Meiosis. Last time… n We saw how cells divide by mitosis to produce genetically identical daughter cells 2n parent cell --> 2 daughter cells, both 2n.

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.

+

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

FERTILIZATION

MITOSIS

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