9/15/2011
Genetics and Cellular Function
Mitosis?
Mitosis: division of cells that results
in daughter cells with the same
the genetic information that the
original cell had.
• DNA replication and the cell cycle
• Mitosis
46
46
46
Diploid 2n
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Diploid 2 n
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DNA Replication
DNA Replication and Cell Cycle
• before cell divides, it must must duplicate its
DNA (replication)
- so each new cell gets exact copy of DNA
• Recall Law of Complementary Base Pairing –
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DNA Replication
DNA Replication
• When cells divide, DNA replicates itself and identical
copies go to 2 daughter cells
• Helicases break hydrogen bonds to produce 2 free strands
of DNA
DNA Primase: Installs RNA primer
– DNA polymerase binds to primer and starts adding
complementary copies to original DNA strand
• Using A-T, C-G pairing rules
• Thus each copy is composed of 1 new strand and 1
old strand (called semiconservative replication)
• Original DNA sequence is preserved
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2. Primase sets up
short sequence of
RNA nucleotides
1. Helicase breaks
H bonds
3. DNA Polymerase
adds complimentary
nucleotides
4. RNA Primer is
replaced with DNA
nucleotides
5. DNA ligase joins
segments together
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DNA Replication
Primase
DNA template
Continuous
Incoming
nucleotides
Primase!!!!
Installs RNA
Primer
(e)
RNA primer
DNA
polymerase III
Parental DNA
(b)
RNA primer Newly made DNA
DNA polymerase
Discontinuous
Replication
fork
DNA
polymerase I
(a)
RNA primer
being replaced
by DNA
nucleotides
DNA helicase
Key
A
T
C
G
(c)
Gap in
replication
(d)
DNA ligase
Figure 4-7
4.14
Key:
Okazaki fragments
Completed
daughter strand
(a)
= Adenine
= Thymine
= Cytosine
= Guanine
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Replication
fork
DNA
polymerase III
Old
(template)
strand
Newly
made
strand
Leading
strand
New strand
forming
Lagging
strand
Old (template) strand
DNA of one
chromatid
DNA polymerase III
Errors and Mutations
• DNA polymerase does make mistakes
– But:
– checks new base pairs and tends to fix mistakes
– result is only 1 error per 1 billion bases replicated
• mutations - changes in DNA structure due to
replication errors or environmental factors (radiation,
viruses, chemicals)
– some mutations = no problem/some kill the cell, turn
it cancerous or cause genetic defects in future
generations
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Cell Cycle
Cell Cycle
• Interphase is subdivided
into:
– G1 - cell performs
normal physiological
roles
– S - DNA is replicated in
preparation for division
– G2 - chromatin
condenses prior to
division
• Most cells of body are in interphase - the
non-dividing stage of life cycle
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Madre
Padre
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Kinetochore
Centromere
Sister
chromatids
Homologous Chromosomes
Mitosis
Mitosis
1
• cell division in all body cells except the eggs and
sperm
Prophase
Chromosomes condense
and nuclear envelope
breaks down. Spindle
fibers grow from centrioles.
Centrioles migrate to
opposite poles of cell.
Aster
• Functions of mitosis
2
– growth of all tissues and organs after birth
– replacement of cells that die
– repair of damaged tissues
Metaphase
Chromosomes lie along midline
of cell. Some spindle fibers
attach to kinetochores.
Fibers of aster attach
to plasma membrane.
Spindle fibers
Centriole
3
• 4 phases of mitosis
Chromatids
– prophase, metaphase, anaphase, telophase
Kinetochore
Cleavage furrow
Anaphase
Centromeres divide in two.
Spindle fibers pull sister
chromatids to opposite poles
of cell. Each pole (future
daughter cell) now has an
identical set of genes.
Telophase
Chromosomes gather
4 at each pole of cell.
Chromatin decondenses.
New nuclear envelope
appears at each pole.
New nucleoli appear
in each nucleus.
Mitotic spindle
vanishes.
Daughter
cells in
interphase
Figure 4.16
Nuclear envelope
re-forming
Chromatin
Nucleolus
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Mitosis: Prophase
Mitosis: Prophase
• chromosomes shorten and thicken coiling into compact rods
• 46 chromosomes
– two chromatids per chromosome
– one molecule of DNA in each chromatid
• nuclear envelope disintegrates and releases chromosomes into the
cytosol
1 Prophase
Chromosomes condense
and nuclear envelope
breaks down. Spindle
fibers grow from centrioles. 1
Centrioles migrate to
opposite poles of cell.
• centrioles sprout elongated microtubules – spindle fibers
– push centrioles apart as they grow
– pair of centrioles lie at each pole of the cell
• some spindle fibers grow toward chromosomes and attach to the
kinetochore on each side of the centromere
• spindle fibers push chromosomes to line up along the midline of cell
Figure 4.16 (1)
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Mitosis: Metaphase
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Mitosis: Anaphase
3
Aster
2
3
2
Metaphase
Chromosomes lie along
midlineof cell. Some spindle fibers
attach to kinetochores.
Fibers of aster attach
to plasma membrane.
Anaphase
Centromeres divide in two.
Spindle fibers pull sister
chromatids to opposite poles
of cell. Each pole (future
daughter cell) now has an
identical set of genes.
Chromatids
Kinetochore
Spindle fibers
Figure 4.16 (2)
Figure 4.16 (3)
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© Ed Reschke
Mitosis: Telophase
Cytokinesis
• cytokinesis – the division of cytoplasm into two
cells
• chromatids cluster on each side of the cell
• rough ER produces new nuclear envelope
around each cluster
– telophase is the end of nuclear division but overlaps
cytokinesis
– early traces of cytokinesis visible in anaphase
• chromatids begin to uncoil and form chromatin
• achieved by motor protein myosin pulling on
microfilaments of actin in the terminal web of
cytoskeleton
• mitotic spindle breaks up and vanishes
• each nucleus forms nucleoli
– indicating it has already begun making RNA and
preparing for protein synthesis
• creates the cleavage furrow around the equator
of cell
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• cell eventually pinches in two
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Figure 3.32: The stages of mitosis, p. 102.
Centrioles
(two pairs)
Condensed Early mitotic
chromatin spindle
Figure 3.32: The stages of mitosis (continued), p. 103.
Fragments of
Pair of
centrioles nuclear
envelope
Metaphase
plate
Polar
microtubules
Contractile
ring at
cleavage
furrow
Nucleolus
forming
Aster
Nucleolus
Nuclear
envelope
Interphase
Plasma
Chromosome, consisting
membrane of two sister chromatids
Early prophase
Kinetochore
Centromere
Kinetochore
microtubule
Late prophase
Spindle
pole
Spindle
Metaphase
Daughter
chromosomes
Anaphase
Nuclear
envelope
forming
Telophase and cytokinesis
Timing of Cell Division
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Cells divide when:
• they have enough cytoplasm for two daughter cells
• they have replicated their DNA
• adequate supply of nutrients
• are stimulated by growth factor
– chemical signals secreted by blood platelets, kidney
cells, and other sources
• neighboring cells die, opening space in a tissue
Cells stop dividing when:
• snugly contact neighboring cells
• when nutrients or growth factors are withdrawn
• contact inhibition – the cessation of cell division in
response to contact with other cells
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