CAMPBELL BIOLOGY IN FOCUS URRY CAIN WASSERMAN MINORSKY
CAMPBELL BIOLOGY IN FOCUS URRY • CAIN • WASSERMAN • MINORSKY • REECE 10 Meiosis and Sexual Life Cycles Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge, Simon Fraser University © 2016 Pearson Education, Inc. SECOND EDITION
Overview: Variations on a Theme § Living organisms are distinguished by their ability to reproduce their own kind § Heredity: the transmission of traits from one generation to the next § Variation: the differences in appearance that offspring show from parents and siblings § Genetics: the scientific study of heredity and variation © 2016 Pearson Education, Inc.
Comparison of Asexual and Sexual Reproduction § Asexual reproduction: , a single individual passes genes to its offspring without the fusion of gametes § Results in a clone § A group of genetically identical individuals from the same parent § Asexual reproduction is less expensive than sexual reproduction § Sexual reproduction: 2 parents give rise to offspring that have unique combinations of genes inherited from the 2 parents § Sexual reproduction is nearly universal among animals § Overall, genetic variation is evolutionarily advantageous © 2016 Pearson Education, Inc.
Figure 10. 5 Haploid gametes (n = 23) Key Haploid (n) Diploid (2 n) Egg (n) Sperm (n) FERTILIZATION MEIOSIS Ovary Testis Diploid zygote (2 n = 46) Multicellular diploid adults (2 n = 46) © 2016 Pearson Education, Inc. Mitosis and development
TERMINOLOGY § Somatic cell: a body cell; excludes sperm or egg cells § Diploid (2 n); 2 sets of chromosomes § Total 46 chromosomes (2 n) in every somatic cell of humans (every species has its own diploid number) § Gametes: sex cells; sperm or egg § Haploid (n); have only 1 set of chromosomes § 23 chromosomes in each human gamete (half of total) § Fertilization: union of 2 gametes to form zygote § Zygote: fertilized egg; diploid (2 n) cell § As you develop from zygote into mature adult, genetic information is passed to all somatic cells by mitosis
Terminology (Con’d) § Gene: unit of inheritance; packaged as chromosomes; contains DNA and histone proteins § Locus: the exact position a gene is found on a chromosome § Karyotype: an ordered display of the pairs of chromosomes from a cell § Homologous Chromosomes or Homologues: the two chromosomes in each pair of your 23 pairs; one homologue is from Mom; the other homologue is from Dad § Sex chromosomes: are called X and Y § Human females have a homologous pair of X chromosomes (XX) § Human males have one X and one Y chromosome § The remaining 22 pairs of chromosomes in human males are called autosomes © 2016 Pearson Education, Inc.
Figure 10. 6 Various Sexual Reproduction Among Organisms Haploid (n) Diploid (2 n) n Gametes n Mitosis n MEIOSIS Haploid multicellular organism (gametophyte) n Diploid multicellular organism (a) Animals © 2016 Pearson Education, Inc. 2 n Mitosis n n Spores FERTILIZATION Zygote Mitosis n MEIOSIS 2 n n Haploid unicellular or multicellular organism Gametes n n n Gametes Diploid multicellular organism (sporophyte) n FERTILIZATION MEIOSIS 2 n Mitosis n 2 n Mitosis (b) Plants and some algae Zygote FERTILIZATION 2 n Zygote (c) Most fungi and some protists
Figure 10. 6 -1 Animals Haploid (n) Diploid (2 n) n Gametes n n MEIOSIS 2 n Diploid multicellular organism © 2016 Pearson Education, Inc. FERTILIZATION Zygote 2 n Mitosis
KARYOTYPE • A photomicrograph of an individual’s somatic cell metaphase chromosomes arranged in sequence • Human karyotypes often made with lymphocytes (one of your 5 WBC) • Used to screen for chromosomal abnormalities
HOMOLOGOUS CHROMOSOMES (HOMOLOGUES) § One homologue is inherited from each parent Mom Homol Sister chromatid Dad homol Sister chroma Tetrad A pair of chromosomes with the same size, centromere position and staining pattern; 22 pair; the 23 rd pair are the sex chromosomes (XX = female, and XY = male)
Meiotic Cell Division Meiotic cell division is essential for sexual reproduction – The combination of genetic material from 2 parents Sexual Reproduction: involves meiosis (synthesis of gametes) & fertilization (union of 2 gametes to form zygote/fertilized egg) • Results in four daughter cells. • Each daughter cell contains half the number of chromosomes as the parent. • Each daughter cell (gamete) is genetically unique • Offspring have a unique combination of genes inherited from both parents • Results in great genetic variability • Offspring vary from each parent & from each other.
Figure 10. 7 Interphase Pair of homologous chromosomes in diploid parent cell Pair of duplicated homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Meiosis I Homologous chromosomes separate Meiosis II Haploid cells with duplicated chromosomes Sister chromatids separate Haploid cells with unduplicated chromosomes © 2016 Pearson Education, Inc.
GENERAL CHARACTERISTICS OF MEIOSIS § Meiosis: special type of cell division that reduces the number of chromosomes by half § Produces haploid sperm cells in testes & haploid egg cells in ovaries § Fertilization…. the union of haploid gametes…. produces diploid offspring § Meiosis has 2 divisions: § Start meiosis with a germ cell = stem cell that will undergo halving of chromosomes to become sperm/egg § DNA replication in S phase of Interphase § Meiosis I: chromosome number is halved by separating the 2 chromosomes of each homologue; P 1, M 1, A 1, T 1/C § No Interphase between Meiosis I and Meiosis II § Meiosis II: chromosome number is still halved, but sister chromatids are now separated: P 2, M 2, A 2, T 2/C
Figure 10. 7 -2 Meiosis I Homologous chromosomes separate Meiosis II Haploid cells with duplicated chromosomes Sister chromatids separate Haploid cells with unduplicated chromosomes © 2016 Pearson Education, Inc.
Details of Meiosis I • Interphase: chromosomes duplicate in S phase; 23 prs from Mom and 23 prs from Dad; still can’t see 46 prs of sisters in G 2 • Prophase I: can now see 46 prs sister chromatids • Homologues pair up/synapsis • Form a tetrad. Have 23 tetrads/23 prs homologues • Do crossing over/chiasmata • Metaphase I: tetrads (homologues & sisters) line up • Mendel’s Independent assortment • Anaphase I: tetrads split up (homologues w/sisters separate • Mendel’s Principle of Segregation • Telophase 1/C: 2 haploid cells form • Chromosomes still double as sister chromatids © 2016 Pearson Education, Inc.
Figure 10. 7 -1 Interphase G 1 Pair of homologous chromosomes in diploid parent cell S G 2 © 2016 Pearson Education, Inc. Pair of duplicated homologous chromosomes Sister chromatids Chromosomes duplicate Diploid cell with duplicated chromosomes
Figure 10. 4 Prophase I Key 2 n = 6 Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) Sister chromatids of one duplicated chromosome Two nonsister chromatids in a homologous pair Centromere Pair of homologous chromosomes (one from each set) Tetrad after Synapsis © 2016 Pearson Education, Inc.
Figure 10. 8 -1 a MEIOSIS I: Separates homologous chromosomes Prophase I Metaphase I Centrosome Kinetochore (with centriole pair) (at centromere) Sister Chiasmata Kinetochore chromatids Spindle microtubules Metaphase plate Fragments of nuclear envelope Pair of homologous chromosomes Centromere © 2016 Pearson Education, Inc.
Chiasmata Tetrad Region of crossing over; exchange of genetic mat
Figure 10. 9 -2 Synaptonemal complex Crossover Chiasmata © 2016 Pearson Education, Inc. Crossover
Figure 10. 8 -1 b MEIOSIS I: Separates homologous chromosomes Telophase I and Cytokinesis Anaphase I Sister chromatids remain attached Homologous chromosomes separate © 2016 Pearson Education, Inc. Cleavage furrow
Figure 10. 8 -1 MEIOSIS I: Separates homologous chromosomes Prophase I Metaphase I Centrosome Kinetochore (with centriole pair) (at centromere) Sister Chiasmata Kinetochore chromatids Spindle microtubules Metaphase plate Pair of homologous chromosomes Fragments of nuclear envelope Centromere © 2016 Pearson Education, Inc. Telophase I and Cytokinesis Anaphase I Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow
Why Do We Need Meiosis I? § To separate homologues § Now pre-gametes are haploid
SUMMARY OF MEIOSIS I § Have 2 pre-sperm or pre-egg cells of 23 pairs of sister chromatids each § Now haploid, but with sisters still attached
Details of Meiosis II § No Interphase between Meiosis 1 and 2 § Meiosis II: separates the sister chromatids & 4 haploid daughter cells with result, each containing single chromosomes § Division in meiosis II also occurs in 4 phases § Prophase II § Metaphase II § Anaphase II § Telophase II and cytokinesis • Similar to mitosis • Suggests that meiosis evolved from mitosis © 2016 Pearson Education, Inc.
Figure 10. 8 -2 MEIOSIS II: Separates sister chromatids Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming © 2016 Pearson Education, Inc.
WHY DO WE NEED MEIOSIS II? § To separate the sister chromatids § Identical to mitosis (except for chiasmata and independent assortment)
SUMMARY OF MEIOSIS II § Now have 4 sperm cells or 4 egg cells with 23 chromosomes each
Figure 10. 8 MEIOSIS I: Separates homologous chromosomes Prophase I Metaphase I © 2016 Pearson Education, Inc. Anaphase I Telophase I and Cytokinesis MEIOSIS II: Separates sister chromatids Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis
Cytoplasmic Division In meiosis, division of the cytoplasm differs between males & females: Male meiosis: • Produces 4 functional sperm cells Female meiosis: • Produces 1 functional egg cell/oocyte & 3 nonfunctional polar bodies
3 Unique Events to Meiosis § All three occur in meiosis l § Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information § Alignment of homologous pairs at the metaphase I plate: Homologous pairs of chromosomes are positioned there in metaphase I § Separation of homologs during anaphase I © 2016 Pearson Education, Inc.
GENETIC VARIATION IN MEIOSIS § Crossing over § During Prophase 1 § Have 2 -3 cross-overs/homologous pair (remember have 23 homologous pairs) § Independent Assortment § During Metaphase 1 § Each gamete has 1 in 8 million possible haploid combinations from Mom and Dad § Random Fertilization § An egg of 1 in 8 million possibilities will be fertilized by a sperm with 1 in 8 million possibilities § Results in a zygote that can have 1 in 64 trillion possible diploid combinations
COMPARISON OF MITOSIS & MEIOSIS Meiosis § Reduction division § Genetic variation § In testes/ovaries to make gametes § Pro 1: synapsis, tetrads, chiasmata § Meta 1: tetrads line up, indep assort § Ana 1: tetrads separate § Preceded by single replication of chromosomes (to make sister chromatids in S phase) § 2 cell divisions: Meiosis 1 and 2 § Produces 4 daughter cells § Each having 23 chromosomes Mitosis § Reproducing Division § No genetic variation § Makes somatic cells § Not applicable § Sister line up; no indep assort § Sisters separate § Same § § § 1 cell division Produces 2 daughter cells Each identical having 46 chromosomes (same as parent)
Figure 10. 10 -2 a © 2016 Pearson Education, Inc.
Figure 10. 10 -2 b © 2016 Pearson Education, Inc.
Figure 10. 11 -s 3 Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 © 2016 Pearson Education, Inc. Combination 3 Combination 4
Figure 10. 12 -s 5 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma site Synapsis and crossing over Centromere Breakdown of proteins holding sister chromatid arms together. TEM Anaphase II Daughter cells Recombinant chromosomes © 2016 Pearson Education, Inc.
Concept 10. 4: Genetic variation produced in sexual life cycles contributes to evolution § Mutations: changes in an organism’s DNA § Are the original source of genetic diversity § Mutations create different versions of genes called alleles § Reshuffling of alleles during sexual reproduction produces genetic variation © 2016 Pearson Education, Inc.
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