Meiosis and Mitosis Flashcards
(50 cards)
How do eukaryotic cells reproduce? What are the general phases of this?
All cells have a cell cycle, which represents the life of a cell from the moment it is created from a parent cell until it divides to produce daughter cells (or dies).
The cell cycle consists of 2 consecutive, continuous phases: the mitotic (M) phase and interphase. Interphase involves 3 stages: G1, S and G2.
What is interphase? How long does it last? Can you see the chromosomes during this phase?
When a cell is in the mitotic phase it is undergoing cell division. By contrast, interphase is not considered part of cell division – rather preparatory.
Interphase sees the cell engaged in its normal metabolic activities whilst also preparing for its subsequent division by activities such as DNA replication and organelle duplication.
Approx 90% of a cell cycle is spent in interphase; for some adult mammalian cells this phase lasts for about 20 hours.
It is not usually possible to see individual chromosomes within the nucleus during interphase; this is only possible in the early mitotic stages once the genetic material has condensed.
What happens during interphase - G1 (first gap)
Cell recovers from previous division
Cell grows and increases in volume; high amount of protein synthesis
Organelles, such as the mitochondria and ribosomes are duplicated
Materials needed for DNA replication are accumulated
G1 checkpoint takes place to check that DNA is suitable for replication
What is interphase - S (synthesis)
DNA is replicated, resulting in two identical copies (or sister chromatids) of each chromosome joined at the centromere. Another name for replicated is synthesised, hence the name of the stage
Cell continues to grow and replicate organelles, the centrosome (in animal cells) is also duplicated
Specialised chromatin structures (such as sister-chromatid cohesions made from cohesin protein complexes) are constructed to prepare chromosomes for separated in M phase
S checkpoint to ensure DNA replication is complete
What is interphase - G2 (2nd gap)
Cell continues to grow and increase in volume; proteins required for chromosome manipulation are produced
Cytoskeletal filaments change; these will later aid in the movement of chromosomes during the mitotic phase and cell shape changes
G2 checkpoint takes place to check that DNA has replicated correctly
What is mitosis? What does it produce? What are the 5 stages?
Process by which one single cell divides to produce 2 genetically identical daughter cells. During mitosis, cells divide their nucleus and replicated chromosomes and their organelles before dividing their cytoplasm to form the 2 daughter cells.
There are 5 stages to mitosis: prophase, prometaphase, metaphase, anaphase and telophase. An additional stage of cytoplasmic division then occurs – cytokinesis.
What is prophase? (nucleolus, chromatins, centrosomes, mitotic spindle)
The nucleolus disappears, the nuclear envelope starts to fragment, and the DNA and associated proteins found within a cell, known as chromatins, start to coil up and condense.
The process of chromatin coiling up makes the DNA molecules more compact until they have condensed to a point at which they become visible through a light microscope.
During prophase, organelles called centrosomes (in animal cells) move to the opposite poles of the cell and make a series of long tubulin filaments, a form of microtubule. These microtubules are polar, with the minus ends embedded in the spindle pole in the centrosome and the plus ends pointing outward.
Collectively, these filaments form the mitotic spindle, the structure that will eventually be responsible for separating and moving the chromosomes into the new daughter cells.
Microtubules are dynamic polymers that constantly grow and shrink: this lengthening and shortening of the mitotic spindle is an essential element of cell division.
What is prometaphase? (kinetochores, cohesin)
Prometaphase follows prophase. During prometaphase, in cells other than fungal cells, the nuclear membrane breaks down fully, releasing DNA into the cytoplasm of the cell.
The microtubules produced during prophase now extend and attach to the chromosome centromeres at regions called the kinetochores in a process called search and capture.
The kinetochores feature molecular motor proteins that along with changes to the microtubules themselves, provide the pulling force to separate and move sister chromatids.
The size and complexity of kinetochores varies among different species. In higher eukaryotic cells they are multi-layered protein complexes that form at the centromere and can each bind 20-40 microtubules. Once these microtubules have attached to the kinetochores, the chromosomes can then begin the movement needed for the subsequent stages of mitosis.
During both prometaphase and metaphase (which follows), the chromosomes undergo dramatic structural changes as they condense, which help with their later separation.
Sister chromatids are held together by a type of protein called cohesin. During these stages, most cohesin complexes are lost from the arms, but remain at the centromeres. As a result, the centromeres become the only locations at which the chromatids are joined.
What is metaphase? What does the metaphase checkpoint?
Metaphase is the middle stage of mitosis, during which the replicated chromosomes line up along the equator of the dividing cell.
The spindle fibres which attached to the kinetochores of the replicated chromosomes during prophase, contract and relax and in doing so they draw the replicated chromosomes to the centre of the cell so that they line up along a region known as the metaphase plate.
Each sister chromatid is attached via the kinetochore to a spindle fibre. Sister chromatids are attached to spindle fibres that originate from opposite poles of the cells.
This arrangement of the chromosomes helps ensure that the 2 new daughter cells will each receive one copy of each of the chromosomes. A form of molecular ‘safety’ check called the metaphase checkpoint also confirms correct attachment of the kinetochores to the spindle, again to help ensure that the separation of chromosomes into the daughter cells happens as it should.
What is anaphase?
During anaphase, the replicated chromosomes are separated by cleavage of the cohesins that remain at the centromeres, through which the sister chromatids remain bound. Consequently, the sister chromatids are no longer physically associated.
The spindle fibre, which connect the kinetochores of the replicated chromosomes with one of the poles of the cell, shorten. Now that the chromosomes are no longer tethered to one another, the shortening of the spindle fibres pull them apart.
The spindle fibres continue to shorten, drawing the chromosome to which they are attached towards one of the poles of the cell. This ensures that each pole recieves the same kind and number of chromosomes as the original parent cell.
What is telophase?
By the time telophase is reached, the 2 complete sets of genetic information are situated at opposite poles of the dividing cell. During telophase these individual, separated chromosomes being to disperse and decondense, reforming the relaxed chromatin; at this point they are no longer visible under a light microscope.
A new nuclear membrane forms around each set of chromosomes and the spindle fibres disappear as they are dismantled. It may also be possible to see the cytoplasm of the cell starting to prepare for division during this stage.
What is cytokinesis?
Cytokinesis is the final stage of cell division, during which the cytoplasm is divided to produce the daughter cells. Cytokinesis proceeds differently in plant cells compared to animal cells.
How does cytokinesis occur in animal cells? (cleavage furrow)
In animal cells, a contractile ring of actin and myosin filaments forms around the centre of the cell, forming a cleavage furrow which becomes smaller in diameter until the cell is pinched into two, forming 2 daughter cells.
Each daughter cell contains a full complement of organelles and one nucleus with a complete set of chromosomes.
How does cytokinesis occur in plant cells?
Plant cells need to separate and rebuild a section of their cell wall, in addition to the cell membrane, to create daughter cells.
This rebuilding happens when modified vesicles from the golgi apparatus form across the previous cell plate until they eventually fuse together and with the plasma membrane. A new cell wall forms between these new membranes and division of the parent cell occurs.
When should mitosis occur? What is G0 state? How is mitosis triggered?
It is important that a cell undergoes mitosis only when new daughter cells are required. Mitosis should occur at the right time and in the right place. Normal, healthy eukaryotic cells will not undergo mitosis unless they receive signals, such as growth factors, telling them to grow and divide.
When they are not undergoing mitosis, cells exist in a non-dividing but viable state called G0. Mitosis can then be intiated by binding of growth factors to cell surface receptors, which span the plasma membrane. This binding triggers the activation of cell signalling pathways, which in turn trigger an increase in the amount of cyclin D proteins present.
What are D-type cyclins? What are checkpoints?
The D-type cyclins are members of one of 2 key groups of proteins that control the various stages of the cell cycle, and which also ensure that the stages occur in the correct order. The key regulatory proteins are the cyclin dependent kinases (CDK’s or CDC’s) and the cyclins.
Control is exerted at certain ‘checkpoints’ within the cell cycle. The checkpoints involves a combination of cyclins and CDKs although the identity of the proteins is specific to each checkpoint.
What happens at Checkpoint Restriction (R) or G1 to S
Checks for cell size, availability of nutrients, signals (positive growth cues), DNA damage
CDK 4,6,2
Cylin D, E
What happens at checkpoint G2?
Checks for DNA damage, DNA replication completeness
CDK 2
Cylcin A
What happens at checkpoint M?
Checks for chromosome attachment to spindle at metaphase plate
CDK 1
Cylin B
How do CDKs and cyclin binding partners work?
The cyclin-dependent kinases (CDKs) are inactive without a cyclin binding partner. So when the growth factor induced expression of the D cyclins raises their concentration, they can bind with either CDK 4 or 6.
CDK 4 and 6 are then able to phosphorylate their target proteins, ultimately leading to the transcription and expression of proteins required for DNA replication during S phase. Cyclin E is also epxpressed at this point, together with its partner CDK 2 it can help the cell to progress into S phase.
When DNA replication is completed, CDK 2 changes cyclin partner, from E to A. This results in a change of protein targets that CDK 2 can phosphorylate. For example, CDK2/cylinA phosphorylates a protein called DP1, causing it to be deactivated. DP1 is an important part of a transcription complex that helps drive S phase. So, by inhibiting this DP1 protein, S phase is brough to an end.
What do CDK 1 and cyclin B do?q
Further checks and DNA repair processes are carried out before the cell enters M phase. Here CDK 1 and cyclin B are important, particularly to check the correct attachment of chromosomes to the spindle. A single unattached kinetochore in the cell can block anaphase. The degradation of cyclin B is the signal for the final part of mitosis to occur.
How do CDKs and cyclins change to ensure stages in the cell cycle occur in the correct order?
The various CDKs are present throughout the cell cycle, whereas the cyclins change in concentration, as their name implies. It is the sequential and cyclic increase and decrease in different cyclin proteins and their binding to the CDKs that ensures the various stages in the cell cycle occur in the correct order – for example that M phase or cell division only occurs after DNA replication in S phase and G2 has taken place.
The cyclins control the activity of their CDK binding partners. However, the progression and control of the cell cycle is not as simple as this. The CDKs need activating by being phosphorylated, and for any inhibitory phosphates being removed, before they can act on their targets.
How are CDKs inhibited? What does this cause?
They are also subject to inhibition by families of proteins collectively known as the cyclon-dependent kinase inhibitors (CDKIs, also sometimes known as CKIs). Mutations in any of these important families of proteins (such as amplification or too much cyclin or CDK, or loss of one of the CDKIs) could result in mitosis occurring when it should not. This could lead to too many cells being produced, one key characteristic of a group of diseases known as cancers.
What is the main error that can occur with mitosis?
Despite there being a number of checks to ensure cell division occurs correctly, the process can still go wrong. One problem that can occur is called non-disjunction, whereby the chromosomes fail to segregate properly. This results in the daughter cells receiving an incorrect number of chromosomes.
