Meiosis and Sexual Life Cycles

Meiosis and Sexual Life Cycles Living organisms are distinguished by their ability to reproduce their own kind Heredity Is the transmission of traits from one generation to the next Variation Shows that offspring differ somewhat in appearance from parents and siblings

Inheritance of Genes Genes are segments of DNA, units of heredity Offspring acquire genes from parents by inheriting chromosomes Genetics is the scientific study of heredity and hereditary variation

Inheritance of Genes Each gene in an organism’s DNA has a specific locus on a certain chromosome We inherit one set of chromosomes from our mother and one set from our father Two parents give rise to offspring that have unique combinations of genes inherited from the two parents - sexual reproduction

Asexual Reproduction In asexual reproduction, one parent produces genetically identical offspring by mitosis Figure 13.2 Parent Bud 0.5 mm

Sexual Reproduction - The Human Life Cycle Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism Fertilization and meiosis alternate in sexual life cycles At sexual maturity the ovaries and testes produce haploid gametes by meiosis Unlike somatic cells, sperm and egg cells are haploid cells, containing only one set of chromosomes During fertilization, sperm and ovum fuse forming a diploid zygote The zygote develops into an adult organism

Meiosis Reduces the chromosome number such that each daughter Cell has a haploid set of chromosomes Ensures that the next generation will have: Diploid number of chromosome Exchange of genetic information (combination of traits that differs from that of either parent)

Meiosis Only diploid cells can divide by meiosis. Prior to meiosis I, DNA replication occurs. During meiosis, there will be two nuclear divisions, and the result will be four haploid nuclei. No replication of DNA occurs between meiosis I and meiosis II.

Meiosis Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous separate Haploid cells with replicated chromosomes Sister chromatids Haploid cells with unreplicated chromosomes Meiosis I Meiosis II Meiosis reduces the number of chromosome sets from diploid to haploid Meiosis takes place in two sets of divisions Meiosis I reduces the number of chromosomes from diploid to haploid Meiosis II produces four haploid daughter cells

Meiosis Phases Meiosis involves the same four phases seen in mitosis prophase metaphase anaphase telophase They are repeated during both meiosis I and meiosis II. The period of time between meiosis I and meiosis II is called interkinesis. No replication of DNA occurs during interkinesis because the DNA is already duplicated.

Prophase I Prophase I occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, the 2 members of each homologous pair of chromosomes line up side-by-side, aligned gene by gene, to form a tetrad consisting of 4 chromatids During synapsis, sometimes there is an exchange of homologous parts between non-sister chromatids. This exchange is called crossing over Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred Prophase I of meiosis Tetrad Nonsister chromatids Chiasma, site of crossing over

Metaphase I At metaphase I, tetrads line up at the metaphase plate, with one 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 Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad 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

Anaphase I In anaphase I, pairs of homologous chromosomes separate One chromosome moves toward each pole, guided by the spindle apparatus Sister chromatids remain attached at the centromere and move as one unit toward the pole Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad 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

Telophase I and Cytokinesis 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 Cytokinesis usually occurs simultaneously, forming two haploid daughter cells In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

Prophase II Meiosis II is very similar to mitosis In prophase II, a spindle apparatus forms In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND Sister chromatids separate Haploid daughter cells forming

Metaphase II At metaphase II, the sister chromatids are 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 Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND Sister chromatids separate Haploid daughter cells forming

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 Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND Sister chromatids separate Haploid daughter cells forming

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 Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND Sister chromatids separate Haploid daughter cells forming

A 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 two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

A 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: Homologous chromosomes physically connect and exchange genetic information At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell. In anaphase II of meiosis, the sister chromatids separate

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

Comparison Mitosis Meiosis Homologous chromosomes do not pair up No genetic exchange between homologous chromosomes One diploid cell produces 2 diploid cells or one haploid cell produces 2 haploid cells New cells are genetically identical to original cell (except for mutation) Meiosis DNA duplication followed by 2 cell divisions Sysnapsis Crossing-over One diploid cell produces 4 haploid cells Each new cell has a unique combination of genes

Sexual Reproduction - The Human Life Cycle Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) During fertilization, sperm and ovum fuse forming a diploid zygote The zygote develops into an adult organism

Spermatocytes to Spermatids Primary spermatocytes undergo meiosis I, forming two haploid cells called secondary spermatocytes Secondary spermatocytes undergo meiosis II and their daughter cells are called spermatids Spermatids are small round cells seen close to the lumen of the tubule Late in spermatogenesis, spermatids are nonmotile Spermiogenesis – spermatids lose excess cytoplasm and form a tail, becoming motile sperm

Spermatogenesis Figure 27.8b, c

Oogenesis Production of female sex cells by meiosis In the fetal period, oogonia (2n ovarian stem cells) multiply by mitosis and store nutrients Primordial follicles appear as oogonia are transformed into primary oocytes Primary oocytes begin meiosis but stall in prophase I From puberty, each month one activated primary oocyte completes meiosis one to produce two haploid cells The first polar body The secondary oocyte The secondary oocyte arrests in metaphase II and is ovulated If penetrated by sperm the second oocyte completes meiosis II, yielding: One large ovum (the functional gamete) A tiny second polar body

Oogenesis