Meiosis: Understanding Cell Division PDF

Topic 11. Meiosis Learning objectives (LOBs) 1. Describe the organisation of the eukaryotic genome in the human karyotype. 2. Define and compare haploid and diploid cells. 3. Define and compare autosomes and sex chromosomes. 4. Define the terms chromatin, sister chromatids, chromosome and hom...

Topic 11. Meiosis Learning objectives (LOBs) 1. Describe the organisation of the eukaryotic genome in the human karyotype. 2. Define and compare haploid and diploid cells. 3. Define and compare autosomes and sex chromosomes. 4. Define the terms chromatin, sister chromatids, chromosome and homologous chromosomes. 5. Describe the process of meiosis and explain its role in gamete production. 6. Compare mitosis to meiosis and distinguish the general purpose of mitosis from the purpose of meiosis. 7. Discuss the mechanisms that contribute to genetic variation. Reading: Chapter 13, Campbell Biology Overview: Reproduction and chromosomal inheritance Living organisms are distinguished by their ability to reproduce their own kind Literally, children do not inherit particular physical traits from their parents Offspring acquire genes from parents by inheriting chromosomes Comparison of Asexual and Sexual Reproduction Asexual reproduction: a single individual passes genes to its offspring without the fusion of gametes => Clone: a group of genetically identical individuals from the same parent Sexual reproduction: two parents give rise to offspring that have unique combinations of genes inherited from both parents Role of meiosis in sexual reproduction Sexual reproduction by fertilization: the union of gametes (the sperm and the egg) The gametes are produced by meiosis The fertilized egg is called a zygote and has 1 set of chromosomes from each parent The zygote then produces somatic cells by mitosis and develops into an adult  Μeiosis The cell division process that produces the Meiosis gametes (sperm and egg) 2n Reduction of chromosome number by half n (production of haploid cells= n chromosomes) Fertilization After fertilization, the zygote is diploid (2n) 2n Mitosis 2n 6 Sets of Chromosomes in Human Cells 2 types of cells in humans: Somatic cells: have two sets of 23 chromosomes = 46 total chromosomes => diploid cells (2n, n = 23 chromosomes) Gametes (sperm and egg): have one set of chromosomes = 23 total => haploid cells (n, n =23 chromosomes) Sets of Chromosomes in Human Cells Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes (diploid, 2n) Karyotype: an ordered display of the pairs of chromosomes from a cell The 2 chromosomes in each pair are called homologous chromosomes (or homologs) Homologous chromosomes are the same length and shape and carry genes controlling the same inherited characters  Homologous chromosomes Homologous chromosomes (homologs): - The chromosome pairs during Meiosis I - They carry the same genes but they may carry different alleles of those genes In each pair, one homologous chromosome is inherited from the father (paternal homolog) and the other one from the mother (maternal homolog) => One allele is inherited from the father and the other one from the mother Homologous chromosomes Figure 13.3b Human Karyotype Pair of homologous 5 m duplicated chromosomes Centromere Sister chromatids Metaphase chromosome Figure 13.3c Human Karyotype 5 m Each somatic cell = 44 autosomes (22 pairs) + 2 sex chromosomes XX (if female) or XY (if male) = 46 chromosomes Sets of Chromosomes in Human Cells Human karyotype: consists of 22 pairs of autosomes and 1 pair of sex chromosomes Autosomes: chromosomes that do not determine the sex Sex chromosomes: determine the sex of the individual and are called X and Y -Human females have a homologous pair of X chromosomes (XX) -Human males have one X and one Y chromosome (XY) (=> non-homologous) Sets of Chromosomes in Human Cells Each homologous chromosomes pair includes 1 chromosome from each parent (1 paternal + 1 maternal chromosome) The 46 chromosomes in a human somatic cell are 2 sets of 23 - 1 set from the mother (23 maternal chromosomes) - 1 set from the father (23 paternal chromosomes) Each replicated chromosome (after DNA replication) => consists of 2 identical sister chromatids Figure 13.4 Sets of Chromosomes in Human Cells Key Maternal set of 2n = 6 chromosomes (n = 3) Paternal set of chromosomes (n = 3) Sister chromatids of one duplicated chromosome Centromere Two non-sister Pair of homologous chromatids in chromosomes a homologous pair (one from each set) Chromosome Sets in the Human Life Cycle At sexual maturity, the gonads (ovaries and testes) produce haploid gametes (egg and sperm) A gamete (sperm or egg) contains a single set of chromosomes => haploid (n) For humans, the haploid number is 23 (n =23) Each set of 23 consists of 22 autosomes and 1 sex chromosome: - In the egg (ovum): the sex chromosome is X - In a sperm cell: the sex chromosome may be either X or Y Figure 13.5 Key Haploid gametes (n = 23) Human Haploid (n) Egg (n) Sexual Diploid (2n) Life Cycle Sperm (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) The Variety of Sexual Life Cycles Sexual reproduction: Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number Three main types of sexual life cycles in eukaryotes depending on timing between meiosis and fertilization In animal sexual life cycles: - Gametes are the only haploid cells and the only cell type produced by meiosis - Gametes undergo no further cell division before fertilization - Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism  The Variety of Sexual Life Cycles Figure 13.6 Key Haploid (n) Haploid unicellular or Haploid multi- Diploid (2n) cellular organism multicellular organism (gametophyte) n Gametes n n Mitosis n Mitosis Mitosis n Mitosis n n n n n MEIOSIS FERTILIZATION Spores n n Gametes Gametes n MEIOSIS FERTILIZATION Zygote 2n MEIOSIS FERTILIZATION 2n 2n Diploid 2n Zygote Diploid 2n multicellular multicellular Mitosis Mitosis organism Zygote organism (sporophyte) (a) Animals (b) Plants and some algae (c) Most fungi and some protists In animals: only In other organisms: diploid cells divide by Both haploid and diploid cells can divide by mitosis mitosis Figure 13.6a Key Haploid (n) Diploid (2n) n Gametes n n MEIOSIS FERTILIZATION 2n Zygote 2n Diploid multicellular Mitosis organism Animals  The Variety of Sexual Life Cycles Depending on the type of organism, either haploid or diploid cells can divide by mitosis - In animals: only diploid cells divide by mitosis - In other organisms (plants, algae, protists, fungi): haploid and diploid cells can divide by mitosis However, in all organisms only diploid cells can undergo meiosis Sexual reproduction: the halving (by meiosis) and doubling (by fertilization) of chromosomes contributes to genetic variation in offspring Meiosis reduces the number of chromosome sets from diploid to haploid Like mitosis, meiosis is preceded by the replication of chromosomes (DNA replication during S phase) Meiosis takes place in two sets of cell divisions called meiosis I and meiosis II The 2 cell divisions result in 4 daughter cells, rather than the 2 daughter cells (in mitosis) Each daughter cell has only half as many chromosomes as the parent cell (haploid)  Meiosis Production of haploid cells from diploid cells  Production of gametes (egg, sperm)  reduction of chromosomal number by half  Each new cell has only 1 chromosome from each pair of homologous chromosomes  4 new haploid cells produced by 2 consecutive meiotic divisions from an initial diploid cell Includes pairing of homologous chromosomes and 2 meiotic divisions The Stages of Meiosis After chromosomes duplicate, two divisions follow: – Meiosis I (reductional division): pairing and separation of homologous chromosomes  Results in 2 haploid daughter cells with replicated chromosomes (2 chromatids/ chromosome) – Meiosis II (equational division): sister chromatids separation => The result is four haploid daughter cells with unreplicated chromosomes (1 chromatid/ chromosome)  Figure 13.7-3 Interphase Chromosome content: 2n Pair of homologous DNA (chromatid) content: 2n chromosomes in diploid parent cell Duplicated pair Chromosomes of homologous duplicate chromosomes Diploid cell with Chromosome content: 2n Sister duplicated DNA (chromatid) content: 4n chromatids chromosomes Meiosis I Meiosis I: separates Chromosome content: n 1 Homologous homologous DNA (chromatid) content: 2n chromosomes separate chromosomes Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids Meiosis II: separate separates sister Chromosome content: n chromatids DNA (chromatid) content: n Haploid cells with unduplicated chromosomes Chromosomal and DNA content during Meiosis I and II Meiosis I : results in two haploid daughter cells with replicated chromosomes (2 chromatids/ chromosome) Explanation: - cells are haploid in terms of chromosome content as they have one chromosome from each pair of homologs - However, they are diploid (2n) in terms of DNA content (chromatids) as they have replicated chromosomes (2 chromatids/ chromosome) - Example: human cells have 23 chromosomes with 2 chromatids each => have 46 chromatids in total (2n)  Number of chromosomes = n (23 in humans)  Number of chromatids (DNA content)= 2n (46 in humans) Chromosomal and DNA content during Meiosis I and II Meiosis II : results in four haploid daughter cells with unreplicated chromosomes (1 chromatid/ chromosome) Explanation: - cells are haploid both in terms of chromosome number and in terms of DNA content (chromatid number) - Chromosomes only have 1 chromatid each  Number of chromosome = n (23 in humans)  Number of chromatids (DNA content)= n (23 in humans) The Stages of Meiosis Meiosis I is preceded by interphase DNA replication during interphase => chromosomes duplicate to form sister chromatids The sister chromatids are genetically identical and joined at the centromere The single centrosome replicates during interphase, forming two centrosomes Figure 13.8 The Stages of Meiosis MEIOSIS I: Separates homologous chromosomes MEIOSIS I: Separates sister chromatids Telophase I and Telophase II and Prophase I Metaphase I Anaphase I Prophase II Metaphase II Anaphase II Cytokinesis Cytokinesis Centrosome Sister chromatids (with centriolepair) remain attached Sister Chiasmata Centromere chromatids (with kinetochore) Spindle Metaphase plate During another round of cell division, the sister chromatids finally separate; Cleavage four haploid daughter cells result, containing unduplicated chromosomes. furrow Homologous Sister chromatids Haploid daughter Homologous Fragments chromosomes separate cells forming chromosomes of nuclear separate envelope Microtubule Each pair of homologous Two haploidcells attached to chromosomes separates. form; each chromosome kinetochore still consists of two Duplicated homologous sister chromatids. Chromosomes line up chromosomes (red and blue) by homologouspairs. pair and exchange segments; 2n = 6 in this example. The Stages of Meiosis I Division in meiosis I occurs in 4 phases: – Prophase I – Metaphase I – Anaphase I – Telophase I and cytokinesis  MEIOSIS I: separation of homologous chromosomes Metaphase I Anaphase I Telophase I and Prophase I Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Sister Chiasmata Centromere chromatids (with kinetochore) Spindle Metaphase plate Cleavage furrow Homologous Homologous Fragments chromosomes chromosomes of nuclear separate envelope Microtubule Each pair of homologous Two haploid attached to chromosomes separates. cells form; each kinetochore chromosome Duplicated homologous Chromosomes line up still consists chromosomes (red and blue) by homologous pairs. of two sister pair and exchange segments; chromatids. 2n = 6 in this example. Prophase I Prophase I: 90% of the time required for meiosis Chromosomes begin to condense Synapsis: pairing of homologous chromosomes (aligned gene by gene) -Each pair of chromosomes forms a tetrad, a group of four chromatids (synaptonemal complex) -Each tetrad usually has one or more chiasmata: X-shaped regions where crossing over occurred Genetic recombination (crossing over): exchange of DNA segments between non-sister chromatids of homologous chromosomes - May occur during synapsis Synaptonemal complex Synaptonemal complex tetrad chiasmata Figure 13.11a Synaptonemal complex Chiasma Centromere TEM Metaphase I Tetrads line up at the metaphase plate, with each homologous chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome Anaphase I Homologous chromosomes pairs separate Each homologous chromosome moves towards the opposite pole Sister chromatids remain attached at the centromere and move as one unit toward the pole Telophase I and Cytokinesis Telophase I: - each half of the cell has a haploid set of chromosomes - each chromosome still consists of two sister chromatids Cytokinesis: - cytoplasm division to forming 2 haploid daughter cells - In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms Meiosis II No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated Meiosis II is very similar to mitosis Division in meiosis II also occurs in four phases: – Prophase II – Metaphase II – Anaphase II – Telophase II and cytokinesis  MEIOSIS II: separation of sister chromatids Telophase II and Prophase II Metaphase II Anaphase II Cytokinesis During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicatedchromosomes. Sister chromatids Haploid daughter separate cells forming Prophase II A spindle apparatus forms In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Metaphase II The sister chromatids are arranged at the metaphase plate If crossing over has occurred in meiosis I, the two sister chromatids of each chromosome may no longer be genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles Anaphase II The sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes towards opposite poles Telophase II and Cytokinesis Telophase II The chromosomes arrive at opposite poles Nuclei reform and the chromosomes decondense Cytokinesis Division of the cytoplasm At the end of meiosis, there are 4 daughter cells, each with a haploid set of unreplicated chromosomes (1 chromatid per chromosome) Each daughter cell is genetically distinct from the others and from the parent cell  Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis reduces the number of chromosomes sets from 2 (diploid) to 1 (haploid), producing cells that differ genetically from each other and from the parent cell In humans: - Μitosis: production of 2 diploid cells from 1 diploid cell - Μeiosis: production of 4 haploid cells from 1 diploid cell  Comparison of Mitosis and Meiosis Three events are unique to meiosis, and all three occur in meiosis l: – Synapsis and crossing over in prophase I: Pairing of homologous chromosomes and exchange genetic material – Metaphase I: alignment of homologous chromosomes pairs (chromatid tetrads) instead of individual replicated chromosomes – Anaphase I: separation of homologous chromosomes instead of sister chromatids Figure 13.9a MITOSIS MEIOSIS Parent cell MEIOSIS I Chiasma Prophase Prophase I Duplicated Chromosome Chromosome duplication duplication Homologous chromosome 2n = 6 chromosome pair Metaphase Metaphase I Anaphase Anaphase I Telophase Daughter Telophase I cells of Haploid meiosis I n=3 2n 2n MEIOSIS II Daughter cells n n n n of mitosis Daughter cells of meiosis II Figure 13.9b Comparison of Mitosis and Meiosis SUMMARY Property Mitosis Meiosis DNA Occurs during interphase before Occurs during interphase before meiosis I begins replication mitosis begins Number of One, including prophase, metaphase, Two, each including prophase, metaphase, anaphase, divisions anaphase, and telophase and telophase Synapsis of Does not occur Occurs during prophase I along with crossing over homologous between non-sister chromatids; resulting chiasmata chromosomes hold pairs together due to sister chromatid cohesion Number of Two, each diploid (2n) and genetically Four, each haploid (n), containing half as many daughter cells identical to the parent cell chromosomes as the parent cell; geneticallydifferent and genetic from the parent cell and from each other composition Role in the Enables multicellular adult to arise from Produces gametes; reduces number of chromosomes animal body zygote; produces cells for growth, repair, by half and introduces genetic variability among the and, in some species, asexual reproduction gametes  Maternal Meiosis Mitosis homolog parental paternal cell parental homolog cell DNA replication prophase I prophase Meiosis I Meta- phase I Homologous chromosomes align at the same level on the metaphase plate Cytokinesis I Anaphase I Meiosis II Homologous chromosomes Metaphase II align along the metaphase plate Metaphase I Anaphase II Anaphase Cytokinesis II Cytokinesis II 47 http://www.youtube.com/watch?v=D1 _-mQS_FZ0 http://www.youtube.com/watch?v=Ba 9LXKH2ztU Mitosis and Meiosis Ι Prophase Metaphase Anaphase Telophase and cytokinesis Sister chromatid separation Mitosis Prophase Metaphase Anaphase Telophase and cytokinesis Meiosis Sister chromatids remain connected Meiosis II Metaphase Anaphase Telophase and cytokinesis Sister chromatid separation Origins of Genetic Variation Among Offspring Sexual reproduction => genetic variation => Contributes to evolution Four mechanisms contribute to genetic variation: 1. Genetic recombination (crossing over): exchange of chromosomal segments 2. Independent assortment of chromosomes: 223 combinations of homologous chromosomes in meiosis => > 8 million gametes 2.Random fertilization: > 64 trillions possible offspring 3. Mutations: create different version of the genes (alleles) 1. Genetic recombination (crossing over) Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene (synapsis) In crossing over, homologous portions of 2 non- sister chromatids exchange places Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome  Genetic recombination non-parental chromosomes Meiosis Before Meiosis Chromosomes in gametes parental chromosomes Genetic recombination (crossing over): Exchange of genetic material between homologous chromosomes Figure 13.11-5 Prophase I Non-sister chromatids of meiosis held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes 2. Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes 223 = 8 388 608 possible gametes Independent Assortment of Chromosomes Figure 13.10-3 Possibility 1 Possibility 2 P1 M1 Two equally P1 M1 probable Diploid Diploid cell cell arrangements of P2 M2 M2 P2 chromosomes at metaphase I P1 M1 P1 M1 Metaphase P2 M2 II M2 P2 Daughter cells (gametes) Combination 1 Combination 2 Combination 3 Combination 4 P1+P2 M1+M2 P1+M2 P2+M1 P=paternal chromosome; M =maternal chromosome; n=2 => chromosome no=4 3. Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) 223 sperms x 223 eggs = approx. 70 trillion possible zygotes The fusion of 2 gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations That is why we are all so unique!!! 4. Mutations Mutations: changes in an organism’s DNA - they are the original source of genetic diversity Mutations create different versions of genes called alleles Reshuffling of alleles during sexual reproduction produces genetic variation  The Evolutionary Significance of Genetic Variation Within Populations Sexual reproduction contributes to the genetic variation in a population, which originates from mutations Natural selection results in the accumulation of genetic variations favored by the environment  Summary Meiosis: - Meiosis I: separation of homologous chromosomes - Meiosis II: separation of sister chromatids Mechanisms contribute to genetic variation: - Genetic recombination (crossing over) - Independent assortment of chromosomes - Random fertilization - Mutations  Chromosomal/DNA content during cell cycle (Mitosis) 2n chromosomes G1 phase 2n chromatids (2n) Diploid (2n) DNA replication 2n chromosomes S phase 4n chromatids G2 phase (4n) Tetraploid Mitotic (4n) spindle chromatids 2n chromosomes Metaphase Tetraploid 4n chromatids cell (4n) 4n chromosomes Anaphase 4n chromatids cell Tetraploid (4n) cytokinesis 2n chromosomes 2n chromatids Daughter cells (2n) Diploid (2n)  Interphase Chromosomal/DNA Figure 13.7-3 content during Meiosis Chromosome content: 2n Pair of homologous DNA (chromatid) content: 2n chromosomes in diploid parent cell Duplicated pair Chromosomes duplicate of homologous (S phase) chromosomes Diploid cell with Chromosome content: 2n Sister duplicated DNA (chromatid) content: 4n chromatids chromosomes Meiosis I Meiosis I: separates Chromosome content: n 1 Homologous homologous DNA (chromatid) content: 2n chromosomes separate chromosomes Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids Meiosis II: separate separates sister Chromosome content: n chromatids DNA (chromatid) content: n Haploid cells with unduplicated chromosomes  Mitosis chromosomal content Cell cycle phase Chromosome Chromatid number number (DNA content) G0 and G1 phase 2n 2n S phase 2n From 2n to 4n G2 phase 2n 4n M phase (Prophase to 2n 4n Metaphase) M phase (Anaphase to 4n 4n Cytokinesis) End of Mitosis (after 2n 2n cytokinesis) Note: at the beginning of S phase the chromatid number is 2n and at the end it is 4n. Therefore during S phase the DNA/chromatid content is between 2n and 4n.  Meiosis chromosomal content Interphase or Meiosis stage Chromosome Chromatid number number (DNA content) G0 and G1 phase 2n 2n S phase 2n From 2n to 4n G2 phase 2n 4n Meiosis I (Prophase I to 2n 4n cytokinesis I) End of Meiosis I (after n 2n cytokinesis) Meiosis II (Prophase II to n 2n metaphase II) Meiosis II (Anaphase II to 2n 2n cytokinesis II) End of Meiosis II (after n n cytokinesis)