egg
Haploid nucleus
Sperm
Haploid nucleus
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Fertilization Results In A Diploid Zygote
Egg
Haploid nucleus
Sperm
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Fertilization Results In A Diploid Zygote
Egg
Haploid nucleus
Sperm
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From Zygote to Embryo
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From Zygote to Embryo
Mitosis
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From Zygote to Embryo
Mitosis
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From Zygote to Embryo
Mitosis
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From Zygote to Embryo
Mitosis
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From Zygote to Embryo
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Twins
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Monozygotic Twins (Identical)

• 1/3 of all twins are identical
• Twins that form from one zygote (one egg
  fertilized by one sperm).
• These twins have identical genes and must be the
  same sex.
• Having identical twins is random, not genetic

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Dizygotic Twins (fraternal)

• 2/3 of all twins are fraternal
• Twins that form from two zygotes (two eggs
  fertilized by two sperm)
• Can be the same sex or different sexes.
• The ability to have fraternal twins is thought to
  be genetic.

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Phenotype
The way an ORGANISM LOOKS
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Genotype
THE GENETIC MAKEUP OF AN ORGANISM
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Dominant and Recessive Traits
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Dominant Traits
Characteristics you can always see if you have
the genes.
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Symbol

• capital letters

A
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Recessive Traits
Characteristics you cannot see if you have a
dominant gene.
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Symbol

• lower case letters

a
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