PowerLecture: Chapter 10

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PowerLectureChapter 10

• Meiosis and Sexual Reproduction

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What you should understand

• The two types of reproduction.
• Why each occur and how.
• What is a gamete and how it is made
• Difference between haploid and diploid and which
  type of cell they occur.
• Why sexual reproduction increases variation
  within a species.

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Impacts, Issues Why Sex

• Asexual reproduction is easier and faster
• Sexual reproduction can be an alternative
  adaption in changing environments

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Differences between types of reproduction
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Asexual Reproduction

• Single parent produces offspring
• All offspring are genetically identical to one
  another and to parent
• Mitosis

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Sexual Reproduction

• Involves
• Meiosis
• Gamete production (egg and sperm)
• Fertilization
• Produces genetic variation among offspring
• Chromosomes are duplicated in germ cells

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Germ cells undergo meiosis and cytoplasmic
division

• Cellular descendents of germ cells (sex cells)
  become gametes (egg and sperm)
• Gametes meet at fertilization

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Gamete Formation

• Gametes are sex cells
• Sperm
• eggs
• Arise from germ cells
• Some plants can have both parts

ovaries
anther
ovary
Figure 10-3Page 156
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FLOWERING PLANT
anther (where cells that give rise to male
gametes originate)
ovules, inside an ovary (where cells that give
rise to female gametes originate)
Fig. 10-3a, p.156
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• Recap of chromosomes

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Homologous Chromosomes Carry Different Alleles

• Cell has two of each chromosome (homologous)
• One chromosome in each pair from mother, other
  from father
• Paternal and maternal chromosomes carry different
  alleles
• Alleles are different forms of a gene
• Examples

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Homologous Chromosomes You get one from each
parent you have 23 pairs of homologous
chromosomes. They carry the same genes but not
necessary the same form of the gene.
Fig. 10-2, p.156
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Sexual Reproduction Shuffles Alleles

• Through sexual reproduction, offspring inherit
  new combinations of alleles, which leads to
  variations in traits
• This variation in traits is the basis for
  evolutionary change

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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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Human Karyotype Explanation
Fig. 10-4, p.157
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What is meiosis?
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Meiosis Two Divisions overview

• Two consecutive nuclear divisions
• Meiosis I
• Meiosis II
• DNA is not duplicated between divisions
• Four haploid nuclei form

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Meiosis I
Each homologue in the cell pairs with its
partner,
then the partners separate
p. 158
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Meiosis II

• The two sister chromatids of each duplicated
  chromosome are separated from each other

two chromosomes (unduplicated)
one chromosome (duplicated)
p. 158
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• What is the difference between meiosis I and
  meiosis II in regards to the chromosomes.

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Meiosis I - Stages
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Prophase I

• Each duplicated chromosome pairs with homologue
• 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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Prophase II

• Microtubules attach to the kinetochores of the
  duplicated chromosomes

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

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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Meiosis

• Meiosis I and II

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Meiosis

• Meiosis step-by-step

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Crossing Over

• Each chromosome becomes zippered to its homologue
• All four chromatids are closely aligned
• Non-sister chromosomes exchange segments (which
  contain genes)

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Crossing Over
Stepped Art
Fig. 10-6, p.160
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Crossing Over
a Both chromosomes shown here were duplicated
during interphase, before meiosis. When prophase
I is under way, sister chromatids of each
chromosome are positioned so close together that
they look like a single thread.
Fig. 10-6a, p.160
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Crossing Over
b Each chromosome becomes zippered to its
homologue, so all four chromatids are tightly
aligned. If the two sex chromosomes have
different forms, such as X paired with Y, they
still get zippered together, but only in a tiny
region at their ends.
Fig. 10-6b, p.160
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Crossing Over
c We show the pair of chromosomes as if they
already condensed only to give you an idea of
what goes on. They really are in a tightly
aligned, threadlike form during prophase I.
d The intimate contact encourages one crossover
(and usually more) to happen at various intervals
along the length of nonsister chromatids.
e Nonsister chromatids exchange segments at the
crossover sites. They continue to condense into
thicker, rodlike forms. By the start of metaphase
I, they will be unzippered from each other.
f Crossing over breaks up old combinations of
alleles and puts new ones together in the cells
pairs of homologous chromosomes.
Fig. 10-6c, p.160
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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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Random Alignment

• During transition between prophase I and
  metaphase I, microtubules from spindle poles
  attach to kinetochores of chromosomes
• Initial contacts between microtubules and
  chromosomes are random
• What does this mean?

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Random Alignment

• 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 ChromosomeCombinations
1
2
3
combinations possible
or
or
or
Fig. 10-7, p.161
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Alignment at metaphase I
Stepped Art
Fig. 10-7, p.161
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Crossing Over

• Crossing over

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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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Animal Life Cycle

• Random alignment

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Oogenesis
three polar bodies (haploid)
first polar body (haploid)
primary oocyte (diploid)
oogonium (diploid)
secondary oocyte (haploid)
ovum (haploid)
Meiosis I, Cytoplasmic Division
Meiosis II, Cytoplasmic Division
Growth
Figure 10-10Page 163
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Oogenesis

• Generalized life cycles

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Spermatogenesis
primary spermatocyte (diploid)
spermato-gonium (diploid )
sperm (mature, haploid male gametes)
secondary spermatocytes (haploid)
spermatids (haploid)
Meiosis I, Cytoplasmic Division
Meiosis II, Cytoplasmic Division
Growth
cell differentiation, sperm formation
Figure 10-9Page 163
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Fig. 10-10, p.163
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Animal Egg Formation

• Sperm formation

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Animal Egg Formation

• Egg formation

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Fertilization

• Male and female gametes unite and nuclei fuse
• Fusion of two haploid nuclei produces diploid
  nucleus in the zygote
• Which two gametes unite is random
• Adds to variation among offspring

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Factors Contributing to Variation among Offspring

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

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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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• Each person of a pair take out a piece of paper
  and you will compare PMAT of mitosis to meiosis.
  Skip the T.
• How many squares do you need to do this?
• When you are done share with another pair.
• When everyone is done we will compare answers

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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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• What are the over arching concepts for mitosis
  and meiosis?
• Focus on number of cells produced, how they are
  produced and chromosome number.

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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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Results of Mitosis and Meiosis

• Comparing mitosis and meiosis