What is interphase?
Long periods of growth and normal cell activities.
What happens during interphase?
Major functions, such as enzyme and hormone
synthesis.
DNA is replicated and checked for errors in the nucleus. S, G2
Protein synthesis occurs in the cytoplasm. G1
Mitochondria grow and divide, increasing in number in the cytoplasm. G1
Chloroplasts in plants and algal cell cytoplasm grow and divide, increasing in number. G1
Normal metabolic processes in the cell occurs, e.g respiration. G1
What are the three phases of interphase?
G1
S
G2
What happens in G1?
The first growth phase - proteins are synthesised and organelles replicate. The cell increases in size.
What happens in S?
Synthesis phase - DNA is replicated in the nucleus.
What happens in G2?
The second growth phase - the cell continues to increase in size, energy stores are increased and the duplicated DNA is checked for errors.
What are the two phases of the cell cycle?
Interphase - G1, S, G2
Mitotic - mitosis, cytokinesis
What happens in the mitotic stage?
Mitosis - the nucleus divides.
Cytokinesis - the cytoplasm divides and two daughter cells are produced.
What is G0?
The phase where the cell temporarily or permanently leaves the cell cycle.
Why might a cell enter G0?
Differentiation - a cell that becomes specialised can no longer divide.
The DNA of a cell may be damaged, so it can no longer divide and becomes senescent. Most cells can only divide a few times before becoming senescent.
As you age, many of your cells become senescent and so can less damage can be repaired.
What types of cells can be stimulated back from G0 to G1?
Lymphocytes in an immune response.
What are checkpoints and why are the used?
It is vital to ensure that a cell only divides when it is the right size, the replicated DNA is error-free or repaired, and the chromosomes are in their correct position during mitosis. This is to ensure that 2 identical daughter cells are made.
Checkpoints are control mechanisms of the cell cycle. They monitor and verify whether the processes at each phase of the cell cycle have been completed before the cell is allowed to move to the next phase.
What is the G1 checkpoint?
At the end of the G1 phase before entry into the S phase. If the cell satisfies the requirements for the S phase, it is triggered to begin DNA replication. If not, it enters a resting state (G0).
What is the G2 checkpoint?
At the end of the G2 phase before the start of the mitotic phase.
In order for this checkpoint to be passed, the cell has to check a number of factors, including whether the DNA has been replicated without error.
If this checkpoint has been passed, the cell initiates the molecular processes that signal the beginning of mitosis.
What is the metaphase checkpoint?
AKA the spindle assembly checkpoint.
This checkpoint is at the point in mitosis where all the chromosomes should be attached to spindles and aligned.
Mitosis cannot proceed until this checkpoint is passed.
Why is mitosis important?
Mitosis ensures that both daughter cells produced when a parent cell divides are genetically identical
Mitosis is necessary when all the daughter cells have to be identical.
This is the case during growth, replacement and repair of tissues in multicellular organisms such as animals, plants, and fungi. Mitosis is also necessary for asexual reproduction, which is the production of genetically identical offspring from one parent.
What are chromatids?
Each DNA molecule (chromosome) is converted into two identical DNA molecules, called chromatids.
The two chromatids are joined together at a region called the centromere.
What are the stages of mitosis?
Prophase
Metaphase
Anaphase
Telophase
Describe prophase?
During prophase, chromatin fibres (complex made up of various proteins, RNA and DNA) begin to coil and condense to form chromosomes.
The nucleolus disappears. The nuclear membrane begins to break down.
Protein microtubules form spindle-shaped structures linking the poles of the cell. The fibres forming the spindle are necessary to move the chromosomes into the correct positions before division.
In animal cells and some plant cells, two centrioles migrate to opposite poles of the cell. The centrioles are cylindrical bundles of proteins that help in the formation of the spindle.
The spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre of the cell.
By the end of prophase the nuclear envelope has disappeared.
Describe metaphase?
During metaphase the chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate, and then held in position.
Describe anaphase?
The centromeres holding together the pairs of chromatids in each chromosome divide during anaphase.
The chromatids are separated - pulled to opposite poles of the cell by the shortening spindle fibres.
The characteristic V’ shape of the chromatids moving towards the poles is a result of them being dragged by their centromeres through the liquid cytosol.
Describe telophase?
In telophase the chromatids have reached the poles and are now called chromosomes. The two new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them.
The chromosomes start to uncoil and the nucleolus is formed.
Cell division - or cytokinesis, begins.
Describe cytokinesis in animal cells?
In animal cells a cleavage furrow forms around the middle of the cell. The cell-surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming two cells.
Describe cytokinesis in plant cells?
Plant cells have cell walls so it is not possible for a cleavage furrow to be formed. Vesicles from the Golgi apparatus begin to assemble in the same place as where the metaphase plate was formed. The vesicles fuse with each other and the cell surface membrane, dividing the cell into two.
New sections of cell wall then form along the new sections of membrane.
