Mitosis and Meiosis

• PROPHASE

  • Individual chromosomes have condensed to the point of being visible as discrete objects under the light microscope
• PROPHASE

  • The chromosomes are composed of two sister chromatids (since the S phase is already done)
• PROPHASE

  • Two centrosomes separate from each other toward the opposite poles of the nucleus
• PROPHASE

  • The centrosomes function as microtubule-organizing center where microtubules are assembled and anchored
• PROPHASE

  • The region between the two centorosomes begins to fill with microtubules destined to form mitotic spindles
• PROPHASE

  • A dense starburst of microtubules called aster forms in the immediate vicinity of each chromosome
• PROMETAPHASE

  • Marked by the fragmentation of the membranes of the nuclear envelope
• PROMETAPHASE

  • The breakdown of nuclear envelope allows spindle microtubules to make contact with the chromosomes (paired chromatids)
• PROMETAPHASE

  • The spindle microtubules are destined to attach in the centromere which is a constricted area where the two members of each chromatid pair are held together
• PROMETAPHASE

  • Some of the spindle microtubules attach to the kinetochore
• METAPHASE

  • The fully condensed chromosomes all become aligned at the metaphase plate (which is a plane equidistant between the two poles of the mitotic spindle)
• METAPHASE

  • The two sister chromatids are actively tugged toward the opposite poles
• ANAPHASE

  • Usually the shortest phase of mitosis
• ANAPHASE

  • The two sister chromatids abruptly separate and begin moving toward the opposite spindle poles
• ANAPHASE

  • Anaphase A: movement of chromosomes toward the spindle poles
  • Anaphase B: movement of the two spindle poles away from each other
  • Anaphase A and B are two kinds of movements during anaphase and may occur simultaneously
• TELOPHASE & CYTOKINESIS

  • The chromosomes have arrived at the spindle poles
  • The chromosomes uncoil into extended fibers typical of interphase chromatin
• TELOPHASE & CYTOKINESIS

  • Nucleoli develop at the nucleolar organizing sites on the DNA
  • Spindle disassembles and nuclear enveloped form around the two groups of chromosomes
  • Occurs simultaneously with cytokinesis (division of cytoplasm)
• OTHER TYPES OF SPINDLE MICROTUBULES

  • astral microtubules
  • polar microtubules
• POLARITY OF MICROTUBULES

  • Minus (-) end is at the chromosome
  • Plus (+) end is pointing away from the entrosome
• a chromosome is a paired structure that contains a shorter arm (p-arm) and a longer arm (q-arm) joined by a centromere.
• a sister chromatid is also a paired structure but contains two chromosomes joined together by cohesin molecules.
• Identify the stages of mitosis

  A) Interphase

  B) Telophase

  C) Prophase

  D) Interphase

  
• Identify the stages of mitosis

  A) Prophase

  B) Prophase

  C) Metaphase

  D) Anaphase

  E) Metaphase

  
• Substages of Prophase I

  • Leptotene - Replicated chromosomes begin to condense and become visible with a light microscope
• Substages of Prophase I

  • Zygotene
  • Continued condensation makes individual chromosomes distinguishable
  • Homologous chromosomes recognize each other and pair up via synapsis, thereby forming bivalents, also called tetrad (each bivalent has 4 chromatids, two derived from each chromosome)
• Substages of Prophase I

  • Pachytene
  • The homologous chromosomes have completely aligned
  • Crossing over occurs, which involves physical exchange of chromosome pieces
  • The connection that results from crossing over is called chiasma
• Substages of Prophase I

  • Diplotene
  • The homologous chromosomes of each bivalent begin to separate from each other
  • ▪ The synaptonemal complex has largely disappeared
• Substages of Prophase I

  • Diakinesis
  • The chromosomes recondense to their maximally condensed state
  • The centromeres of the homologous chromosomes separate further
  • The chiasmata become the remaining attachment between the homologous chromosomes
• Synaptonemal Complex

  • An elaborate protein structure resembling a zipper that holds homologous chromosomes in tight apposition during synapsis
  • The lateral elements of the synaptonemal complex start to attach to individual chromosomes during leptotene
  • The central element which actually joins the homologous chromosomes together does not form until zygotene
• Prophase I

  • Spindle apparatus is complete
  • The chromatids are attached via kinetochore microtubules
• Metaphase I

  • The bivalents (tetrads) are organized at the metaphase plate
  • Take note that the dyads are aligned in a double row (unlike in mitosis which has a single row only)
  • Homologous chromosomes are solely held together solely by chiasmata
• eachAnaphase I

  • The members of each pair of homologous chromosomes separate from ach other and start migrating toward opposite spindle poles, pulled by their respective kinetochore microtubules
  • However, the connection that holds the sister chromatids together does not break
• Telophase I & Cytokinesis

  • Marked by arrival of haploid set of chromosomes at each spindle pole
  • Sometimes, nuclear envelopes form around the chromosomes before cytokinesis
  • In most cases, the chromosomes do not decondense before meiosis II begins
• Meiosis II

  • Resembles mitosis (hence equational division)
  • A brief interphase may occur before meiosis II but it does not involve DNA replication because each chromosome already consists of a pair of replicated sister chromatids
• Prophase II

  • Very brief stage only; if detectable at all, similar to mitotic prophase
• Metaphase II

  • The kinetochores of sister chromatids now face opposite directions, allowing sister chromatids to separate and move to opposite spindle poles in anaphase II
• Anaphase II

  • Sister chromatids separate and move to opposite spindle poles
• Telophase II & Cytokinesis

  • Formation of 4 daughter cells, each containing a haploid set of chromosomes (no sister chromatid)
  • The haploid daughter cells produced by meiosis II contain a random mixture of paternal and maternal chromosomes
  • Moreover, each of these chromosomes is composed of a mixture of DNA sequences created by crossing over during prophase I
  • Spindle disassembles and nuclear envelope forms
• In either case when nondisjunction occurs during meiosis I or meiosis II, one daughter cell receives two of the same type of chromosome, whereas the other receives none
• Depending on the chromosome involved, nondisjunction can be lethal or lead to lifelong consequences