Cell cycle and mitosis

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Chapter 2
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Cell cycle and mitosis
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S phase

• DNA is synthesized in S phase of interphase
• No DNA synthesis in mitosis or meiosis

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Mitosis

• For growth and repair
• Start with 1 diploid cell and end with 2 diploid
  cells
• Controlled by contraction and elongation of
  spindle fibers - microtubules

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Stages of mitosis

• http//www.cellsalive.com/mitosis.htm
• Centrioles used in cell division made up of
  microtubules
• Part of the centrosome
• Form spindle fibers

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Mitosis vs. meiosis

• Diploid number 6
• Mitosis
• 12 chromatids joined as 6 pairs
• Meiosis
• 3 pairs of homologous chromosomes

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Meiosis

• Involved in sexual reproduction
• To create genetic diversity
• Start with 1 diploid and end with 4 haploid cells
• Haploid cells fuse during fertilization to
  produce diploid cells
• in diploid organisms, chromosomes exist as
  homologous chromosomes one from each parent

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Stages of meiosis

• http//www.cellsalive.com/meiosis.htm

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Meiosis

• Pairing of homologous chromosomes
• Meiosis I homologous chromosomes separate
• Meiosis II sister chromatids separate similar
  to mitosis
• Meiosis crossing over (prophase I) and
  alignment of chromosomes on the equatorial plate
  create genetic diversity
• What you end up with is different from what you
  started with

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Substages of prophase I
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Look at textbook web site

• http//www.aw-bc.com/klug/
• Chapter 2 meiosis
• Part 1 animation

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Prophase I longest stage

• Leptonema
• Chromosomes become visible
• Pairing of homologous chromosomes
• Chromomeres condensations that resemble beads
  on a string

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Prophase I

• Zygonema
• Homologous chromosomes start to pair
• forms a bivalent complex
• two pairs of sister chromatids or four chromatids
• Centriolles begin moving to opposite sites
• Pachynema
• Coiling and shortening of chromosomes continues
• Tetrads visible each bivalent contains 4
  chromatids
• Synapsis homologous chromosomes more closely
  paired

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Prophase I

• Diplonema
• Chiasma chromatids intertwined
• Nonsister chromatids undergone genetic exchange
  crossing over
• Centrioles move to the poles
• Diakinesis
• Terminalization chiasmata move towards the ends
  of the tetrad
• formation of spindle fibers, nuclear envelope
  breaks down

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Comparison of mitosis and meiosis
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Independent orientation of chromosomes in meiosis
leads to varied offspring
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Centromeres hold 2 sister chromatids together

• p short arm, above the centromere
• q long arm, below the centromere

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Karyotype
Humans have 23 pairs of chromosomes 22 pairs of
autosomes and 1 pair of sex chromosomes Nondisjun
ction diseases chromosomes do not separate
appropriately
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Nondisjunction results from failures in meiosis I
or meiosis II
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Differences between the formation of sperm and
eggs
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Spermatogenesis

• The formation of sperm
• Occurs in the seminiferous tubules of the testes
• Takes 65 to 70 days
• Begin with 1 diploid cell and produce 4 haploid
  cells at the end
• All sperm cells receive an equal amount of
  genetic material and cytoplasm

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Primary spermatocytes

• These are the basic cells, where process begins
• Diploid (2n)
• Produced continuously by mitosis

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Spermatogenesis

• Secondary Spermatocytes
• Produced after meiosis I
• These are haploid
• Spermatids
• haploid
• Produced after meiosis II
• Spermatozoa (sperm)
• haploid
• Produced after differentiation

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Spermatogenesis vs. Oogenesis
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Oogenesis

• Formation of eggs (ova) occurs in the ovaries
• Human females born with all the eggs that they
  will have
• Final eggs produced have equal amounts of genetic
  material but different amounts of cytoplasm

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Primary oocyte

• These are present at birth
• In prophase I
• Stay dormant until sometime after puberty
• Diploid

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First polar body and secondary oocyte produced
after meiosis I

• First polar body is smaller - has less cytoplasm
• Secondary oocyte is released by ovulation
• Happens roughly every 28 days

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Fertilization triggers meiosis II

• Secondary oocyte
• Becomes Ootid second polar body
• Ootid differentiates into an ovum

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Spermatogenesis vs. oogenesis

• Spermatogenesis
• 4 mature gametes develop
• Oogenesis
• 1 mature ovum polar bodies that degenerate
• Oogenesis
• Female born with all cells (arrested at prophase
  I) that will develop into mature ovum
• Sperm cells develop throughout life