Telomeres & Mitosis

• Each cell has its own precisely timed cycle to determine when (or if) it’ll divide
• For cells that are actively dividing, there are 2 distinct phases:

  1. Interphase (cell prepares to divide)2. M phase (cell divides)
• Interphase has 3 sub-phases

  1. G1 phase (cell grows in size and preps for S phase)
  2. S phase (cell replicates its genetic info inprep for dividing in M phase)
  3. G2 phase (more growth, final prep for M phase)
• The Synthesis Phase

  • In eukaryotes, there’s one final step to DNA replication/S phase:
  • The restoration of the telomeres
• A chromosome is simply how DNA is organizedand stored
• DNA replication during S Phase involves replication of each and every chromosome in the cell
• In bacteria, that means only 1 chromosome

  • Once the replication forks come together on the other side of the circle, replication is complete
• Eukaryotic genome organization

  • Eukaryotic genomes are spread across several chromosomes.
  • Eukaryotic chromosomes are linear.
• Eukaryotic genome organization

  • Unlike a circular chromosome, linear chromosomes have ends that can “fray”
  • To prevent fraying, ends of chromosomes are capped by repetitive DNA sequences called telomeres.
• Why telomeres?

  • For leading strand, only 1 primer is required to start synthesis
  • But on the lagging strand... Multiple RNA primers are required for synthesis
• Why telomeres?

  • Replication continues smoothly to the end of chromosome, the whole template is replicated
• Why telomeres?

  • The final primer is added about 100 nucleotides from the 3’ end of the template
  • When it’s removed, new daughter strand shortened by ~100 nucleotides b/c DNA polymerase can’t fill it in
• Why telomeres?

  • Next round of replication, shortened template results in shorted chromosome
  • If this were allowed to continue for several rounds of replication, the DNA would eventually be nibbled away to nothing
• Telomeres are non-coding repeated sequence at the end of each chromosome

  • In humans, the telomere consists of the sequence 5’-TTAGGG-3’ repeated around 1500-3000 times
• Telomeres are maintained by an enzyme called telomerase
• Telomerase contains a piece of RNA complementary to telomere repeat, used as template for building more repeats on the template (parent) strand
• Telomerase activity differs in different cell types

  • It is fully active in germ cells (sex cells) that produce eggs or sperm and in stem cells.
• Telomerase activity differs in different cell types

  • It is almost inactive in adult somatic cells. In these cells, mitotic division can occur about 50 times before the telomeres become so short that the cells stop dividing
• In cancer cells, activating telomerase allows cells to divide without telomeres shortening
• As humans, each of our cells has 23 different chromosomes

  • We are also diploids, meaning we have 2 sets of each chromosome (1 from mom, 1 from dad)
• • Each numbered chromosome contains adifferent set of genes
  • Collectively, our two sets of chromosomes 1-23 form our entire genome
• All of them (46 total!) get replicated during S phase so that when the cell divides in two, each new cell can have one full set of chromosomes
• Goal of mitosis: Arrange each of the 46 sister chromatid pairs in such a way that when the cell divides in ½, each daughter cell ends up with one full set of 46 chromosomes (i.e., 23 pairs of homologous chromosomes)
• For starters, sister chromatid pairs (i.e., replicated chromosomes) must be easy to distinguish from each other and move around cell
• Mitosis takes place in 5 “stages”:

  1. Prophase
  2. Prometaphase
  3. Metaphase
  4. Anaphase
  5. Telophase
• Prophase

  • Tidy up replicated chromosome noodles into that “x” shape and start assembling the infrastructure for moving them around ( themitotic spindle)
• Prophase

  • Each sister chromatid pair gets condensed
• Prophase

  • The cell’s cytoskeleton is used to move each chromatid pair into position
  • Two microtubule assembling structures (centrosomes) start rapidly polymerizing microtubule fibers, forming the mitotic spindle
  • The centrosomes begin to migrate towards opposite poles of the cell
• Prometaphase

  • Free chromosomes from nucleus, then attach them to spindle microtubules
• Prometaphase (1)

  • Nuclear membrane breaks down
  • Spindle microtubules go through cycles of growth and contraction as they explore the region of the cell no longer occupied by the nuclear membrane.
• Prometaphase (2)

  • Prometaphase occurs when spindle microtubules encounter and then attach tochromosomes at the centromere
• The mitotic spindle attaches to the centromere connecting two sister chromatids via the kinetochores
• • Two protein complexes called kinetochores flank each side of the centromere
  • Each kinetochore is associated with one of the two sister chromatids and forms the site of attachment for a single spindle microtubule.
• This arrangement (Kinetochores) ensures that each sister chromatid is attached to a microtubule radiating from one of the poles of the cell.
• Metaphase

  • Spindle microtubules align chromosomes along a single plane in the middle of the cell
• Anaphase

  • Centromere splits apart and the spindle microtubules connected to kinetochores on each sister chromatid shorten, reeling each chromatid from the pair towards opposite poles of the cell.
• Anaphase

  • A complete set of 46 chromosomes (one set of 23 from mom, one set of 23 from dad) is now at each pole of the cell.
• Telophase

  • Spindle microtubules break down and nuclear membrane re-forms
  • Chromosomes decondense, marking the end of telophase and mitosis
• Cytokinesis (in animal cells)

  • Division of the cytoplasm and formation of genetically identical daughter cells
• Cytokinesis (in animal cells)

  • As mitosis ends, cytokinesis begins and parent cell divides into two identical daughter cells