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Meiosis and Sexual Life Cycles
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Slide 20

At sexual maturity, the ovaries and testes produce haploid gametes

At sexual maturity, the ovaries and testes produce haploid gametes

Gametes are the only types of human cells produced by meiosis, rather than mitosis

Meiosis results in one set of chromosomes in each gamete

Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number

Slide 21

Fig. 13-5

Fig. 13-5

Key

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Egg (n)

Sperm (n)

MEIOSIS

FERTILIZATION

Ovary

Testis

Diploid

zygote

(2n = 46)

Mitosis and

development

Multicellular diploid

adults (2n = 46)

Slide 22

The Variety of Sexual Life Cycles

The Variety of Sexual Life Cycles

The alternation of meiosis and fertilization is common to all organisms that reproduce sexually

The three main types of sexual life cycles differ in the timing of meiosis and fertilization

Slide 23

In animals, meiosis produces gametes, which undergo no further cell division before fertilization

In animals, meiosis produces gametes, which undergo no further cell division before fertilization

Gametes are the only haploid cells in animals

Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism

Slide 24

Fig. 13-6

Fig. 13-6

Key

Haploid (n)

Diploid (2n)

n

n

Gametes

n

n

n

Mitosis

MEIOSIS

FERTILIZATION

MEIOSIS

2n

2n

Zygote

2n

Mitosis

Diploid

multicellular

organism

(a) Animals

Spores

Diploid

multicellular

organism

(sporophyte)

(b) Plants and some algae

2n

Mitosis

Gametes

Mitosis

n

n

n

Zygote

FERTILIZATION

n

n

n

Mitosis

Zygote

(c) Most fungi and some protists

MEIOSIS

FERTILIZATION

2n

Gametes

n

n

Mitosis

Haploid multi-

cellular organism

(gametophyte)

Haploid unicellular or

multicellular organism

Slide 25

Fig. 13-6a

Fig. 13-6a

Key

Haploid (n)

Diploid (2n)

Gametes

n

n

n

2n

2n

Zygote

MEIOSIS

FERTILIZATION

Mitosis

Diploid

multicellular

organism

(a) Animals

Slide 26

Plants and some algae exhibit an alternation of generations

Plants and some algae exhibit an alternation of generations

This life cycle includes both a diploid and haploid multicellular stage

The diploid organism, called the sporophyte, makes haploid spores by meiosis

Slide 27

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