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Biology Concepts and Applications | 9e

Starr | Evers | Starr

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Chapter 12

Meiosis and Sexual Reproduction

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• Alleles & sexual

reproduction

• Process of meiosis

• How is variation

introduced?

• Mitosis vs. Meiosis

Meiosis & Sexual Reproduction

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12.1 Why Sex?

• In asexual reproduction, a single individual

gives rise to offspring that are identical to

itself and others (clones)

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• In sexual

reproduction, two

individuals mix their

genetic material

12.1 Why Sex?

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Homologous Chromosomes

5

Chromosomes with the same length, shape

and genes

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• Forms of a gene

with slightly

different DNA

and hence

slightly different

products

Introducing Alleles

Genes occur in pairs on

homologous

chromosomes.

The members of each pair

of genes may be identical,

or they may differ slightly,

as alleles.

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Meiosis – process of nuclear division that

halves the chromosome number – basis for

sexual reproduction

MEIOSIS II

Pair of

homologous chromosomes in diploid parent cell

A pair of duplicated homologous chromosomes

Sister chromatids

INTERPHASE BEFORE MEIOSIS MEIOSIS I

Gametes and the Life Cycle of Humans

•Haploid Cell

•A cell with only

one set of

chromosomes

•Diploid Cell

•A cell with two

sets of

chromosomes

•Zygote

•A fertilized egg

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Meiosis I

Prophase I

Telophase I

& Cytokinesis

Metaphase I Anaphase I

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Meiosis II: Starts with 2 haploid cells

Prophase II

Telophase I

& Cytokinesis Metaphase I Anaphase I

Prophase II Metaphase II Anaphase II Telophase II

& Cytokinesis

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Meiosis and Genetic Variability

• Crossing over – Prophase I

• Chromosome Segregation (Independent

Assortment) – Metaphase I

• Random Fertilization

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Prophase I of meiosis Duplicated pair of homologous chromosomes

Homologous (nonsister) chromatids exchange corresponding segments.

Site of crossing over

Metaphase I

Sister chromatids remain joined at their centromeres.

Metaphase II

Spindle

Gametes

Recombinant chromosomes combine genetic information from different parents.

Recombinant chromosomes

Figure 12-8 p199

3

1

2

Chromosome Segregation – Metaphase I

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Random Fertilization

Human egg (female gamete) surrounded by

sperm (male gametes)

Mitosis Meiosis

Ploidy of Parent

Cell (Haploid or

Diploid?)

# of Divisions?

# of Daughter

Cells?

Ploidy of Daughter

Cells?

Do Homologous

Chromosomes

separate? (If so,

when?)

Do Sister Chromatids

separate? (If so,

when?

Function(s)

Mitosis vs. Meiosis