Meiosis and Sexual Reproduction Chapter 9

Why Sex?

9.1 Genes and Alleles Genes Alleles 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

ovules inside an anther (where ovary (where sexual sexual spores that give rise to sperm form) ovules inside an ovary (where sexual spores that give to eggs form) Flowering plant Fig. 9.3a, p.140

(where sperm originate) ovary (where eggs develop) testis (where sperm originate) ovary (where eggs develop) Human male Human female Fig. 9.3b-c, p.140

Animation: Reproductive organs CLICK HERE TO PLAY

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

9.2 What Meiosis Does Meiosis Fertilization 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

germ cell germ cell 2n each chromosome duplicated during interphase n MEIOSIS I separation of homologues MEIOSIS II separation of sister chromatids gametes gametes 2n diploid number restored at fertilization zygote Fig. 9.12, p.150

Animation: Meiosis I and II CLICK HERE TO PLAY

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

Homologous Chromosomes

9.3 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)

Meiosis I The first nuclear division Each duplicated chromosome lines up with its homologous partner The two homologous chromosomes move apart, toward opposite spindle poles

Prophase I Chromosomes condense and align tightly with their homologues Each homologous pair undergoes crossing over Microtubules form the bipolar spindle One pair of centrioles moves to the other side of the nucleus

Prophase I (cont.) Nuclear envelope breaks up Microtubules growing from each spindle pole penetrate the nuclear region Microtubules tether one or the other chromosome of each homologous pair

Prophase I

Metaphase I Microtubules from both poles position all pairs of homologous chromosomes at the spindle equator

Metaphase I

Anaphase I Microtubules separate each chromosome from its homologue, moving to opposite spindle poles Other microtubules overlap midway between spindle poles, slide past each other to push poles farther apart As anaphase I ends, one set of duplicated chromosomes nears each spindle pole

Anaphase I

Telophase I Two nuclei form All chromosomes are still duplicated Typically, the cytoplasm divides All chromosomes are still duplicated Each still consists of two sister chromatids

Telophase I

Meiosis II The second nuclear division Sister chromatids of each chromosome are pulled away from each other Each is now an individual chromosome

Prophase II

Metaphase II

Anaphase II and Telophase II In anaphase II, one chromosome of each type is moved toward opposite spindle poles Occurs in both nuclei formed in meiosis I By the end of telophase II, there are four haploid nuclei, each with unduplicated chromosomes

Anaphase II

Telophase II

Prophase I Metaphase I Anaphase I Telophase I Meiosis I newly forming microtubules of the spindle one pair of homologous chromosomes spindle equator (midway between the two poles) plasma membrane breakup of nuclear envelope centrosome with a pair of centrioles, moving to opposite sides of nucleus Prophase I Metaphase I Anaphase I Telophase I Chromosomes were duplicated earlier, in interphase. Prior to metaphase I, one set of microtubules had tethered one chromosome of each type to one spindle pole and another set tethered its homologue to the other spindle pole. One of each duplicated chromosome, maternal or paternal, moves to a spindle pole; its homologue moves to the opposite pole. One of each type of chromosome has arrived at a spindle pole. In most species, the cytoplasm divides at this time. Fig. 9.5a, p.142

Anaphase I Telophase I Prophase I Metaphase I Meiosis I Stepped Art Fig. 9-5a, p.142

Fig. 9.5b, p.142

Prophase II Metaphase II Anaphase II Telophase II Meiosis II there is no DNA replication between the two divisions Prophase II Metaphase II Anaphase II Telophase II In each cell, one of two centrioles moves to the opposite side of the cell, and a new bipolar spindle forms. By now, microtubules from both spindle poles have finished a tug-of-war. The sister chromatids of each chromosome move apart and are now individual, unduplicated A new nuclear envelope encloses each parcel of chromosomes, so there are now four nuclei. Fig. 9.5b, p.142

Anaphase II Telophase II Prophase II Metaphase II Meiosis 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

Key Concepts: STAGES OF MEIOSIS (cont.) Meiosis sorts out a reproductive cell’s chromosomes into four haploid nuclei Haploid nuclei are distributed to daughter cells by way of cytoplasmic division

Animation: Meiosis step-by-step CLICK HERE TO PLAY

9.4 Meiosis Introduces Variation in Traits Two events in meiosis cause variation in traits in sexually reproducing species Crossing over during prophase I of meiosis Chromosome shuffling during metaphase I of meiosis

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

a A maternal chromosome (purple) and paternal chromosome (blue) were duplicated earlier, during interphase. They become visible in microscopes early in prophase I, when hey star to condense to threadlike form. b Each chromosome and its homologous partner zipper together, so all four chromatids are tightly aligned. mom’s allele A dad’s allele a mom’s allele A mom’s allele A mom’s allele B dad’s allele b mom’s allele B dad’s allele b c Here is a simple way to think about crossing over. (Chromosomes are still c ondensed and threadlike, and each is tightly aligned with its homologous partner.) d Their intimate contact promotes crossing over at different places along the length of nonsister chromatids. e At the crossover site, paternal and maternal chromatids exchange corresponding segments. f Crossing over mixes up maternal and paternal alleles on homologous chromosomes. Fig. 9.6, p.144

Fig. 9.6a, p.144

Fig. 9.6b, p.144

Fig. 9.6c, p.144

Fig. 9.6d, p.144

Fig. 9.6e, p.144

Fig. 9.6f, p.144

Animation: Crossing over CLICK HERE TO PLAY

Metaphase I: Chromosome Shuffling Homologous chromosomes align randomly during metaphase I Microtubules can harness either a maternal or paternal chromosome of each homologous pair to either spindle pole Either chromosome may end up in any new nucleus (gamete)

Chromosome Shuffling: Random Alignment

combinations possible 1 2 3 combinations possible or or or Fig. 9.7, p.145

combinations possible 1 2 3 combinations possible or or or Stepped Art Fig. 9-7, p.145

Animation: Random alignment CLICK HERE TO PLAY

Key Concepts: CHROMOSOME RECOMBINATION AND SHUFFLING During meiosis, each pair of maternal and paternal chromosomes swaps segments and exchanges alleles Pairs get randomly shuffled, so forthcoming gametes end up with different mixes of maternal and paternal chromosomes

9.5 From Gametes to Offspring Multicelled diploid and haploid bodies are typical in life cycles of plants and animals Plants Sporophyte: A multicelled plant body (diploid) that makes haploid spores Spores give rise to gametophytes (multicelled plant bodies in which haploid gametes form)

From Gametes to Offspring Animals Germ cells in the reproductive organs give rise to sperm or eggs Fusion of a sperm and egg at fertilization results in a zygote

Comparing Plant And Animal Life Cycles

Fig. 9.8a, p.146

DIPLOID HAPLOID multicelled gametophyte meiosis multicelled sporophyte (2n) zygote (2n) DIPLOID fertilization meiosis HAPLOID gametes spores (n) (n) multicelled gametophyte (n) meiosis meiosis a Plant life cycle Fig. 9.8a, p.146

Fig. 9.8b, p.146

multicelled body DIPLOID HAPLOID meiosis multicelled body (2n) zygote (2n) DIPLOID fertilization meiosis HAPLOID gametes (n) b Animal life cycle Fig. 9.8b, p.146

Animation: Generalized life cycles CLICK HERE TO PLAY

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

Sperm Formation in Animals

secondary spermatocytes (haploid) primary spermatocyte (diploid) sperm (mature, haploid male gametes) secondary spermatocytes (haploid) primary spermatocyte (diploid) diploid male germ cell spermatids (haploid) a Growth b Meiosis I and cytoplasmic division c Meiosis II and cytoplasmic division Fig. 9.9, p.147

Animation: Sperm formation CLICK HERE TO PLAY

Egg Formation in Animals

three polar bodies (haploid) first polar body (haploid) oogonium (diploid female germ cell) primary oocyte (diploid) secondary oocyte (haploid) ovum (haploid) a Growth b Meiosis I and cytoplasmic division c Meiosis II and cytoplasmic division Fig. 9.10a, p.147

Animation: Egg formation CLICK HERE TO PLAY

Key Concepts: SEXUAL REPRODUCTION IN LIFE CYCLES In animals, gametes form by different mechanisms in males and females In most plants, spore formation and other events intervene between meiosis and gamete formation

9.6 Comparing Mitosis and Meiosis Both mitosis and meiosis require bipolar spindle to move and sort duplicated chromosomes Some mechanisms of meiosis resemble those of mitosis, and may have evolved from them Example: DNA repair enzymes function in both

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

Comparing Mitosis and Meiosis

Comparing Mitosis and Meiosis

Comparing Mitosis and Meiosis

Prophase I Metaphase I Anaphase I Telophase I In a diploid (2n) germ cell, duplicated chromosomes now condense. The bipolar spindle forms and tethers the chromosomes. Crossovers occur between homologues. Each maternal chromosome and its paternal homologue are randomly aligned midway between the two spindle poles. Either one may get attached to either pole. Homologous partners separate and move to opposite poles. There are two clusters of chromosomes. New nuclear envelopes may form and the cytoplasm may divide before meiosis II begins. Fig. 9.11a, p.148

cell, the duplicated chromosomes now condense. Bipolar spindle forms Mitosis Prophase Metaphase Anaphase Telophase In a diploid (2n) body cell, the duplicated chromosomes now condense. Bipolar spindle forms and tethers the chromosomes. All chromosomes aligned at the spindle equator. Sister chromatids of each chromosome moved to opposite spindle poles. Two diploid (2n) nuclei form. After cytoplasmic division, there are two diploid body cells. Fig. 9.11b, p.149

no interphase and no DNA replication between the two nuclear divisions Prophase II Metaphase II Anaphase II Telophase II All chromosomes still duplicated. New spindle forms in each nucleus, tethers chromosomes to spindle poles. All chromosomes aligned at the spindle equator. Sister chromatids of each chromosome moved to opposite spindle poles. Four haploid (n) nuclei form. After cytoplasmic division, haploid cells function as gametes or spores. Fig. 9.11c, p.149

Key Concepts: MITOSIS AND MEIOSIS COMPARED Recent molecular evidence suggests that meiosis originated through mechanisms that already existed for mitosis and, before that, for repairing damaged DNA

Animation: Comparing mitosis and meiosis CLICK HERE TO PLAY

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