D2.1 Cell & Nuclear Division Flashcards
Cytokinesis
Cytokinesis occurs when a cell reaches a certain size and needs to split into two.
Animal cells: cytokinesis involves an inward pinching of the fluid plasma membrane to form a cleavage furrow.
Plant cells: cytokinesis involves the formation of a cell plate. Cell plate is built by vesicles that collect midway between the two poles of the cell and lay down a cell membrane and cell wall which then expand outwards towards the sides of the cell from a central region.
(This is because plant cells have a relatively rigid cell wall)
Equal & unequal cytokinesis
Equal cytokinesis occurs for most instances of cell division
- necessary for the process of growth and repair
- each cell receives a full copy of the parent cell’s DNA and some of the essential organelles
Unequal cytokinesis occurs when there is unequal sharing of the parent cell’s resources
- Eg: Oogenesis
Produces 4 haploid cells, but 3 out of 4 cells donate their cytoplasm and organelles to the fourth cell and are not used as eggs because they are too small to produce a viable zygote
This provides the zygote with the resources it needs to survive until it is implanted in the wall of the uterus
- Eg: Yeast cells budding
A small cell is generated from the parent cell. When the daughter cell becomes big enough to survive on its own, cytokinesis closes the cell membranes and each cell is an independent organism.
Role of mitosis in eukaryotes
- necessary for growth of organisms, the development of embryos and tissue repair
- necessary for asexual reproduction
- ensures a full copy of the nucleus is made before cytokinesis occurs
all genetic information is preserved. daughter cells have the same chromosomes and the same genome as the parent cell.
Role of meiosis in eukaryotes
- results in 4 daughter cells each has a nucleus containing only half of the parent’s DNA
- each of the four have a different combination of genetic information –> random combinations of chromosomes helps generate genetic variety. animals cannot thrive with extra copies of chromosomes.
Def: Haploid & Diploid
Haploid: Cells with 1 set of chromosomes
Diploid: Cells with 2 sets of chromosomes
Def: Homologous chromosomes
- a pair of chromosomes that are structurally identical but not genetically identical
- one of the pair comes from the mother, the other from the father
- carry the same genes in the same order (same loci) but the type of each gene may be different
Def: Centrosome
Location of the cell that includes a pair of centrioles and the surrounding cytoplasm, which contains proteins that aid in the assembly of spindle microtubules
aka Microtubule Organising Centre (MTOC)
Def: Centrioles
- A cytoplasmic, barrel-shaped organelle which is only found in animal cells and exist as a cylindrical pair
- Each member of the pair is composed of nine triplets of microtubules arranged in a ring
- The 2 cylinders are perpendicular to each other
Def: Spindle fibre
An organised system of microtubules that attaches to the centromere region of a chromosome and draws the sister chromatids to opposite poles during eukaryotic cell division.
Def: Centromere
Def: Kinetochore
Centromere: Location where two sister chromatids join and where spindle fibres attach to
Kinetochore: A complex, fibrous structure formed by proteins & centromere. Microtubules are attached here.
What must happen before mitosis & meiosis?
DNA Replication.
- Happens during the S phase of the cell’s life.
- A complete copy of the cell’s DNA is made. The replicated DNA is arranged in a chromosome as two sister chromatids. The banding patterns of each sister chromatid match from top to bottom.
Condensation of chromatin to chromosomes
Necessary in order to prevent the strands from tangling up and breaking.
Condensed structure makes sure that different DNA molecules can be transported together instead of being spread out around the nucleus.
Involves the many nucleosomes within the chromatin stacking up in an organised structure, allowing the chromatin to coil and supercoil to form chromosome.
Movement of chromosomes
- Centrosome makes microtubule spindle fibres that are needed to guide the chromosomes to the right place before the cell can divide
- Microtubules can be constructed and disassembled as needed. They have directionality as one side has a positive charge and one side has a negative charge.
- Specialised molecules called motor proteins push or pull objects around the cell, using microtubules as “tracks”.
- A motor protein attaches to two microtubules and get one to slide past the other. ATP is used to produce a conformational change that moves the microtubules.
Process of movement of chromosomes
- When a cell is ready to separate its chromosomes, the motor proteins between the overlapping microtubules become active
- Action of two microtubules sliding past each other pushes the two poles of the centrosome away from each other
- As the sister chromatids are attached to the poles via opposite-facing microtubules, they will be pulled away from each other. Each sister chromatid will be transported to different half of the cells.
- Once the chromatids have separated, microtubules are dismantled and pieces of track are recycled.
Types of microtubules
- Kinetochore
- Attach to centromere of a chromosome - Astral
- Reach out from the centrosomes - Overlap
- Not attached to chromosomes but pass between them
- Motor proteins are sandwiched between 2 microtubules
Cell cycle
Interphase
- G1
(accumulates proteins & nucleotides)
- S
(semi-conservative DNA rep)
- G2
(organelles divide)
M phase
- Mitosis / Meiosis
- Cytokinesis
Mitosis - PMAT
Prophase
- Chromatin fibres become more tightly coiled to form chromosomes
- Nuclear envelope disintegrates and nucleolus disappears
- Centrosome forms mitotic spindles
- Centrosomes move towards the opposite poles
Metaphase
- Chromosomes move to and align on the metaphase plate
- Centrosomes are at the opposite ends of the pole
- Spindle fibres attach to centromere
Anaphase
- Centromere divides into 2
- Spindle fibres pull the two sister chromatids apart to opposite poles of the cell with the centromere moving first
Telophase
1. A set of chromosome has moved to each pole
2. Chromosomes uncoil
3. Spindle fibres disappear
4. Nucleoli reappears and nuclear envelope is reformed
5. Cell elongates in preparation for cytokinesis
Meiosis (reduction division) - PMAT I & II
Gametes cannot contain a full set of chromosomes to avoid the offsprings from accumulating too many chromosomes
Prophase 1
1. Chromatin condenses, coils and supercoils into chromosomes
2. Nuclear envelope disintegrates and nucleolus disappears
3. Homologous chromosomes are attracted to each other and pair up, forming a bivalent / tetrad.
4. A synapsis is formed, parts of the non-sister chromatid in the homologous pair twist and break then recombine. This is crossing over. Recombinant chromosomes are formed.
5. Spindle fibre forms from centrosomes.
6. Centrosomes move to opposite poles of the cell
Metaphase 1
- The bivalents line up along cell’s metaphase plate in random orientation.
- The centrosomes have reached the opposite poles of the cell
- Spindle fibres attach to centromere
Anaphase 1
- Spindle fibres pull the chromosomes to the opposite poles of the cell
Telophase 1
- Spindle fibres disintegrate
- Chromosomes uncoil into chromatin
- Nucleolus reappears and nuclear envelope reforms
— cytokinesis happens at the same time as telophase 1, forming 2 haploid daughter cells—
Prophase 2
- DNA coils and condenses into chromosomes
- Nuclear envelope disintegrates, nucleolus disappears
- New spindle fibre forms at right angles to the original spindle in P1
Metaphase 2
- Individual chromosomes line up along the metaphase plate in random assortment
- Spindle fibres from opposite poles attach to each of the sister chromatids at the centromeres
Anaphase 2
1. Centromere of each chromosome splits, releasing each sister chromatid as a single chromosome
2. The spindle fibres pull individual chromosomes to opposite poles of the cell
Telophase 2
1. Chromosomes reached opposite poles of the cell
2. Chromosomes de-condense and uncoil into chromatin
3. Spindle fibres disappear
4. Nuclear envelope forms around each of the daughter nuclei
5. 2 daughter cells elongates in preparation for cytokinesis
Down syndrome
- trisomy 21
- has 47 instead of 46 chromosomes
- extra chromosome arises from a phenomenon known as non-disjunction
- probability increases with increased age of mother
Non-disjunction
2 possibilities:
1) Homologous chromosomes fail to separate at Anaphase I due to incorrect spindle attachment
2) Two sister chromatids fail to separate at Anaphase II due to centromere not dividing
More common in egg
Leads to either the addition or reduction in the standard number of chromosomes
How does meiosis generate genetic diversity?
- Crossing over
- prophase I
- sections of 2 non-sister chromatids in a homologous pair twist and break at the chiasma, exchanging positions
- form recombinant chromosomes
- allows DNA from maternal chromosome to mix with DNA from paternal chromosomes - Random orientation & independent assortment
- metaphase I & metaphase II
- the order and orientation at which the chromosomes line up along the centre of the cell is highly random
- which chromosome/chromatid ends up at which pole is down to chance - Fertilisation
- each male and female gamete is genetically different from all others
- any female gamete can fuse with any male gamete
Cell proliferation in plants
Meristematic cells = undifferentiated cells that can divide rapidly
- Allows growth in plants
- later differentiate and play a specific role for the plant
- Mitosis takes place in meristematic tissue for growth to take place
- Zone of cell division is where new, undifferentiated cells are found
Apical meristematic cells
- root tips and branch tips
- enables a plant to lengthen
- majority are undergoing mitosis
Lateral meristematic cells
- occurs in stem tissue
- enables a stem to widen
Cell proliferation in animals
- Zygote as an undifferentiated cell
- will undergo mitosis to make copies of itself until it forms many thousands of cells - the thousands of cells will organise themselves into layers and a hollow sphere to form an embryo
- Skin cells
- to replace death and damaged skin cells
- for organism to grow
- skin grows in the bottom layer and dead cells flake off the top layer
- skin has to be repaired to prevent excessive loss of blood and infection
Cyclins & their role in controlling the cell cycle
Cyclins are a group of proteins that control the cell’s progression through the cell cycle
- Cyclins bind to cyclin-dependent protein kinases (CDKs) and phosphorylate them.
- The CDKs function at checkpoints in the cell cycle. Kinases act as enzymes, causing the cell to move from G1 to S phase and from G2 to M phase.
(cell auto moves from S to G2 phase when DNA rep is completed) - Cyclin levels change during the cell cycle. When a particular cyclin concentration is low, it binds to so few CDKs that checkpoint cannot be reached. When cyclin concentration reaches a threshold, the cell can move to the next checkpoint.
- Eg: G1 cyclin tells he cell to grow and get ready For DNA rep. It is introduced in G1 phase. // Mitotic cyclin tells the cell to start making microtubules that will form spindle fibres for mitosis. When it reaches its threshold just before mitosis, it tells the cell that it is time to separate chromosomes. After anaphase, the cyclin is broken down so that the cell can move on to telophase and cytokinesis.
- The activated kinases also phosphorylate other proteins that perform specific functions in the cell cycle.
- Some cells will pause during G1 and enter G0. G0 is a non-growing phase and certain cells stay in this phase for different periods of time.
- Nerve and muscle cells never go beyond G0.
