Within multicellular organisms, not all cells retain the ability to divide.

The idea

Differentiation makes cells specialised, and many differentiated cells no longer divide. A mature mammalian red blood cell loses its nucleus, so it has no nuclear DNA to replicate and cannot divide. A phloem sieve tube element also loses its nucleus at maturity; it retains mitochondrial and plastid DNA but lacks nuclear DNA, so it cannot divide. Neurones and mature skeletal muscle fibres are terminally differentiated: they leave the cell cycle and enter a non-dividing state called G0. Xylem vessel elements are dead at maturity. Some differentiated cells, such as hepatocytes, can re-enter the cell cycle to repair tissue, but division is mostly restricted to specific populations: stem cells in bone marrow, intestinal crypts and the basal layer of the skin, and plant meristems at root and shoot tips.

What earns the marks

• Naming a cell that cannot divide and giving the structural reason, for example a mature red blood cell lacks a nucleus and so cannot replicate its DNA.
• Explaining that terminally differentiated cells leave the cell cycle and enter a non-dividing state known as G0.
• Naming a site where actively dividing cells are found, such as a plant meristem, bone marrow or an intestinal crypt.
• Linking the retention of the ability to divide to the growth, repair or replacement of specific tissues.
• Distinguishing differentiation (becoming specialised) from division (producing more cells), for example a neurone is specialised but does not divide.

Common mistakes and how to put them right

• Stating red blood cells do not divide without giving the structural reason, which is that they lack a nucleus.

  Put it right: State that the nucleus is absent, so nuclear DNA cannot be replicated.

• Saying a phloem sieve tube element has no DNA at all.

  Put it right: It lacks a nucleus and nuclear DNA but retains mitochondrial and plastid DNA; the absence of nuclear DNA is why it cannot divide.

• Assuming every cell in a multicellular organism is actively progressing through the cell cycle, whereas many enter G0.

  Put it right: State that many differentiated cells have left the cycle.

• Sampling a mature region of a plant root rather than the meristematic tip when a dividing cell population is required.

  Put it right: Use the root tip, where meristematic cells are dividing.

• Confusing differentiation, which is becoming specialised, with division, which is producing more cells.

  Put it right: Keep the two processes distinct.

Exam tips

• When asked why a specific tissue does not repair, name the cell, state it is terminally differentiated or lacks a nucleus, and give the consequence.
• Learn one animal and one plant site of active division (for example bone marrow and root tip meristem) to use as concrete examples.