Mitosis Flashcards
(29 cards)
What is binary fission?
Prokaryotes dividing into 2 cells.
Chromosome replicates, single circular DNA in this case.
2 daughter chromosomes divided into 2 daughter cells.
Describe the cell during interphase.
Nucleus evident.
Nuclear membrane intact.
During S phase in interphase, chromosomes replicate to form sister chromatids.
Describe the cell during prophase.
Chromosomes start to condense.
Cytoskeleton fibers are organized in a different way from interphase.
Describe the cell during prometaphase.
The nuclear membrane breaks down.
It disaggregated into vesicles, which will be recycled to reconstitute new nuclear membranes.
Describe the cell during metaphase.
Chromosomes line up at the equatorial plane (mitotic plane) of the cell, they align very precisely.
They wait there for a signal to move.
When chromosomes are most compact, thus used when studying chromosomes.
Describe the cell during anaphase.
2 chromatids composing each chromosome moves apart to opposite poles.
Describe the cell during telophase.
2 daughter cells are forming.
Both nuclei recomposing.
Nuclear membranes form by recycling material derived from previous as well as synthesis of new molecules.
Last phase of mitosis, but 2 daughter cells not produced yet, another phase needed to divide mother cells completely into daughter cells - cytokinesis (separate from mitosis).
What is the cleavage furrow?
the position in the mother cell where it will separate into daughter cells, this comes after telophase.
What are condensin proteins?
Condensin proteins keep chromosomes in a tightly condensed state during mitosis.
Condensin decorates DNA, then an enzyme (topoisomerase II) induces positive superhelical tension into DNA, hydrolyzing ATP.
What are cohesin proteins?
Cohesin keeps the 2 sister chromatids adherent to each other.
This complex is pre-assembled during the S phase.
During anaphase, this is then degraded to allow sister chromatids to separate. This is done by the enzyme separin.
What are mitotic spindles composed of?
Mitotic spindles are composed of microtubules.
Specific types of microtubules have specific roles during mitosis and meiosis. This is used in chromosomal separation.
What are the 3 types of microtubules?
Astral microtubules - ‘rays’ starting from centrioles. Their role is to set the position and orientation of the spindle along the cell division axis. Also tightly connected to the cytoskeletal fibers of the cell.
Kinetochore (chromosomal microtubules) - only ones among 3 types which directly interact with chromosomes. They bind and carry chromosomes during anaphase.
Polar microtubules - not attached to chromosomes, they force centrosomes towards the poles so that the spindle can be diffused in the whole volume of the cell.
Describe the structure of kinetochore.
The centromere is always represented by constitutive heterochromatin, its role is structural. The kinetochore is bound to this. This has an inner and outer plate and intermediary region called inner zone. All these regions are made of proteins, and the inner region is always associated with DNA.
In the heterochromatic region, H3 histone is specialised typical of only the kinetochore. The outer plate is actively associated with the + end of microtubules. This part is assembled during mitosis, in prometaphase.
Another component at the contact between outer plate and microtubules - fiber called fibrous corona. They are used as a bridge to bind the outer plate, the microtubules and motor proteins.
The sudden movement of chromosomes at anaphase is not the only one. They are moved earlier before anaphase.
Describe the process of chromosomal separation.
Microtubules are shortened at both ends, which drags the bound chromosomes.
There’s a sliding of polar microtubules, and a traction of cytoplasmic dyneins over astral microtubules.
Why doesn’t the cell start anaphase until all the chromosomes are aligned?
This is important for a healthy result of mitosis, if a chromosome is not aligned, it will not be divided so both copies of the chromosome will remain in 1 cell.
What is the spindle checkpoint?
There is a moment of delay called spindle checkpoint during metaphase to check that all the chromosomes are correctly aligned at the metaphysical plate.
How are sister chromatids separated during anaphase?
There are 2 sister chromatids held together by cohesin. During anaphase, this cohesin backbone must be degraded. The separase enzyme will perform this function. This is present in its inactive form, kept there by the protein securin.
As soon as all chromosomes are present at the metaphysic plate, a specific complex made of a Cdc20 and APC starts phosphorylating and sending securin to the protostome. Consequently, separase becomes activated and begins degrading securin to separate the sister chromatids.
What happens in cytokinesis?
In animal cells, the mother cell is squeezed in the middle from a contraction to divide into 2 daughter cells. A cleavage furrow produces this, which is made of a contractile ring composed of microfilaments.
In plant cells, the building of a new cell wall separates the 2 daughter cells in the middle of the mother cell.
Describe cytokinesis in animal cells.
Contractile ring in the middle which contracts to divide the mother cell into 2 daughter cells.
Actin and myosin filaments make up this ring-like structure. These proteins are also used in muscle contraction.
These filaments use ATP to contract.
In some specialised cells, this division will be asymmetrical, for instance stem cells.
Ca2+ is needed for this function.
Describe cytokinesis in plant cells.
Builds a new inner cell plate between 2 daughter cells using vesicles from the Golgi complex.
They start laying at the midzone of the mother cell along the equatorial plate and start fusing.
A new membrane is formed.
These membranes start releasing a content, which is a component of a cell wall.
How does the cell know to divide symmetrically?
The correct positioning for cells to divide symmetrically is possible because signals from the mitotic spindle arrive (In both cases).
Otherwise cytokinesis would be completely random without signals telling the cell where to divide.
Describe the main features of meiosis.
Nuclear division is like mitosis.
Produces new nuclei from the mother nucleus.
Process which produces haploid nuclei starting from a diploid nucleus.
Describe the subphases of prophase 1.
Laptonema: Chromosomes start to condense.
Zygonema: homologous chromosomes start pairing.
Pachynema: chromosomes are completely paired.
Diplonema: chromosomes start separating, leaving sites that are still bound called chiasmata.
Diakinesis: chromosomes condense further and become attached to spindle fibers.
When does crossing over happen?
This happens when the chromosomes are completely aligned - zygonema.
A synaptonemal complex keeps 2 homologous chromosomes tightly attached so that the tips on both ends are precisely aligned.
