Albia Dugger Miami Dade College Chapter 12 Meiosis and Sexual Reproduction.
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Transcript of Albia Dugger Miami Dade College Chapter 12 Meiosis and Sexual Reproduction.
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Albia Dugger • Miami Dade College
Chapter 12Meiosis and Sexual
Reproduction
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12.1 Why Sex?
• In sexual reproduction, offspring arise from two parents and inherit genes from both
• Asexual reproduction produces genetically identical copies of a parent (clones)
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Advantages of Sexual Reproduction
• An adaptive trait can spread more quickly through a sexually reproducing population than through an asexually reproducing one
• Collectively, offspring of sexual reproducers have a better chance of surviving the effects of a harmful mutation that arises in the population
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Sexual Reproduction in Animals and Plants
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Meiosis and Sexual Reproduction
• Sexual reproduction generates new combinations of traits in fewer generations than does asexual reproduction
• The process inherent to sexual reproduction that gives rise to this variation is meiosis, a nuclear division mechanism that halves the chromosome number
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12.2 Meiosis Halves the Chromosome Number
• Sexual reproduction mixes up alleles from two parents
• Meiosis, the basis of sexual reproduction, is a nuclear division mechanism that occurs in immature reproductive cells of eukaryotes
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Homologous Chromosomes
• Somatic (body) cells of humans and other sexually reproducing organisms are diploid
• One chromosome of each pair is maternal, and the other is paternal
• Except for a pairing of nonidentical sex chromosomes, homologous chromosomes carry the same set of genes
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Introducing 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
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Figure 12-2a p190
A Corresponding colored patches in this fluorescence micrograph indicate corresponding DNA sequences in a homologous chromosome pair. These chromosomes carry the same set of genes.
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Figure 12-2b p190
Genes occur in pairs on homologous chromosomes.
The members of each pair of genes may be identical, or they may differ slightly, as alleles.
B Homologous chromosomes carry the same series of genes, but the DNA sequence of any one of those genes might differ just a bit from that of its partner on the homologous chromosome.
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ANIMATED FIGURE: Genetic terms
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Gametes and Germ Cells
• Sexual reproduction involves the fusion of reproductive cells (gametes) from two parents
• In plants and animals, gametes form inside special reproductive structures or organs
• Division of immature reproductive cells (germ cells) gives rise to gametes
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Figure 12-3a p190
Reproductive organs of a human male
testis (where sperm originate)
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Figure 12-3b p190
Reproductive organs of a human female
ovary (where eggs develop)
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Figure 12-3c p190
Reproductive organs of a flowering plant
ovary (where sexual spores that give rise to egg cells form)
anther (where sexual spores that give rise to sperm cells form)
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ANIMATED FIGURE: Reproductive organs
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What Meiosis Does
• Meiosis in animal germ cells gives rise to eggs (female gametes) or sperm (male gametes)
• Gametes have a single set of chromosomes, so they are haploid (n): Their chromosome number is half of the diploid (2n) number
• Meiosis of a human germ cell (2n) produces gametes with 23 chromosomes: one of each pair (n)
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Two Divisions in Meiosis
• Meiosis partitions the chromosomes of one diploid nucleus (2n) into four haploid (n) nuclei
• In meiosis I, each duplicated homologous chromosome is separated from its partner
• In meiosis II, sister chromatids are separated
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Meiosis Halves the Chromosome Number
C Sister chromatids separate in meiosis II. The now unduplicated chromosomes are packaged into four new nuclei.
B Homologous partners separate. The still-duplicated chromosomes are packaged into two new nuclei.
Stepped Art
A In meiosis I, each duplicated chromosome in the nucleus pairs with its homologous partner.
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Fertilization
• The diploid chromosome number is restored at fertilization, when two haploid gametes (one egg and one sperm) fuse to form a zygote, the first cell of a new individual
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Take-Home Message: Sexual Reproduction Increases Variation in Heritable Traits
• Corresponding genes on homologous chromosomes vary in sequence as alleles
• Alleles are the basis of traits; sexual reproduction mixes up alleles from two parents
• Meiosis is the basis of sexual reproduction in eukaryotes; it precedes the formation of gametes or spores
• Meiosis halves the diploid (2n) chromosome number, to the haploid number (n); when two gametes fuse at fertilization, the chromosome number is restored; the zygote has one set of chromosomes from each parent
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12.3 Visual Tour of Meiosis
• Meiosis halves the chromosome number
• During meiosis, chromosomes of a diploid nucleus become distributed into four haploid nuclei
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Summary of Steps in Meiosis
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Meiosis
Meiosis I One diploid nucleus to two haploid nuclei
Prophase I. Homologous chromosomes condense, pair up, and swap segments. Spindle micro- tubules attach to them as the nuclear envelope breaks up.
plasma membrane spindle
nuclear envelope breaking up
centrosome
1 Metaphase I. The homologous chromosome pairs are aligned midway between spindle poles.
one pair of homologous chromosomes
2 Anaphase I. The homologous chromosomes separate and begin heading toward the spindle poles.
3 Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope forms around each cluster, so two haploid (n) nuclei form.
4
Stepped Art
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MeiosisMeiosis II Two haploid nuclei to four haploid nuclei
Anaphase II. All sister chromatids separate. The now unduplicated chromo -somes head to the spindle poles.
7 Telophase II. A cluster of chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form.
8 Metaphase II. The (still duplicated) chromosomes are aligned midway between poles of the spindle.
6 Prophase II. The chromosomes condense. Spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up.
No DNA replication
5
Stepped Art
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3D ANIMATION: Meiosis
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Take-Home Message:
What happens to a cell during meiosis?
• During meiosis, the nucleus of a diploid (2n) cell divides twice.
• Four haploid (n) nuclei form, each with a full set of chromosomes—one of each type
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ANIMATION: Meiosis I and II
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12.4 How Meiosis Introduces Variation in Traits
• Crossovers and the random sorting of chromosomes into gametes result in new combinations of traits among offspring
• Along with fertilization, these events contribute to the variation among the offspring of sexually reproducing species
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Crossing Over in Prophase I
• Crossing over is the process by which a chromosome and its homologous partner exchange heritable information in corresponding segments
• Crossing over occurs during condensation in prophase I
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A Here, we focus on only two of the many genes on a chromosome. In this example, one gene has alleles A A and a; the other has alleles B and b.
Stepped Art
B Close contact between homologous chromosomes promotes crossing over between nonsister chromatids. Paternal and maternal chromatids exchange corresponding pieces.
C Crossing over mixes up paternal and maternal alleles on homologous chromosomes.
Figure 12-6 p194
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ANIMATED FIGURE: Crossing over
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Chromosome Segregation
• 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 1023 (8,388,608) possible combinations of homologous chromosomes
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The four possible alignments of three pairs of chromosomes in a nucleus at metaphase I.
1
Stepped Art
Resulting combinationsof maternal and paternal chromosomes in the two nuclei that form at telophase I.
2
Resulting combinations of maternal and paternal chromosomes in the four nuclei that form at telophase II. Eight different combinations are possible.
3
Figure 12-7 p195
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ANIMATED FIGURE: Random alignment
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Take-Home Message: How does meiosis introduce variation in combinations of traits?
• Crossing over is recombination between nonsister chromatids of homologous chromosomes during prophase I; it makes new combinations of parental alleles
• Homologous chromosomes can be attached to either spindle pole in prophase I, so each homologue can end up in either one of the two new nuclei
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12.5 From Gametes to Offspring
• Aside from meiosis, the details of gamete formation and fertilization differ among plants and animals
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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
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Figure 12-8a p196
mitosis
zygote (2n)multicelled
sporophyte (2n)
Diploid
Fertilization
Haploid
Meiosis
gametes (n) spores (n)
multicelled gametophyte (n)
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Figure 12-8b p196
mitosis
zygote (2n) multicelled body (2n)
Diploid
Fertilization
Haploid
Meiosis
gametophyte (n)
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Figure 12-8c p196
multicelled sporophyte (2n)
multicelled body (2n)
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ANIMATED FIGURE: Generalized life cycles
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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
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Sperm Formation in Animals
male germ cell
1
2 3
sperm
4
Stepped Art
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ANIMATED FIGURE: Sperm formation
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Egg Formation in Animals
female germ cell
5 6
egg7
Stepped Art
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ANIMATED FIGURE: Egg formation
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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
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Fertilization
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Take-Home Message: How does meiosis fit into the life cycle of plants and animals?
• Meiosis and cytoplasmic division precede the development of haploid gametes in animals and spores in plants
• The union of two haploid gametes at fertilization results in a diploid zygote
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12.6 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
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Comparing Mitosis and Meiosis
Meiosis I One diploid nucleus to two haploid nuclei
Prophase I • Chromosomes condense. • Homologous chromosomes pair.• Crossovers occur (not shown). • Spindle forms and attaches
chromosomes to spindle poles. • Nuclear envelope breaks up.
Metaphase I • Chromosomes
align midway between spindle poles.
Anaphase I • Homologous
chromosomes separate and move toward at spindle poles.
Telophase I • Chromosome clusters arrive
opposite spindle poles.• New nuclear envelopes form. • Chromosomes decondense.
Stepped Art
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Comparing Mitosis and Meiosis
Stepped Art
Anaphase • Sister chromatids
separate and move toward at spindle poles.
Mitosis One diploid nucleus to two diploid nuclei
Prophase • Chromosomes condense.• Spindle forms and attaches
chromosomes to spindle poles. • Nuclear envelope breaks up.
Metaphase • Chromosomes
align midway between spindle poles.
Telophase • Chromosome clusters arrive
opposite spindle poles.• New nuclear envelopes form.• Chromosomes decondense
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Comparing Mitosis and Meiosis
Stepped Art
Telophase II • Chromosome clusters arrive
at spindle poles. • New nuclear envelopes form. • Chromosomes decondense.
Meiosis II Two haploid nuclei to four haploid nuclei
Prophase II • Chromosomes condense. • Spindle forms and attaches
chromosomes to spindle poles. • Nuclear envelope breaks up.
Metaphase II • Chromosomes
align midway between spindle poles.
Anaphase II • Sister chromatids
separate and move toward opposite spindle poles.
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Evidence in BRCA Genes
• Molecular products of BRCA genes monitor and repair breaks in DNA, for example during DNA replication prior to mitosis
• These same molecules monitor and fix breaks in homologous chromosomes during crossing over in prophase I of meiosis
• Mutations that affect these molecules can affect the outcomes of both mitosis and meiosis
• Sexual reproduction probably originated by mutations that affected mitosis
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Mouse Cell Nuclei During Meiosis
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Take-Home Message: Are the processes of mitosis and meiosis related?
• Meiosis may have evolved by the remodeling of existing mechanisms of mitosis