Mitosis and Meiosis

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Chapter 2

• Mitosis and Meiosis

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Genetic Material

• For almost all organisms it is DNA with the
  exception of a few viruses that use RNA
• To transfer the genetic material to new cells or
  offspring, eukaryotes use 2 processes
• Mitosis 2 cells with same chromosomes
• Meiosis 4 cells with ½ the chromosomes
• Both use similar mechanisms but have different
  outcomes

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Cell Structure Tied to Genetic Function

• Many cellular components play a role in cell
  division
• mitochondria and chloroplasts are unique in they
  have their own DNA
• Eukaryotes and prokaryotes are different in how
  they replicate
• Gene expression is what allows for specific
  variation in a given cell

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Cell Structure

• Plasma membrane
• Plants have cell wall and animals have cell coats
  that aid in biochemical recognition
• Nucleus chromatin and chromosomes
• Nucleolus rRNA synthesis and ribosome formation
• nucleolus organizing region (NOR)
• Other membranous organelles

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Bacterial Structure

• No membranous organelles
• Nucleoid region that DNA is in the cell
• may be attached to the plasma membrane
• DNA does not undergo compaction into chromosomes
• No nucleolus but does have genes for rRNA

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Additional Structures

• From the plasma membrane to nuclear membrane is
  the cytoplasm
• Remove the membrane bound organelles from the
  cytoplasm cytosol
• colloidal material that contains proteins,
  ribosomes and cytoskeleton
• Cytoskeleton consists of system of tubules and
  filaments
• microtubules, actin and intermediate filaments
• aids in cell shape, motility and anchoring
  various organelles
• Endoplasmic reticulum compartmentalizes the
  cytoplasm and increases surface area
• SER fatty acid synthesis and phospholipids
• RER studded with ribosomes for protein synthesis

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More Additional Structures

• Mitochondria and chloroplasts cellular
  respiration and photosynthesis, respectively
• both can duplicate themselves have DNA a little
  different than in nucleus
• Endosymbiont Hypothesis
• Centrioles found in centromere and associated
  with spindle fibers that function in mitosis and
  meiosis
• derived from basal body
• spindle fibers play a role in the movement of
  chromosomes as thy separate - microtubules

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Chromosomes

• Distinctive shapes and lengths when visible
• Each has a condensed or constricted region called
  the centromere which helps establish the general
  appearance
• Centromeres are placed at different points along
  the chromosome depending on the placement,
  create arms of different lengths

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Shorter arm is always placed above the centromere
and called the p (petite) arm Longer arm is
always placed below the centromere and called the
q arm Must know name and location of the
centromere
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Observations of Mitosis

• Somatic cells (all but sex cells) have identical
  numbers of chromosomes diploid or 2n
• Nearly all chromosomes exist in pairs
• pairs are homologous have identical features
• exception is in human males have one of each
  sex chromosomes, females have 2 Xs, X and Y act
  as homologous pairs during meiosis
• Exceptions
• bacteria and viruses usually have 1 chromosome
• some organisms live in a haploid state (1n)

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Karyotype

• Pairing of chromosomes to look at structure
• See as a double structure sister chromatids
  that are connected at centromere
• will divide during mitosis

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Haploid Numbers

• n genetic information that constitutes the
  genome genes and non-coding DNA
• Contain identical genes on chromosome pair
• genes are present at a locus (loci)
• identical in genetic potential
• 1 from ? and 1 from ? - called biparental
  inheritance
• In the population, many different alternative
  forms called alleles
• During meiosis, only 1 from each homologous pair
  is placed in the gamete or spore
• fertilization forms a diploid zygote

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Mitosis

• Critical to all eukaryote cells, in protozoans
  and some fungi and algae use as the basis of
  asexual reproduction
• Partitions chromosomes during nuclear division or
  karyokinesis follows exact copying of DNA
• Karyokinesis is followed by cytokinesis which
  causes the cell to divide into 2 new daughter
  cells
• half the size of parent cell and needs to grow
  but the nucleus is about the same as parent

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Cell Cycle

• Occurs from the completion of one division to the
  beginning of the next division
• Interphase is the step between cell division that
  gets the cell ready to divide
• includes the S phase DNA replication
• 2 gap phases (G1 and G2) one before and after
  DNA replication
• time of intense metabolic activity to get ready
  for cell division

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Time Line

• Mitosis occupies a small portion of the cell
  cycle
• S phase and G2 are fairly constant
• G1 is the most variable as cells can stay here
  until signals for cell division comes
• may actually spin out to G0 which is a
  metabolically active stage without cell division

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Phases of the Cell Cycle
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Interphase

• No visible chromosomes
• Mitosis happens after G1, S and G2 finished
• Not really individual steps but subdivided into
  discrete stages for ease of learning
• M phase consists of prophase, prometaphase,
  metaphase, anaphase and telophase

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Prophase

• ½ the time of mitosis
• 2 centrioles move to opposite ends of cells
  outside the nuclear membrane 1 new and 1 old
• Spindle fibers organized by centriole and run
  between the centrioles animals have but not in
  plants, fungi or some algae
• Nuclear membrane disintegrates
• Chromosomes become visible double structure
  joined at the centromere each called a
  chromatid, genetically identical, randomly
  dispersed where nucleus was

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Prometaphase and Metaphase

• Prometaphase is a period of chromosome movement
• Metaphase is strictly configuration after
  migration
• Migration of chromosomes to equatorial plate
  metaphase plate perpendicular to axis formed by
  the spindle fibers
• movement caused by spindle fibers attaching to
  kinetochore which is associated with centromere
• kinetochore is multi-layer plates of proteins
  that microtubules attach to and pull chromosomes
  to the pole

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Anaphase

• Shortest stage
• Sister chromatids disjoin and migrate to opposite
  poles centromere must split to create the
  daughter chromosome
• dependent on centromere-spindle fiber attachment
• use motor proteins to move uses ATP hydrolysis
  and are called molecular motors
• Migration leads with centromere
• Critical step to make sure that each new cell
  gets a full complement of chromosomes

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Telophase

• Final stage
• Cytokinesis is the significant event partitions
  the cytoplasm
• plants make a cell plate that forms at the
  metaphase plate which eventually becomes a cell
  wall
• animals have a constriction in the center making
  a cell furrow
• In late telophase, the reversal of prophase takes
  place
• Cell re-enters interphase

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Meiosis

• Process by which we reduce the number of
  chromosomes from 2n to n
• sexual reproduction will reconstitute the 2n
  number
• Each gamete or spore will have only 1 copy of
  each chromosome
• allows for genetic diversity as each time a sex
  cell goes thru meiosis will result in a different
  assortment of chromosomes
• also can add diversity by crossing over
  swapping of portions of adjacent chromosomes in
  the homologous pair
• it is part mom and part dad - mosaic
• Mitosis maintains the 2n quantity of genetic
  material

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Mitosis vs. Meiosis
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Overview of Meiosis

• 2 divisions with several stages
• Homologous chromosomes form pairs synapse
• initially called a bivalent which eventually
  becomes a tetrad (4 chromatids as both
  chromosomes have been duplicated)
• Division 1 reductional division go from 2n to
  n number of chromosomes
• tetrads separate forming dyads
• Division 2 equational division chromosome
  number stays equal but the sister chromatids
  split to make monad
• potentially forms 4 cells

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Prophase I

• Chromatin condenses into chromosomes
• Homologous chromosomes undergo synapsis
• Crossing-over of synapsed homologous chromosomes
• Broken into 5 additional stages for ease of
  discussion

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5 Stages of Prophase I

• Leptonema leptotene chromatin condenses,
  still linear, have chromomeres that is thought to
  aid in homolog recognition
• Zygonema zygotene chromosomes continue to
  shorten and thicken undergo rough-pairing,
  synaptonemal complex forms to create bivalent (
  n number)
• Pachynema pachytene continue to shorten and
  coil, closer pairing and formation of the tetrad
• Diplonema diplotene can see the 2 sister
  chromatids, chromosomes intertwine at the chiasma
  get crossing-over between non-sister chromatids
  of a tetrad
• Diakinesis chromosomes are pulled further apart
  but chiasma stays connected, nucleolus and
  nuclear membrane disappear, centromeres attach to
  spindle fiber

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Remainder of Meiosis I

• Use similar mechanisms as seen in mitosis
• Metaphase I thickened, short chromosomes,
  terminal chiasmata visible, tetrads interact with
  spindle fibers, alignment is random at metaphase
  plate
• Anaphase I sister chromatids held by
  centromeres and tetrad is separated and moved to
  the poles in process called disjunction, each
  gets n number of chromosomes
• occasionally non-disjunction occurs with an
  abnormal splitting of the tetrad
• Telophase I nuclear membrane forms around the
  dyads, undergo a short interphase without DNA
  replication
• some cells enter Meiosis II after anaphase I

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Meiosis II

• Separate sister chromatids to gamete or spore
• Prophase II sister chromatids held by
  centromere
• Metaphase II centromeres are positioned at the
  metaphase plate
• Anaphase II chromatids pulled to separate poles
• Telophase II chromosome is a monad and after
  cytokinesis you have 4 haploid cells that may
  have increased genetic diversity

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Meiosis
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Spermatogenesis

• Makes ? gametes, in the testes
• Spermatogonium large undifferentiated germ cell
  that enlarges to primary spermatocyte
• Meiosis I make the secondary spermatocytes that
  have haploid number of dyads
• Meiosis II makes 2 spermatids from each 2
  spermatocyte
• Spermatids undergo developmental changes
  (spermatogenesis) to become spermatozoa or sperm
• all are equal size

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Oogenesis

• Make ? gamete or spore in the ovary, each
  receives equal amounts of chromosomes but unequal
  amounts of cytoplasm
• almost all cytoplasm from primary oocyte goes to
  secondary oocyte
• used to nourish growing zygote
• cell with little to no cytoplasm is the 1st polar
  body which may or may not divide again
• Mature ovum is produced after meiosis II, ootid
  (differentiates to ovum) and 2nd polar body
• Meiosis I and II may not be continuous in
  humans, meiosis I occurs in embryonic ovary and
  arrests in prophase I and completes at ovulation,
  second division occurs after fertilization

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Sexual Reproduction

• Meiosis is critical for this and allows for
  genetic diversity
• Plants produce haploid spores that will become
  gametes
• Yeast spend most of life as haploid and divide by
  mitosis
• Plants alternate between sporophyte and
  gametophyte alternation of generations

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Photomicrographs of Chromosomes