Cellular Reproduction
How to preserve genetic information.
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.
Specialized cell membrane proteins signal cell division
when growth factors are present.
Growth factor
binds to receptor
and stimulates
cyclin synthesis.
growth factor
receptor
(plasma
membrane)
cyclin
cyclindependent
kinase
Cdk’s are always
present in the cell.
(cytoplasm)
Cyclin activates
Cdk; active Cdk
stimulates DNA
replication.
Mitosis
• One-celled eukaryotic organisms, and
individual cells in a multi-cellular
organisms, reproduce by mitosis
followed by cytokinesis.
The problem
Eukaryotic cells are
often diploid: that
is, they have two of
each kind of
chromosome.
• 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?
W
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A cell with 14 chromosomes divides by mitosis. How
many chromosomes will each daughter cell have?
25%
1.
2.
3.
4.
25%
25%
2
3
25%
28
14
7
Each daughter cell
will have a different,
unpredictable
number.
1
4
When is DNA replicated?
33%
33%
33%
1. Just before cell
division.
2. In preparation for
transcription and
translation.
3. Any time.
1
2
3
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.
Warning: Confusing terminology ahead!
After cell
Before cell division,
division, the
a strand of DNA is a During cell division,
single strand is a
chromosome. (Think two identical copies
chromosome
of it as a oneof a DNA strand link
again. (Again,
chromatid
together into a twothink of it as a
chromosome.)
chromatid
one-chromatid
chromosome.
chromosome.)
Normal cell
processes, including
protein synthesis
(transcription and
translation).
G1: cell
growth and
differentiation
G0: nondividing
G2: cell
growth
interphase
The cell prepares
for division, which
includes making
specialized proteins
(more transcription
and translation).
S: synthesis
of DNA;
chromosomes
are duplicated
Under certain
circumstances, cell
may return to
cell cycle.
DNA is replicated
ONLY during Sphase. Once
replication has
happened, the cell
MUST divide.
Animated cell cycle at http://cellsalive.com
Prior to Mitosis, DNA is replicated during the S-phase of
the cell cycle.
Chromosomes appear late in G2 phase, just prior to mitosis.
How many cells in this slide of an onion root tip are
actually dividing? How can you tell?
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?
DNA (2 nm diameter)
histone proteins
nucleosome: DNA wrapped
around histone proteins
(10 nm diameter)
coiled nucleosomes
(30 nm diameter)
protein
scaffold
chromosome:
coils gathered onto
protein scaffold
(200 nm diameter)
DNA coils
A strand (double helix) of DNA wraps around histone
proteins to form chromosomes. This protects DNA from
damage during cell division.
genes
centromere
telomeres
The structure of a condensed chromosome (before
pairing).
• 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.)
W
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gene 1
same alleles
gene 2
different 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.
sister chromatids
centromere
Identical (sister) chromatids pair up during Prophase,
and join at a pinched-in point called the centromere.
sister
chromatids
duplicated
chromosome
(2 DNA double
helices)
The chromosome at the end of Prophase consists of two
strands of condensed DNA. Each sister chromatid carries
exactly the same information.
Which of these are genetically
identical?
33%
33%
33%
1. Homologous
chromosomes
2. Sister chromatids
3. Neither of these
1
2
3
Which of these will pair up to
form chromosomes in mitosis?
33%
33%
33%
1. Homologous
chromosomes
2. Sister chromatids
3. Chromatids pair at
random, so it’s
impossible to
predict.
1
2
3
MITOSIS: Prophase
condensing
chromosomes
beginning of
spindle formation
Notice that these cells in prophase
have barely visible chromosomes as
DNA begins to condense.
MITOSIS: Metaphase
spindle
microtubules
Chromosomes, with their paired identical chromatids,
move to the center of the cell.
MITOSIS: Anaphase
"free" spindle
fibers
Identical chromatids separate from one another and migrate
to opposite poles of the cell.
MITOSIS: Telophase
chromosomes
nuclear envelope
extending
re-forming
Telophase completes Mitosis. Both poles of the cell now
have identical DNA, and the cell can divide in half.
MITOSIS:
Cytokinesis
After Mitosis has
finished sorting the
chromosomes,
cytokinesis takes
place, dividing the cell
into two new cells.
INTERPHASE
Before S phase, the
cell was diploid (two
copies of each
chromosome). After
cytokinesis, are the
cells diploid or
haploid?
The process of cytokinesis
1 Microfilaments form 2 The microfilament
a ring around the
ring contracts,
cell's equator.
pinching in the
cell's “waist.”
3 The waist
completely pinches
off, forming two
daughter cells.
Cytokinesis in plant cells
Golgi apparatus
cell wall
plasma
membrane
carbohydratefilled vesicles
1 Carbohydrate-filled
vesicles bud off the
Golgi apparatus and
move to the equator
of the cell.
2 Vesicles fuse to
form a new cell
wall (red) and
plasma membrane
(yellow) between
daughter cells.
3 Complete
separation of
daughter cells.
In which phase do
chromosomes condense?
25%
1.
2.
3.
4.
25%
25%
2
3
25%
Prophase
Metaphase
Anaphase
Telophase
1
4
In which phase do sister chromatids
separate from one another?
25%
1.
2.
3.
4.
25%
25%
2
3
25%
Prophase
Metaphase
Anaphase
Telophase
1
4
In which phase do chromosomes line up in the
middle of the cell?
25%
1.
2.
3.
4.
25%
25%
2
3
25%
Prophase
Metaphase
Anaphase
Telophase
1
4
Meiosis
• Meiosis is cell division that involves the
reduction of chromosomes in a cell.
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.
n
2n
meiotic
cell division
2n
2n
n
fertilization
diploid
parental
cells
haploid
gametes
diploid
fertilized
egg
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.
homologous
chromosomes
Meiosis is reduction
division. It begins with a
diploid cell and
produces haploid cells.
Why does it produce
four haploid cells?
sister
chromatids
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
G0: nondividing
G2: cell
growth
interphase
S: synthesis
of DNA;
chromosomes
are duplicated
Under certain
circumstances, cell
may return to
cell cycle.
MEIOSIS I
Homologous chromosomes
pair and cross over.
Homologous chromosomes
line up in pairs.
Homologous chromosomes
move to opposite poles.
paired homologous
chromosomes
chiasma
spindle
microtubule
(a) Prophase I
recombined
chromosomes
(b) Metaphase I
(c) Anaphase I
(d) Telophase I
First half of meiosis: separation of homologous
chromosomes.
Prophase I
Homologous chromosomes
pair and cross over.
paired homologous
chromosomes
chiasma
spindle
microtubule
Notice that four strands — maternal and paternal
chromosomes and their identical sister chromatids —
join into a single unit, called a tetrad.
protein strands
joining duplicated
chromosomes
direction of
“zipper”
formation
Protein strands “zip” the homologous chromosomes
together.
While in tetrads, homologous chromosomes often swap
ends, further mixing up genetic information.
recombination
enzymes
chiasma
chiasma
Recombination
enzymes bind
to the joined
chromosomes.
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 and protein
zippers leave.
chiasmata remain,
helping to hold
homologous
chromosomes
together.
Metaphase I
Homologous chromosomes
line up in pairs.
Tetrads line up in the center
of the cell.
recombined
chromosomes
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.
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.
Anaphase II
Metaphase II
In both cells,
chromosomes line
up in Metaphase II
so that sister
chromatids can
separate in
Anaphase II.
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.
In prophase I of meiosis:
1. Sister chromatids
pair up.
2. Homologous
chromatids pair up.
3. Homologous
chromosomes pair
up.
4. Nothing pairs up.
25%
1
25%
25%
2
3
25%
4
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.
33%
1
33%
2
33%
3
The end product of meiosis is:
25%
25%
25%
2
3
25%
1. Two identical cells.
2. Two non-identical
cells.
3. Four identical cells.
4. Four non-identical
cells.
1
4
Each cell produced by meiosis is:
25%
1.
2.
3.
4.
25%
25%
2
3
25%
Diploid
Haploid
Tetraploid
A random ploidy
1
4
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.