Interphase

Interphase is the stage of the cell cycle between one cell division and the next. During interphase the cell grows, makes new proteins and organelles, and copies all of its DNA so that it is ready to divide. It has three parts, G1, S and G2, and a dividing cell spends most of its time in interphase.

What happens during interphase?

A cell that divides goes round a repeating cell cycle: interphase, then mitosis, then cytokinesis. Mitosis and cytokinesis are short. Interphase is the long preparation in between, and it is split into three phases. The times below are for fast-dividing mammalian cells grown in a lab, which take about 24 hours for one cycle.

Adding them up, interphase takes 9 + 10 + 4.5 = 23.5 of the 24 hours, about 98% of the cycle. That is why, when you look at dividing tissue under a microscope, almost every cell you see is in interphase.

G1 phase

G stands for gap, from the days when interphase looked like a quiet gap between divisions. In fact G1 is busy. The cell gets bigger and makes the sub-cellular structures it will need to share between two daughter cells, such as ribosomes and mitochondria. It also makes the building blocks for copying its chromosomes.

S phase

S stands for synthesis, meaning DNA synthesis. Every chromosome is copied by semi-conservative replication, and the two copies stay joined at the centromere as sister chromatids. The number of chromosomes doesn't change, but the amount of DNA doubles. A human cell in G2 still has 46 chromosomes, but each one has two chromatids, so it holds 92 chromatids. The DNA replication guide covers how the copying works.

G2 phase

In G2 the cell keeps growing, tops up its energy stores and makes the proteins it needs to move chromosomes during mitosis. It also checks that every chromosome has been copied completely and without damage before it goes any further.

DNA content during interphase

The DNA in a cell stays constant through G1, doubles steadily during S, and stays doubled through G2 and mitosis. It halves only when the cell divides and each daughter cell takes one copy. In the graph, x is the time in hours since the start of G1 and y is the DNA per cell in relative units, for the 24-hour cycle in the table.

Is interphase part of mitosis?

No. Interphase is part of the cell cycle, but it is not a stage of mitosis. Mitosis is the division of the nucleus, and it has four stages: prophase, metaphase, anaphase and telophase. Interphase comes before prophase, and cytokinesis comes after telophase. The mitosis guide covers those four stages.

Interphase is sometimes called a resting stage, but it isn't resting at all: it is when the cell does almost all of its growing and all of its DNA copying. It only looks quiet because the chromosomes are spread out and can't be seen.

Try one: identify a stage of the cell cycle, then press Mark to check your answer in the Cell Biology course.

What does interphase look like under a microscope?

• The nucleus is clear and round, with its nuclear envelope intact.
• You can't see separate chromosomes. During interphase the DNA stays as loosely packed chromatin, so the nucleus looks evenly stained or grainy.
• One or more darker spots, the nucleoli, are often visible inside the nucleus. They disappear in prophase.

In a stained root tip squash, the cells with visible, thread-like chromosomes are the ones in mitosis. Everything else is in interphase. The root tip slide in the Virtual Microscope lets you find both and count them.

G0 and the cell cycle checkpoints

Not every cell keeps cycling. AQA's A-level specification notes that within multicellular organisms, not all cells retain the ability to divide. A cell can leave the cycle after G1 and enter G0, where it does its job but doesn't prepare to divide. Some cells sit in G0 for a while, for example when nutrients or growth signals are missing. Others, such as most nerve cells and heart muscle cells, stay there for life.

Interphase also contains checks that stop a faulty cell from dividing:

• G1 checkpoint: is the cell big enough, with enough reserves, and is its DNA undamaged? If not, it can pause, repair or enter G0.
• G2 checkpoint: has all the DNA been copied, and copied without damage? If not, the cycle stops until replication or repair is finished.

A third check, the M or spindle checkpoint, happens during mitosis. When damage can't be fixed, a protein called p53 can make the cell destroy itself. Cancer often involves mutations that break these controls, so cells keep dividing when they shouldn't.

Worked examples

Example 1: time in interphase from a cell count

A student counts 240 cells in a root tip squash. 216 are in interphase and the rest are in mitosis. One cell cycle takes 16 hours. How long does a cell spend in interphase, and in mitosis?

1. Fraction in interphase = 216 ÷ 240 = 0.9, so 90% of cells.
2. The share of cells at a stage matches the share of time spent in it, so interphase lasts 0.9 × 16 = 14.4 hours.
3. Mitosis takes the other 10%: 0.1 × 16 = 1.6 hours = 96 minutes. The mitotic index is 24 ÷ 240 = 0.1.

Example 2: which phase are most cells in?

In the 24-hour cycle in the table, what percentage of cells would you expect to find in S phase at any moment? S lasts 10 of the 24 hours, so 10 ÷ 24 × 100 = 42% (to the nearest whole number). More cells are in S than in any other single phase, because it is the longest.

Example 3: chromosomes and chromatids

A cell from a fruit fly has 8 chromosomes in G1. How many chromosomes and chromatids does it have in G2? S phase copies the DNA but keeps each copy joined, so it still has 8 chromosomes, now with 16 chromatids.

Common mistakes

• Calling interphase a stage of mitosis. It is a stage of the cell cycle. Mitosis is prophase, metaphase, anaphase and telophase.
• Saying the cell is resting. Interphase is when growth and DNA replication happen.
• Saying the chromosome number doubles in S phase. The DNA doubles, but each chromosome now has two chromatids, so the number of chromosomes stays the same.
• Putting DNA replication in G1 or G2. It happens only in S phase.
• Forgetting the organelles. At GCSE, the cell must grow and increase the number of sub-cellular structures, such as ribosomes and mitochondria, as well as copy its DNA.

To practise cell cycle questions alongside other topics, try today's GCSE Biology Higher or A-level Biology: Year 1 short timed paper.

Read next

Biology · CellsThe cell cycle GCSE: the three stages explainedThe cell cycle for GCSE Biology: the three stages, what happens to DNA and organelles, why mitosis matters, exam answers, and what A-level adds.