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CHAPTER 13

CHAPTER 13. MEIOSIS AND SEXUAL LIFE CYCLES. I. OVERVIEW. 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 via genes (DNA)

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CHAPTER 13

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  1. CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES

  2. I. OVERVIEW • Living organisms are distinguished by their ability to reproduce their own kind • Geneticsis the scientific study of heredity and variation • Heredityis the transmission of traits from one generation to the next via genes (DNA) • Variationis demonstrated by the differences in appearance that offspring show from parents and siblings

  3. I. OVERVIEW • Meiosis, sexual reproduction, and heredity are all aspects of the same process. • Meiosis is a modified type of cell division in sexually reproducing organism consisting of two rounds of cell division but only one round of DNA replication. It results in cells with half the number of chromosome sets as the original cell.

  4. II. Concept 13.1: Genes, DNA, and Chromosomes • Your genome is made up of the genes that you inherited from your mother and father A. Inheritance of Genes • Genesare the units of heredity, and are made up of segments of DNA • Gene- a unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA in some viruses). • A gene’s specific location along the chromosome is called the gene’s locus.

  5. II. Concept 13.1: Genes, DNA, and Chromosomes • Your genome is made up of the genes that you inherited from your mother and father A. Inheritance of Genes • Genes are passed to the next generation through reproductive cells called gametes(sperm and eggs) • Each gene has a specific location called a locuson a certain chromosome • Most DNA is packaged into chromosomes • One set of chromosomes is inherited from each parent

  6. Inheritance is possible because of DNA replication and sperm and egg which carry each parent’s genes which are combined at fertilization B. Comparison of Asexual and Sexual Reproduction • In asexual reproduction, one parent produces genetically identical offspring by mitosis • Any genetic difference would be due to mutation • Does not involve the formation of gametes (sex cells) • A cloneis 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 • Involves the formation of gametes

  7. Inheritance is possible because of DNA replication and sperm and egg which carry each parent’s genes which are combined at fertilization B. Comparison of Asexual and Sexual Reproduction • In asexual reproduction, one parent produces genetically identical offspring by mitosis Asexual Reproduction- a single individual is the sole parent and passes copies of all its genes to its offspring. As a result, the offspring are an exact copy of themselves (a clone). • Any genetic difference would be due to mutation • Does not involve the formation of gametes (sex cells) • A cloneis a group of genetically identical individuals from the same parent

  8. Inheritance is possible because of DNA replication and sperm and egg which carry each parent’s genes which are combined at fertilization B. Comparison of Asexual and Sexual Reproduction 3. In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents. Offspring of sexual reproduction vary genetically to their siblings and both parents. • Involves the formation of gametes

  9. Asexual Reproduction

  10. III. Concept 13.2: Sexual Life Cycles • Alife cycle is the sequence of stages in the reproductive history of an organism from conception to the production of its own offspring A. Sets of Chromosomes in Human Cells • Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes • A karyotypeis an ordered display of the pairs of chromosomes from a cell (usually a wbc) in metaphase • Karyotype- a display of the chromosome pairs of a cell arranged by size and shape. • 22 pairs of autosomes • 1 pair of sex chromosomes

  11. Karyotype

  12. Karyotype

  13. Female Karyotype

  14. Male Karyotype

  15. III. Concept 13.2: Sexual Life Cycles • Alife cycle is the sequence of stages in the reproductive history of an organism from conception to the production of its own offspring • Sets of Chromosomes in Human Cells 3. The two chromosomes in each pair are called homologous chromosomes, or homolog

  16. Homologous chromosomes have the same length, centromere position, staining pattern and carry genes controlling the same inherited characters • The sex chromosomes are X and Y and are also known as allosomes • 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 • Each pair of homologous chromosomes includes one chromosome from each parent

  17. 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) • In a cell in which DNA synthesis has occurred, each chromosome is replicated • Each replicated chromosome consists of two identical sister chromatids

  18. 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 autosomesand 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

  19. B. Behavior of Chromosome Sets in the Human Life Cycle • Fertilizationis the union of gametes (the sperm and the egg) • The fertilized egg is called a zygoteand has one set of chromosomes from each parent • The zygote produces somatic cells by mitosis and develops into an adult • At sexual maturity, the ovaries and testes produce haploid gametes • Gametes are the only types of human cells produced by meiosis, rather than mitosis

  20. Meiosis results in one set of chromosomes in each gamete • Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number

  21. Human Life Cycle

  22. C. The Variety of Sexual Life Cycles • The alternation of meiosis and fertilization is common to all organisms that reproduce sexually • The three main types of sexual life cycles differ in the timing of meiosis and fertilization: animals, plants and algae, and fungi and protists • Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis • However, only diploid cells can undergo meiosis • In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring

  23. 1. Animals • In animals, meiosis produces gametes, which undergo no further cell division before fertilization • Gametes are the only haploid cells in animals • Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism

  24. Life Cycle of Animals

  25. 2. Plants and some Algae • Plants and some algae exhibit an alternation of generations • This life cycle includes both a diploid and haploid multicellular stage • The diploid organism, called the sporophyte, makes haploid sporesby meiosis • Each spore grows by mitosis into a haploid organism called a gametophyte • A gametophyte makes haploid gametes by mitosis • Fertilization of gametes results in a diploid sporophyte

  26. Life Cycle of Plants and some Algae

  27. 3. Most Fungi and Some Protists • The only diploid stage is the single-celled zygote; there is no multicellular diploid stage • The zygote produces haploid cells by meiosis • Each haploid cell grows by mitosis into a haploid multicellular organism • The haploid adult produces gametes by mitosis

  28. Life Cycle of Most Fungi and Some Protists

  29. IV. Concept 13.3: Meiosis—Reduction Division A. Overview • 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 • See page 256 (Be familiar with the ways that mitosis and meiosis are alike and different)

  30. B. 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 reduction division • In the second cell division (meiosis II), sister chromatids separate • Meiosis II results in four haploid daughter cells with unreplicated chromosomes

  31. Meiosis I is preceded by interphase, in which chromosomes are replicated to form sister chromatids • The sister chromatids are genetically identical and joined at the centromere • The single centrosome replicates, forming two centrosomes

  32. 8. Phases of Meiosis I and Meiosis II • Meiosis I Prophase I Metaphase I Anaphase I Telophase I • Meiosis II Prophase II Metaphase II Anaphase II Telophase II

  33. C. Interphase • Precedes meiosis • Chromosomes replicate as in mitosis • In animal cells, centriole pairs duplicate

  34. D. Meiosis I • Reduces chromosome number by one-half • Four phases: • Prophase I -Chromosomes become visible as long, thin, single threads -Chromosomes begin to contract which continues throughout Prophase I

  35. --homologous chromosomes undergo synapsis (pairing gene by gene) --involves formation of synaptonemal complex (protein structure that aids in the chromosomal pairing) --doubled chromosomes appear as tetrads (refers to doubled chromosomes in Prophase I) --crossing over occurs

  36. --pairing between homologs becomes less tight and they appear to repel each other --chiasmata (visible points where crossovers occurred earlier) appear

  37. PROPHASE I

  38. MEIOSIS

  39. Prophase I (continued) • Centriole pairs move apart and spindle microtubules form between them • Nuclear envelope and nucleoli disappear • Chromosomes begin moving to the metaphase plate • Occupies more than 90% of time required for meiosis

  40. 2. Metaphase I • Tetrads are aligned on the metaphase plate

  41. 3. Anaphase I Homologous chromosomes separate • Chromosomes move toward the poles by the spindle apparatus • Chromosomes now referred to as dyads (half of a tetrad, refers to the chromosomes in Anaphase I)

  42. 4. Telophase I and Cytokinesis • Spindle apparatus continues to separate homologous chromosome pairs (dyads) until the chromosomes reach the poles. • Cytokinesis occurs as in mitosis. • In some species, the nuclear envelope and nucleoli reappear, and the daughter cells enter a period of interkinesis before Meiosis II. In other species, the daughter cells immediately prepare for Meiosis II. • NO DNA replication occurs before Meiosis II

  43. TELOPHASE I and CYTOKINESIS

  44. E. Meiosis II 1. Prophase II • If cells entered interkinesis, nuclear envelope and nucleoli disappear. • Spindle apparatus forms • Chromosomes (dyads) begin to move towards the Metaphase II plate

  45. 2. Metaphase II • Chromosomes (dyads) align on the metaphase plate

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