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VIII. CELL REPRODUCTION
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
Reproduction is selected for,
by definition.
X
X
X
X
- WHY??
Bigger is better….
As cells increase in size, they decrease in efficiency.
- WHY??
Bigger is better….
So selection favors growth…
But as cells increase in size, they decrease in efficiency.
SA/V = 6
SA/V = 3
SA/V = 1.5
The “surface area to volume ratio” decreases as something
increases in size….
SA/V = 6
SA/V = 3
SA/V = 1.5
The “surface area to volume ratio” decreases as something
increases in size….
The surface area – the membrane – limits the rate of supply of
nutrients to the cell.
The volume – where all the enzymes are – represents potential
production and ‘demand’ for nutrients.
SA/V = 6
SA/V = 3
SA/V = 1.5
So, as something gets larger, the volume increases more than the
surface area… and the demand for nutrients (to meet peak
productivity) grows faster than the rate at which the more slowly
increasing SA can supply them. So, supply fails to meet demand,
and the cell cannot meet peak productivity… it becomes less
efficient.
- WHY??
Bigger is better….
So selection favors growth…
But as cells increase in size, they decrease in efficiency.
So, to get bigger, most organisms increase cell number, not cell
size….
- WHY??
And with many cells, cell specialization can increase efficiency at
the organismal level.
LE 12-5
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
INTERPHASE
G1
S
(DNA synthesis)
G2
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
1. Interphase:
a. G1: high metabolic activity (protein synthesis)
chromosomes diffuse; one DNA double helix per chromosome
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
1. Interphase:
Some cell types are "stuck" in this stage when they mature... it is
only "stem cells" that keep dividing. In some tissues, all stem cells
eventually mature, so the tissue can't regenerate (neurons)
LE 12-15
G0
G1 checkpoint
G1
If a cell receives a go-ahead
signal at the G1 checkpoint,
the cell continues on in the
cell cycle.
G1
If a cell does not receive a
go-ahead signal at the G1
checkpoint, the cell exits the
cell cycle and goes into G0, a
nondividing state.
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
1. Interphase
a. G1
b. S
Chromosome
duplication
(including DNA
synthesis)
Centromere
Sister chromatids
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
1. Interphase
a. G1
b. S
c. G2
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
1. Interphase
2. Mitosis
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
- Replisomes land at "origin"
- They have a variety of enzymes that work together,
including:
HELICASE that unzips the double helix.
PRIMASE, an RNA POLYMERASE, that starts replication
DNA POLYMERASES that make DNA.
- HELICASE separates strands
5'
3'
3'
5'
- HELICASE separates strands
- PRIMASE lays down RNA from 5'
3' (like transcription)
3'
5'
3'
3'
5'
3'
- HELICASE separates strands
- PRIMASE lays down RNA from 5'
3' (like transcription)
- DNA POLYMERASE - 'knocks' primase off and adds DNA bases to
the free 3' OH group....
5'
3'
3'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
So, into each 'replication fork',there is "continuous
synthesis" on one strand
5'
3'
"FORK"
"FORK"
3'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
What about the other strands?
5'
3'
3'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
What about the other strands? PROBLEM!!!
Only 5' phosphates available.....so they can't be extended
5'
5'
3'
3'
5'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
The process must start again....
5'
5'
3'
Primase makes RNA
Primase makes RNA
3'
3'
3'
5'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
The process must start again....
DNA POLY adds DNA
5'
5'
3'
3'
DNA POLY adds DNA
5'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
And what happens when more DNA is opened up????
5'
5'
3'
3'
5'
3'
5'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
Continuous synthesis can continue from 3' end of DNA
5'
5'
3'
3'
5'
3'
5'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
But Primase must begin the process on "lagging" strand
5'
5'
3'
3'
5'
3'
5'
5'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
...then DNA POLY can add DNA bases to "primer"
5'
5'
3'
3'
5'
3'
5'
5'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
And this happens at the other fork...... DNA is opened...
3'
5'
5'
3'
3'
5'
5'
5'
3'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
Continuous synthesis occurs from the free 3' end
3'
5'
5'
3'
3'
5'
5'
5'
3'
3'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
Discontinuous synthesis on "lagging strand" (Primase, etc)
3'
5'
5'
3'
3'
5'
5'
5'
5'
3'
3'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
So, BOTH STRAND HAVE "OKAZAKI FRAGMENTS...."
3'
5'
5'
3'
3'
5'
5'
5'
5'
5'
3'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
NOW WHAT??? We have RNA in the DNA double helix
3'
5'
5'
3'
3'
5'
5'
5'
5'
5'
3'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
Repair DNA Polymerases cut RNA out and add DNA to 3'
3'
5'
3'
3'
3'
3'
3'
3'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
3. DNA Repair
Repair DNA Polymerases cut RNA out and add DNA to 3'
3'
5'
3'
3'
3'
3'
3'
3'
5'
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
3. DNA Repair
Final linkage between DNA frag's made by LIGASE
3'
5'
3'
3'
3'
5'
3'
5'
3'
5' 3'
5'
H2O
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
3. DNA Repair
Final linkage between DNA frag's made by LIGASE
3'
5'
3'
3'
3'
5'
5'
3'
5'
3'
REPLICATION COMPLETE
H2O
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
1. Initiation
2. Replication at the fork
3. DNA Repair
Semi-conservative replication
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
C. Mitosis
LE 12-6aa
INTERPHASE
Centrosomes
(with centriole pairs
Chromatin
(duplicated)
PROPHASE
Early mitotic
spindle
PROMETAPHASE
Aster
Centromere
Nucleus
Nuclear
envelope
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Fragments
of nuclear
envelope
Kinetochore
Nonkinetochore
microtubules
Kinetochore
microtubule
LE 12-6ba
METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Centrosome at
one spindle pole
TELOPHASE
Cleavage
furrow
Daughter
chromosomes
Nuclear
envelope
forming
Nucleolus
forming
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
C. Mitosis
G2 OF INTERPHASE
PROPHASE
PROMETAPHASE
LE 12-6da
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
C. Mitosis
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
LE 12-10
VIII. CELL REPRODUCTION
Overview: Why Reproduce?
A. The Cell Cycle
B. DNA Replication
C. Mitosis
Nucleus
Chromatin
condensing
10 µm
Chromosomes
Nucleolus
Prophase. The
chromatin is condensing.
The nucleolus is
beginning to disappear.
Although not yet visible
in the micrograph, the
mitotic spindle is starting
to form.
Cell plate
Prometaphase. We
now see discrete
chromosomes; each
consists of two identical
sister chromatids. Later
in prometaphase, the
nuclear envelope will
fragment.
Metaphase. The spindle is
complete, and the
chromosomes, attached
to microtubules at their
kinetochores, are all at
the metaphase plate.
Anaphase. The
chromatids of each
chromosome have
separated, and the
daughter chromosomes
are moving to the ends of
the cell as their
kinetochore microtubules shorten.
Telophase. Daughter
nuclei are forming.
Meanwhile, cytokinesis
has started: The cell
plate, which will divide
the cytoplasm in two, is
growing toward the
perimeter of the parent
cell.
LE 12-9a
100 µm
Cleavage furrow
Contractile ring of
microfilaments
Daughter cells
Cleavage of an animal cell (SEM)
LE 12-9b
Vesicles
forming
cell plate
Wall of
parent cell
Cell plate
1 µm
New cell wall
Daughter cells
Cell plate formation in a plant cell (TEM)