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EW Title Meiosis 1. Define the term gene. 2. What is the difference between sexual and asexual reproduction?

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Presentation on theme: "EW Title Meiosis 1. Define the term gene. 2. What is the difference between sexual and asexual reproduction?"— Presentation transcript:

1 EW Title Meiosis 1. Define the term gene. 2. What is the difference between sexual and asexual reproduction?

2 Chapter 13~ Meiosis and Sexual Life Cycles

3 Heredity 4 Heredity: the transmission of traits from one generation to the next 4 Variation: Inherited differences among individuals of the same species. 4 Genetics: The study of heredity and hereditary variations. 4 Gene: A segment of DNA that codes for a particular trait 4 Locus: Location on a chromosome for a particular gene (It is consistent throughout a species).

4 Karyotype 4 An ordered image of chromosomes starting with the longest chromosome.

5 LE 13-4 Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosomes Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere

6 Heredity 4 Asexual reproduction: clones 4 Sexual reproduction: variation 4 Human life cycle: 23 pairs of homologous chromosomes (46); 1 pair of sex and 22 pairs of autosomes] gametes are haploid (1N)/ all other cells are diploid (2N); fertilization (syngamy) results in a zygote 4 Meiosis: cell division to produce haploid gametes

7 Important Terms to Know 4 Autosomes: –Chromosomes that do not determine the sex of an organism (1-22). 4 Diploid Cells: –Cells that have 2 sets of chromosomes Haploid Cells: Cells that have 1 set of chromosomes

8 Animal Life Cycle

9 Plants/some algae 4 Alternation of generations: 2N sporophyte, by meiosis, produces 1N spores; spore divides by mitosis to generate a 1N gametophyte; gametes then made by mitosis which then fertilize into 2N sporophyte

10 Fungi/some algae 4 meiosis produces 1N cells that divide by mitosis to produce 1N adults (gametes by mitosis)

11 Meiosis 4 Preceded by chromosome replication, but is followed by 2 cell divisions (Meiosis I & Meiosis II) 4 4 daughter cells; 1/2 chromosome number (1N); variation

12 Meiosis 4 Preceded by chromosome replication, but is followed by 2 cell divisions (Meiosis I & Meiosis II) 4 4 daughter cells; 1/2 chromosome number (1N); variation

13 Prophase I 4 Prophase I typically occupies more than 90% of the time required for meiosis 4 Chromosomes begin to condense 4 In synapsis, homologous chromosomes loosely pair up, aligned gene by gene 4 In crossing over, nonsister chromatids exchange DNA segments 4 Each pair of chromosomes forms a tetrad, a group of four chromatids 4 Each tetrad usually has one or more chiasmata, X- shaped regions where crossing over occurred

14 LE 13-8ab Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

15 Metaphase I 4 At metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole 4 Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad 4 Microtubules from the other pole are attached to the kinetochore of the other chromosome

16 LE 13-8ab Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

17 Anaphase I 4 In anaphase I, pairs of homologous chromosomes separate 4 One chromosome moves toward each pole, guided by the spindle apparatus 4 Sister chromatids remain attached at the centromere and move as one unit toward the pole

18 LE 13-8ab Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

19 Telophase I and Cytokinesis 4 In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids 4 Cytokinesis usually occurs simultaneously, forming two haploid daughter cells 4 In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms 4 No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

20 LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

21 Prophase II 4 Meiosis II is very similar to mitosis 4 In prophase II, a spindle apparatus forms 4 In late prophase II (not shown in the art), chromosomes (each still composed of two chromatids) move toward the metaphase plate

22 LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

23 Metaphase II 4 At metaphase II, the sister chromatids are arranged at the metaphase plate 4 Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical 4 The kinetochores of sister chromatids attach to microtubules extending from opposite poles Animation: Metaphase II Animation: Metaphase II

24 LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

25 Anaphase II 4 At anaphase II, the sister chromatids separate 4 The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

26 LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

27 Telophase II and Cytokinesis In telophase II, the chromosomes arrive at opposite poles 4 Nuclei form, and the chromosomes begin decondensing 4 Cytokinesis separates the cytoplasm 4 At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes 4 Each daughter cell is genetically distinct from the others and from the parent cell

28 LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

29 A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell 4 Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell 4 The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

30 Meiosis vs. Mitosis 4 Synapsis/tetrad/chiasmata (prophase I) 4 Homologous vs. individual chromosomes (metaphase I) 4 Sister chromatids do not separate (anaphase I) 4 Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes.

31 Origins of Genetic Variation, I 4 Independent assortment: homologous pair of chromosomes position and orient randomly (metaphase I) and nonidentical sister chromatids during meiosis II 4 Combinations possible: 2 ; with n the haploid number of the organism n

32 Origins of Genetic Variation, II 4 Crossing over (prophase I): the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes) 4 Random fertilization: 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!


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