cell division Flashcards
Cell Cycle
the cell is an ordered set of events which culminates in the division of a cell into two daughter cells. It can be divided into two main phases (Interphase and M phase)
Interphase
1) G1 - First intermediate gap stage in which the cell grows and prepares for DNA replication
2) S - synthesis stage in which DNA is replicated
3) G2 - second intermediate gap stage in which the cell finishes growing and prepares for cell division
M phase
1) Mitosis - nuclear division, whereby DNA is separated into two identical nuclei
2) Cytokinesis - Cytoplasmic division, whereby cellular contents are segregated and the cell splits into two
Events occuring in Interphase
1) DNA replication (DNA is copied during the S phase of interphase)
2) Organelle duplication (organelles duplicated for daughter cells)
3) Cell growth (Cytoplasmic volume increase prior to division)
4) Transcription/translation - proteins and enzymes synthesised
5) Obtain nutrients - vital cellular materials must be present
6) Respiration - ATP production is needed to drive division process
Chromatin
Dna is usually loosely packed within the nucleus as a unravelled chromatin
in this unravelled form, the DNA is accessible to transcriptional machinate and so genetic information can be translated
DNA is organised as chromatin in all non - dividing cells and throughout the process of interphase
Chromosome
DNA is temporarily tightly packed into a chromosome prior to division (via supercoiling)
In this form the DNA can be easily segregated yet is inaccessible to transcriptional machinery
DNA is organised as chromosomes during the process of mitosis
Chromosome versus Chromatid
Chromosome = condensed form of DNA which is visible during mitosis. During the S phase the chromosomes contain two identical DNA strands
Chromatids = the identical strands (DNA) are called sister chromatids and are held together by the Centromere. when the Chromatids spereate during mitosis, independent chromosomes are formed with individual DNA
Mitosis
Mitosis is the process of nuclear division whereby duplicated DNA molecules are arranged into two separate nuclei. It occurs during interphase and is divided into 4 stages: prophase, metaphase, anaphase, telophase
Before Mitosis
1) DNA is present as uncondensed chromatin
2) DNA is contained within a clearly defined nucleus
3) Centrosomes and other organelles have been duplicated
4) Cell is enlarged in preparation for division
Prophase
1) DNA supercoils and chromosomes condense (becoming visible under microscope)
2) Chromosomes are comprised of genetically identical sister chromatids (joined at a centromere)
3) Paired centrosomes move to the opposite poles of the cell and form microtubule spindle fibres
4) The nuclear membrane breaks down and the nucleus dissolves
Metaphase
1)Microtubule spindle fibres from both centrosomes connect to the centromere of each chromosome
2) Microtubule depolymerisation causes spindle fibres to shorten in length and contract
3) This causes chromosomes to align along the centre of the cell (equatorial plane or metaphase plate)
Anaphase
1) Continued contraction of the spindle fibres causes genetically identical sister chromatids to separate
2) Once the chromatids separate, they are each considered an individual chromosome in their own right
3) The genetically identical chromosomes move to the opposite poles of the cell
telophase
1) Once the two chromosome sets arrive at the poles, spindle fibres dissolve
2) Chromosomes decondense (no longer visible under light microscope)
3) Nuclear membranes reform around each chromosome set
4) Cytokinesis occurs concurrently, splitting the cell into two
Cytokinesis
Cytokinesis is the process of cytoplasmic division, where the cell splits into two identical daughter cells.
Cytokinesis occurs concurrently with the final stage of mitosis (telophase) and is different in plant and animal cells
Animal cells (cytokinesis)
1) After anaphase microtubule filaments form a concentric ring around the cells centre
2) The microfilaments constrict to form a cleavage furrow, which deepens from the periphery towards the centre
3) When the furrow meets in the centre the cell is formed into two cells
4) Its describes as Centripetal (outside to centre)
Plant cells (cytokinesis)
1) After anaphase carbohydrate rich vesicles form in a row at the centre of the cell
2) the vesicles fuse together and an early cell plate begins to form within the middle of the cell
3) the cell plate extends outwards and fuses with the cell wall dividing the cell into two distinct daughter cells
4) Its described as Centrifugal (centre to outside)
Mitotic index
The mitotic index is a measure of the proliferation status of a cell population. It is an important prognostic tool for predicting the response of cancer cells to chemotherapy
Identifying Mitotic cells
Cells undergoing mitosis will lack a clearly defined nucleus and possess visibly condensed chromosomes.
Prophase - Chromosomes condensed but still confined to a nuclear region
Metaphase - chromosomes aligned along the equator of the cell
Anaphase - two distinct clusters of chromosomes apparent at poles of the cell
Telophase - two nuclear regions present within a single cell
Calculating Mitotic index
ratio between the number of cells in mitosis and the total number of cells
Cyclins
Cyclins are a family of regulatory proteins that control the progression of the cell cycle.
Cylclins activate cyclin dependent kinases (CDKs) which control cell cycle processes through phosphorylation
Mechanism of Cyclin action
1) Cyclin and CDK form a complex which will bind to a target protein and modify it via phosphorylation
2) The phosphorylated target protein will trigger a specific event
3) After the event, the cyclin is degraded and the CDK becomes inactive again
Cyclin expression patterns
Different cyclins specifically bind to and activate different classes of cyclin dependent kinases. Cyclin levels will peak when their target protein is required
Tumours
Tumours are abnormal cell growths resulting from uncontrolled cell division and can only occur in any tissue or organ.
Diseases caused by the growth of tumours are collectively known as cancers
Mutagens
A mutagen is an agent that changes the genetic material of an organism
