3.2.2 - Cell division

The cell cycle

A cell copies its DNA before it divides, so the time between divisions is anything but rest.

Every new cell comes from a pre-existing cell, but not all cells retain the ability to divide. Many mature specialised cells leave the cycle, whereas stem cells and meristematic cells (found in plant root and shoot tips) can divide. Cells that do divide pass through a repeating sequence called the cell cycle.

Read the diagram from left to right for its three main parts:

1. Interphase (G1, S and G2): the cell grows, carries out normal metabolic activity and prepares for division.
2. Mitosis: the nucleus divides.
3. Cytokinesis: the cytoplasm divides to form two separate cells.

Box sizes do not represent duration.

What happens in interphase

Interphase is not a resting phase. It is a period of intense activity:

• In G1, the cell grows and synthesises proteins and organelles.
• In S phase, DNA is replicated, so each chromosome is copied.
• In G2, the cell continues to grow and synthesises the proteins needed for division.

Under a microscope you see an intact nucleus but no separate chromosomes, because the DNA is dispersed as less-condensed chromatin. DNA is present throughout, so chromosomes becoming visible later does not mean they have just been made.

Counting chromosomes and DNA

By the end of interphase, each chromosome consists of two identical sister chromatids. Each chromatid contains one DNA molecule with the same sequence, apart from copying errors, and the two remain joined at the centromere.

Count chromosomes by their centromeres, not by the arms drawn: one X shape is one replicated chromosome, not two homologous chromosomes. Homologous chromosomes are the corresponding maternal and paternal chromosomes and may carry different alleles (versions of a gene). Mitosis separates sister chromatids, not homologous pairs.

For a cell with four chromosomes before replication:

Replication doubles DNA quantity without changing chromosome number, because the copies stay joined at one centromere. In anaphase the separated chromatids become chromosomes, so the count in the still-undivided cell temporarily doubles. Each new nucleus then receives the original chromosome number.

Relative DNA mass follows the same logic. If G1 DNA mass is 6 arbitrary units per cell, it rises to 12 through S, remains 12 through G2 and early mitosis, then returns to 6 per daughter cell after division.

Mitosis and cytokinesis

Every stage of mitosis looks the way it does for a reason, and those reasons are how you identify it.

Cytokinesis then usually follows, so a eukaryotic cell produces two daughter cells, each with identical copies of the DNA made during replication. The key event is the separation of the two chromatids of each chromosome.

The four stages of mitosis

Mitosis is a continuous process, but it is usually described in four stages. Follow the blue chromosome across this simplified animal cell, which starts with two replicated chromosomes.

1. Prophase. Chromosomes condense: coiling packs the DNA into short, thick structures that take up stain strongly, so they become visible under the light microscope as two sister chromatids joined at the centromere. Spindle fibres, protein strands that stretch between the poles, begin to form. In animal cells, centrioles (small protein cylinders that organise the spindle) move to opposite poles; root tip cells form a spindle without centrioles. The nuclear envelope breaks down, so spindle fibres can attach to centromeres in late prophase.
2. Metaphase. Chromosomes line up at the equator of the cell. Spindle fibres from opposite poles are attached at the centromeres of the sister chromatids, so one copy of each chromosome can be moved to each pole.
3. Anaphase. The centromeres divide, which frees the sister chromatids from each other. Spindle fibres shorten and pull them to opposite poles, a movement that requires energy from ATP. The centromere leads and the arms trail behind, which gives the V shape you see under the microscope. Once separated, each chromatid is described as a chromosome.
4. Telophase. Chromosomes reach the poles, uncoil and become less visible. A nuclear envelope forms around each group, giving two genetically identical nuclei.

At anaphase the still-undivided cell holds four chromosomes, two moving to each pole, so each daughter nucleus receives two, the same as the parent.

Cytokinesis

Cytokinesis divides the cytoplasm and usually overlaps late mitosis or follows it, producing two separate daughter cells.

• In animal cells, the cell-surface membrane pinches inward.
• In plant cells, vesicles form a cell plate that develops into a new cell wall, so a cell plate beginning to form marks cytokinesis.

Recognising stages in a root tip squash

AQA questions often show stained cells from a plant root tip and ask for the stage. Focus on the arrangement of chromosomes and whether the nucleus is intact.

Interphase is usually the most common stage in a root tip squash, because cells spend much longer in interphase than in mitosis.

Growth, repair and cell numbers

Mitosis produces genetically identical cells, and three of its uses depend on that.

• Growth: a zygote (fertilised egg) divides repeatedly to produce the many cells of a multicellular organism.
• Repair and replacement: damaged or worn-out cells can be replaced with cells that have the same genes.
• Asexual reproduction: some organisms produce offspring by mitosis, so the offspring are genetically identical to the parent apart from any mutation.

Retaining a complete copy of the genome allows replacement cells to express the genes needed for the tissue's function.

Predicting cell numbers

If every cell divides once in each cycle and none dies, the number of cells doubles each cycle.

Standard form writes a number as $a\times10^b$, with $1\leq a\lt10$.

A correct number left out of standard form can lose a mark when a question asks for it. Reverse the calculation by dividing by $2^n$ to estimate a starting population.

This ideal model rests on the assumption that every cell completes every cycle; real tissues include non-dividing cells, cell death and different cycle lengths. It therefore gives a prediction, not a guaranteed tumour growth rate.

Uncontrolled division and cancer

Tumours form when the controls on mitosis fail, and many cancer drugs slow division.

Mitosis is a controlled process: genes and cell-signalling molecules regulate when a cell divides. If control is lost, cells may divide too often and form a tumour.

Tumours may be:

• Benign: contained in one place and not invasive.
• Malignant: invasive and able to spread to other parts of the body. Malignant tumours are cancers.

How cancer drugs slow division

Many cancer treatments are directed at controlling the rate of cell division. Drug treatment of this kind is called chemotherapy:

• Some drugs prevent DNA replication, so the cell cannot make the second copy of its DNA that mitosis needs.
• Some drugs prevent spindle fibres forming, so chromosomes cannot separate properly.

Either way, the rate of cell division falls, so the tumour grows more slowly. Cells that divide often are hit hardest, because they are more likely to be replicating DNA or forming a spindle while the drug is present.

These treatments can also affect normal tissues in which cells divide frequently, such as the gut lining, bone marrow, and hair follicles.

Binary fission

Prokaryotes have no nucleus to divide, yet each daughter cell still receives a full copy of the circular DNA.

Prokaryotes do not divide by mitosis because they do not have a nucleus or linear chromosomes. Instead:

1. The circular DNA replicates.
2. Plasmids replicate independently of the circular DNA.
3. The cytoplasm divides to produce two daughter cells. Each receives one copy of the circular DNA and a variable number of plasmids, because a cell can hold many plasmid copies and they need not be shared out equally.

Binary fission involves no chromosome condensation, spindle fibres or nuclear envelope.

Cover the table, then list the differences from memory.

How new virus particles are made

Viruses do not divide: an infected host cell builds the new particles.

Viruses are non-living and do not undergo cell division. A virus has no ribosomes or complete metabolic machinery, so after infection the host cell replicates the particles:

1. Attachment proteins bind the virus to a complementary receptor on a host cell.
2. The viral nucleic acid is injected into the host cell. Some viruses use another route: HIV's envelope fuses with the cell membrane (3.2.4b).
3. Viral nucleic acid directs the host cell to make viral nucleic acid and proteins, using host ribosomes, energy and raw materials. Depending on the virus, host and/or viral enzymes copy its genome.
4. New virus particles are assembled. Copying the genome alone is insufficient: capsid and attachment proteins must also be made and assembled with that genome.
5. The new viruses leave the host cell, often when the cell lyses (bursts).

So a suitable host cell is necessary, and viral replication differs from the doubling of a bacterium.