Meiosis and Sexual Reproduction Chapter 10
Impacts, Issues: Why Sex? An adaptive trait tends to spread more quickly through a sexually reproducing population than through an asexually reproducing one
Fig. 10-1b, p. 154
10.1 Introducing Alleles Asexual reproduction produces genetically identical copies of a parent (clones) Sexual reproduction introduces variation in the combinations of traits among offspring
Genes and Alleles Genes are regions in an organism s DNA that encode information about heritable traits In sexual reproduction, pairs of genes are inherited on pairs of chromosomes, one maternal and one paternal Alleles are different forms of the same gene Offspring of sexual reproducers inherit new combinations of alleles, the basis of traits
Chromosome Pair: Maternal and Paternal
10.1 Key Concepts Sexual vs Asexual Reproduction In asexual reproduction, one parent transmits its genetic information to offspring In sexual reproduction, offspring typically inherit information from two parents who differ in their alleles Alleles are different forms of the same gene; they specify different versions of a trait
10.2 What Meiosis Does Meiosis A nuclear division mechanism that precedes cytoplasmic division of immature reproductive cells in sexually-reproducing eukaryotic species
Halving the Diploid Number A diploid cell has two nonidentical copies of every chromosome (except XY sex chromosomes) Humans have 23 pairs of homologous chromosomes Meiosis in germ cells halves the diploid number of chromosomes (2n) to the haploid number (n), producing haploid gametes Eggs and sperm have 23 unpaired chromosomes
Gamete Production Gametes are produced in specialized reproductive structures or organs
Fig. 10-3a, p. 156
anther (where sexual spores that give rise to sperm form) ovules inside an ovary (where sexual spores that give rise to eggs form) a Flowering plant Fig. 10-3a, p. 156
Fig. 10-3b, p. 156
testis (where sperm originate) b Human male Fig. 10-3b, p. 156
Fig. 10-3c, p. 156
ovary (where eggs develop) c Human female Fig. 10-3c, p. 156
Restoring the Diploid Number Human gametes (eggs and sperm) have 23 chromosomes one of each homologous pair The diploid number (23 pairs) is restored at fertilization, when two haploid gametes fuse and form a diploid zygote, the first cell of a new individual
Human Chromosomes: Homologous Pairs
Two Divisions, Not One In meiosis, DNA is replicated once and divided twice (meiosis I and meiosis II), forming four haploid nuclei In meiosis I, each duplicated homologous chromosome is separated from its partner In meiosis II, sister chromatids are separated
Two Divisions, Not One Meiosis I Meiosis II
each chromosome in the cell pairs with its homologous partner then the partners separate p. 157
one chromosome (duplicated) two chromosomes (unduplicated) p. 157
10.3 Visual Tour of Meiosis
10.3 Visual Tour of Meiosis
Meiosis I plasma membrane newly forming microtubules of the spindle one pair of homologous chromosomes breakup of nuclear envelope centrosome with a pair of centrioles, moving to opposite sides of nucleus A Prophase I B Metaphase I C Anaphase I D Telophase I Fig. 10-5a, p. 158
Fig. 10-5a (1), p. 158
Fig. 10-5a (1), p. 158
Fig. 10-5a (2), p. 158
Fig. 10-5a (2), p. 158
Fig. 10-5a (3), p. 158
Fig. 10-5a (3), p. 158
Fig. 10-5a (4), p. 158
Fig. 10-5a (4), p. 158
Fig. 10-5b (1), p. 159
Fig. 10-5b (2), p. 159
Fig. 10-5b (3), p. 159
Fig. 10-5b (4), p. 159
Meiosis I plasma membrane newly forming microtubules of the spindle one pair of homologous chromosomes breakup of nuclear envelope centrosome with a pair of centrioles, moving to opposite sides of nucleus A Prophase I B Metaphase I C Anaphase I D Telophase I Stepped Art Fig. 10-5a, p. 158
Meiosis II There is no DNA replication between the two nuclear divisions. E Prophase II F Metaphase II G Anaphase II H Telophase II Stepped Art Fig. 10-5b, p. 159
Animation: Meiosis step-by-step
10.2-10.3 Key Concepts Stages of Meiosis Meiosis reduces the chromosome number Meiosis occurs only in cells set aside for sexual reproduction Meiosis sorts a reproductive cell s chromosomes into four haploid nuclei, which are distributed to descendent cells by cytoplasmic division
10.4 How Meiosis Introduces Variation in Traits Crossovers and the random sorting of chromosomes in meiosis introduce novel combinations of alleles into gametes, resulting in new combinations of traits among offspring
Crossing Over in Prophase I Crossing over The process by which a chromosome and its homologous partner exchange heritable information in corresponding segments Occurs during condensation in prophase I
Crossing Over Between Homologous Chromosomes
Fig. 10-6a, p. 160
Fig. 10-6b, p. 160
A A a a B Here, we focus on only two genes. One gene has alleles A and a; the other has alleles B and b. B B b b Fig. 10-6b, p. 160
Fig. 10-6c, p. 160
crossover C Close contact between the homologous chromosomes promotes crossing over between nonsister chromatids, so paternal and maternal chromatids exchange segments. Fig. 10-6c, p. 160
Fig. 10-6d, p. 160
D Crossing over mixes up paternal and maternal alleles on homologous chromosomes. A A a a B B b b Fig. 10-6d, p. 160
Animation: Crossing over
Segregation of Chromosomes into Gametes Homologous chromosomes can be attached to either spindle pole in prophase I, so each homologue can be packaged into either one of the two new nuclei Random assortment produces 10 23 (8,388,608) possible combinations of homologous chromosomes
Random Assortment
A Alignment in nucleus at metaphase I B Alignments in two nuclei at metaphase II C Nuclei of the four resulting gametes Possible lineup #1 Possible lineup #2 Possible lineup #3 Possible lineup #4 Fig. 10-7, p. 161
A Alignment in nucleus at metaphase I B Alignments in two nuclei at metaphase II C Nuclei of the four resulting gametes Possible lineup #1 Possible lineup #2 Possible lineup #3 Possible lineup #4 Stepped Art Fig. 10-7, p. 161
Animation: Random alignment
10.4 Key Concepts: Chromosome Recombinations and Shufflings During meiosis, each pair of maternal and paternal chromosomes swaps segments Then, each chromosome is randomly segregated into one of the new nuclei Both processes lead to novel combinations of alleles and traits among offspring
10.5 From Gametes to Offspring Aside from meiosis, the details of gamete formation and fertilization differ among plants and animals
Gamete Formation in Plants Sporophytes Diploid bodies with specialized structures that form spores (haploid cells) that give rise to gametophytes through mitosis Gametophytes A multicelled haploid body inside which one or more gametes form
Life Cycles of Plants and Animals
Fig. 10-8a, p. 162
mitosis zygote (2n) multicelled sporophyte (2n) fertilization DIPLOID HAPLOID meiosis gametes (n) mitosis multicelled gametophyte (n) mitosis spores (n) a Plant life cycle Fig. 10-8a, p. 162
Fig. 10-8b, p. 162
mitosis zygote (2n) multicelled body (2n) fertilization DIPLOID HAPLOID meiosis b Animal life cycle gametes (n) Fig. 10-8b, p. 162
Gamete Formation in Animals Males Meiosis of primary spermatocytes produces four haploid spermatids, which mature into sperm Females Meiosis of a primary oocyte forms cells of different sizes; the secondary oocyte gets most of the cytoplasm and matures into an ovum (egg); other cells (polar bodies) get little cytoplasm and degenerate
Sperm Formation in Animals
sperm (mature, haploid male gametes) secondary spermatocytes (haploid) diploid male germ cell A Growth primary spermatocyte (diploid) B Meiosis I and cytoplasmic division spermatids (haploid) C Meiosis II and cytoplasmic division Fig. 10-9, p. 163
sperm (mature, haploid male gametes) secondary spermatocytes (haploid) diploid male germ cell A Growth primary spermatocyte (diploid) B Meiosis I and cytoplasmic division spermatids (haploid) C Meiosis II and cytoplasmic division Stepped Art Fig. 10-9, p. 163
Animation: Sperm formation
Egg Formation in Animals
Fig. 10-10 (left), p. 163
first polar body (haploid) three polar bodies (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. 10-10 (left), p. 163
first polar body (haploid) three polar bodies (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 Stepped Art Fig. 10-10, p. 163
Animation: Egg formation
More Shufflings at Fertilization Chance combinations of maternal and paternal chromosomes through fertilization produce a unique combination of genetic information Fertilization The fusion of two haploid gametes (sperm and egg) resulting in a diploid zygote
10.5 Key Concepts: Sexual Reproduction in Context of Life Cycles Gametes form by different mechanisms in males and females In most plants, spore formation and other events intervene between meiosis and gamete formation
10.6 Mitosis and Meiosis An Ancestral Connection? Though they have different results, mitosis and meiosis are fundamentally similar processes Meiosis may have evolved by the remodeling of existing mechanisms of mitosis
Comparing Mitosis and Meiosis
Fig. 10-11a (1), p. 165
Fig. 10-11a (2), p. 165
Fig. 10-11a (3), p. 165
Fig. 10-11b, p. 165
one diploid nucleus two diploid nuclei Prophase Metaphase Anaphase Telophase Chromosomes condense. Nuclear envelope breaks up. Bipolar spindle forms; it attaches chromosomes to spindle poles. Chromosomes align midway between spindle poles. Sister chromatids separate as they are pulled toward spindle poles. Chromosome clusters arrive at spindle poles. Chromosomes decondense. New nuclear envelopes form. Fig. 10-11b, p. 165
Fig. 10-11c, p. 164
one diploid nucleus two haploid nuclei Prophase I Chromosomes condense. Homologous chromosomes pair. Bipolar spindle forms; it attaches chromosomes to spindle poles. Crossovers occur. Nuclear envelope breaks up. Metaphase I Anaphase I Telophase I Chromosomes align midway between spindle poles. Homologous chromosomes separate as they are pulled toward spindle poles. Chromosome clusters arrive at spindle poles. New nuclear envelopes form. Chromosomes decondense. Fig. 10-11c, p. 164
Fig. 10-11d, p. 165
two haploid nuclei four haploid nuclei Prophase II Metaphase II Anaphase II Telophase II Chromosomes condense. Bipolar spindle forms; it attaches chromosomes to spindle poles. Nuclear envelope breaks up. Chromosomes align midway between spindle poles. Sister chromatids separate as they are pulled toward spindle poles. Chromosome clusters arrive at spindle poles. New nuclear envelopes form. Chromosomes decondense. Fig. 10-11d, p. 165
10.6 Key Concepts Mitosis and Meiosis Compared Meiosis may have originated by evolutionary remodeling of mechanisms that already existed for mitosis, and before that, for repairing damaged DNA
Animation: Comparing mitosis and meiosis
Animation: Generalized life cycles
Animation: Meiosis
Animation: Meiosis I and II
Animation: Reproductive organs
Video: Why sex?