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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