Chapter 13
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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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Chapter 13

CHAPTER 13

MEIOSIS AND SEXUAL LIFE CYCLES


I overview

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


I overview1

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.


Ii concept 13 1 genes dna and chromosomes

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.


Ii concept 13 1 genes dna and chromosomes1

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


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  • 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


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    • 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


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    • 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


  • Asexual reproduction

    Asexual Reproduction


    Iii concept 13 2 sexual life cycles

    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


    Karyotype

    Karyotype


    Karyotype1

    Karyotype


    Female karyotype

    Female Karyotype


    Male karyotype

    Male Karyotype


    Iii concept 13 2 sexual life cycles1

    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


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    • 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


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    • 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


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    • 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


    B behavior of chromosome sets in the human life cycle

    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


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    • Meiosis results in one set of chromosomes in each gamete

    • Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number


    Human life cycle

    Human Life Cycle


    C the variety of sexual life cycles

    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


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    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


    Life cycle of animals

    Life Cycle of Animals


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    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


    Life cycle of plants and some algae

    Life Cycle of Plants and some Algae


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    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


    Life cycle of most fungi and some protists

    Life Cycle of Most Fungi and Some Protists


    Iv concept 13 3 meiosis reduction division

    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)


    B stages of meiosis

    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


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    • 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


    8 phases of meiosis i and meiosis ii

    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


    C interphase

    C. Interphase

    • Precedes meiosis

    • Chromosomes replicate as in mitosis

    • In animal cells, centriole pairs duplicate


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    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


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    --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


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    --pairing between homologs becomes less tight and they appear to repel each other

    --chiasmata (visible points where crossovers occurred earlier) appear


    Prophase i

    PROPHASE I


    Meiosis

    MEIOSIS


    Prophase i continued

    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


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    2. Metaphase I

    • Tetrads are aligned on the metaphase plate


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    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)


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    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


    Telophase i and cytokinesis

    TELOPHASE I and CYTOKINESIS


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    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


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    2. Metaphase II

    • Chromosomes (dyads) align on the metaphase plate


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    3. Anaphase II

    • Centromeres of sister chromatids separate

    • Monads (half of a dyad, refers to chromosomes in Anaphase II) move toward opposite poles


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    4. Telophase II

    • Nuclei form at opposite poles of the cell

    • Cytokinesis occurs producing four haploid daughter cells


    Animal meiosis

    Animal Meiosis


    F comparison of meiosis i and mitosis

    F. Comparison of Meiosis I and Mitosis


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    F. Meiosis II is virtually identical to mitosis.


    V concept 13 4 genetic variation contributes to evolution

    V. Concept 13.4: Genetic Variation Contributes to Evolution

    • Mutations (changes in an organism’s DNA) are the original source of genetic diversity

    • Mutations create different versions of genes called alleles

    • Reshuffling of alleles during sexual reproduction produces genetic variation


    A origins of genetic variation among offspring

    A. Origins of Genetic Variation Among Offspring

    1. The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

    • Three mechanisms contribute to genetic variation:

      • Independent assortment of chromosomes

      • Crossing over

      • Random fertilization


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    3. Independent Assortment of Chromosomes

    • Homologous pairs of chromosomes orient randomly at Metaphase I of Meiosis

    • In independent assortment, 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, where n is the haploid number

    • For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes


    Independent assortment

    Independent Assortment


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    4. Crossing Over

    • Crossing over produces recombinant chromosomes, which combine genes inherited from each parent

    • Crossing over begins in Prophase I, as homologous chromosomes pair up gene by gene (synapsis)

      • Synapsisinvolves the formation of synaptonemal complex (a protein structure that brings chromosomes into close association)

  • In crossing over, homologous portions of two nonsisterchromatids trade places

  • Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome


  • Crossing over

    Crossing Over


    5 random fertilization

    5. Random Fertilization

    • Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)

    • Fertilization- the union of haploid gametes to produce a zygote.

      • Meiosis- gametes (sex cells) reproduce by meiosis

    • The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations

    • Each zygote has a unique genetic identity


    B genetic variation and evolution

    B. Genetic Variation and Evolution

    • Natural selection (Darwin’s theory of evolution) results in the accumulation of genetic variations favored by the environment

    • Sexual reproduction contributes to the genetic variation in a population, which originates from mutations


    You should now be able to

    You should now be able to:

    • Distinguish between the following terms: somatic cell and gamete; autosome and sex chromosomes; haploid and diploid

    • Describe the events that characterize each phase of meiosis

    • Describe three events that occur during meiosis I but not mitosis

    • Name and explain the three events that contribute to genetic variation in sexually reproducing organisms


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