Lecture 6 BIOL L100 Indiana University Southeast David Partin, Instructor

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Lecture 6BIOL L100 Indiana University
SoutheastDavid Partin, Instructor
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Todays Agenda

• Announcements/Questions
• Lecture 6 Meiosis/Sexual Reproduction
• Video clip
• One Minute Paper
• Lab

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Chapter 10

• Meiosis and Sexual Reproduction

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Lets talk about SEX!

• Scientifically speaking, of course

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• Sexual reproduction involves
• the fusion of male and female gametes.
• The resulting cell is called a
• zygote.

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Background terminology

• Haploid vs. Diploid
• Definitions
• In humans, only gametes are haploid.
• All other human cells are diploid somatic cells

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Haploid or diploidWhy does it matter?

• Remember, only gametes are haploid. And there is
  a very important reason why
• The new organism will form from the zygote. The
  zygote needs 2 copies of each chromosome, and NO
  MORE.
• 1 copy of moms DNA and 1 copy of dads
• More than 2 copies of ANY chromosome is BAD NEWS!
  (e.g. trisomy 21)

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All of our cells (except mature gametes) have 2
copies of each chromosome.
One copy of each chromosome is maternal, and the
other copy is paternal.
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Meiosis I is reductional division!
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Meiosis Preview
future sperm or egg cell
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Importance of meiosis

• Meiosis keeps the number of chromosomes constant
  generation after generation.
• Without meiosis, the number of chromosomes would
  continue to increase each generation.
• Each generation 1 sperm (23 chromosomes) 1
  egg (23 chromosomes) 1 zygote (46 chromosomes)
• The new zygote grows by mitosis. Each new cell
  has 46 chromosomes.

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But waitIts not that simple!

• 46!!
• You dont
• get moms
• chromosome
• 8 or dads
• chromosome
• 8
• You get a mix
• of the 2!

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That mix is accomplished by genetic
recombination.

• 3 methods of genetic recombination
• 1. crossing over
• 2. independent assortment
• 3. fertilization

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Methods of genetic recombination1. crossing over
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Homologous chromosomes align.
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Methods of genetic recombination2. independent
assortment
1 2 3
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Methods of genetic recombination3. fertilization

• When the gametes fuse at fertilization, the
  chromosomes donated by each parent are combined.
  In humans, this means that (223)2, or
  70,368,744,000,000 chromosomally different
  zygotes are possible for every couple, if no
  crossing over occurs.
• If crossing over occurs only once, then (423)2,
  or 4,951,760,200,000,000,000,000,000,000
  genetically different zygotes are possible for
  every couple.

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Lets take a short break before we continue the
lecture

• When we come back
• steps of meiosis
• comparing mitosis vs. meiosis
• human life cycle
• intro to stem cells

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Steps of Meiosis I
Reductional Division !
In humans, n23, so 2n46.
n23 in humans
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Steps of Meiosis II
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Meiosis vs. Mitosis
(All 46)
(Line up in 23 pairs.)
(All 46)
(23 in each)
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Stem cells

• Stem cells are cells that are undifferentiated
• Translation they havent been assigned a job
  yet. No specific genes turned on.
• These cells can be influenced to become ANY TYPE
  OF CELL, given the right cues.
• Found in embryos, also to some degree in adult
  tissues.

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Which one of these drawings represents metaphase
I? How do you know?
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Any questions?
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Take a short break if you need one!

• Whats next?
• Video clip
• One-Minute Paper
• Lab

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Preparation for Lecture 7

• Read Ch 11
• Keep up with Oncourse!