Meiosis and Sexual Reproduction

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Chapter 10

• Meiosis and Sexual Reproduction

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Why Sex
Fig. 10-1b, p.154
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Why sex?

• Asexual

• Sexual

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Why sex?

• Asexual
• Easier, faster
• Big population
• Indentical
• Bits can make whole indv.
• No new combos
• All inherit the same info
• Clones
• parthogenesis

• Sexual
• Changing env
• More variety
• New combos
• Involves meiosis (gametes) and fertilization
• allele

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Modes of Reproduction

• Sexual reproduction
• Meiosis, gamete formation, and fertilization
• Offspring show genetic variation
• Asexual reproduction
• Single parent produces offspring
• Offspring are genetically identical

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Cost of Sexual Reproduction
Fig. 43-2c, p.756
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43.1 (p. 756)Cost of Sexual Reproduction

• Specialized cells and structures must be formed
• Special courtship, and parental behaviors can be
  costly
• Timing of gamete formation and mating
• Nurturing developing offspring, either in egg or
  body, requires resources from mother

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10.2 What Meiosis Does

• Meiosis nuclear division that divides parental
  c-some by half in specialized reproductive
  cells
• Ex anther, ovules

anther
ovary
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Homologous Chromosomes Carry Different Alleles

• Homologous c-some same shape, length and
  assortment of genes, line up with each other
• Paternal and maternal chromosomes can carry
  different alleles

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Chromosome Number

• Sum total of chromosomes in a cell
• Germ cells are diploid (2n)
• Gametes are haploid (n)
• Meiosis halves chromosome number

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Meiosis Two Divisions

• Two consecutive nuclear divisions
• Meiosis I aligns with partner
• Meiosis II sister chromatids separate
• DNA is not duplicated between divisions
• Four haploid nuclei form

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10.3 Tour of MeiosisProphase I

• Each duplicated chromosome pairs with homologue
  (synapse)
• Homologues swap segments (crossing over)
• Each chromosome becomes attached to spindle

Fig. 10-5, p. 158
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Metaphase I

• Chromosomes are pushed and pulled into the middle
  of cell
• The spindle is fully formed

Fig. 10-5, p. 158
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Anaphase I

• Homologous chromosomes segregate
• The sister chromatids remain attached

Fig. 10-5, p. 158
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Telophase I

• The chromosomes arrive at opposite poles
• Usually followed by cytoplasmic division

Fig. 10-5, p. 158
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Meosis II Prophase II

• Microtubules attach to the kinetochores of the
  duplicated chromosomes
• Attach to one chromatid of each chromosome

Fig. 10-5, p. 158
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Metaphase II

• Duplicated chromosomes line up at the spindle
  equator, midway between the poles

Fig. 10-5, p. 158
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Anaphase II

• Sister chromatids separate to become independent
  chromosomes
• Attachments break

Fig. 10-5, p. 158
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Telophase II

• The chromosomes arrive at opposite ends of the
  cell
• A nuclear envelope forms around each set of
  chromosomes
• Four haploid cells

Fig. 10-5, p. 158
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10.4 Factors Contributing to Variation among
Offspring

• Crossing over during prophase I
• Independent assortment
• Random alignment of chromosomes at metaphase I
• Random combination of gametes at fertilization

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Crossing Over

• Each chromosome becomes zippered to its homologue
• All four chromatids are closely aligned
• Nonsister chromosomes exchange segments

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Effect of Crossing Over

• After crossing over, each chromosome contains
  both maternal and paternal segments
• Creates new allele combinations in offspring

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Independent Assortment

• Microtubules from spindle poles attach to
  kinetochores of chromosomes randomly, between
  Prophase I and Metaphase I

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Randomness cont.

• Either the maternal or paternal member of a
  homologous pair can end up at either pole
• The chromosomes in a gamete are a mix of
  chromosomes from the two parents

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Possible Chromosome Combinations

• As a result of random alignment, the number of
  possible combinations of chromosomes in a gamete
  is
• 2n
• (n is number of chromosome types)

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Fertilization

• Which two gametes unite is random
• Adds to variation among offspring

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Life Cycles

• Plant

• Animal

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Plant Life Cycle
sporophyte
zygote
diploid
fertilization
meiosis
haploid
spores
gametes
gametophytes
Fig. 10-8a, p.162
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Animal Life Cycle
multicelled body
zygote
diploid
meiosis
fertilization
haploid
gametes
Fig. 10-8b, p.162
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44.2Spermatogenesis

• Spermatogonium (2n) divides by mitosis to form
  primary spermatocyte (2n)
• Meiosis produces haploid spermatids
• Spermatids mature to become sperm

movie
Figure 44.4 Page 775
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Other Testicular Cells

• Sertoli cells
• Line the seminiferous tubules
• Nourish the developing sperm
• Leydig cells
• Lie between the seminiferous tubules
• Secrete testosterone

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Male Hormonal Control
Hypothalamus
GnRH
Inhibin
Anterior Pituitary
LH
FSH
Sertoli Cells
Leydig Cells
Testes
Testosterone
Formation and Development of Sperm
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44.1Oocytes Arrested in Meiosis I

• Girl is born with primary oocytes already in
  ovaries
• Each oocyte has entered meiosis I and stopped
• Meiosis resumes, one oocyte at a time, with the
  first menstrual cycle

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Ovarian Cycle
secondary oocyte
first polar body

• Follicle grows and matures
• Ovulation occurs
• Corpus luteum forms

antrum
corpus luteum
primordial follicle
Figure 44.8 Page 778
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A transparent and somewhat elastic layer, the
zona pellucida, starts forming around the primary
oocyte.
Primary oocyte, not yet released from meiosis I.
A cell layer is forming around it. A follicle
consists of the cell layer and the oocyte.
A fluid-filled cavity (antrum) starts forming in
the follicles cell layer.
Mature follicle. Meiosis I is over. The secondary
oocyte and first polar body are now formed.
primordial follicle
first polar body
secondary oocyte
The corpus luteum breaks down when the woman
doesnt get pregnant.
A corpus luteum forms from remnants of the
ruptured follicle.
Ovulation. Mature follicle ruptures and releases
the secondary oocyte and the first polar body.
Fig. 44-8b, p.778
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Female Hormonal Control
Hypothalamus
GnRH
Rising estrogen stimulates surge in LH
Anterior pituitary
Progesterone, estrogens
LH
FSH
Ovary
follicle growth, oocyte maturation
Estrogen
Corpus luteum forms
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Mitosis Meiosis Compared

• Mitosis
• Functions
• Asexual reproduction
• Growth, repair
• Occurs in somatic cells
• Produces clones

• Meiosis
• Function
• Sexual reproduction
• Occurs in germ cells
• Produces variable offspring

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Prophase vs. Prophase I

• Prophase (Mitosis)
• Homologous pairs do not interact with each other
• Prophase I (Meiosis)
• Homologous pairs become zippered together and
  crossing over occurs

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Anaphase, Anaphase I, and Anaphase II

• Anaphase I (Meiosis)
• Homologous chromosomes separate from each other
• Anaphase/Anaphase II (Mitosis/Meiosis)
• Sister chromatids of a chromosome separate from
  each other

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Results of Mitosis and Meiosis

• Mitosis
• Two diploid cells produced
• Each identical to parent
• Meiosis
• Four haploid cells produced
• Differ from parent and one another

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An Ancestral Connection

• Was sexual reproduction a giant evolutionary step
  from aseuxal reproduction?
• Giardia intestinalis
• Chlamydomonas
• Recombination mechanisms are vital for
  reproduction of euk cells may have evolved from
  DNA repair mechanisms in prok ancestors