Mitosis and the cell cycle

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Mitosis and the cell cycle Biology 130 Lecture 6
In cell theory we say that cells come from other
cells. This lecture is about how cells reproduce.
It is not about reproduction of complex,
multicellular organisms, nor is it about sex,
recombination of genes etc.
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Life cycle versus cell cycle
Multicellular individual
Syngamy or fertilization
1N
Meiosis
Zygote
Multicellular individual
2N
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• Mitosis is the replication of the nucleus of
  eukaryotic cells
• Cytokinesis is the division of the cell into two
• Together mitosis and cytokinesis can make a
  complex multicellular organism

Multicellular individual
1N
Syngamy or fertilization
Meiosis
Zygote
Multicellular individual
2N
Meiosis is the reduction event that occurs during
alternation of generations
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Cell reproduction

• Cell reproduction in prokaryotes
• Cell reproduction in eukaryotes
• Cell cycle
• Mitosis (reproduction of the nucleus)
• Cytokinesis (reproduction of the rest of the
  cell)
• The emphasis in this lecture is on cell
  reproduction. This may or may not involve
  reproduction of the organism.

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Cell reproduction in prokaryotes - Binary Fission

• Simple process
• Attach circular chromosome to cell membrane
• Duplicate the chromosome
• Make sure the duplicates are moved into new
  developing cells
• Fission of the cell into two

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Cytokinesis is the division of the cell into two
Mitosis is the division of the nucleus into two
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Cell Cycle
M
G1
DNA X 1
G2
DNA X 2
During the S phase, the cells DNA is reproduced
in preparation for cell duplication
S
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Cell Cycle
M
G1
DNA X 1
G2
DNA X 2
How can you make more DNA without making more
chromosomes?
This reproduction of the DNA is not a doubling
of the number of chromosomes, the number of
chromosomes stays the same.
S
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Chromosome nomenclature
Long arm
One chromosome
One chromatid
Centromere
Short arm
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Chromosomes
During S phase, the chromosomes are replicated
One chromosome (replicated)
Two chromatids
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Chromosomes
In diploid phases of life cycles, there are pairs
of chromosomes. The members of each pair are
called homologous chromosomes.
Long arm
Two chromosomes
One chromatid per chromosome
Centromere
Short arm
Second pair of homologous chromosomes, one
chromatid each
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Chromosomes
During S phase of the cell cycle, all chromosomes
are replicated
Two chromatids per chromosome
Two chromosomes, four chromatids
Second pair of homologous chromosomes, two
chromatids each
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The stages of cell division
The orientation of the division plane is
determined by a band of cytoskeletal elements
called the preprophase band.
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The stages of cell division
The nucleus migrates to the middle of the cell. A
phragmosome (layer of cytoplasm) forms in the
plane where cell division will occur.
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The stages of cell division
Chromosomes become visible, nucleus breaks down.
This is prophase
Microtubules form a mitotic spindle. Animals
have centrioles, plants do not. This is
metaphase
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The stages of cell division
Metaphase
During mitosis, the homologous chromosomes do
not come together in pairs. Each chromosome
behaves independently, regardless of the
action of the homolog.
Anaphase
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The stages of cell division
Telophase
During telophase the nucleus reforms, the
chromosomes become indistinct.
Late Telophase
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The stages of cell division
Telophase
Mitosis is normally associated with cytokinesis
A cell plate forms at the phragmoplast.
Late Telophase
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The stages of cell division
Interphase
G1
G2
The cell will expand during G1 and the
chromosomes will be replicated during the S
phase.
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Stages of Mitosis (PMAT)

• Prophase
• Chromosomes become distinct
• Centrosome divides and migrates to opposite ends
  of the cell
• Nuclear envelope breaks down
• Metaphase
• Chromosomes line up on the cell metaphase plate
• Anaphase
• Telophase

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Stages of Mitosis (PMAT)

• Prophase
• Metaphase
• Anaphase
• Chromosomes crawl along the microtubules of the
  mitotic spindle
• Telophase
• Chromosomes become less distinct
• Nucleolus and nuclear envelope reappear (are
  formed?)

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Figure 12.7 Testing a hypothesis for chromosome
migration during anaphase
Experiment to test whether chromosomes crawl or
are pulled to opposite ends of the cell
undergoing mitosis
Mitosis is usually followed by cytokinesis
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Figure 12.8 Cytokinesis in animal and plant cells
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Cytokinesis

• By cleavage in animals
• By building a new wall at a cell plate that forms
  between the two halves of the cell
• A band of microtubules that can be seen before
  prophase (preprophase band) determines where the
  new wall will be built
• If cytokinesis does not occur, multinucleate
  cells result, can occur in animals, plants, and
  fungi
• Common in fungi
• Coenocytic fungi can have thousands of nuclei in
  one long cell

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Figure 12.11 A hypothesis for the evolution of
mitosis
There are variations on the mitosis theme that
can be seen in some algae
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Cell Cycle
M
G1
G2
Checkpoints
Most human cells have exited the cell cycle and
are considered to be in a G0 state. This helps
control cells so they do not become cancerous
G0
S
Plants can also get cancer
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Cell Cycle Control

• Because cancer is a failure of cell cycle
  control, there is substantial research on this
  topic
• A protein called cyclin is important in cell
  cycle regulation
• Interacts with a cyclin dependent kinase to
  signal when cells should move past certain
  checkpoints
• Also important are
• Density-dependent inhibition
• Anchorage dependence for cell division

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Figure 12.14 Molecular control of the cell cycle
at the G2 checkpoint
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Apoptosis - When cells commit suicide

• One of the ways the body prevents cancer is to
  send a signal for cancerous cells to commit
  suicide
• This signal can be stimulated by a protein called
  p53. Cancer nearly always involves loss of
  function of p53.
• Among other things, apoptosis signals result in
  release of cytochrome c from mitochondria