The mitotic cell cycle

Chromosome structure: DNA, histones, chromatids and the centromere

A chromosome is a single molecule of DNA associated with proteins, and in a eukaryotic cell it is linear. The DNA double helix is wound around histone proteins to form a compact, coiled structure. This packaging is what allows a DNA molecule several centimetres long to be stored inside a nucleus only a few micrometres across. When the cell is not dividing the material is spread out and is called chromatin; when the cell divides it condenses so tightly that the individual chromosomes can be seen with a light microscope.

Before a chromosome divides it is replicated, and the two copies are called sister chromatids. They are identical copies of the same DNA and are held together at a narrow region called the centromere. The centromere is not just a join: it is the point at which spindle fibres attach, so it controls how the chromosome is moved during mitosis. The two ends of each chromatid are protected by telomeres, lengths of repetitive, non-coding DNA.

Learn the five parts the syllabus names, and be able to say what each one is for. DNA carries the genetic information in its base sequence; histone proteins package it and help to control which genes are active; sister chromatids are the two identical copies made by replication; the centromere attaches the chromosome to the spindle; and the telomeres protect the ends. Note also that a replicated chromosome made of two sister chromatids still counts as one chromosome, not two, because the chromatids are joined at a single centromere.

Why mitosis matters: growth, replacement, repair and asexual reproduction

Mitosis is nuclear division that produces two daughter nuclei, each with the same number of chromosomes and the same genes as the parent nucleus. Because the DNA was replicated during interphase and the copies were then separated exactly, the daughter cells are genetically identical to each other and to the parent cell. That genetic uniformity matters: every cell of one organism needs the same set of instructions so that the body works as a single unit.

Growth of a multicellular organism happens because cells divide, not because each individual cell gets bigger. A human begins as one fertilised egg and reaches billions of cells by mitosis, and a plant grows in the same way from the dividing cells of its root and shoot tips.

Replacement and repair use the same process for a different purpose. Cells wear out and die: the lining of the small intestine is replaced every few days, the epidermis of the skin is constantly shedding cells from its surface, and red blood cells are replaced continuously from stem cells in the bone marrow. When skin is cut, mitosis in the basal layer of the epidermis and in the dermis closes the wound, and a broken bone is repaired by the division of cells in the periosteum.

Asexual reproduction produces a new organism from a single parent by mitosis alone, so the offspring are clones. A potato plant produces tubers, a strawberry plant produces runners, and a gardener can grow a whole new plant from a cutting. Asexual reproduction is quick and needs no second parent, but it creates no genetic variation, so a population of clones can all be destroyed by the same disease or the same change in conditions.

• Growth: an increase in the number of cells, each one genetically identical to the parent cell.
• Replacement: worn out or dead cells, such as the epidermis and the lining of the small intestine.
• Repair: the healing of a wound or a fracture by cell replacement.
• Asexual reproduction: tubers, runners, cuttings and micropropagation all produce clones.
• The point of mitosis is not only making new cells but making them genetically identical.

The mitotic cell cycle: interphase, mitosis and cytokinesis

The mitotic cell cycle is the sequence of events between one division and the next. It has two main parts: interphase, when the cell grows and replicates its DNA, and the division itself, which consists of mitosis (division of the nucleus) followed by cytokinesis (division of the cytoplasm). Interphase is much the longest part of the cycle, so in a photograph of dividing tissue most of the cells will be in interphase.

Interphase has three phases, and the syllabus expects you to name them. In G1 the cell grows, transcribes genes and synthesises proteins and organelles, and carries out its normal function. In S the DNA is replicated, so every chromosome becomes two sister chromatids, and histone proteins are made at the same time; the amount of DNA in the nucleus therefore doubles, although the number of chromosomes does not change. In G2 the cell grows further, checks the newly made DNA for damage, and makes the proteins needed for the spindle. Animal cells also duplicate their centrioles during interphase.

Mitosis follows interphase, and cytokinesis then divides the cytoplasm so that two daughter cells form, each entering its own interphase. The cycle is controlled: checkpoints at the end of G1, at the end of G2 and at metaphase allow division to be halted if the DNA is damaged or if the chromosomes are not correctly attached to the spindle. Only cells that pass those checks go on to divide.

Read a graph of DNA content with care. The amount of DNA per cell is at its base level during G1, doubles during S phase, stays doubled through G2, mitosis and telophase, and falls back to the base level at cytokinesis when the cell splits in two. The number of cells does not increase until cytokinesis has happened: mitosis on its own leaves two nuclei in one cell.

Telomeres: preventing the loss of genes from the ends of chromosomes

A linear DNA molecule has a problem at its ends. DNA polymerase can only add nucleotides to an existing 3' end, so it cannot start a new strand from nothing. When the short RNA primer at the very end of a strand is removed, that stretch cannot be replaced, so the new DNA molecule is slightly shorter than the template it was copied from. If the trimmed sequence were part of a gene, the cell would lose genetic information every time it divided.

Telomeres solve that problem. Each telomere is a long run of short, repetitive, non-coding DNA sequences with associated proteins, sitting at the very end of every chromatid. Because the sequence is repetitive and codes for nothing, the shortening happens inside the telomere instead of inside a gene, and the buffer is long enough to last through many rounds of division. The telomere also stops the end of the chromosome being mistaken for a break and joined to another chromosome.

The enzyme telomerase can rebuild the buffer. It carries its own short RNA template and uses it to extend the DNA end, so repeats are added back. Telomerase is active in germ cells, in many stem cells and in the great majority of cancer cells, but it is not active in most ordinary body cells. That is why telomeres shorten with every division in somatic cells and eventually become short enough that the cell stops dividing and may die, which is one reason a normal body cell can only divide a limited number of times.

Named case: dyskeratosis congenita is a rare inherited condition caused by mutations in genes for the telomerase complex, including DKC1. Telomeres are shorter than normal from an early age, and the bone marrow fails to replace blood cells properly. It shows how much tissues that divide often depend on the telomere buffer.

• Telomeres are repetitive, non-coding DNA sequences at the ends of chromatids.
• They act as a buffer that is eroded instead of the genes, because DNA polymerase cannot copy the very end of a linear molecule.
• Telomerase carries an RNA template and adds repeats back; it is active in germ cells and many stem cells, and in most cancer cells.
• In most body cells telomeres shorten with each division, which is linked to the limited number of times a cell can divide.
• A telomere also protects the chromosome end from being treated as a break and rejoined incorrectly.

Stem cells: cell replacement and tissue repair by mitosis

A stem cell is an unspecialised cell that can divide by mitosis to produce copies of itself and can differentiate into one or more other cell types. Because they keep dividing, stem cells provide a permanent supply of replacement cells for tissues that lose cells all the time.

Stem cells are classified by how many cell types they can become. Totipotent cells, such as the zygote and the cells of a very early embryo, can form a whole organism. Pluripotent embryonic stem cells, taken from the inner cell mass of a blastocyst, can form any of the body cell types but not the placenta. Multipotent adult or tissue stem cells are restricted to a related group of cells: the haematopoietic stem cells of the bone marrow give rise to red blood cells, all the white blood cell types and platelets. In plants, the meristems at root and shoot tips are regions of undifferentiated dividing cells that produce every plant tissue.

Cell replacement in an adult depends entirely on these cells dividing by mitosis. The cells at the base of the epidermis divide continuously to replace the cells shed from the surface, the lining of the small intestine is renewed from stem cells in the crypts, and the whole population of blood cells is replaced several times a year from the bone marrow. Stem cells in a tissue usually divide rarely, and the division is often asymmetric: one daughter remains a stem cell and the other differentiates, which keeps the supply going without producing too many cells.

Because a stem cell divides by mitosis, the cells it produces are genetically identical to it apart from any mutation. That is why a transplant of a patient's own stem cells is not rejected by the immune system, and why stem cells from a donor are treated as foreign tissue unless the donor is a close match.

• Self-renewal: a stem cell divides by mitosis to make more stem cells.
• Differentiation: the daughter cells become specialised cells of one or more types.
• Haematopoietic stem cells in bone marrow replace the whole blood cell population.
• Meristems at plant root and shoot tips supply all the new plant cells.
• A bone marrow transplant relies on donated stem cells dividing by mitosis to rebuild the patient's blood system.

When control is lost: uncontrolled cell division and tumours

The cell cycle is controlled at checkpoints by signalling proteins, including cyclins and the kinases they activate. If DNA is damaged, the cycle stops at the G1 or the G2 checkpoint so the damage can be repaired; if the spindle checkpoint does not detect correct attachment of every chromosome, anaphase is delayed. This control is what keeps the number of cells in each tissue at the right level.

Uncontrolled division begins with a mutation in a gene that controls the cycle. A change in a proto-oncogene can turn it into an oncogene that stimulates division when it should not, and a change in a tumour suppressor gene, such as TP53 which codes for the p53 protein, can remove a brake on the cycle. The mutated cell divides more often than it should, and if the change is passed on to the daughter cells they do the same, so the number of cells increases without limit.

The result is a tumour, an abnormal mass of cells formed by uncontrolled cell division. A benign tumour grows by cell division but stays as a compact, encapsulated mass that does not invade the surrounding tissue or spread. A malignant tumour invades and destroys the tissue around it, and its cells can break away and travel in the blood or lymph to form secondary tumours elsewhere - this spread is called metastasis, and it is what makes a tumour a cancer. Malignant tumours also compete with normal tissue for nutrients, so a patient may lose weight and become weak.

Anything that damages DNA and so causes mutation increases the risk of losing this control: tobacco smoke, ionising radiation including ultraviolet light, and some viruses. Human papillomavirus infects cells of the cervix and its proteins interfere with tumour suppressor proteins, which is why a persistent HPV infection is the main cause of cervical cancer and why vaccination against HPV reduces the risk. Treatments are aimed at dividing cells: the chemotherapy drug vinblastine, obtained from the rosy periwinkle, stops the spindle from working. Because such drugs affect every fast-dividing tissue, patients lose their hair and their bone marrow is temporarily suppressed.

Chromosome behaviour: prophase, metaphase, anaphase and telophase

Prophase. The chromatin condenses until the chromosomes are visible as two sister chromatids held together at the centromere. The nuclear envelope breaks down into small vesicles, so the nucleus is no longer enclosed. The spindle, made of microtubules, forms from the two poles of the cell: in animal cells the centrioles have already duplicated and now move apart and organise the spindle, while plant cells, which have no centrioles, still form a spindle from the pole regions. The nucleolus disappears.

Metaphase. Spindle fibres attach to the centromere of each chromosome, with fibres from each pole attached to opposite sides. The chromosomes are moved until they lie along the equator of the cell, also called the metaphase plate, each still made of two chromatids. The spindle checkpoint confirms that every chromosome is attached before anaphase is allowed to begin.

Anaphase. The centromeres split, so the two sister chromatids of each chromosome separate. The spindle fibres shorten and draw each chromatid towards the pole they are attached to, and the poles themselves move further apart, so the chromatids are carried to opposite ends of the cell. Each separated chromatid is now a daughter chromosome in its own right.

Telophase. The daughter chromosomes reach the two poles and lose their condensed packing, becoming chromatin again. A new nuclear envelope forms around each set from the vesicles of the old one, the nucleolus reappears and the spindle breaks down. The cell now contains two nuclei with identical sets of chromosomes, and cytokinesis divides the cytoplasm between them.

Cytokinesis in plant and animal cells, and identifying stages in photographs

In an animal cell a cleavage furrow forms at the equator during cytokinesis. A ring of actin and myosin microfilaments just inside the cell surface membrane contracts and draws the membrane inwards, until the cell is pinched into two. The cell surface membrane is directly involved, the cell becomes rounded as it divides, and the two new cells stay joined for a while by a narrow bridge that eventually breaks.

A plant cell cannot do this, because the rigid cellulose cell wall cannot be pulled inwards. Instead, vesicles from the Golgi apparatus carry wall material to the equator and line up there. The vesicles fuse to form a cell plate, which grows outwards until it meets the existing cell wall. The plate becomes the middle lamella between the two cells, and each side of it is finished with a new cell surface membrane and a new cell wall, so each daughter cell has a complete wall of its own.

Plant and animal mitosis also differ at the spindle. An animal cell has a pair of centrioles at each pole, which organise the microtubules of the spindle. A plant cell has no centrioles, but microtubules are still organised into a spindle from the pole regions, so a spindle is present in both. Every other part of the process, including the behaviour of the nuclear envelope, is the same.

When you are given a photomicrograph, a diagram or a prepared slide, identify the stage from the chromosomes. In prophase the chromosomes are visible as distinct threads or rods, with no plate and no separation. In metaphase they form a clear line across the middle of the cell, and each chromosome looks like an X because its chromatids are still joined at the centromere. In anaphase the two sets are moving apart, and the separating chromatids look V-shaped, with the point of the V leading towards the pole. In telophase the two sets have reached opposite ends and each is becoming a nucleus again, and a cell plate or furrow may be visible. Count several fields of view, because a single field can mislead you: the stage you see most often is the stage that lasts longest, and that is interphase.