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Unit 3. Chapter 9: Meiosis and Sexual Reproduction. Impact, Issues: Why Sex?. Asexual Reproduction: quick and efficient Does not require the participation of a partner BUT the offspring are all clones – no variation

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Unit 3

Unit 3

Chapter 9: Meiosis and Sexual Reproduction

Impact issues why sex
Impact, Issues: Why Sex?

  • Asexual Reproduction: quick and efficient

    • Does not require the participation of a partner

    • BUT the offspring are all clones – no variation

  • Sexual Reproduction: most costly, but also most responsive to changing conditions

    • Male and female partners must find each other and exchange genetic material

    • The variation introduced by sex has selective advantages

Asexual reproduction
Asexual Reproduction

  • One parent passes a duplicate of its genes (stored in DNA molecules) to its offspring

  • Offspring can only be genetically identical __________ of the parent

  • Bacteria:

Sexual reproduction
Sexual Reproduction

  • Two Parents

  • Each parent contributes one gene for each trait

    • Offspring has a pair of genes on a pair of chromosomes

    • One chromosome of a pair is maternal, and the other is paternal

  • Offspring differ from its parents and each other

  • Sexual reproduction includes: meiosis, formation of mature reproductive cells called gametes, and fertilization


  • Genes for each trait come in slightly different forms called ___________

  • Alleles are unique molecular forms of the same gene; they specify different versions of a trait

  • Originally produced by mutations

  • Example: gene for eye color

    • Different alleles would include:

Sexual reproduction1
Sexual Reproduction

  • Meiosis shuffles the alleles during gamete formation

  • Fertilization produces offspring with unique combination of alleles

  • The variation generated by sexual reproduction allows for natural selection to occur and is the basis for evolutionary change


  • Meiosis is a nuclear division process that divides the parental chromosome number in half

    • In animals: gametes form by meiosis of germ cells

    • Plants: spores

  • Begin with diploid (2n) germ cells and produces ____________ gametes (n)

    • Humans:

  • In 2n cells, there are TWO chromosomes of each type, which are called _______________ chromosomes

    • Homologous chromosomes line up during meiosis, including sex chromosomes

  • Each gamete produced by meiosis has one of each pair of homologous chromosomes

Homologous chromosomes
Homologous Chromosomes

The same length, size, and genes except for the non-identical sex chromosomes (X and Y)

Where gametes form
Where Gametes Form

Figure 9.3, pg 140


  • In most multicelled species, gametes form from cells in reproductive structures or organs

  • Fertilization is the fusion of two gamete nuclei

    • Restores the parental chromosome number

  • Forms a ____________ , the first cell of a new individual

Two divisions not one
Two Divisions, Not One!

  • Mitosis and meiosis are similar, but different

  • Similarities between meiosis and mitosis:

    • Chromosomes are duplicated during interphase to form sister chromatids held together at the centromere

    • Chromosomes are moved by spindle fibers

  • Meiosis is different in that it has two series of divisions: ______________ and _______________

Meiosis i
Meiosis I

  • Meiosis I is the ______________________________

  • Each duplicated chromosome lines up with its homologous partner

  • Homologous chromosomes are separated

    • The two homologous chromosomes move apart toward opposite spindle poles

  • Each of the two daughter cells receives a haploid number of chromosomes

  • After meiosis I, each chromosome is ________ ____________________

Meiosis ii
Meiosis II

  • The _______________________________

  • In meiosis II, the sister chromatids of each chromosome separate and the cytoplasm divides again

    • Once pulled away from each other, each sister chromtid is now an individual chromosome

  • Results in FOUR _________ cells (n)

    • Each have one unduplicated chromosome

Meiosis i and meiosis ii
Meiosis I and Meiosis II

Figure 9.12, pg 150

Prophase i
Prophase I

  • Chromosomes condense and align tightly with their homologues

  • Each homologous pair undergoes ____________ _____

  • Microtubules form the bipolar spindle

  • One pair of centrioles moves to the other side of the nucleus

  • Nuclear envelope breaks up

    • Microtubules growing from each spindle pole enter the nuclear region

  • Microtubules tether one or the other chromosome of each homologous pair

Prophase i1
Prophase I

Figure 9.5, pg 142

Metaphase i
Metaphase I

  • Microtubules from both poles position all pairs of homologous chromosomes at the spindle equator

Figure 9.5, pg 142

Anaphase i
Anaphase I

  • Microtubules separate each chromosome from its homologue

  • As in mitosis, other microtubules that overlap at the equator slide past each other to push the poles farther apart

  • At the end of anaphase I, one set of duplicated chromosomes nears each spindle pole

Figure 9.5, pg 142

Telophase i
Telophase I

  • Two nuclei form

  • In most species, the cytoplasm divides

    Each chromosome still consists of TWO _________ ______________

    (remains duplicated)

Figure 9.5, pg 142

Prophase ii
Prophase II

  • There is no DNA replication between the two nuclear division

  • Prophase II is the beginning of meiosis II

  • One of the two centrioles moves to the opposite side of the cell

Figure 9.5, pg 143

Metaphase ii
Metaphase II

  • Chromosomes are aligned at the equator

  • Sister chromatids are attached to spindle fibers from opposite poles

Figure 9.5, pg 143

Anaphase ii
Anaphase II

  • One chromosome of each type is moved toward opposite spindle poles

Figure 9.5, pg 143

Telophase ii
Telophase II

  • Each step of Meiosis II occurs in BOTH nuclei formed in meiosis I

  • By the end of telophase II, there are four haploid nuclei, each with unduplicated chromosomes

  • Figure 9.5, pg 143

    Haploid daughter cells
    Haploid Daughter Cells

    • Once the cytoplasm divides after meiosis,

    • One or all may serve as gametes or, in plants, as spores that lead to gamete-producing bodies