Stay alert....feast on this lecture because it is all about you

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Stay alert....feast on this lecture because it is
all about you!
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Chapter 13 Meiosis Sexual Life Cycles

• Introduction to Heredity
• Genes, DNA Chromosomes
• Sexual Asexual Reproduction
• Sexual Life Cycle Humans
• Meiosis Role in Sexual Life Cycles
• Variety of Sexual Life Cycles

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Chapter 13 Meiosis Sexual Life Cycles

• Meiosis A Detailed Look
• Chromosome number reduced
• Comparison Mitosis Meiosis

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Chapter 13 Meiosis Sexual Life Cycles

• Origins of Genetic Variation.
• Sexual Sources of Genetic Variation
• Independent assortment of chromosomes
• Crossing over
• Random variation

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Like Begats Like

• Offspring resemble their parents more than they
  do less closely related individuals.
• This continuity of traits from one generation to
  the next is called HEREDITY (heir).

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Like Begats Like
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Like Begats Like

• Along with inherited similarity there is also
  variation.
• So how are these similarities and variations
  selected at a fundamental level?

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Like Begats Like

• Mechanisms of heredity and variation eluded
  biologists until the development of genetics in
  the last century (1900s).
• GENETICS is the scientific study of heredity
  variation.

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Genes, DNA Chromosomes

• Parents endow their offspring with coded
  information in the form of hereditary units
  called GENES (not Levis 501 JEANS) Genes vs.
  Jeans

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Genes, DNA Chromosomes

• Our genes program the emergence of specific
  traits as we develop from fertilized eggs into
  adults.

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Genes, DNA Chromosomes

• Genes are made of DNA, which is a polymer of
  four different kinds of monomers called
  NUCLEOTIDES.
• Review Chapters 1 5.

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Genes, DNA Chromosomes

• Inherited information is passed along in the
  form of each genes specific nucleotide sequence.

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Genes, DNA Chromosomes

• The language is abstract...the sequence (word or
  sentence) does not resemble what is actually made.

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Genes, DNA Chromosomes

• Cells translate genetic sentences into big
  noses and other features that bear no
  resemblence to genes.

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Genes, DNA Chromosomes

• Inheritance has its chemical basis in the precise
  replication of DNA, which produces copies of
  genes that can be passed along from parents to
  offspring.

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Genes, DNA Chromosomes

• The cellular vehicles for these genes are
  sperm (males) ova (females)

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Genes, DNA Chromosomes

• Biological order based on heritable programs in
  the form of DNA is one of the uniflying themes in
  biology (e.g. biology megatheme)

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Genes, DNA Chromosomes

• DNA in eukaryotes is subdivided into chromosomes
  located within the nucleus....each species has a
  characteristic chromosome number (e.g. you know
  we have 46 except in reproductive cells).

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Genes, DNA Chromosomes

• Each chromosome is composed of a single DNA
  molecule that is very long.
• Along with proteins, the DNA is folded and coiled.

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Genes, DNA Chromosomes

• Each chromosome has hundreds or thousands of
  genes each occupying a specific region on the
  chromosome (e.g. locus, not locust).

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Chromosomal Behavior

• Our genetic endowment consists of whatever genes
  happened to be part of the chromosomes we
  inherited from our parents.

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Chromosomal Behavior

• How does sexual reproduction pass chromosomes
  from parent to offspring???????
• This is what your parents couldnt tell
  you.........

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Sexual Asexual Reproduction A Comparison

• Strictly speaking, like begats like only to
  organisms that reproduce asexually.

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Asexual Reproduction

• A single individual is the sole parent and
  passes on all its genes to its offspring exact
  copy clone
• Remember Dolly was a clone...was it asexual

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Asexual Reproduction

• Differences in asexual reproduction are due to
  mutations, which are rare changes in DNA.
• Sexual reproduction usually results in much
  greater variation than for asexual reproduction.

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Sexual Reproduction

• Two parents give rise to offspring..they have a
  unique combination of genes from both parents.

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Sexual Reproduction

• What mechanisms generate this genetic variation?
• The key is the activity of chromosomes during
  the sexual cycle (e.g. megatheme)

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Sexual Life Cycles Humans

• Life cycle refers to generation-to-generation
  sequence of stages in the reproductive history of
  an organism, from conception to production of its
  own offspring.

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Sexual Life Cycles Humans

• Each somatic cell (other than sperm or egg) has
  46 chromosomes.

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Sexual Life Cycles Humans

• Appearance of each chromosome is used to
  distinguish them (banding, centromere
  position/size)

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Sexual Life Cycles Humans

• Karyotypes are displays of an individuals
  somatic-cell metaphase chromosomes arranged in a
  standard sequence to determine if there are
  abnormalities (e.g. lymphocytes examined)

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Karyotyping
X Y Chromosomes
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Sexual Life Cycles Humans

• Homologous Chromosomes a pair of chromosomes
  that have the same size, centromere position and
  staining pattern.

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Sexual Life Cycles Humans

• Homologous AUTOSOMES carry the same genetic
  loci however, human sex chromosomes carry
  different loci even though they pair during
  prophase of Meiosis I........more later!

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Sexual Life Cycles Humans

• Human females have a pair of homologous X
  chromosomes (XX), but males have one X and one Y
  chromosome (XY).

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Sexual Life Cycles Humans

• We have 22 pairs of autosomes and 1 pair of sex
  chromosomes.

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Karyotype Human Lymphocytes22 Autosomal Pairs,
1 Sex Pair (XY)
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Sexual Life Cycles Humans

• Somatic cells in humans and most other animals
  are diploid (2 sets of 23 chromsomes, one
  maternal (mommy) one paternal (daddy) set
• (2n46, humans)

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Sexual Life Cycles Humans

• A cell with a single chromosome set is a haploid
  cell
• n 23 in humans

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Sexual Life Cycles Humans

• The union of haploid sperm with haploid ovum is
  called fertilization or syngamy. Syntogether
  gamygamete

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Sexual Life Cycles Humans

• Via mitosis, the zygotes genes are passed along
  to all somatic cells....
• Thus, all somatic cells are diploid.

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• The only cells in the body not produced by
  mitosis are gametes.

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Sexual Life Cycles Humans

• Sexually reproducing cells carry out a process
  that halves the chromosome number in the gametes,
  compensating for the doubling that occurs at
  fertilization...this cell division is MEIOSIS
  (Fig. 13.3)

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Human Life Cycle Alternates Meiosis with
Fertilization
n
n
Meiosis
2n
Mitosis
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Sexual Life Cycles

• Although the alternation of meiosis
  fertilization is common to all sexually
  reproducing organisms, the timing of the events
  in the life cycle may vary. Review Fig. 13.4

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Meiosis Reduces Chromosome Number Diploid to
Haploid

• Meiosis, like mitosis, is preceded by the
  replication of chromsomes.
• This is followed by two consecutive cell
  divisionsmeiosis I meiosis II

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Meiosis Reduces Chromosome Number Diploid to
Haploid

• Cell divisions produce 4 daughter cells instead
  of 2 as in mitosis.
• Resulting daughter cells have half the number of
  chromosomes are different from their parent
  cells.

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How Meiosis Reduces Chromosome Number (Fig. 13.5)
Homologues Separate Meiosis I
Sister Chromatids Separate Meiosis II
Chromosomes Replicate Only One Time
Two Haploid Daughter Cells
Four Haploid Daughter Cells Half of Parental DNA
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Stages of Meiotic Cell Division
Meiosis I
Interphase I
Prophase I
Metaphase I
Anaphase I
Separates Homologues
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Meiosis I
Meiosis II Chromatids Separated
HAPLOID
Telophase I Cytokinesis
Anaphase II
Telophase II Cytokinesis
Prophase II
Metaphase II
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MEIOSIS MITOSIS COMPARISON (Fig. 13.7, p. 234)

• Chromosome number is reduced by half in meiosis,
  but not mitosis.
• Mitosis produces identical daughter cells
  meiosis produces cells different from parents and
  each other.

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MEIOSIS MITOSIS COMPARISON

• Meiosis Synapsis occurs, pairing homologous
  chromosomes (tetrads)
• Cross-over region is called chiasmata, and
  nonsister chromatids exchange genetic material.

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MEIOSIS MITOSIS COMPARISON

• Homologous pairs (meiosis) align on plate, rather
  than as individuals (mitosis).

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MEIOSIS MITOSIS COMPARISON

• Anaphase I of meiosis, sister chromatids remain
  together (Bowtie intact), rather than separating
  (mitosis).

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MEIOSIS MITOSIS COMPARISON

• Meiosis II similar to mitosis but without
  replication, thus the number of chromosomes is
  halved. Bowties broken separate chromatids,
  haploid cells.

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MEIOSIS
MITOSIS
Parent cell
Chiasma
Crossing over
Chromosomes
Tetrads
Daughter cells of Meiosis I
2n
2n
n
n
n
n
Daughter cells of Meiosis II
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SEXUAL SOURCES OF VARIATION Three Major Sources
(pp. 235-37)

• Independent Assortment of Chromosomes
  Orientation of homologous chromosome pair
  relative to the two poles is random. See Fig.
  13.8(see Meiosis I Metaphase I).

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Metaphase of Meiosis IITwo equally probable
arrangements of chromosomes (Diploid, 2n 4)
Blue Chromosome from DAD Yellow is from MOM
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
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SEXUAL SOURCES OF VARIATION

• The number of combinations possible for gametes
  formed by meiosis starting with two homologous
  pairs of chromosomes is four or 2n.
• n haploid number

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SEXUAL SOURCES OF VARIATIONIndependent
Assortment

• Haploid number (n) for humans is 23, thus the
  possible number of maternal and paternal
  chromosomes is 223 or about 8 million. Or, 8
  million possible assortments of combinations of
  heads or tails for simultaneous tossing of 23
  coins.

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SEXUAL SOURCES OF VARIATION Source Number 2

• Crossing Over (Fig. 13.9)Occurs during prophase
  I of meiosis I. SYNAPTONEMAL COMPLEX (protein)
  brings nonsister chromosomes into close proximity
  for precise pairing and alignment of genes.

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Crossing Over Fig. 13.9

• Meiotic Prophase I

Tetrad

• Meiotic Metaphase I

Chiasma

• Meiotic Anaphase I

• Meiotic Metaphase II

• Meiotic Anaphase II

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SEXUAL SOURCES OF VARIATION

• Homologous portions of nonsister chromotids trade
  places. For us, this occurs two or three times
  per chromosome pair. This is an important
  source of genetic variation.

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SEXUAL SOURCES OF VARIATION Source Number 3

• Random fertilization A human ovum, representing
  8 million combinations, combines with a sperm,
  representing 8 million different possibilities to
  produce a zygote with any of 64 TRILLION diploid
  combinations (8 X 8 million).

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Sexual Sources of VariationSummary

• Independent Assortment of Chromosomes
• Crossing Over
• Random Fertilization

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Chapter 13 OverviewMeiosis Sexual Life Cycles

• Genes, DNA Chromosomes
• Sexual Asexual Reproduction (see next slide)
• Sexual Life Cycle Humans
• Variety of Sexual Life Cycles

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Asexual vs. Sexual Reproduction

• Asexual Reproduction
• Single individual is sole parent
• Single parent passes all genes to offspring
• Offspring genetically identical to parent
• Result is a clone. Mutation is only source of
  variation (rare).

• Sexual Reproduction
• Two parents give rise to offspring
• Each parent passes on half its genes to offspring
• Offspring have unique combination of genes from
  both parents
• Results in greater genetic variation offspring
  vary genetically from parents and siblings.

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Chapter 13 Meiosis Sexual Life Cycles

• A Detailed Look at Meiosis
• Comparison of Mitosis Meiosis
• Sexual Sources of Genetic Variation

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Chapter 13 Summary

• Review Study Outline
• Take Self-Study Quiz
• Review BOLDFACE TERMS
• Read Chapter 14
• Ask questions of Recitation TA
• Use Biology Tutor (no charge!)