The Cell Cycle

1
The Cell Cycle

• Chapter 12

2
The cell cycle

• Why do cells need to divide??
• Growth (did you know you started as a cell
  smaller than a period on the end of a sentence?)
• Reproduction for unicellular organisms
• Repair
• Replacing old/dead cells

3
The Cell Cycle

• The life of a cell from its origin to division
  into two new daughter cells

4
What needs to be passed on to daughter cell?

• DNA new cell needs an exact copy of the genetic
  material. Why?
• Organelles and cytoplasm the new cell needs all
  of the organelles to function

5
Copying DNA

• An important step in cell division, then, is
  making an exact copy of the cells DNA
• Splitting that DNA evenly so each new cell has
  identical copies
• A typical human cell has 2-3 meters of DNA
  (250,000 times greater than cells diameter)
• 3 billion base pairs
• Error rate 1 per 100 million bases

6
DNA review

• Really close up DNA has this structure

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DNA is organized into chromosomes

• Double helix DNA molecule
• Wrapped around proteins called Histones
• Super-coiled (tightly wrapped) DNA and protein
  complex is called chromatin

8
Preparing to divide

• After DNA duplication, chromatin condenses
• It is coiled and folded to make a smaller package
• These are called chromosomes

9
DNA to chromosomes

• Once DNA is this compact, cell is ready for
  Mitosis (dividing)

10
Chromosome vocabulary

• A duplicated chromosome (looks kind of like an x)
  consists of
• 2 sister chromatids
• Narrow at their centromeres
• Identical copies of the chromosomes DNA

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• These are sets of two sister chromatids
• Why is there two of everything?
• Male or female?

12
Cell Cycle

• Phases of a cells life
• Interphase
• Cell grows
• Replicates chromsomes
• Mitotic phase
• Cell divides chromsomes
• Mitosis
• Cell divides cytoplasm and organelles
• Cytokinesis

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Interphase

• Interphase can be divided into subphases
• G1 phase (gap 1)
• Cells increase in size, make new organelles and
  proteins
• G1 checkpoint determines cell is ready to go to
  next phase
• S phase (DNA synthesis)
• DNA replication takes place
• G2 phase (gap 2)
• Cell continues to grow, make new organelles and
  proteins
• G2 checkpoint before Mitosis

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• Interphase is 90 of cell life cycle

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G2 the end of Interphase
Chromsomes blue, microtubulesgreen, filaments red

• Nucleus is well defined
• Chromsomes are duplicated
• DNA is loosly packaged in long chromatin fibers
• Prepares for Mitosis
• Produces proteins and organelles
• Centrosomes/centriole formed

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Mitosis

• The second part of the cell cycle is Mitosis
  dividing the cell and its contents in half.
• Mitosis
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase

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Prophase

• Chromatin condenses
• Chromosomes visible with light microscope!
• Fibers extend from centromeres/centrioles
• Centrioles move to opposite poles of cell
• Fibers (microtublues) cross cell to form mitotic
  spindle
• Phase ends with breakdown of nuclear membrane

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Prometaphase

• Proteins attach to centromeres creating
  kinetochores
• Microtubules attach at kinetochores, at
  centromere (center) of chromsomes
• Chromosomes begin moving

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The mitotic spindle

• Is an apparatus of microtubules that controls
  chromosome movement during mitosis
• Attaches to the chromosome through a kinetochore
  proteins attached to the chromosome at its center

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Metaphase

• Spindle fibers align chromosomes along the middle
  of the cell
• Meta middle
• Helps to ensure chromosomes separate properly
• So each new nucleus receives exactly one copy of
  each chromosome

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Anaphase

• Sister chromatids separate at kinetochores
• Move to opposite poles
• Pulled at centromeres
• Kinetochore microtubles shorten
• At the end, the two ends of the cell have
  equivalent and complete sets of chromsomes

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Telophase

• Chromsomes arrive at opposite poles
• Nucleus on each reforms
• Chromsomes disperse
• Spindle fibers disperse
• Last stage of Mitosis, Cytokinesis begins

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Cytokinesis

• Animals
• Cleavage furrow forms
• Tightens to split cell in to two
• Like tightening a draw string
• Cell splits into two daughter cells

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Cytokinesis

• Plants
• Vesicles move to equator line, line up to fuse
  and form 2 membranes cell plate
• New cell wall is laid down between the membranes
• New cell wall fuses with existing cell wall

25

• Cell division animation
• Cell division video

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Evolution of Mitosis

• Mitosis in eukaryotes likely evolved from binary
  fission in bacteria
• Intermediates between these two processes exist.