Cell Cycle

• There are two principal phases of the cell cycle:

  • M phase – the short time during which cells divide

  • Interphase – the longer period between M phases

  
• G1 (Gap 1) - The first gap phase that occurs immediately following mitosis or cytokinesis. It is characterized by an increase in size and metabolic activity of the cell.
• During S phase, dna replication results in doubling of the dna in each nucleus, and occurs after G1 phase
• During s phase in interphase, sister chromatids are produced for each chromosome
• G0 is the resting phase of the cell cycle, where the cell is not dividing. It is still specialized.
• M phase is where karyokinesis occurs; partitioning of dna in to daughter cell nuclei. Cytokinesis also happens which is the partitioning of the cytoplasmic contents into daughter cells.
• Centromeres are specialized DNA sequences where the sister chromatids are joined together, which becomes visible in prophase. The centromeres bind protein called kinetochores.

  
• • Kinetochore microtubules are attached to centromeres of each chromatid and are involved in chromosome movement.
  • Polar microtubules extend to the opposite poles of the centrosome and help with elongation and cell stability.
  • Astral microtubules grow toward the membrane of the cell and help with cell stability too.

  
• At the end if metaphase:

  • The kinetochore microtubules from opposite centrosomes line up the chromosomes at the metaphase plate of the cell.
  • Chromosomes are condensed more than 10,000 fold compare to the previous phase, prophase.

  
• Sister chromatid cohesion is the tension created by the pull of the kinetochore microtubules.

  • The protein cohesion between the sister chromatids holds them together to prevent premature separation.
  • Cohesin is a 4subunit protein that coats the sister chromatids, especially at the centromeres

  
• Animal cells have two centrosomes which move to the opposite poles of the cell.

  • Centrosomes are where spindle fiber microtubules are formed which have a negative end that is at the centrosome and a positive end that grows away from the centrosome.

  
• In anaphase:

  • A: sister chromatids separate due to the enzyme separase cleaving the polypeptides in cohesin. Kinetochore microtubules begin to depolymerize at their positive end, moving individual chromatids toward the centrioles.
  • Disjunction = separation of sister chromatids
  • B: polar microtubules lengthen making the cell have an elongated shape.

  
• In telophase:

  • Nuclear membranes reassemble around the chromosomes at each pole.
  • Decondensation returns chromosomes to their interphase state.
  • Two identical nuclei occupy the elongated cell, which will divide into two daughter cells by cytokinesis.

  
• Cytokinesis:

  • Begins in anaphase, and ends in telophase.
  • In animals cells, a ring of actin creates a cleavage furrow around the circumference of the cell; this pinches the cell into two.
  • In plant cells, a new cell wall (cell plate) is constructed along the cellular midline.
  • In both animal + plant cells, cytokinesis divides the cytoplasmic fluid and organelles between the daughter cells.

  
• • In meiosis 1, homologous chromosomes separate from one another, reducing the diploid number of chromosomes to the haploid number.
  • In meiosis 2, sister chromatids separate from one another to produce four haploid gametes, each with one chromosome of every original diploid pair.
• In meiosis 1, prophase 1 is subdivided into...

  Leptotene, zygotene, pachytene, diplotene, and diakinesis
• Leptotene and Zygotene:

  • Chromosome condensation begins in leptotene stage
  • The meiotic spindle forms as microtubules extend out from centrosomes
  • The nuclear envelope disintegrates during zygotene
  • Homologous chromosomes undergo synapsis

  
• Synaptonemal Complex



  • formed between homologous chromosomes.
  • It is a tri-layer protein structure that binds non-sister chromatids of homologous chromosomes.
  • Non-sister chromatids = belong to different members of a homologous pair.

  
• Pachytene

  • A stage in prophase after leptotene + zygotene where chromosome condensation continues.
  • Tetrads = paired homologs with 4 chromatids.
  • Recombination nodules can be seen at intervals in the synaptonemal complex between homologs.
  • These are aggregates of enzymes and proteins needed for crossing over.
• Diplotene

  • Chromosomes continue to condense in diplotene, and the synaptonemal complex begins to dissolve.
  • Homologs pull apart slightly, revealing chiasmata at locations where crossing over has occurred in pachytene.
  • Cohesin protein is present between sister chromatids, to resist the pulling forces of kinetochore microtubules.
• Diakinesis

  Kinetochore microtubules move synapsed chromosome pairs toward the metaphase plate. Here, homologs align side by side

  
• Metaphase I

  • After prophase 1 ends, in metaphase 1 the chiasmata between homologs are resolved; this completes crossing over.
  • Homologs are aligned on opposite sides of the metaphase plate
  • Kinetochore microtubules attach to both sister chromatids of one homolog; kinetochore microtubules from the opposite pole do the same for the other homolog