Meiosis and Sexual Reproduction

Genes and Alleles

Genes• Sequences of DNA that encode heritable traits

Alleles • Slightly different forms of the same gene• Each specifies a different version of gene product

Sexual and Asexual Reproduction

Asexual reproduction (1 parent)• Offspring inherit parent’s genes• Clones (identical copies of parent)

Sexual reproduction (2 parents)• Offspring differ from parents and each another• Different combinations of alleles • Different details of shared traits

Sexual Reproduction

Meiosis, gamete formation, and fertilization occur in sexual reproduction

Meiosis and fertilization shuffle parental alleles• Offspring inherit new combinations of alleles

Where Gametes Form

Key Concepts: SEXUAL VS. ASEXUAL REPRODUCTION

By asexual reproduction, one parent alone transmits its genetic information to offspring

By sexual reproduction, offspring typically inherit information from two parents that differ in their alleles

Alleles are different forms of the same gene; they specify different versions of a trait

What Meiosis Does

Meiosis • Nuclear division mechanism that precedes

gamete formation in eukaryotic cells• Halves parental chromosome number

Fertilization• Fusion of two gamete nuclei • Restores parental chromosome number• Forms zygote (first cell of new individual)

Meiosis and Fertilization

Homologues

Sexual reproducers inherit pairs of chromosomes• 1 from maternal parent, 1 from paternal parent

The pairs are homologous (“the same”) • Except nonidentical sex chromosomes (X and Y)• Same length, shape, genes

All pairs interact at meiosis• One chromosome of each type sorts into gametes

Tour of Meiosis

All chromosomes are duplicated during interphase, before meiosis

Two divisions, meiosis I and II, divide the parental chromosome number by two

Each forthcoming gamete is haploid (n)

Prophase I

Metaphase I

Anaphase I

Telophase I

Prophase II

Metaphase II

Anaphase II

Telophase II

Telophase IAnaphase IMetaphase IProphase I

Meiosis I

Stepped Art

Fig. 9-5a, p.142

Meiosis II

Prophase II Metaphase II Anaphase II Telophase II

Stepped Art

Fig. 9-5b, p.142

Haploid Daughter Cells

When cytoplasm divides, four haploid cells result

One or all may serve as gametes or, in plants, as spores that lead to gamete-producing bodies

Key Concepts:STAGES OF MEIOSIS

Diploid cells have a pair of each type of chromosome, one maternal and one paternal

Meiosis, a nuclear division mechanism, reduces the chromosome number

Meiosis occurs only in cells set aside for sexual reproduction

Prophase I: Crossing Over

Nonsister chromatids of homologous chromosomes undergo crossing over• They exchange segments at the same place

along their length

Each ends up with new combinations of alleles not present in either parental chromosome

Crossing Over

Fig. 9.6d, p.144

Fig. 9.6e, p.144

Fig. 9.6f, p.144

Comparing Plant And Animal Life Cycles

Introducing Variation in Offspring

Three events cause new combinations of alleles in offspring: • Crossing over during prophase I (meiosis)• Random alignment of maternal and paternal

chromosomes at metaphase I (meiosis)• Chance meeting of gametes at fertilization

All three contribute to variation in traits

a Growth b Meiosis I andcytoplasmic division

c Meiosis II and cytoplasmic division

spermatids (haploid)

secondary spermatocytes

(haploid)

primary spermatocyte

(diploid)

Fig. 9.9, p.147

diploid malegerm cell

sperm (mature,haploid male

gametes)

Egg Formation in Animals

Differences in Mitosis and Meiosis

Mitosis maintains parental chromosome number• Duplicates genetic information• Occurs in body cells

Meiosis halves chromosome number• Introduces new combinations of alleles in

offspring• Occurs only in cells for sexual reproduction