Meiosis

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Meiosis

• A presentation by
• Ms. Edmondson

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Part 1 Introduction
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Mitosis Reminder

• Mitosis is the process of cell division.
• One cell divides to produce two identical cells.

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So what is Meiosis?

• Meiosis is a special kind of cell division.
• It occurs when one cell divides twice to produce
  four different cells

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Mitosis Meiosis

• Cell divides once
• Two genetically identical cells are made.
• Daughter cells have same genetic information.
• Occurs in body cells

• Cell divides twice
• Four genetically different cells are made.
• Daughter cells have half of the genetic
  information.
• Occurs in sex cells

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Vocabulary

• Gene- Basic unit of heredity
• Diploid- A cell with two copies of each
  chromosome.
• Haploid- A cell with one copy of each
  chromosome.
• Gamete- A sperm or egg cell
• Zygote- Fertilized egg

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Why dont all cells use mitosis?

• Lets look at an example!

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Fertilization

• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg

n46
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Sowhy dont all cells use mitosis?

• During sexual reproduction
• Organisms get a set of chromosomes from their mom
  and a set of chromosomes from their dad.
• What would happen if a human babys cells got 46
  chromosomes from their mom and 46 chromosomes
  from their dad?
• They would end up with 92 chromosomes!
• Meiosis allows gametes to have only half of the
  genetic information available ( of chromosomes)
• Based on this information, how many chromosomes
  should be present in gametes?
• 23 chromosomes

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Mitosis vs. Meiosis

• Occurs in body cells
• Results in 2 daughter cells each with 46
  chromosomes (in humans)
• These chromosomes are called diploid

• Occurs in sex cells
• Results in 4 daughter cells (gametes) each with
  23 chromosomes
• (in humans)
• These chromosomes are called haploid

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Part 2 Haploid vs. Diploid
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Diploid

• Diploid cells have two copies of each chromosome,
  one from mom and one from dad.
• We write diploid as 2n.

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Haploid

• Haploid cells have one copy of each chromosome.
• We write haploid as 1n

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Review Question

• Remember, just because humans need 23 pairs of
  chromosomes, all organisms do not.
• A cell containing 20 chromosomes (diploid) at the
  beginning of meiosis would, at its completion,
  produce cells containing how many chromosomes?

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Answer

• 10 chromosomes!

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Question

• A cell containing 40 chromosomes at the beginning
  of meiosis would, at its completion, produce
  cells containing how many chromosomes?

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Answer

• 20 chromosomes!

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What does meiosis look like?

• Diploid
• Having two copies of each chromosome
• 2n diploid
• Haploid
• Having one copy of each chromosome
• 1n haploid

2n
Meiosis I
Meiosis 2
1n
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Practice!

• Lets do some haploid diploid practice!

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Part 3 Homologous Chromosomes
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Homologous Shoes?

1. Lets start by having everyone take off their
   shoes!
2. Nowhold them in the air.
3. Other than the stench, what do you notice?

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Homologous Shoes?
Homologous Chromosomes
Homologous Chromosomes
Homologous Chromosomes
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Homologous Chromosomes

• Homologous Chromosomes - Pair of chromosomes
  (maternal and paternal) that are similar in shape
  and size.
• Humans have 23 pairs of chromosomes
• 22 pairs of autosomes
• 1 pair of sex chromosomes
• XX- female
• XY- male

Homologous Shoes
Homologous Chromosomes
left
right
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Homologous Chromosomes

eye color gene
eye color gene
hair color gene
hair color gene
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Tetrads
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Part 4Phases of Meiosis
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Interphase I

• Similar to what we learned when we studied
  mitosis
• G1 Growth
• S-phase DNA is replicated
• G2 Growth and preparation for meiosis
• Each replicated chromosome consist of two
  identical sister chromatids attached at their
  centromeres

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Stages of Meiosis

• Meiosis involves 2 divisions
• Meiosis I and Meiosis II
• Meiosis I- Reduces the number of chromosomes in
  each daughter cell in half
• Meiosis II- Very similar to mitosis.

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Meiosis Stages

• Meiosis goes through mitosis twice.
• There are only a few differences and I will point
  those out as we go along.

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Interphase I

• Nucleus and nucleolus visible.

chromatin
nuclear membrane
cell membrane
nucleolus
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Meiosis I (four phases)

• Cell division that reduces the chromosome number
  by one-half.
• Four phases
• 1.) Prophase I
• 2.) Metaphase I
• 3.) Anaphase I
• 4.) Telophase I

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Prophase I

• Longest and most complex phase of meiosis
• (90) of the cells time is spent in this phase!
• Chromosomes condense.
• There are two major differences between prophase
  of mitosis and prophase I of meiosis occur in
  this phase
• 1.) Synapsis
• 2.) Crossing Over

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Prophase I - Synapsis

• Synapsis Homologous chromosomes come together to
  form a tetrad.
• Tetrad 2 replicated chromosomes four
  chromatids 2 sister 2 nonsister chromatids

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Prophase I Crossing Over

• Segments of non-sister chromatids break and
  reattach to the other chromatid.
• Crossing over (variation) may occur between
  non-sister chromatids at the chiasmata.
• Chiasmata (chiasma) are the sites of crossing
  over.

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Crossing Over - Variation

variation
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Prophase I the end result

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Prophase Mitosis vs. Meiosis

• Nuclear membrane degenerates

• Nuclear membrane degenerates
• Homologous chromosomes pair off into tetrads
• Crossing over occurs between like parts of
  non-sister chromatids

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Metaphase I

• Shortest phase
• Tetrads align on the metaphase plate.
• Independent Assortment Occurs
• 1. Placement of homologous pair is random.

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Metaphase I


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Metaphase Mitosis vs. Meiosis

• Single chromosomes line up along the metaphase
  plate
• Spindle fibers emerge from the both centrioles
  and attach to both sides of the centromere

• Tetrads line up along the metaphase plate
• Spindle fibers emerge from both centrioles and
  attach to one side of the centromere

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Anaphase I

• Homologous chromosomes separate and move towards
  the poles.
• Sister chromatids remain attached at their
  centromeres.

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Anaphase I
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Anaphase Mitosis vs. Meiosis

• Sister chromatids are pulled to opposite poles of
  the cell

• Sister chromatids move together to the same pole
  of the cell
• Homologous chromosomes are pulled to opposite
  poles of the cell

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Telophase I

• Each pole now has haploid set of chromosomes.
• Cytokinesis may or may not occur and two haploid
  daughter cells are formed.

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Telophase I
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Telophase Mitosis vs. Meiosis

• Each daughter cell is diploid
• This is the end of mitosis

• Each daughter cell is haploid

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Meiosis II

• No interphase II
• (No more DNA replication)
• Has four stages
• 1.) Prophase II
• 2.) Metaphase II
• 3.) Anaphase II
• 4.) Telophase II
• Remember Meiosis II is similar to mitosis

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Prophase II

• Same as prophase in mitosis
• Nuclear membrane degenerates
• New spindle fibers form

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Metaphase II

• Same as metaphase in mitosis
• Chromosomes line up along metaphase plate
• Spindle fibers bind to each side of the centromere

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Anaphase II

• Same as anaphase in mitosis
• Spindle fibers shorten and pull sister chromatids
  apart

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Telophase II

• Same as telophase in mitosis.
• Chromsosomes aggregate into nuclei
• Nuclear membranes reform.
• Remember Four haploid daughter cells called
  gametes (sperm or egg) are formed

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Telophase II
Cytokinesis
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Variation

• Important to populations.
• Question
• Based on our notes today are the three ways for
  organisms to get lots of genetic variation?

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Answer

• 1. Crossing over (Prophase I)
• 2. Independent assortment (Metaphase I)
• 3. Random fertilization
• Remember Variation is good!

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Part 5 Types of Meiosis
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Two Kinds of Meiosis!

• 1.) Spermatogenesis
• - Occurs in males
• - Results in four different sperm cells
• 2.) Oogenesis
• -Occurs in females
• -Results in one egg (ova) and 3 polar bodies

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Spermatogenesis
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Oogenesis
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Sex Chromosomes
XX chromosome - female
XY chromosome - male
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A Karyotype
Is this karyotype from a male or a female?
HINT!
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Ms. Edmondsons Claymation