Chapter 13: Meiosis and Sexual Life Cycles

November 25, 2007

Like begets like.

1 An Introduction to Heredity

1.1 O�spring acquire genes from parents by inheriting

chromosomes

1. DNA is polymer of four nucleotide monomers.

2. Transmission of traits based molecularly in precise replication of DNA.

3. Locus � gene's speci�c location along chromosome.

1.2 Like begets like, more or less: a comparison of asexual

and sexual reproduction

1. Asexual reproduction

(a) Single-celled eukaryotes: mitosis.

(b) Budding (Hydra)�cells of bud derived from mitosis.

(c) Occasionally mutation.

2. Sexual reproduction: genetic variation key.

2 The Role of Meiosis in Sexual Life Cycles

2.1 Fertilization and meiosis alternate in life cycles

2.1.1 The Human Life Cycle

1. Somatic cell is not sex cell; 46 light-microscope-visible chromosomes.

2. Two chromosomes of each type�homologous chromosomes. Samegenes at same loci.

3. X, Y are sex chromosomes. Others are autosomes.

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Figure 1: Variety of life cycles

4. Gametes have single set of 22 autosomes plus X or Y.

(a) Haploid (n) � humans, n = 23.

(b) Fertilization = syngamy � form diploid zygote.

5. Meiosis halves chromosomal number. Fertilization restores diploidness.

2.1.2 The Variety of Sexual Life Cycles

Three types of life cycles due to temporal variation of events.

1. Animals: gametes are only haploid cells; all other cells diploid.

2. Most fungi and some protists: meiosis occurs before o�spring develop.

(a) Meiosis produces haploid cells. Adult is haploid.

(b) Gametes produced by mitosis.

(c) Only diploid stage is zygote.

3. Alternation of generations in plants and some algae:

(a) Diploid and haploid multicellular stages.

(b) Sporophyte � multicellular diploid. Produces haploid spores.

(c) Spores mitose (NOT fertilize) into multicellular haploid gameto-

phyte.

(d) Haploid gametophyte produce gametes by mitosis.

(e) Sporophyte and gametophyte reproduce each other.

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2.2 Meiosis reduces chromosome number from diploid to

haploid: a closer look

1. Sister chromatids are NOT homologous pairs!

(a) Homologous pairs are di�erent chromosomes.

(b) Sister chromatids are two parts of a replicated chromosome.

2. Meiosis I produces two haploid cells with replicated chromosomes.

3. Meiosis II produces four haploid cells with unreplicated chromosomes.

1. Interphase�each chromosome duplicates into sister chromatids attachedat centromeres.

2. Prophase I�homologues pair up.

(a) Synapsis: synaptonemal complex attaches homologues.

(b) Each homologues pair is treated as tetrad�four chromatids.

(c) Chiasmata�crossing over. Hold chromosomes together.

(d) Like mitosis: spindle puts stu� at metaphase plate.

(e) Takes 90% of time; can take days.

(f) Only time of chromosome replication.

3. Metaphase I�Tetrads line up.

4. Anaphase I�Homologous chromosomes separate. Sister chromatids stilltogether, unlike mitosis.

5. Telophase I and cytokinesis�two haploid cells with replicated chromo-somes form.

6. Anaphase II�centromeres of sister chromatids now separate.

7. Telophase II and cytokinesis�four daughter cells with haploid unrepli-cated chromosomes.

2.2.1 Mitosis and Meiosis Compared

Three unique events to meiosis all occur in Meiosis I:

1. Synapsis in prophase I: synaptonemal complex (protein zipper) holdshomologous chromosomes.

(a) Treated as tetrad.

(b) Chiasmata are X-shaped regions of crossing of non-sister but ho-mologous chromatids.

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Figure

2:Interphase

andMeiosisI

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Figure

3:Telophase

I,CytokinesisI,andMeiosisII

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Figure 4: Comparison of mitosis and meiosis

2. In metaphase I, homologous pairs instead of individual chromosomes lineup.

3. Meiosis I separates homologous pairs, not sister chromatids.

Meiosis II virtually identical to mitosis, but does not replicate.

3 Origins of Genetic Variation

3.1 Sexual life cycles produce genetic variations among

o�spring

Three mechanisms.

3.1.1 Independent Assortment of Chromosomes

1. Orientation of homologous pairs relative to cell poles are random.

(a) Can receive maternal or paternal for any chromosome.

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2. Each homologous pair positioned independently.

3. 2n possibilities where n = 2N number.

3.1.2 Crossing Over

1. Produces recombinant chromosomes combining genes from two par-ents.

2. Begins early in prophase I before synaptonemal complex forms.

3. After synaptonemal complex disappears, crosses visible as chiasmata.

4. Humans cross average 2�3 per chromosome.

5. Recombinant chromatids can no longer identical twins at metaphase II.

3.1.3 Random fertilizations

Zygote is union of two gametes. Therefore,

2n × 2n = 22n

possibilities without crossing over.

• Three mechanisms reshu�e genes, but mutations ultimately create diver-sity.

3.2 Evolutionary adaptation depends on a population's

genetic variation.

1. Darwin realized heritable variation is what makes evolution possible.

2. Mendel (1822-84) published theory of inheritance that had no impact until15 years after Darwin died.

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