Cell cycle

 There are two main checkpoints, the G1/S checkpoint, also called the restriction point, and the G2/M checkpoint. There are other checkpoints, e.g. there is one halfway through mitosis and one in early G1.

The purpose of the checkpoint is:

• To prevent uncontrolled division that would lead tumours (cancer).
• To detect and repair damage to DNA, e.g. from UV.

Because the molecular events that control the cell cycle happen in a specific sequence, they also ensure that:

• The cycle cannot be reserved.
• The DNA is only duplicated once during each cell cycle.

Phase of cell cycle and checkpoints

• A checkpoint chemical triggers condensation of chromatin.

• Halfway through the cycle, the metaphase checkpoint ensures that the cell is ready to complete mitosis.

Events within the cell

• Cell growth stops.

• Nuclear division (mitosis) consisting of stages: prophase, metaphase, anaphase and telophase.

• Cytokinesis ( cytoplasmic division).

Phase of cell and checkpoints

• 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.

Events within the cell

• In this phase, cells may undergo apoptosis (programmed cell death), differentiation or senescence.

• Some types of cells (e.g. neurones) remain in this stage for a very long time or indefinitely.

Phase of cell cycle and checkpoints

• Ensures that the cell is ready to enter the S phase and begin DNA synthesis.

Events within the cell

• 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 supressor) gene helps control this phase.

Phase of cell cycle and checkpoints

• Because the chromosomes are unwound and the DNA is spread out, every molecule of DNA is replicated.

There is a specific sequence to the replication of genes:

• Housekeeping genes- These are active in all cells, so are duplicated first.
• Genes that are normally inactive in specific cells are replicated last.

Events within the cell

• Once the cell has entered this phase, it's 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 susceptable to mutagenic agents, this reduces the chances of spontaneous mutations happening.

Phase of cell cycle and checkpoints

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

Events within the cell

• Cells grow.

Cytokinesis- Cytoplasmic division following nuclear division, resulting in two new daughter cells.

Interphase- Phase of cell cycle where the cell is not dividing; it is subdivided into growth and synthesis phases.

Mitosis- Type of nuclear division that produces daughter cells genetically identical to eachother and the parent cell.

Chromatids- Replicates of chromosomes.

Haploid- Having only one set of chromosomes.

Homologous chromosomes- Matching chromosomes, containing the same genes at the same places (loci). They may contain different alleles for some of the genes.

Meiosis- Type of nuclear division that results in the formation of cells containing half the number of chromosomes of the parent cell.

All living organisms need to produce genetically identical daughter cells, by mitosis, for the following reasons:

• Asexual reproduction

• Growth

• Tissue repair

Growth repair

- All multicellular organisms grow by producing more cells that are genetically identical to each other and to the parent cell from which they arose by mitosis.

Tissue repair

- Wounds heal when growth factors, secreated by platelets and macrophages (white blood cells) and damaged cells of the blood-vessel walls, stimulate the proliferation ( rapid replication) of endothelial and smooth muscle cells to repair damaged blood vessels. 

Asexual reproduction

•  Single-celled organisms divide by mitosis to produce new individuals.

• Some plants e.g.strawberry, reproduce asexually by forming new plantlets on the end of stolons (runners).

• Fungi, such as single celled yeast, can reproduce asexually by mitosis.

• Asexual reproduction is rare in animals but female sharks kept in captivity without males have produced female offspring genetically identical to themselves.

• Aphids may sometimes produce eggs, by mitosis, that do not need fertilising.

• The chromosomes that have replicated during the S phase of interphase and consist of two identical sister chromatids, now shorten and thicken as DNA super coils.

• The nuclear envelope breaks down.

• The centriole in animal cells 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 (similar to lines of longitude on a 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 the centromere goes first, the chromatids, now called chromosomes, assume a "V" shape.

• The separated chromosomes reach the poles.

• A nuclear envelope forms around each set of chromosomes.

• The cell now contains two nuclei each genetically identical to eachother and to the parent cell from which they arose.

Once mitosis is complete, the cell splits into two, so that each new cell contains a nucleus.

• In animal cells, the plasma membrane folds inwards and "nips in" the cytoplasm.

• In plant cells, an end plate forms where the equator of the spindle was, and new plasma membrane and cellulose cell- wall material are laid down on either side along this end plate.

Two new daughter cells are now formed. They are genetically identical to eachother and to the parent cell.

• Sexual reproduction increases genetic variation because it involves the combining of genetic material from two ( ususally) unrelated individuals of the same species, by the process of fertilisation.

• Genetic variation within a population increases its chances of survival when the environment changes, as some individuals will have characteristics that enable them to be better adapted to the change.

• In many organisms the body cells are diploid. For sexual reproduction to occur they must produce haploid gametes, so that when two gamete nuclei fuse during fertilisation, a diploid zygote is produced wth the normal number of chromosomes.

• Meiosis occurs in the gonads (ovaries and testies).

• Cells undergo interphase before they undergo meiosis.

• In your body cells there are 46 chromosomes.

• 23 from mum in egg nucleus, 23 from dad in sperm nucleus.

• These can form matching pairs - one maternal and one paternal chromosome containing the same genes at the same places on the chromosome.

• These matching pairs are called homologous chromosomes.

• Although they have the same genes, they may contain different alleles (variants) for the genes.

Before meiosis, during the S phase of interphase, each chromosome was duplicated as its DNA replicated, after which each chromosome consists of two sister chromatids. In meiosis, the chromosomes pair up in their homologous pairs.

• There are two divisions in meiosis, and in each division there are 4 stages.

• In the first meiotic division, the four stages are: prophase 1, metaphase 1, anaphase 1 and telophase 1.

• The cell may then enter a short interphase, before embarking on the second meiotic division that also has four stages: prophase 2, metaphase 2, anaphase 2, telophase 2.

• This takes place in a plane at right angles to that of meiosis 1.

• At the end of the second division, cytokinesis may occur.

• The chromatin condenses and each chromosome supercoils. In this state, they can take up stains and can be seen with a light microscope.

• The nuclear envelope breaks down, and the spindle threads of tubulin protein form from the centriole in animal cells.

• The chromosomes come together in homologous pairs.

• Each member of the pair consists of two chromatids.

• Crossing over occurs where non-sister chromatids wrap around each other and may swap sections so that alleles are shuffled.

• The pairs of homologous chromosomes, still in their crossed over state, attach along the equator if the spindle.

• Each attaches to a spindle thread by its centromere.

• The homologous pairs are arranged randomly, with the members of each pair facing opposite poles of the cell. The arrangement is independent assortment.

• The way they line up in metaphase determines how they will segregate independently when pulled apart during anaphase.

• The members of each pair of homologous chromosomes are pulled apart by motor proteins that drag them along the tubulin threads of the spindle.

• The centromeres dont divide, and each chromosome consists of two chromatids.

• The crossed-over areas separate from eachother, resulting in swapped areas of chromosome and allele shuffling.

• In most animal cells, two new nuclear envelopes form around each set of chromosomes, and the cell divides by cytokinesis. There is a short interphase when the chromosomes uncoil.

• Each new nucleus contains half the original number of chromosomes, but each chromosome consists of two chromatids.

• In most plant cells they skip this stage.

• If the nuclear envelopes have reformed, then they now break down.

• The chromosomes coil and condense, each one consisting of two chromatids.

• The chromatids of each chromosome are no longer identical, due to the crossing over in prophase 1.

• Spindles form.

• The chromosomes attach, by their centromere, to the equator of their spindle.

• The chromatids of each chromosome are randomly assigned.

• The way that they are arranged will determine how the chromatids separate during anaphase.

• The centromeres divide.

• The chromatids of each chromosome are pulled apart by motor proteins that drag them along the tubulin threads of the spindle, towards opposite poles.

• The chromatids are therefore randomly segregated.

• Nuclear envelopes from around each of the 4 haploid nuclei.

• In animals, the two cells now divide to give 4 haploid cells.

• In plants, a tetrad of four haploid cells is formed.

• Crossing over during prophase 1 shuffles alleles.

• Independent assortment of chromosomes in anaphase 1 leads to random distribution of maternal and paternal chromosomes in each pair.

• Independent assortment of chromatids in anaphase 2 leads to further random distribution of genetic material.

• Haploid gametes are produced, which can undergo random fusion with gametes derived from another organism of the same species.