Biol 160 - Chapter 10: Meiosis - PDF

Chapter 10 Meiosis: the basis of sexual reproduction 1 Chapter 10 At a Glance  10.2 How Does Meiotic Cell Division Produce Haploid Cells?  10.3 How do meiosis and union of gametes produce genetically variable offspring?...

Chapter 10 Meiosis: the basis of sexual reproduction 1 Chapter 10 At a Glance  10.2 How Does Meiotic Cell Division Produce Haploid Cells?  10.3 How do meiosis and union of gametes produce genetically variable offspring? 2 3 4 5 6 7 How Does Meiotic Cell Division Produce Haploid Cells?  Meiosis separates homologous chromosomes, producing haploid daughter nuclei  Meiosis is a specialized cell division process that produces haploid gametes  Each gamete receives one member of each pair of homologous chromosomes  Meiosis consists of one round of DNA replication, followed by two rounds of nuclear divisions - One round of DNA replication produces two chromatids in each duplicated chromosome - Because diploid cells have pairs of homologous chromosomes, with two chromatids per homologue, a single round of DNA replication creates four chromatids for each type of chromosome 8 How Does Meiotic Cell Division Produce Haploid Cells?  Meiosis consists of one round of DNA replication, followed by two rounds of nuclear divisions (continued)  The first nuclear division, meiosis I, separates the pairs of homologues, with each daughter nucleus receiving one - Each daughter nucleus is haploid, even though each homologue it receives had two chromatids  The second nuclear division, meiosis II, separates the chromatids and parcels one chromatid into each of two more daughter nuclei  At the end of meiosis, there are four haploid daughter nuclei, each with one copy of each homologous chromosome  Meiotic cell division normally produces four haploid cells from a single diploid parent cell 9 10 How Does Meiotic Cell Division Produce Haploid Cells?  Fusion of gametes keeps the chromosome number constant between generations  Meiosis reduces the chromosome number by half, producing haploid (n) gametes (eggs and sperm)  Fusion of the gametes (fertilization) combines the two haploid chromosome sets to produce a diploid (2n) zygote  If halving of the chromosome number did not occur in gametes, sexual reproduction would double the chromosome number in each new generation, leading to inviability 11 How Does Meiotic Cell Division Produce Haploid Cells? 12 13 How Does Meiotic Cell Division Produce Haploid Cells?  Meiosis I separates homologous chromosomes into two haploid daughter nuclei  Meiosis has many similarities to mitosis - The phases of meiosis have the same names as the equivalent phases in mitosis, followed by a “I” or a “II” to distinguish the two nuclear divisions that occur in meiosis - Cytokinesis accompanies the nuclear divisions - As in mitosis, the chromosomes are duplicated during interphase prior to meiosis - As in mitosis, the sister chromatids of each chromosome are attached to each other at the centromere when meiosis begins 14 How Does Meiotic Cell Division Produce Haploid Cells?  During prophase I, homologous chromosomes pair up and exchange DNA  In prophase I, homologous chromosomes pair up - In mitosis, homologous chromosomes move independently of each other  Proteins bind chromatids of the maternal and paternal homologues together so that they align precisely along their entire lengths - A maternal homologue was contributed by the egg; a paternal homologue, by the sperm  Crossing over is a mutual exchange of corresponding chromatid sections (and, therefore, DNA) between maternal and paternal homologues - Crossing over begins when enzymes cut through the DNA of the paired homologues and then graft cut ends back together, often switching maternal and paternal ends - The binding proteins and enzymes then depart, leaving crosses or chiasmata (singular, chiasma), where the maternal and paternal chromosomes have exchanged parts 15 How Does Meiotic Cell Division Produce Haploid Cells?  During prophase I, homologous chromosomes pair up and exchange DNA (continued)  If the exchanged segments carry different alleles, genetic recombination has occurred - The genes on one homologue are thus combined with an allele from the other homologue, and the combination may be totally new  In human cells, each pair of homologues usually forms two or three chiasmata during prophase I  The arms of the homologues remain temporarily entangled at the chiasmata - This keeps the two homologues together until they are pulled apart during anaphase I  As during the prophase of mitosis, spindle microtubules assemble, the nuclear envelope breaks down, and microtubules capture chromosomes 16 Crossing Over 17 How Does Meiotic Cell Division Produce Haploid Cells?  During metaphase I, paired homologous chromosomes line up at the equator of the cell  Spindle microtubules attach to the kinetochore regions of duplicated chromosomes in prophase I - Each member of a homologous pair is attached by microtubules to the opposite pole from its homologue  Duplicated homologous chromosomes are pulled into a line perpendicular to the spindle  Chromosomes line up as pairs of replicated homologous chromosomes - During mitosis, a chromosome’s homologues line up at the equator independently of each other 18 How Does Meiotic Cell Division Produce Haploid Cells?  During metaphase I, paired homologous chromosomes line up at the equator of the cell (continued)  Which member of a homologous pair is connected by microtubules to which pole is random - This means that a daughter cell can receive any combination of maternal and paternal homologues - This random combining of maternal and paternal homologues (and, therefore, alleles) is called independent assortment  Genetic recombination and independent assortment are responsible for the genetic diversity of the haploid cells produced by meiosis 19 How Does Meiotic Cell Division Produce Haploid Cells?  During anaphase I, homologous chromosomes separate  Whole duplicated chromosomes of each homologous pair separate - In anaphase of mitosis, the sister chromatids separate and move to opposite poles - In anaphase I of meiosis, the sister chromatids remain attached and move as a homologue unit  One duplicated chromosome of each homologous pair moves to each pole, pulled by microtubules 20 How Does Meiotic Cell Division Produce Haploid Cells?  During telophase I, two haploid clusters of duplicated chromosomes form  The cluster of chromosomes at each pole contains one duplicated member of each pair of homologous chromosomes - Each cluster therefore contains the haploid number, even through each homologue in it consists of two sister chromatids  The spindle microtubules disappear  Cytokinesis usually occurs during telophase I  Nuclear envelopes may re-form  Telophase I is usually followed immediately by meiosis II, with little or no intervening interphase  The chromosomes do not replicate between meiosis I and meiosis II  Two haploid cells result from meiosis I 21 22 How Does Meiotic Cell Division Produce Haploid Cells?  Meiosis II separates sister chromatids into four daughter nuclei  Meiosis II occurs in each of the two haploid cells produced during meiosis I  During meiosis II, the sister chromatids of each duplicated chromosome separate, in a process that is virtually identical with mitosis  Because the cell is haploid, there is no homologue in the cell as there is in mitosis 23 How Does Meiotic Cell Division Produce Haploid Cells?  Meiosis II separates sister chromatids into four daughter nuclei (continued)  Meiotic prophase II  If decondensed, the chromosomes recondense  Spindle microtubules re-form and capture duplicated chromosomes  The kinetochores of chromatids in each chromosome are attached by microtubules to opposite poles  Meiotic metaphase II  Duplicated chromosomes line up singly, perpendicular to the spindle, as they do in mitosis 24 How Does Meiotic Cell Division Produce Haploid Cells?  Meiosis II separates sister chromatids into four daughter nuclei (continued)  Meiotic anaphase II  Chromatids separate and move to opposite poles  Meiotic telophase II  Cytokinesis occurs, nuclear membranes re-form, and the chromosomes decondense  The two nuclear divisions of meiosis produce four haploid cells from a single diploid cell 25 How Does Meiotic Cell Division Produce Haploid Cells? 26 How Does Meiotic Cell Division Produce Haploid Cells? 27 How Do Meiosis and Sexual Reproduction Produce Genetic Variability?  Genetic variability among organisms is essential for survival in a changing environment  Mutations produce new variation but are relatively rare occurrences  The genetic variability that occurs from one generation to the next results almost entirely from meiosis and sexual reproduction 28 How Do Meiosis and Sexual Reproduction Produce Genetic Variability?  Shuffling of homologues creates novel combinations of chromosomes  The randomized line up and separation of homologous chromosomes in meiotic metaphase I and anaphase I increase variation  Which of the two homologues from each chromosome ends up at a particular pole (and, thus, in a particular daughter cell) is completely random - The number of possible chromosome combinations is 2n, where n = number of homologous pairs  In humans, with 23 pairs of chromosomes, this independent assortment of homologues can create gametes with 8 million (223) possible combinations of chromosomes 29 Random Separation of Homologous Pairs of Chromosomes Produces Genetic Variability 30 How Do Meiosis and Sexual Reproduction Produce Genetic Variability?  Crossing over creates chromosomes with novel combinations of genes  Because sexual reproduction can bring together homologous chromosomes from two vastly diverse populations, crossing over between these homologues can introduce allelic combinations that are completely new  The resulting gametes are potentially very valuable to the evolution of the organism 31 How Do Meiosis and Sexual Reproduction Produce Genetic Variability?  Fusion of gametes adds further genetic variability to the offspring  Each gamete, by independent assortment alone, can have 2n possible combinations - Fusion of gametes from two individuals, therefore, can create 2n x 2n possible combinations - Gametes from two humans could produce about 64 trillion different combinations  Fusion, taken together with independent assortment and crossing over, assures that each human individual is absolutely genetically unique 32