BIOL 1201 Lecture Notes - Lecture 4: Spindle Apparatus, Dna Replication, Sister Chromatids

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Biology Exam 4 Notes
Cell Cycle
The life of a cell
o Represented as a cycle circle
Stages of Cell Cycle
o Starting point is cytokinesis
The end of cell division the end of the life of one cell and the
beginning of the life of a new cell
o The first stage after cytokinesis in a cell is G1
G stands for gap so also known as Gap 1
o After G1, the next stage is S
S stands for Synthesis - stood for the synthesis of DNA which is now
referred to as DNA replication
The S phase is when we have replication so the DNA is going to
replicate during the S phase
o After S, the next phase is G2 (Gap 2)
o Finally there is Cell Division
Cell division ends with the division of the cytoplasm Cytokinesis
And then you go back into G1 and restart the cycle
Interphase
o Everything from the beginning of G1 all the way around to the end of G2
o After interphase, there is some form of cell division
What determines when a cell divides?
When in the cell cycle, in G1 there is a signal (checkpoint)
o The cell can either pass the signal or get stopped at
that point and go to G0
o If goes to G0 (G naught), the cell will not divide and
will stay there until it dies
o Start at beginning in G1
In interphase
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Go into S where DNA is synthesized and then go into G2 and
then go into cell division
In this case cell division is mitosis but it could also be meiosis
Components of Cell Division Regulation
Cyclin
o A protein that varies in concentration depending on the cell cycle
o As we go around the cell cycle, the amount of cyclin accumulates
o G1: Little or no cyclin present at all
o S: low cyclin by the end of the S phase
o G2: relatively high levels of cyclin
o At the end of cytokinesis, all of the cyclin is destroyed
o How fast cyclin accumulates determines the rate of cell division
Lot of cyclin fast cell division
Not a lot slow cell division
CDK: Cyclin Dependent Kinase
o Remains in constant concentration
Always present
Needs cyclin
o Lot of cyclin joins with CDK creates MPF molecule
MPF: Maturation Promoting Factor
o Cyclin and CDK together form this
o The presence of this factor triggers the cell to undergo mitosis
o Presence of MPF triggers cell division
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Chromosome Structure
In eukaryotes, the chromosomes are composed of DNA and histone proteins
o DNA is wrapped on histone proteins
During interphase, the DNA is relaxed or extended
During cell division, the DNA is supercoiled and condensed
o All coiled up in a tight structure chromosomes we think of are actually
chromosomes during cell division
Unreplicated chromosomes: chromosome can be made of 1 molecule of DNA
Replicated chromosomes: chromosome can be made of 2 identical molecules of DNA
o Composition of replicated chromosome
2 sister chromatids
Centromere in the middle
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