Cellular Reproduction

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Cellular Reproduction

• How to preserve genetic information.

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When and why do cells divide?

• Cells divide when there is a chemical signal to
  do so.
• Skin cells may divide in response to crowding.
  Certain cells send out a chemical signal that
  tells neighboring cells to divide.
• Cells may divide in response to an injury, to
  mend damaged tissue.
• Growth factors can signal cell division in
  children to lengthen bones and add other tissues.

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Specialized cell membrane proteins signal cell
division when growth factors are present.
growth factor
Growth factor binds to receptor and
stimulates cyclin synthesis.
growth factor receptor
(plasma membrane)
cyclin
Cyclin activates Cdk active Cdk stimulates
DNA replication.
cyclin- dependent kinase
(cytoplasm)
Cdks are always present in the cell.
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Cancers begin when something goes wrong with the
system controlling cell growth and division.
Normal G1 to S control
Mutated growth factor receptor gene
Mutated cyclin gene
growth factors receptor
growth factors mutated receptor always on
growth factors receptor
cyclin synthesis Cdk
cyclin synthesis Cdk
cyclin synthesis always on Cdk
phosphorylates Rb
phosphorylates Rb
phosphorylates Rb
Rb
Rb
Rb
P
P
P
DNA replication
uncontrolled DNA replication
uncontrolled DNA replication
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Binary Fission

• Bacteria and other prokaryotes reproduce by
  simple binary fission.
• The single ring-shaped chromosome is duplicated,
  and the cell divides in half.

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cell division by binary fission
cell growth and DNA replication
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1
3
attachment site
cell wall
plasma membrane
circular DNA
New plasma membrane is added between the
attachment points, pushing them further apart.
The circular DNA double helix is attached to the
plasma membrane at one point.
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4
The plasma membrane grows inward at the middle of
the cell.
The DNA replicates and the two DNA double helices
attach to the plasma membrane at nearby points.
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The parent cell divides into two daughter cells.
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Mitosis

• One-celled eukaryotic organisms, and individual
  cells in a multi-cellular organisms, reproduce by
  mitosis followed by cytokinesis.

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The problem
Eukaryotic cells are often diploid that is, they
have two of each kind of chromosome.
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W O R K T O G E T H E R

• Why do eukaryotic cells have TWO of each
  chromosome?
• If a cell with two of each chromosome divides,
  how many chromosomes should end up in each of the
  two daughter cells?

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A cell with 14 chromosomes divides by mitosis.
How many chromosomes will each daughter cell have?

1. 28
2. 14
3. 7
4. Each daughter cell will have a different,
   unpredictable number.

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When is DNA replicated?

1. Just before cell division.
2. In preparation for transcription and translation.
3. Any time.

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Overview of Mitosis

• After DNA is replicated, it is condensed into
  chromosomes and identical copies are sorted in
  the process of mitosis.
• Mitosis assures that the two daughter cells have
  exactly the same DNA.

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Warning Confusing terminology ahead!
After cell division, the single strand is a
chromosome again. (Again, think of it as a
one-chromatid chromosome.)
Before cell division, a strand of DNA is a
chromosome. (Think of it as a one-chromatid
chromosome.)
During cell division, two identical copies of a
DNA strand link together into a two-chromatid
chromosome.
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telophase and cytokinesis
G1 cell growth and differentiation
anaphase
metaphase
prophase
mitotic cell
division
G0 nondividing
G2 cell growth
Under certain circumstances, cell may return
to cell cycle.
interphase
S synthesis of DNA chromosomes are duplicated
Animated cell cycle at http//cellsalive.com
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Prior to Mitosis, DNA is replicated during the
S-phase of the cell cycle.
Chromosomes appear late in G2 phase, just prior
to mitosis.
If you wanted to count the onion root tip cells
in this picture that are in mitosis, what feature
would be in the cell that tells you they are in
mitosis?
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INTERPHASE
nuclear envelope
chromatin
nucleolus
centriole pairs
Late Interphase
Can we tell if a cell in Interphase is in G1, S,
or G2 of the cell cycle?
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DNA (2 nm diameter)
histone proteins
nucleosome DNA wrapped around histone
proteins (10 nm diameter)
coiled nucleosomes (30 nm diameter)
chromosome coils gathered onto protein
scaffold (200 nm diameter)
protein scaffold
DNA coils
A strand (double helix) of DNA wraps around
histone proteins to form chromosomes. This
protects DNA from damage during cell division.
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genes
centromere
telomeres
The structure of a condensed chromosome (before
pairing).
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W O R K T O G E T H E R

• Make a prediction during chromosome formation,
  which will pair up?
• Homologous chromatids that carry the same genes
  but different alleles?
• Sister chromatids that carry the same genes and
  the same exact alleles?
• (Hint the end goal is to get identical genetic
  information into the daughter cells.)

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gene 1
gene 2
different alleles
same alleles
Homologous chromosomes are those that carry the
same genes but may have slightly different
information (such as dominant or recessive
versions of a gene). Homologous chromosomes do
not pair together. Chromosomes only pair with
their identical sister chromatids.
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sister chromatids
centromere
Identical (sister) chromatids pair up during
Prophase, and join at a pinched-in point called
the centromere.
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duplicated chromosome (2 DNA double helices)
sister chromatids
The chromosome at the end of Prophase consists of
two strands of condensed DNA. Each sister
chromatid carries exactly the same information.
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Which of these are genetically identical?

1. Homologous chromosomes
2. Sister chromatids
3. Neither of these

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Which of these will pair up to form chromosomes
in mitosis?

1. Homologous chromosomes
2. Sister chromatids
3. Chromatids pair at random, so its impossible to
   predict.

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MITOSIS Early Prophase
condensing chromosomes
beginning of spindle formation
Notice that these cells in prophase have barely
visible chromosomes as DNA begins to condense.
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W O R K T O G E T H E R

• In prophase, the identical chromatids are paired
  into chromosomes. The end goal is to get each
  identical chromatid to opposite ends of the cell.
  Where should the cell place the chromosomes now
  to achieve that goal?

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MITOSIS Late Prophase
pole
kinetochore
pole
As prophase progresses, the chromosomes become
more and more visible as they condense.
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MITOSIS Metaphase
spindle microtubules
Chromosomes, with their paired identical
chromatids, move to the center of the cell.
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W O R K T O G E T H E R

• In metaphase, the chromosomes are lined up in the
  middle of the cell. Predict what has to happen
  next to get the chromatids to opposite poles of
  the cell.

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MITOSIS Anaphase
"free" spindle fibers
Identical chromatids separate from one another
and migrate to opposite poles of the cell.
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MITOSIS Telophase
nuclear envelope re-forming
chromosomes extending
Telophase completes Mitosis. Both poles of the
cell now have identical DNA, and the cell can
divide in half.
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MITOSIS Cytokinesis
After Mitosis has finished sorting the
chromosomes, cytokinesis takes place, dividing
the cell into two new cells.
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INTERPHASE
Before S phase, the cell was diploid (two copies
of each chromosome). After cytokinesis, are the
cells diploid or haploid?
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W O R K T O G E T H E R

• Predict How does cytokinesis differ between
  animal and plant cells?

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The process of cytokinesis
2 The microfilament ring contracts, pinching in
the cell's waist.
1 Microfilaments form a ring around the cell's
equator.
3 The waist completely pinches off, forming
two daughter cells.
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Cytokinesis in plant cells
Golgi apparatus
cell wall
plasma membrane
carbohydrate- filled vesicles
1 Carbohydrate-filled vesicles bud off the Golgi
apparatus and move to the equator of the cell.
3 Complete separation of daughter cells.
2 Vesicles fuse to form a new cell wall (red)
and plasma membrane (yellow) between daughter
cells.
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In which phase do chromosomes condense?

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

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In which phase do sister chromatids separate from
one another?

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

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In which phase do chromosomes line up in the
middle of the cell?

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

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Meiosis

• Meiosis is cell division that involves the
  reduction of chromosomes in a cell.

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The problem

• When diploid organisms reproduce sexually, two
  cells must fuse and share genetic information.
• The end result of sexual reproduction is a new
  diploid organism that has genetic information
  from both parents.

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W O R K T O G E T H E R

• How can two cells from diploid parents fuse to
  make a diploid offspring?

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2n
n
meiotic cell division
2n
2n
n
fertilization
haploid gametes
diploid fertilized egg
diploid parental cells
The cells from the parents must be haploid if
their offspring is to be diploid.
While diploid cells hold two copies of each
chromosome (one from each parent), haploid sex
cells hold one copy of each chromosome.
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sister chromatids
Meiosis is reduction division. It begins with a
diploid cell and produces haploid cells. Why
does it produce four haploid cells?
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Meiosis also involves the cell cycle, and takes
place after S phase of the cell cycle. DNA is
replicated before meiosis.
G1 cell growth and differentiation
mitotic cell
division
G0 nondividing
G2 cell growth
Under certain circumstances, cell may return
to cell cycle.
interphase
S synthesis of DNA chromosomes are duplicated
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MEIOSIS I
Homologous chromosomes move to opposite poles.
Homologous chromosomes pair and cross over.
Homologous chromosomes line up in pairs.
paired homologous chromosomes
recombined chromosomes
chiasma
spindle microtubule
(a) Prophase I
(b) Metaphase I
(c) Anaphase I
(d) Telophase I
First half of meiosis separation of homologous
chromosomes.
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Prophase I
Homologous chromosomes pair and cross over.
paired homologous chromosomes
spindle microtubule
chiasma
Notice that four strands maternal and paternal
chromosomes and their identical sister chromatids
join into a single unit, called a tetrad.
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protein strands joining duplicated chromosomes
direction of zipper formation
Protein strands zip the homologous
chromosomes together.
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While in tetrads, homologous chromosomes often
swap ends, further mixing up genetic information.
recombination enzymes
chiasma
chiasma
Recombination enzymes snip chromatids apart and
reattach the free ends. Chiasmata (the sites of
crossing over) form when one end of the paternal
chromatid (yellow) attaches to the other end of a
maternal chromatid (purple).
Recombination enzymes bind to the
joined chromosomes.
Recombination enzymes and protein zippers
leave. chiasmata remain, helping to
hold homologous chromosomes together.
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Metaphase I
Homologous chromosomes line up in pairs.
Tetrads line up in the center of the cell.
recombined chromosomes
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Comparing Metaphase of Mitosis with Metaphase I
of Meiosis
duplicated chromosomes
MEIOSIS I Homologous chromosomes are paired.
Each pair of chromatids has a single functional
kinetochore.
MITOSIS Homologous chromosomes are not paired.
Each chromatid has a functional kinetochore.
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Anaphase I
Homologous chromosomes move to opposite poles.
Because homologous chromosomes separate (instead
of identical sister chromatids), each pole of the
cell gets a full set of chromosomes but different
genetic information.
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W O R K T O G E T H E R

• Stop and think
• Why is tetrad formation important to reaching the
  end goal of meiosis?
• How is crossing-over directly related to tetrad
  formation?

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MEIOSIS II
(e) Prophase II
(f) Metaphase II
(g) Anaphase II
(h) Telophase II
(i) Four haploid cells
Meiosis II begins immediately after Meiosis I,
with a short rest in between (no interphase in
between). In Meiosis II, sister chromatids
separate from one another.
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Anaphase II
Metaphase II
In both cells, chromosomes line up in Metaphase
II so that sister chromatids can separate in
Anaphase II.
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Telophase II
End
The result of meiosis is four haploid cells. Each
has one copy of each chromosome, which may carry
different versions of the same genes. Each gamete
(sex cell) can have different genetic information.
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In prophase I of meiosis

1. Sister chromatids pair up.
2. Homologous chromatids pair up.
3. Homologous chromosomes pair up.
4. Nothing pairs up.

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In anaphase I of meiosis

1. Tetrads split, homologous chromosomes migrate to
   opposite poles.
2. All four chromatids migrate to four quarters of
   the cell.
3. Chromatids are randomly distributed.

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The end product of meiosis is

1. Two identical cells.
2. Two non-identical cells.
3. Four identical cells.
4. Four non-identical cells.

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Each cell produced by meiosis is

1. Diploid
2. Haploid
3. Tetraploid
4. A random ploidy

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W O R K T O G E T H E R

• How is Meiosis II similar to Mitosis?
• How is it different?

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Recap

• Mitosis divides one diploid cell and produces two
  diploid daughter cells. It is cell division used
  for growth and cell replacement.
• Meiosis divides one diploid cell into four
  haploid cells. It is used in reproduction.