Biology Paper 2 - 2.6 Cell Division, Cell Diversity and Cell Diffentiation

A checkpoint chemical triggers condensation of chromatin. Halfway through the cycle, the metaphase checkpoint ensures that the cell is ready to complete mitosis.
Cell growth stops. Nuclear division (mitosis) consisting of stages: prophase, metaphase, anaphase and telophase. Cytokinesis (cytoplasmic division).

A resting phase triggered during early G1 at the restriction point, by a checkpoint chemical. Some cells, e.g. epithelial cells lining the gut, do not have this phase.
In this phase, cells may undergo apoptosis (programmed cell death), differentiation or senescence. Some types of cells (e.g. neurones) remain in this phase for a very long time or indefinitely.

A G1 checkpoint control mechanism ensures that the cell is ready to enter the S phase and begin DNA synthesis.
Cells grow and increase in size. Transcription of genes to make RNA occurs. Organelles duplicate. Biosynthesis, e.g. protein synthesis, including making the enzymes needed for DNA replication in the S phase. The p53 (tumour suppressor) gene helps control this phase.

Because the chromosomes are unwound and the DNA is diffuse, every molecule of DNA is replicated. There is a specific sequence to the replication of genes: housekeeping genes - those which are active in all types of cells, are duplicated first. Genes that are normally inactive in specific types of cells are replicated last.
Once the cell has entered this phase, it is committed to completing the cell cycle. DNA replicates. When all chromosomes have been duplicated, each one consists of a pair of identical sister chromatids. This phase is rapid, and because the exposed DNA base pairs are more susceptible to mutagenic agents, this reduces the chances of spontaneous mutations happening.

Special chemicals ensure that the cell is ready for mitosis by stimulating proteins that will be involved in making chromosomes condense and in formation of the spindle.
Cells grow.

Asexual reproduction.
Growth.
Tissue repair.

The chromosomes that have replicated during the S phase of interphase and consist of two identical sister chromatids, now shorten and thicken as the DNA supercoils.
The nuclear envelope breaks down.
The centriole in animal cells (normally found within a region of the cell called a centrosome) divides and the two new daughter centrioles move to opposite poles (ends) of the cell.
Cytoskeleton protein (tubulin) threads form a spindle between these centrioles. The spindle has a 3D structure and is rather like lines of longitude on a virtual globe. In plant cells, the tubulin threads are formed from the cytoplasm.

The pairs of chromatids attach to the spindle threads at the equator region.
They attach by their centromeres.

The centromere of each pair of chromatids splits.
Motor proteins, walking along the tubulin threads, pull each sister chromatid of a pair in opposite directions, towards opposite poles.
Because their centromere goes first, the chromatids, now called chromosomes, assume a V shape.

The separated chromosomes reach the poles.
A new nuclear envelope forms around each set of chromosomes.
The cell now contains two nuclei each genetically identical to each other and to the parent cell from which they arose.

Animal cell: the plasma membrane folds inwards and ‘nips in’ the cytoplasm.
Plant cell: an end plate forms where the equator of the spindle was, and new plasma membrane and cell-wall material are laid down on either side along this end plate.

Many chloroplasts to absorb light for photosynthesis.

Has thin cell walls, so carbon dioxide can easily enter.

Large surface area for absorption.
Thin, permeable cell wall, for entry of water and ions.
Cytoplasm contains many mitochondria to provide energy for active transport of ions.

Found in pairs, with a gap between to form a stoma.
In the light, guard cells take up water and become turgid.
Thin outer walls and thickened inner wall force them to bend outwards, opening the stoma – allowing the leaf to exchange gases for photosynthesis.
To close, water moves out of the guard cells by osmosis.
Stomatal closure is stimulated by abscisic acid.
(Stomatal closure happens at night, too much water loss by transpiration or as a defence to invasion by pathogens).

Packing and supporting tissue, e.g. cortex and pith.

Fills spaces between other tissue.

Tissue thickened with cellulose cell walls to provide support and structure.

Tissue thickened with lignified cell walls to provide strength.

In the meristems – parts of the plant where growth can take place by mitosis.

Stem cells of the vascular cambium divide and differentiate into xylem and phloem.

Cytokinesis starts at the centre of the plant cell and grows outwards.
Cytoplasm is divided by the formation of a cell plate.