Meiosis

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Meiosis

• Honors Biology

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What is the function of meiotic cell division?
How does this form the basis of genetics and
evolution?

• Outcome 4 haploid cells (gametes)
• Crossing over prophase I
• Homologous chromosomes
• Sister chromatids

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Hereditary Similarity and Variation

• Heredity is the transmission of traits from one
  generation to the next
• Variation shows that offspring differ in
  appearance from parents and siblings
• Genetics is the scientific study of heredity and
  variation

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Genes

• Genes are the units of heredity
• Genes are segments of DNA
• Each gene has a specific locus on a certain
  chromosome
• One set of chromosomes is inherited from each
  parent (humans 23 from each parent)

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Sexual/Asexual

• Sexual reproduction- two parents give rise to
  offspring that have unique combinations of genes
  inherited from the two parents
• Asexual reproduction- one parent produces
  genetically identical offspring by mitosis

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Karyotype

• karyotype -ordered display of the pairs of
  chromosomes from a cell
• homologous chromosomes (homologues)
• Both chromosomes in a pair. They carry genes
  controlling the same inherited characteristics
• Alleles-

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LE 13-3
Pair of homologous chromosomes
5 µm
Centromere
Sister chromatids
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LE 13-4
Key
Hey, thats my chromosome!
Maternal set of chromosomes (n 3)
2n 6
Paternal set of chromosomes (n 3)
Two sister chromatids of one replicated chromosome
s
Centromere
Two nonsister chromatids in a homologous pair
Pair of homologous chromosomes (one from each set)
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Homologous Chromosomes

• Each pair of homologous chromosomes includes one
  chromosome from each parent
• For humans, the diploid number is 46 (2n 46)
  23 Pairs.
• Gametes are haploid cells, containing only one
  set of chromosomes

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Meiosis twice as nice

• Meiosis I
• Synapsis
• Tetrad Formation
• Crossing over
• Prophase I
• Homologs split

• Meiosis II
• same as mitosis
• Sister chromatids split
• 4 unique cells result
• haploid

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LE 13-7
Interphase
Homologous pair of chromosomes in diploid parent
cell
Chromosomes replicate
Homologous pair of replicated chromosomes
Sister chromatids
Diploid cell with replicated chromosomes
Meiosis I
Homologous chromosomes separate
Haploid cells with replicated chromosomes
Meiosis II
Sister chromatids separate
Haploid cells with unreplicated chromosomes
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LE 13-8ab
MEIOSIS I Separates homologous chromosomes
PROPHASE I
METAPHASE I
ANAPHASE I
Sister chromatids remain attached
Centromere (with kinetochore)
Chiasmata
Sister chromatids
Metaphase plate
Spindle
Microtubule attached to kinetochore
Homologous chromosomes separate
Tetrad
Homologous chromosomes (red and blue) pair
and exchange segments 2n 6 in this example
Pairs of homologous chromosomes split up
Tetrads line up
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• Beginning of telophase I- each half of the cell
  has a haploid set of chromosomes
• each chromosome still consists of two sister
  chromatids
• Cytokinesis usually occurs simultaneously,
  forming two haploid daughter cells

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LE 13-8b
MEIOSIS II Separates sister chromatids
TELOPHASE I AND CYTOKINESIS
TELOPHASE II AND CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
Cleavage furrow
Haploid daughter cells forming
Sister chromatids separate
Two haploid cells form chromosomes are still
double
During another round of cell division, the sister
chromatids finally separate four haploid
daughter cells result, containing single
chromosomes
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Crossing Over

• Because of crossing over in Prophase I, the two
  sister chromatids of each chromosome are no
  longer genetically identical
• Anaphase I Homologous chromosomes separate.
  sister chromatids stay together
• Anaphase II sister chromatids separate
• The sister chromatids of each chromosome are now
  two newly individual chromosomes.

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Four for the Price of One

• At the end of meiosis, there are four daughter
  cells, each with a haploid set of unreplicated
  chromosomes
• Each daughter cell is genetically distinct from
  the others and from the parent cell

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LE 13-9
MITOSIS
MEIOSIS
Chiasma (site of crossing over)
Parent cell (before chromosome replication)
MEIOSIS I
Propase
Prophase I
Chromosome replication
Chromosome replication
Tetrad formed by synapsis of homologous chromosome
s
Duplicated chromosome (two sister chromatids)
2n 6
Tetrads positioned at the metaphase plate
Chromosomes positioned at the metaphase plate
Metaphase I
Metaphase
Anaphase
Sister chromatids separate during anaphase
Anaphase I
Homologues separate during anaphase
I sister chromatids remain together
Telophase
Telophase I
Haploid n 3
Daughter cells of meiosis I
2n
2n
MEIOSIS II
Daughter cells of mitosis
n
n
n
n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II
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Genetic variation contributes to Evolution

• Mutations create different versions of genes
• Mechanisms for variation in Sexual
• Independent Segregation of alleles
• Independent assortment of chromosomes
• Crossing over (prophase I)
• Random fertilization

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Independent Assortment

• Independent assortment-each pair of chromosomes
  sorts maternal and paternal homologues into
  daughter cells independently of the other pairs
  (not all dads chroms to 1 daughter! And moms to
  the other)
• For humans (n 23), there are more than 8
  million (223) possible combinations of
  chromosomes

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Crossing over to the other side

• Crossing over produces recombinant chromosomes,
  which combine genes inherited from each parent
• In crossing over, homologous portions of two
  nonsister chromatids trade places
• Prophase I

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LE 13-11
Prophase I of meiosis
Nonsister chromatids
Tetrad
Chiasma, site of crossing over
Metaphase I
Metaphase II
Daughter cells
Recombinant chromosomes
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Match.cell

• Random fertilization adds to genetic variation
  because any sperm can fuse with any ovum
• The fusion of gametes produces a zygote with any
  of about 64 trillion diploid combinations
• Crossing over adds even more variation

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