Eukaryotic cells that do retain the ability to divide show a cell cycle.

The idea

A cell that can divide passes round a cycle made of two very unequal parts. Interphase occupies most of the time and splits into three. In G₁ the cell grows, makes organelles and synthesises the proteins and enzymes needed for replication. In S phase the DNA is replicated semi-conservatively, so that each chromosome comes to consist of two identical sister chromatids joined at a centromere. In G₂ the cell continues to grow and the replicated DNA is checked. Then comes the much shorter mitotic phase: nuclear division by mitosis, followed immediately by cytokinesis, the division of the cytoplasm, giving two genetically identical daughter cells that can enter the cycle again. Interphase is sometimes carelessly called a resting phase; it is in fact the most metabolically active part of the cycle.

What earns the marks

• Interphase being the longest stage and being made up of G₁, S and G₂.
• The linear DNA being replicated during S phase so that each chromosome becomes two identical sister chromatids.
• Mitosis followed by cytokinesis producing two genetically identical daughter cells.
• Placing the cell cycle in eukaryotic cells, with replication of linear DNA, in contrast to binary fission and circular DNA in prokaryotes.
• Growth and protein synthesis during G₁ and G₂ preparing the cell for division.

Common mistakes and how to put them right

• Calling interphase a resting stage, or saying nothing happens during it.

• Drawing the cycle with mitosis taking up most of the time.

• Listing interphase as one of the stages of mitosis rather than of the cell cycle.

• Attributing a cell cycle to bacteria, which divide by binary fission instead.

• Saying the DNA replicates during mitosis, when replication occurs in S phase of interphase.

Exam tips

• If a table asks you to tick features against mitosis and binary fission, remember replication of linear DNA and eukaryotic cells belong to the cell cycle, replication of circular DNA and prokaryotic cells to binary fission, and two daughter cells to both.
• Quote the sub-stages G₁, S and G₂ by name; interphase alone is often not enough for a describe mark.
• Use the proportions of the cycle to explain why most cells in a stained root tip are in interphase.