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Lecture 8

01:447:380 Lecture Notes - Lecture 8: Interphase, Mitosis, Meiosis

Course Code
R Michaelis

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Lecture 7: Cell cycle + mitosis + meiosis
I. Cell cycle and you
a. You began life as a fertilized egg
b. many rounds of replication and division to build your approx. 50 trillion-celled body
c. Cell differentiate as they replicate
d. You have several hundred different types of tissues in your body
II. Cell cycle
a. G1: Stockpile nucleotides, ATP, enzymes to replicate DNA
b. S: Replicate DNA
c. G2: Synthesize microtubules and other proteins needed for cell division, stockpile ATP
and enzymes for mitosis/meiosis
d. M: Mitosis or Meiosis
i. Good sisters Give massages
e. A cell goes into G0 if it senses a lack of growth factors, nutrients or other molecules
that are necessary for the cell cycle to continue
f. In G0, the cell stops replicating and dividing
III. During interphase: the cell checks several things about itself
a. G1:
i. Phosphorylation of certain proteins
1. Example; retinoblastoma (RM protein)

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ii. AT THE END of G1 the cell makes sure it has stockpiled all the enzymes it needs
to replicate DNA before it goes into S phase
b. G2
i. Cell must accumulate a certain level of a complex called the mitosis promoting
factor (MPF) and activate it by dephosphorylation to enter the M phase
ii. At the end of G2, the cell checks for DNA damage; the presence of DNA damage
keeps the MPF inactive and halts the cell in G2
iii. The cell also checks the length of the telomeres
1. The telomeres shorten every cell cycle, and telomere length is an
indicator of cell age (more on this later)
a. Girlfriend #2 is promoting herself with MPF before going into
Mom phase, checks if things are ok if not keeps the MPF (most
popular female) and stops the situation from going any further
c. M phase (metaphase):
i. Cell checks every chromosome is properly aligned at the equatorial plane
ii. Checks that every centromere has been contacted by a spindle fiber
1. Mom checks everything is in order
iii. If everything is aligned well: MPF is inactivated so the cell can finish mitosis
1. “Most popular female” – feeling - is shut down
iv. If the chromosomes/spindles have not aligned properly, this inhibits inactivation
of the MPF and keeps the cell in mitosis
d. If The Cell Cannot Fix A Problem: It Goes Into Apoptosis
i. The cell has mechanisms to fix some of the problems that cause the cell cycle to
ii. If the cell fails to correct the problems detected at any of its checkpoints, the
cell commits suicide, by turning on genes that induce apoptosis, or
programmed cell death
iii. The cell turns on genes that destroy the cellthe figure illustrates the process
called membrane blebbing : BLEB BLEB BLEB DEATHHHHHHH
i. Spindle fibers not yet organized
ii. Chromatin is decondensed
iii. No discernable chromosomes in the nucleus
iv. Nuclear membrane is intact
1. Chromatin condenses into chromosomes migrates to equatorial plate
a. Early stages it is just about becoming condensed
2. (S phase replication each chromosome has 2 sister chromatids)
3. Centrosomes are beginning to move toward opposite poles of the
dividing cell
1. Centrosomes move to the poles
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