Biochemistry and Molecular Biology II – Lecture 9 + 10: The eukaryotic cell cycle
Recombination – Lecture by Colin Logie
22-05-2018 + 28-05-2018
Endosymbionts: Mitochondria and chloroplasts. Contain their own DNA.
Meiosis (gamete formation):
- Is a special (double) cell cycle where a diploid cell (2n) makes 4 haploid cells (1n)
- There is one DNA replication round (2n 4n)
- Followed by two chromosome segregation steps; meiosis I and II
- Gametes (egg + sperm) will fuse to form a zygote
Meiosis requires two equal karyotypes:
- This allows a spindle to be built, where the microtubules from the centrosomes bind the
centromere of one of the homolog pairs.
Mitotic cell division (vegetative division):
- Is when diploid cells segregate 2 copies of each chromosome
- The point is to make two daughter cells with the same genetic make-up as the parental
cell, via transmitting maternal and paternal genetic
information to each daughter cell.
Mitosis consists out of four phases:
1. Mitosis
2. Gap 1
3. Synthesis
4. Gap 2
Mitosis is divided in 4 sub-phases:
- Prophase
- Metaphase
- Anaphase
- Telophase
*PMAT
,Cytokinesis: The formation of two separate cells, caused by the contractile (myosin) ring
getting smaller, causing septation.
In plants: a new cell wall is made.
Short overview mitosis:
- Prophase: during prophase, the nuclear membrane is dismantled and the chromosomes
are condensed to a size below the diameter of the dividing cell to permit their
segregation.
- Metaphase: during metaphase the centromeres are caught by microtubule, originating
from the two centrosomes and align at the equatorial plane.
- Anaphase: during anaphase the protein bridges that tie the sister chromatids to each
other are destroyed, signaling the beginning of sister chromatid separation towards the
poles of the daughter cells.
- Telophase: during telophase the cell divides into two cells, the nuclear envelopes form
around the segregated chromosomes and the chromosomes undergo decondensation
and are ‘licensed’ to replicate.
Regulation of the eukaryotic cell cycle:
- The mitotic cell is dominant when fusing mitotic + G1-phase cells together, causing pre-
maturely condensed chromosomes and nuclear membrane break down in G1 cells,
due to the factors which are present in mitotic cells.
- Similarly, fusion of S-phase cells + G1-cells cause G1 cell chromosome replication,
meaning that S-phase cells are also dominant over G1 cells.
- Exception: G2 cells cannot be induced to replicate their chromosomes upon fusion with
an S-phase cell, indicating that G2 cell chromosomes are not competent to be
replicated.
MPF is active in mitosis (during meiosis I and II):
MPF: Maturation promoting factor, a factor which is present in eggs during meiosis (I and II)
phase. this is the factor that is able to induce mitotic events in oocytes.
MPF consists out of two proteins:
1. Kinase: phosphoryl transferase
2. Mitotic cyclin
The common amino acid structure is as follows:
[S or T] PX [R or K]
1. Serine/Threonine 2. Proline 3. Any a.a. 4. Arginine or Lysine
*Cyclin levels fluctuate synchronously with the appearance and disappearance of MPF-activity!
Cyclin levels also oscillate in enucleated cells (cells of which the nucleus has been removed),
raising the question whether mRNA (translation) is important. experiments show that RNAse
treatment shows that mRNA is necessary for cyclin oscillation. Cyclin B mRNA is sufficient.
, A ‘destruction box’ directs periodic cyclin B degradation:
The destruction box of cyclin B is an 8 amino acid sequence.
- The box is necessary for destruction.
o Deletion of the box stabilizes cyclin B.
- The box is also sufficient for destruction.
o Fusion of the box to another protein causes cyclical degradation of this protein.
Removal of the cyclin B destruction box causes an arrest in mitosis (anaphase):
Therefore, cyclin B destruction is necessary to initiate late mitotic events (following anaphase)
telophase and cytokinesis.
Cyclins need to be destroyed, in order to leave mitosis. Otherwise mitosis constantly takes
place, due to MPF. In order to destroy these cyclins the destruction box is needed.
Polyubiquitylation causes protein degradation:
- Ubiquitin is an abundant, 76aa long protein found in all eukaryotes.
- Ubiquitylation requires the sequential action of 3 enzymes;
1. E1 = ubiquitin activating enzyme
2. E2 = ubiquitin conjugating enzyme
3. E3 = ubiquitin ligase
- Polyubiquitylation is a signal that targets proteins to the proteasome where the proteins
are degraded into peptides.
The proteasome:
There are 4 rings of 7 proteases that make up the
proteasome
- The proteasome consists out of a cap at
both ends which recognize polyubiquitin,
recycle the ubiquitins and unfold the bound
protein to direct it into the core of the
proteasome.
- The interior of the proteasome is lined with
the active sites of the 28 proteases, which
destroy the protein.
What are the substrates of the highly regulated Cdk/MPF?:
- Structural proteins
- Positive regulators of mitosis
- Negative regulators of mitosis
- Proteins active during S-phase.
Recombination – Lecture by Colin Logie
22-05-2018 + 28-05-2018
Endosymbionts: Mitochondria and chloroplasts. Contain their own DNA.
Meiosis (gamete formation):
- Is a special (double) cell cycle where a diploid cell (2n) makes 4 haploid cells (1n)
- There is one DNA replication round (2n 4n)
- Followed by two chromosome segregation steps; meiosis I and II
- Gametes (egg + sperm) will fuse to form a zygote
Meiosis requires two equal karyotypes:
- This allows a spindle to be built, where the microtubules from the centrosomes bind the
centromere of one of the homolog pairs.
Mitotic cell division (vegetative division):
- Is when diploid cells segregate 2 copies of each chromosome
- The point is to make two daughter cells with the same genetic make-up as the parental
cell, via transmitting maternal and paternal genetic
information to each daughter cell.
Mitosis consists out of four phases:
1. Mitosis
2. Gap 1
3. Synthesis
4. Gap 2
Mitosis is divided in 4 sub-phases:
- Prophase
- Metaphase
- Anaphase
- Telophase
*PMAT
,Cytokinesis: The formation of two separate cells, caused by the contractile (myosin) ring
getting smaller, causing septation.
In plants: a new cell wall is made.
Short overview mitosis:
- Prophase: during prophase, the nuclear membrane is dismantled and the chromosomes
are condensed to a size below the diameter of the dividing cell to permit their
segregation.
- Metaphase: during metaphase the centromeres are caught by microtubule, originating
from the two centrosomes and align at the equatorial plane.
- Anaphase: during anaphase the protein bridges that tie the sister chromatids to each
other are destroyed, signaling the beginning of sister chromatid separation towards the
poles of the daughter cells.
- Telophase: during telophase the cell divides into two cells, the nuclear envelopes form
around the segregated chromosomes and the chromosomes undergo decondensation
and are ‘licensed’ to replicate.
Regulation of the eukaryotic cell cycle:
- The mitotic cell is dominant when fusing mitotic + G1-phase cells together, causing pre-
maturely condensed chromosomes and nuclear membrane break down in G1 cells,
due to the factors which are present in mitotic cells.
- Similarly, fusion of S-phase cells + G1-cells cause G1 cell chromosome replication,
meaning that S-phase cells are also dominant over G1 cells.
- Exception: G2 cells cannot be induced to replicate their chromosomes upon fusion with
an S-phase cell, indicating that G2 cell chromosomes are not competent to be
replicated.
MPF is active in mitosis (during meiosis I and II):
MPF: Maturation promoting factor, a factor which is present in eggs during meiosis (I and II)
phase. this is the factor that is able to induce mitotic events in oocytes.
MPF consists out of two proteins:
1. Kinase: phosphoryl transferase
2. Mitotic cyclin
The common amino acid structure is as follows:
[S or T] PX [R or K]
1. Serine/Threonine 2. Proline 3. Any a.a. 4. Arginine or Lysine
*Cyclin levels fluctuate synchronously with the appearance and disappearance of MPF-activity!
Cyclin levels also oscillate in enucleated cells (cells of which the nucleus has been removed),
raising the question whether mRNA (translation) is important. experiments show that RNAse
treatment shows that mRNA is necessary for cyclin oscillation. Cyclin B mRNA is sufficient.
, A ‘destruction box’ directs periodic cyclin B degradation:
The destruction box of cyclin B is an 8 amino acid sequence.
- The box is necessary for destruction.
o Deletion of the box stabilizes cyclin B.
- The box is also sufficient for destruction.
o Fusion of the box to another protein causes cyclical degradation of this protein.
Removal of the cyclin B destruction box causes an arrest in mitosis (anaphase):
Therefore, cyclin B destruction is necessary to initiate late mitotic events (following anaphase)
telophase and cytokinesis.
Cyclins need to be destroyed, in order to leave mitosis. Otherwise mitosis constantly takes
place, due to MPF. In order to destroy these cyclins the destruction box is needed.
Polyubiquitylation causes protein degradation:
- Ubiquitin is an abundant, 76aa long protein found in all eukaryotes.
- Ubiquitylation requires the sequential action of 3 enzymes;
1. E1 = ubiquitin activating enzyme
2. E2 = ubiquitin conjugating enzyme
3. E3 = ubiquitin ligase
- Polyubiquitylation is a signal that targets proteins to the proteasome where the proteins
are degraded into peptides.
The proteasome:
There are 4 rings of 7 proteases that make up the
proteasome
- The proteasome consists out of a cap at
both ends which recognize polyubiquitin,
recycle the ubiquitins and unfold the bound
protein to direct it into the core of the
proteasome.
- The interior of the proteasome is lined with
the active sites of the 28 proteases, which
destroy the protein.
What are the substrates of the highly regulated Cdk/MPF?:
- Structural proteins
- Positive regulators of mitosis
- Negative regulators of mitosis
- Proteins active during S-phase.
