Meiosis

1
Meiosis

• Meiosis is the type of cell division for that
  produces the reproductive cells (egg or sperm)
  which are also known as gametes.
• Two important characteristics of meiosis is that
  it reduces the number of chromosomes to half of
  the parent cell and it creates genetic diversity.

2
Somatic Cells vs. Gametes

• Somatic cells are body cells (nerve cells,
  muscle cells, epithelial cells).
• These cells all have different appearances and
  functions but have the same genetic information.
• Gametes are the reproductive cells - the egg
  or the sperm.
• Gametes only have half as many chromosomes as
  somatic cells.

3
If Meiosis Did NOT Occur

• If two parental cells combined that had the
  parents full set of 46 chromosomes
• The first generation of offspring would have 92
  chromosomes.
• The next generation of offspring would have 184
  chromosomes

4
Homologous Chromosomes

• Most cells in a human (and in most eukaryotic
  organisms) contain two copies of each chromosome.
  
• These chromosomes are not identical (one is from
  the mother and one from the father), but they
  contain genes for the same traits. These pairs
  are called homologous chromosomes.
• Homologous chromosomes may contain different
  forms of a gene for the same trait.

Brown eyes
Green eyes
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• Humans have 23 pairs of chromosomes, for a
  total of 46 chromosomes in each somatic (body)
  cell.

Autosomal chromosomes Chromosomes 1-22, which
form homologous pairs in both males and females.
Sex chromosomes Chromosome 23. Determines
gender (XX female, XY male). For males, this
pair is not homologous (different length and type
of genes).
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Diploid and Haploid Cells

• Diploid cells have homologous pairs of
  chromosomes.
• Human somatic (body) cells are diploid. (di
  2)
• Haploid cells just have one copy of each
  chromosome.
• Gametes (egg and sperm) cells are haploid.

When fertilization occurs (fusion of egg and
sperm) the diploid number is restored.
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• Diploid cells are represented by 2n and haploid
  by n, where n is the number of different
  chromosomes.
• Examples

Organism Gamete (n) Body cell (2n)
Human 23 46
Pea Plant 7 14
Fruit Fly 4 8
Dog 39 78
Indian Fern 630 1260
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Chromosome number indifferent organisms

• http//morgan.rutgers.edu/morganwebframes/level1/p
  age2/ChromNum.html

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Pair-Share

• Compare and contrast mitosis with meiosis.
• What are homologous chromosomes? What kind of
  cells have homologous chromosomes?
• What do the terms diploid and haploid mean? What
  kind of cells are diploid? What kind of cells
  are haploid?

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Interpreting Genetics Diagrams

• Homologous chromosomes are indicated by length.
• Sister chromatids (identical copies produced
  during S phase of interphase) are shown connected
  by centromere.
• Genes on the chromosome are represented with
  letters. Example R/r represents gene for pea
  seed shape
• Different forms of the same gene are represented
  with capital versus lower case letters.
• Example R represents round shape form of the
  gene, r represents wrinkled shape form of the
  gene.

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Overview of Meiosis

• In meiosis, one replication of chromosomes is
  followed by 2 cell divisions, to produce 4
  haploid cells.

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Phases of Meiosis

• As with mitosis, meiosis is preceded by an
  interphase period,
• during which the chromosomes are duplicated.

The two sister chromatids are identical
copies. The homologous pairs have the same genes,
but could have different alleles (forms of the
gene). At the start of meiosis, there are 4
copies of each gene.
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Prophase I

• During Prophase I,
• the chromosomes
• condense to become
• visible.
• HOMOLOGOUS PAIRS
• JOIN TOGETHER TO
• FORM TETRADS (4 STRANDS).
• (Homologous pairs never form tetrads during
  mitosis).

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Metaphase I

• In metaphase I,
• the HOMOLOGOUS
• PAIRS line up along
• the middle of the
• spindle.

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Anaphase I HOMOLOGOUS PAIRS separate.
Telophase I Nuclei reform, producing 2 haploid
daughter cells. But each cell still has 2
sister chromatids for each chromosome.
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• Meiosis II separates the sister chromatids.

Prophase II
Metaphase II
Telophase/ cytokinesis II
Anaphase II
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Meiosis VideoClick once image to start
http//www.youtube.com/watch?vD1_-mQS_FZ0
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Meiosis Animation

• http//www.cellsalive.com/meiosis.htm

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Genetic Variation from Meiosis

• Meiosis is important for providing genetic
  variation because it allows for genetic
  recombination- new combinations of genes.

Parental combinations
Recombinants
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Sources of Genetic Variation from Meiosis

• Independent Assortment
• Each homologous pair lines up independently on
  the spindle in Meiosis I. So all of the possible
  arrangements of chromosomes are equally likely.

In humans there are 223 possible chromosome
combinations from each parent.
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If an organism has gene forms Y and y on one
homologous pair, and R and r on a different
homologous pair, then due to independent
assortment the gametes produced will be 25 YR,
25 yr, 25 Yr and 25 yR
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Independent Assortment VideoClick once image to
start
http//www.youtube.com/watch?vkZyK2eWhpfQ
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http//www.sumanasinc.com/webcontent/animations/co
ntent/independentassortment.html
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Sources of Genetic Variation from Meiosis cont.

• Crossing Over
• During prophase I, strands of non-sister
  chromatids can be exchanged while the homologous
  pair are joined in tetrads.

Crossing over allows recombination of genes on
the same chromosome.
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Crossing Over VideoClick once on image to start
http//www.youtube.com/watch?vop7Z1Px8oO4
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• Crossing over animation

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Comparing Mitosis and Meiosis
MITOSIS MEIOSIS
Purpose
Location
of DNA replications
of cell divisions
/type of cells produced

• Growth
• and Repair

Gamete Production
Throughout body
Only in germ cells
1
1
2
1

• 4 haploid cells

2 diploid cells
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Comparing Mitosis and Meiosis VideoClick once on
image to start
http//www.youtube.com/watch?vBa9LXKH2ztU
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Comparing Mitosis and Meiosis Cont.

• NOVA site comparing mitosis and meiosis.

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Mnemonic for Meiosis
M E I O S I S
akes
ggs
n
varies
perm
n
permatocytes (precursor cells for male gametes)
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Spermatogenesis produces four equal sperm cells
through meiosis. A greater quantity of sperm is
produced, but each sperm has less stored
energy. Oogenesis (egg production) meiotic
division produces 1 larger egg which receives
most of the organelles and cytoplasm of the
parent cell. The other three cells produced in
the meiotic division are smaller polar bodies.
Fewer eggs need to be produced so more energy is
invested per egg (mitochondria etc.).