Slooze Worm Mutagenesis

The Slooze worm is a creature that lives in the heavily polluted waters of Lake Asbestozene.

The Slooze worm's genome is one double-stranded DNA molecule composed of five nucleotide base pairs. Each of the Slooze worm's eight cells contains one copy of the genome. (A cell containing only one copy of its genome is called a haploid cell. If it had two copies, the cell would be diploid.)

In order for the Slooze worm to grow from one cell into an eight-celled organism, it must divide three times.

Cell Division 1

Before a cell divides, it must copy its DNA, in a process called "DNA replication."

First, the double-stranded DNA must be separated into two single-stranded molecules.

Then the cell makes an exact copy of its DNA (red), using the original DNA (black) as a template. To do this, the cell follows the rules of base-pairing, which means that A nucleotides pair with Ts, and G nucleotides pair with Cs.

The result is two copies of the same DNA molecule, each of which goes to a new cell during cell division.

Cell Division 2

Lake Asbestozene contains many harmful environmental agents called mutagens. Mutagens can damage nucleotides, causing them to look different to the cell. As a result of their new look, the damaged nucleotides pair with the wrong partner during DNA replication.

Before the second round of cell division, a mutagen from Lake Asbestozene damages the first G nucleotide in one cell (purple G inside of orange star). The damage makes this G look more like an A, so when the cell replicates its DNA, the damaged G pairs with a T nucleotide, instead of with the usual C nucleotide.

After the double-stranded DNA is copied, each new DNA molecule goes to a new cell during cell division. This results in one mutated cell and three normal cells.

Cell Division 3

No new DNA damage occurs, and the four cells divide to make eight new cells, creating a mature Slooze worm.

Question: How many of the eight cells will have mutations after the third cell division? Assume that the purple G (in the orange star) still looks like an A nucleotide to the cell, and pairs with other Ts. Note that the mismatched T still looks the same to the cell, and will pair correctly with other A nucleotides.