Meiosis Bio 11 Tuesday, October 7, 2008

Which one of the following does not occur during mitotic anaphase?   A) The centromeres of each chromosome divide.   B) Sister chromatids separate.   C) The chromatid DNA replicates.   D) Daughter chromosomes begin to move toward opposite poles of the cell.   E) All of the choices occur during mitotic anaphase.  

Asexual Reproduction is a primary means of propagation for many species

Sexual reproduction generates diversity Variety in offspring improves chances of adaptation to changing environments

The offspring of sexual reproduction inherit parental traits Offspring inherit DNA from each parent Offspring are not the same DNA is from the same source

Meiosis is the process of cell division that allows gamete formation LE 8-13 Haploid gametes (n = 23) n Egg cell Meiosis is the process of cell division that allows gamete formation Gametes are haploid (n), somatic cells are diploid (2n) n Sperm cell Meiosis Fertilization Diploid zygote (2n = 46) 2n Multicellular diploid adults (2n = 46) Mitosis and development

Chromosomes come in pairs For diploid organims (2n) One chromosome in a paired set comes from each parent #’s 1-22 are autosomes X and Y are sex chromosomes

Homologous Chromosomes LE 8-12 Homologous Chromosomes Centromere Sister chromatids

Homologous chromosomes Pairs of chromosomes are homologous The site for particular genes are called loci (singular: locus) Identical strands of the same chromosome are sister chromatids Loci

Haploid multicellular organism (gametophyte) Haploid multicellular LE 13-6 Key Haploid Diploid Haploid multicellular organism (gametophyte) Haploid multicellular organism Gametes n n Mitosis n Mitosis Mitosis n Mitosis n n n n n n Spores n MEIOSIS FERTILIZATION n Gametes Gametes n MEIOSIS FERTILIZATION MEIOSIS FERTILIZATION Zygote 2n 2n 2n 2n Diploid multicellular organism (sporophyte) Zygote 2n Diploid multicellular organism Mitosis Mitosis Zygote Animals Plants and some algae Most fungi and some protists

Overview of Meiosis Meiosis consists of 2 major stages, Meiosis I and Meiosis II Cell copies DNA once, divides twice, creating 4 haploid cells In meiosis I, homologous pairs of chromosomes are grouped, recombined, and then segregated into two intermediate cells In meiosis II, those cells are divided

The Stages of Meiosis In (meiosis I), homologous chromosomes separate Meiosis I results in two haploid daughter cells with replicated chromosomes In the second cell division (meiosis II), sister chromatids separate Meiosis II results in four haploid daughter cells with unreplicated chromosomes

Microtubules attached to kinetochore Centrosomes (with centriole LE 8-14a MEIOSIS I : Homologous chromosome separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Microtubules attached to kinetochore Centrosomes (with centriole pairs) Metaphase plate Sister chromatids remain attached Sites of crossing over Spindle Nuclear envelope Sister chromatids Tetrad Centromere (with kinetochore) Homologous chromosomes separate Chromatin

(Interphase precedes meiosis, of course) LE 13-8aa INTERPHASE MEIOSIS I: Separates homologous chromosomes (Interphase precedes meiosis, of course) METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Nuclear envelope Chromatin Chromosomes duplicate

Division in meiosis I occurs in four phases: Prophase I Metaphase I Anaphase I Telophase I

Prophase I >90% of the time required for meiosis Chromosomes condense Synapsis: homologous chromosomes pair up Crossing over: nonsister chromatids exchange DNA segments Tetrad: four chromatids Chiasmata: X-shaped regions where crossing over occurred

Crossing-Over in Prophase I

LE 8-18a TEM 2,200 Tetrad Chiasma Centromere

Crossing-over in Prophase I

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

Animation: Metaphase I Tetrads line up at the metaphase plate Microtubules from one pole are attached Microtubules from the other pole are attached to the kinetochore of the other chromosome Animation: Metaphase I

Independent Assortment in Metaphase I Random alignment of maternal/paternal chromosomes at the metaphase plate Produces genetic variability within populations

Independent Assortment in Metaphase I

Independent Assortment in Metaphase I

Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis 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- 1, where n is the haploid number For humans (n = 23), there are more than 8 million (222) possible combinations of chromosomes

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

Anaphase I homologous chromosomes separate One chromosome moves toward each pole Sister chromatids remain attached at the centromere

Meiosis II Resembles mitosis in its mechanisms However, only half the chromosomes are present in meiosis II compared with mitosis

Haploid daughter Sister chromatids cells forming separate LE 8-14b MEIOSIS I : Sister chromatids separate TELOPHASE I TELOPHASE I PROPHASE I METAPHASE I ANAPHASE I AND CYTOKINESIS AND CYTOKINESIS Cleavage furrow Sister chromatids separate Haploid daughter cells forming

MEIOSIS II: Separates sister chromatids LE 13-8b MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow Haploid daughter cells forming Sister chromatids separate 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

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

Spindle Microtubules bind to the Kinetochore Prometaphase marks the binding of spindle fibers to kinetochore Chromosomes will be pulled apart in anaphase

MEIOSIS II: Separates sister chromatids LE 13-8b MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow Haploid daughter cells forming Sister chromatids separate 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

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

MEIOSIS II: Separates sister chromatids LE 13-8b MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow Haploid daughter cells forming Sister chromatids separate 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

Which one of the following statements is false?   A) A typical body cell is called a somatic cell.   B) Gametes are haploid cells.   C) Somatic cells are diploid.   D) Gametes are made by mitosis.  E) A zygote is a fertilized egg.  

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

Nondisjunction in meiosis I Normal meiosis II Normal meiosis II LE 8-21a Nondisjunction in meiosis I Normal meiosis II Normal meiosis II Gametes n + 1 n + 1 n  1 n  1 Number of chromosomes

Normal meiosis I Nondisjunction in meiosis I Gametes n + 1 n  1 n n LE 8-21b Normal meiosis I Nondisjunction in meiosis I Gametes n + 1 n  1 n n Number of chromosomes

LE 8-21c Egg cell n + 1 Sperm cell Zygote 2n + 1 n (normal)

Downs syndrome is caused by a nondisjunction (trisomy) at chromosome 21

Infants with Down syndrome LE 8-20c 90 80 70 60 Infants with Down syndrome (per 1,000 births) 50 40 30 20 10 20 25 30 35 40 45 50 Age of mother

Figure 8.20A

Hypotonic Fixative Packed red and solution white blood cells LE 8-19 Hypotonic solution Packed red and white blood cells Fixative White blood cells Stain Centrifuge Blood culture Fluid Centromere Sister chromatids Pair of homologous chromosomes 2,600

Comparing mitosis and meiosis

Property Mitosis Meiosis DNA replication Divisions Synapsis and crossing over Daughter cells, genetic composition Role in animal body

Property Mitosis Meiosis DNA replication During interphase Divisions One Two Synapsis and crossing over Do not occur In prophase I Daughter cells, genetic composition Two diploid, identical to parent cell Four haploid, different from parent cell and each other Role in animal body Produces cells for growth and tissue repair Produces gametes

(before chromosome replication) LE 13-9 MITOSIS MEIOSIS Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Propase Prophase I Chromosome replication Chromosome replication Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) 2n = 6 Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase Metaphase I Anaphase Sister chromatids separate during anaphase Homologues separate during anaphase I; sister chromatids remain together Anaphase I Telophase Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II

Review of Meiosis

Spermatogenesis vs. Oogenesis

Meiosis One spermatocyte produces 4 sperm One oocyte produces 1 viable egg and 3 non-viable polar bodies

Variation from sexual reproduction contributes to evolution Reshuffling of different versions of genes (alleles) during sexual reproduction produces genetic variation

Key Maternal set of chromosomes Possibility 1 Possibility 2 LE 13-10 Key Maternal set of chromosomes Possibility 1 Possibility 2 Paternal set of chromosomes Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4

LE 13-11 Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations Crossing over adds even more variation Each zygote has a unique genetic identity

Evolutionary Significance of Genetic Variation Within Populations Natural selection results in accumulation of genetic variations favored by the environment Sexual reproduction contributes to the genetic variation in a population, which ultimately results from mutations

Explain how junior and sis are not identical.

duplicated chromosomes) LE 8-17a Coat-color genes Eye-color genes C E Brown Black C E C E Meiosis c e c e c e White Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes

LE 8-18b Coat-color genes Eye-color genes C E (homologous pair of c e Tetrad (homologous pair of chromosomes in synapsis) c e Breakage of homologous chromatids C E c e Joining of homologous chromatids C E Chiasma c e Separation of homologous chromosomes at anaphase I C E C e c E c e Separation of chromatids at anaphase II and completion of meiosis C E Parental type of chromosome C e Recombinant chromosome c E Recombinant chromosome c e Parental type of chromosome Gametes of four genetic types

C E Tetrad c e C E c e Coat-color genes Eye-color genes LE 8-18ba Coat-color genes Eye-color genes C E Tetrad (homologous pair of chromosomes in synapsis) c e Breakage of homologous chromatids C E c e Joining of homologous chromatids

C E c e C E C e c E c e Joining of homologous chromatids Chiasma LE 8-18bb Joining of homologous chromatids C E Chiasma c e Separation of homologous chromosomes at anaphase I C E C e c E c e Separation of chromatids at anaphase II and completion of meiosis

Separation of chromatids at anaphase II and completion of meiosis LE 8-18bc Separation of chromatids at anaphase II and completion of meiosis C E Parental type of chromosome C e Recombinant chromosome c E Recombinant chromosome c e Parental type of chromosome Gametes of four genetic types

Table 8.22

Review of mitosis and meiosis

LE 8-un149 INTERPHASE (cell growth and chromosome duplication) S (DNA synthesis) G1 G2 Cytokinesis Mitosis (division of cytoplasm) (division of nucleus) Genetically identical “daughter cells” MITOTIC PHASE (M)

Mitosis and development LE 8-un149s Haploid gametes (n = 23) n Egg cell n Sperm cell Meiosis Fertilization HUMAN LIFE CYCLE Multicellular diploid adults Diploid zygote 2n (2n = 46) (2n = 46) Mitosis and development

Sister chromatids Sister chromatids Homologous pair of chromosomes LE 8-un1 Sister chromatids Homologous pair of chromosomes Sister chromatids