Meiosis
Mar 19, 2016
Meiosis
Last time…
We saw how cells divide by mitosis to produce genetically identical daughter cells
• 2n parent cell --> 2 daughter cells, both 2n
Why Mitosis?
• Growth (new cells)
• Cell replacement (skin, wound healing)
• Asexual reproduction- produces genetically identical offspring
Specialized cells called gametes from 2 different individuals fuse together to form a one-celled zygote, which develops into the offspring of the next generation
If gametes were formed by mitosis, the chromosome number would double each generation
In sexually reproducing organisms…
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Sperm egg
fertilization
One-celled zygote
Cell division (mitosis)And development
gametes
Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms
Meiosis reduces the chromosome number by half, enabling sexual recombination to occur.
• Meiosis of diploid cells produces haploid daughter cells, which may function as gametes.
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Sperm egg(1n) (1n)
fertilization
One-celled Zygote--> 2n
Cell division (mitosis)And development
gametes
meiosis
2n
During meioisis, chromosome number is reduced
In meiosis I, homologous pairs split
In meiosis II, sister chromatids split
Femalegamete (1n) Fertilization
Diploid offspringcontains homologouspairs of chromosomes
Malegamete (1n)
A full complement of chromosomes is restored during fertilization.
Figure 13.7 The stages of meiotic cell division: Meiosis I
Figure 13.7 The stages of meiotic cell division: Meiosis II
Figure 13.8 A comparison of mitosis and meiosis
Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms
Meiosis and fertilization introduce genetic variation in three ways:
• 1. Crossing over between homologous chromosomes at prophase I.
Crossing over can lead to new combinations of
alleles on a chromosome
Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms
Meiosis and fertilization introduce genetic variation in three ways:
• 2. Independent assortment of homologous pairs at metaphase I:
• Each homologous pair can orient in either of two ways at the plane of cell division.
• The total number of possible outcomes = 2n (n = haploid number of chromosomes).
The results of alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I
Eye color
Allele thatcontributes to browneyes (B)
Allele thatcontributes to blueeyes (b)
Maternalchromosome
Paternalchromosome
Hair color
allele thatcontributes to black hair(r)
allele thatcontributes to red hair (R)
Maternalchromosome
Paternalchromosome
Hypothetical example
OR
Brown eyesBlack hair(Br)
Blue eyesRed hair(bR)
Blue eyesBlack hair(br)
Brown eyesRed hair(BR)
During metaphase of meiosis I, tetrads can line up two different ways before the homologs separate.
Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms
Meiosis and fertilization introduce genetic variation in three ways:
• 3. Random chance fertilization between any one female gamete with any other male gamete.
Br br+Bbrr
br bR+bbRr
OR
Brown eyes, black hair
Blue eyes, red hair
2. Crossingover duringmeiosis I.
1. Parent cellwith four chromosomes.
3. Homologs separate.(Pairing of chromosomes depends on independentassortment.)
4. Gametes produced by meiosis II.
5. Offspring produced by selfing (only some of the possibilities shown.)
EVEN SELF-FERTILIZATION LEADS TO GENETICALLY VARIABLE OFFSPRING
Meiosis Occurs in Different Places in the Life Cycles of Different Organisms
Diploid life cycles: diploid phase predominates; meiosis directly produces gametes.
Haploid life cycles: haploid phase predominates; meiosis occurs in the zygote.
Alternation of generations: Haploid and diploid phases alternate with one another.
Diploid adult
MITOSIS FERTILIZATION
MEIOSIS
Haploidgametes
Diploidzygote
Diploid dominant
FERTILIZATIONMITOSIS
MITOSISMEIOSIS
Diploid cell
Haploid cell
Haploid gametes
Haploid adult
Haploid dominant
Diploid plant
Diploid cell
Haploid cells
Haploid gametes Haploid
plant
MITOSIS
MEIOSISMITOSIS
FERTILIZATIION
MITOSIS
Alternation of generations