Download Meiosis What is an “n”?

1/5/2012
Meiosis
What is an “n”?
• Haploid (n):
• Number of chromosomes in a
gamete.
• Diploid (2n):
• Number of chromosomes in a
somatic cell
• One copy from mother, one
copy from father.
• Homologous pairs of
chromosomes
1
1/5/2012
Two Types of Diploid Cells In
Sexual Organisms
Somatic
• Body Cells
• Diploid
• Divide by mitosis to
make exact copies
• In every tissue except
reproductive tissue
GERM
• “Germinating”
(reproductive) cells
• Diploid
• (Most) Divide by meiosis
to make gametes (sex
cells) (sperm & ova)
• In testes & ovaries
Mitosis vs. Meiosis
•Asexual Reproduction
•Sexual Reproduction
• Body cells (somatic)
• Sex cells (gamete)
• DNA replicates once
• DNA replicates once
• # cell divisions: 1
• # cell divisions: 2
• 2 identical daughter
cells
• Makes Diploids (2N)
• 4 different daughter
cells
• Makes Haploids (1N)
2
1/5/2012
Meiosis is part of SEXUAL Reproduction
(requiring 2 parents)
In Meiosis: ONE diploid, germ
cell divides to become FOUR
• (*circle*) … diploid /
• (*circle*) … somatic /
haploid
gamete
(4) daughter cells
3
1/5/2012
In order to go from diploid to
haploid, Meiosis Involves Two
Divisions
Meiosis I
•
Splits homologous pairs of
chromosomes
Meiosis II
•
Splits sister chromatids apart
Time To Name The Phases
•
PROPHASE I
•
METAPHASE I
•
ANAPHASE I
•
TELOPHASE I
•
PROPHASE II
•
METAPHASE II
•
ANAPHASE II
•
TELOPHASE II
4
1/5/2012
Take out 2 Tables
PROPHASE I
METAPHASE I
ANAPHASE I
TELOPHASE I
&
CYTOKINESIS
WHAT IT
LOOKS LIKE
WHAT’S
GOING ON
Take Out 2 Tables
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
&
CYTOKINESIS
WHAT IT
LOOKS LIKE
WHAT’S
GOING ON
5
1/5/2012
PROPHASE I
• Nuclear envelope disintegrates
• Centrioles migrate, produce spindle fibers
pair up
CROSS OVER!
PROPHASE I
CROSSING OVER
Exchange of segments of genes between
homologous chromosomes
6
1/5/2012
METAPHASE I
of
chromosomes line up along
metaphase plate
ANAPHASE I
pulled apart to opposite
poles
7
1/5/2012
TELOPHASE I & CYTOKINESIS
• Chromosomes (no longer homologous pairs!)
prepare for meiosis II
• Cytoplasm begins to split into first 2 (of 4)
daughter cells
TELOPHASE I & CYTOKINESIS
NOTE:
At the end of meiosis I, daughter
cells are already
!
(because homologous pairs were separated)
8
1/5/2012
2 haploid daughter cells then go
through MEIOSIS II
In Meiosis II, 2 haploid cells will separate sister chromatids to make 4 gametes!
PROPHASE II
• Centrioles migrate, produce spindle
fibers
9
1/5/2012
METAPHASE II
• Chromosomes line up along
metaphase plate
ANAPHASE II
• Sister chromatids pulled apart to
opposite poles
10
1/5/2012
TELOPHASE II & CYTOKINESIS
• Nuclear envelopes reform around
• Cytoplasm splits to separate the
• Each of the 4 daughter cells is a new gamete with
genetic information different from either parent!
Is This Meiosis I or Meiosis II?
Why?
11
1/5/2012
Is This Meiosis I or Meiosis II?
Why?
Meiosis I
Meiosis II
Check Point: When do the cells become haploid?
12
1/5/2012
Mitosis v. Meiosis
,
13
1/5/2012
Finally, Fertilization
• Male and female
gametes unite
• Fusion of two
haploid nuclei
produces diploid
nucleus
• Egg + Sperm = Zygote (new life in one diploid
cell)
Meiosis and Sexual Reproduction
Guarantee Variation and Diversity!
In Three Ways
1. Independent Assortment
•
Homologous Chromosomes separate randomly
2. Crossing Over
•
Homologous pairs exchange gene segments
3. Random Fertilization
•
Which sperm meets which egg? (game of chance)
14