Lecture 7: Meiosis PDF

Lecture 7: Meiosis BIO1101 Biology Department College of Science IMSIU Overview: Variations on a Theme • Living organisms are distinguished by their ability to reproduce their own kind • Genetics is the scientific study of heredity and variation • Heredity is the transmission of traits from one...

Lecture 7: Meiosis BIO1101 Biology Department College of Science IMSIU Overview: Variations on a Theme • Living organisms are distinguished by their ability to reproduce their own kind • Genetics is the scientific study of heredity and variation • Heredity is the transmission of traits from one generation to the next • Variation is demonstrated by the differences in appearance that offspring show from parents and siblings Copyright © Pearson Education Ltd. Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes • In a literal sense, children do not inherit particular physical traits from their parents • It is genes that are actually inherited Copyright © Pearson Education Ltd. Inheritance of Genes • Genes are the units of heredity, and are made up of segments of DNA • Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs) • Each gene has a specific location called a locus on a certain chromosome • Most DNA is packaged into chromosomes • One set of chromosomes is inherited from each parent Copyright © Pearson Education Ltd. Comparison of Asexual and Sexual Reproduction • In asexual reproduction, one parent produces genetically identical offspring by mitosis • A clone is a group of genetically identical individuals from the same parent • In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Copyright © Pearson Education Ltd. Sets of Chromosomes in Human Cells • Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes • A karyotype is an ordered display of the pairs of chromosomes from a cell • The two chromosomes in each pair are called homologous chromosomes, or homologs • Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters Copyright © Pearson Education Ltd. Fig. 13-3 APPLICATION TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome Fig. 13-3b TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome • The sex chromosomes are called X and Y • Human females have a homologous pair of X chromosomes (XX) • Human males have one X and one Y chromosome • The 22 pairs of chromosomes that do not determine sex are called autosomes Copyright © Pearson Education Ltd. • Each pair of homologous chromosomes includes one chromosome from each parent • The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father • A diploid cell (2n) has two sets of chromosomes • For humans, the diploid number is 46 (2n = 46) Copyright © Pearson Education Ltd. • In a cell in which DNA synthesis has occurred, each chromosome is replicated • Each replicated chromosome consists of two identical sister chromatids Copyright © Pearson Education Ltd. Fig. 13-4 Key 2n = 6 Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Centromere Pair of homologous chromosomes (one from each set) • A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) • For humans, the haploid number is 23 (n = 23) • Each set of 23 consists of 22 autosomes and a single sex chromosome • In an unfertilized egg (ovum), the sex chromosome is X • In a sperm cell, the sex chromosome may be either X or Y Copyright © Pearson Education Ltd. Behavior of Chromosome Sets in the Human Life Cycle • Fertilization is the union of gametes (the sperm and the egg) • The fertilized egg is called a zygote and has one set of chromosomes from each parent • The zygote produces somatic cells by mitosis and develops into an adult Copyright © Pearson Education Ltd. • At sexual maturity, the ovaries and testes produce haploid gametes • Gametes are the only types of human cells produced by meiosis, rather than mitosis • Meiosis results in one set of chromosomes in each gamete • Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number Copyright © Pearson Education Ltd. Fig. 13-5 Key Haploid gametes (n = 23) Haploid (n) Egg (n) Diploid (2n) Sperm (n) MEIOSIS Ovary FERTILIZATION Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid • Like mitosis, meiosis is preceded by the replication of chromosomes • Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II • The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis • Each daughter cell has only half as many chromosomes as the parent cell Copyright © Pearson Education Ltd. The Stages of Meiosis • In the first cell division (meiosis I), homologous chromosomes separate • Meiosis I results in two haploid daughter cells with replicated chromosomes; it is called the reductional division • In the second cell division (meiosis II), sister chromatids separate • Meiosis II results in four haploid daughter cells with unreplicated chromosomes; it is called the equational division Copyright © Pearson Education Ltd. Fig. 13-7-1 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Fig. 13-7-2 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Fig. 13-7-3 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes Fig. 13-8 Metaphase I Prophase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids remain attached Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Homologous chromosomes Fragments of nuclear envelope Telophase I and Cytokinesis Anaphase I Microtubule attached to kinetochore Cleavage furrow Sister chromatids separate Haploid daughter cells forming • Division in meiosis I occurs in four phases: – Prophase I – Metaphase I – Anaphase I – Telophase I and cytokinesis Copyright © Pearson Education Ltd. Fig. 13-8a Prophase I Metaphase I Centrosome (with centriole pair) Sister chromatids Telophase I and Cytokinesis Anaphase I Sister chromatids remain attached Centromere (with kinetochore) Chiasmata Spindle Metaphase plate Homologous chromosomes separate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore Cleavage furrow Prophase I • Prophase I typically occupies more than 90% of the time required for meiosis • Chromosomes begin to condense • In synapsis, homologous chromosomes loosely pair up, aligned gene by gene Copyright © Pearson Education Ltd. • In crossing over, nonsister chromatids exchange DNA segments • Each pair of chromosomes forms a tetrad, a group of four chromatids • Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred Copyright © Pearson Education Ltd. Metaphase I • In 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 Copyright © Pearson Education Ltd. Fig. 13-8b Prophase I Metaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore 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 Copyright © Pearson Education Ltd. 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 Copyright © Pearson Education Ltd. • 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 Copyright © Pearson Education Ltd. Fig. 13-8c Telophase I and Cytokinesis Anaphase I Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow • Division in meiosis II also occurs in four phases: – Prophase II – Metaphase II – Anaphase II – Telophase II and cytokinesis • Meiosis II is very similar to mitosis Copyright © Pearson Education Ltd. Fig. 13-8d Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming Prophase II • In prophase II, a spindle apparatus forms • In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Copyright © Pearson Education Ltd. Metaphase II • In 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 Copyright © Pearson Education Ltd. Fig. 13-8e Prophase II Metaphase II Anaphase II • In anaphase II, the sister chromatids separate • The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles Copyright © Pearson Education Ltd. Telophase II and Cytokinesis • In telophase II, the chromosomes arrive at opposite poles • Nuclei form, and the chromosomes begin decondensing Copyright © Pearson Education Ltd. • 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 Copyright © Pearson Education Ltd. Fig. 13-8f Anaphase II Telephase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming Fig. 13-9b SUMMARY Property Mitosis Meiosis DNA replication Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins Number of divisions One, including prophase, metaphase, anaphase, and telophase Two, each including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes