Interphase, DNA Content and Counting Chromosomes

Interphase is not a rest

The cell cycle has two unequal parts. Interphase occupies most of the cycle and is divided into $G_1$, in which the cell grows and makes new organelles and proteins, $S$, in which the DNA is replicated so that every chromosome comes to consist of two identical sister chromatids, and $G_2$, in which the cell checks the copied DNA and makes the proteins needed for division. Only after all this does mitosis occur, followed by cytokinesis, the division of the cytoplasm. Describing interphase as a resting stage costs marks: it is the most metabolically active part of the cycle.

Reading a DNA content graph

A graph of DNA per nucleus against time is one of the most reliable ways of identifying stages. The amount is constant through $G_1$, rises steadily through $S$ until it has doubled, stays at the doubled value through $G_2$ and through prophase and metaphase, and then falls vertically back to the original value at anaphase and telophase, when the chromatids are pulled apart into two nuclei. Note carefully what does not change: the number of chromosomes stays the same during $S$ phase, because the two copies remain joined at the centromere and count as one chromosome until they separate.

Chromosomes, chromatids and how to count them

Before $S$ phase a human cell has $46$ chromosomes, each a single DNA molecule. After $S$ phase it still has $46$ chromosomes, but $92$ chromatids and $92$ DNA molecules. At anaphase the centromeres split; each chromatid instantly becomes a chromosome in its own right, so a cell momentarily contains $92$ chromosomes. When the two nuclei form, each has $46$ again. Counting errors in this sequence are the single largest source of lost marks in the chapter.