Meiosis and Sexual Life Cycles

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Each spore grows by mitosis into a haploid organism called a gametophyte

A gametophyte makes haploid gametes by mitosis

Fertilization of gametes results in a diploid sporophyte

Slide 28

Fig. 13-6b

Key

Haploid (n)

Diploid (2n)

n

n

n

n

n

2n

2n

Mitosis

Mitosis

Mitosis

Zygote

Spores

Gametes

MEIOSIS

FERTILIZATION

Diploid

multicellular

organism

(sporophyte)

Haploid multi-

cellular organism

(gametophyte)

(b) Plants and some algae

Slide 29

In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage

The zygote produces haploid cells by meiosis

Each haploid cell grows by mitosis into a haploid multicellular organism

The haploid adult produces gametes by mitosis

Slide 30

Fig. 13-6c

Key

Haploid (n)

Diploid (2n)

Mitosis

Mitosis

Gametes

Zygote

Haploid unicellular or

multicellular organism

MEIOSIS

FERTILIZATION

n

n

n

n

n

2n

(c) Most fungi and some protists

Slide 31

Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis

However, only diploid cells can undergo meiosis

In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring

Slide 32

Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid

Like mitosis, meiosis is preceded by the replication of chromosomes

Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II

The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis

Each daughter cell has only half as many chromosomes as the parent cell

Slide 33

The Stages of Meiosis

In the first cell division (meiosis I), homologous chromosomes separate

Meiosis I results in two haploid daughter cells with replicated chromosomes; it is called the reductional division

In the second cell division (meiosis II), sister chromatids separate

Meiosis II results in four haploid daughter cells with unreplicated chromosomes; it is called the equational division

Slide 34

Fig. 13-7-1

Interphase

Homologous pair of chromosomes