Problem 9 - Campbell Biology Concepts & Connections

Question

A cell containing a single chromosome is placed in a medium containing radioactive phosphate so that any new DNA strands formed by DNA replication will be radioactive. The cell replicates its DNA and divides. Then the daughter cells (still in the radioactive medium) replicate their DNA and divide, and a total of four cells are present. Sketch the DNA molecules in all four cells, showing a normal (nonradioactive) DNA strand as a solid line and a radioactive DNA strand as a dashed line.

Step-by-Step Solution

Answer

Two cells have one solid and one dashed strand, while the other two cells have both strands dashed.

Step 1: Initial DNA Replication

The original cell starts with a single chromosome, represented by a solid line. When the cell replicates its DNA, each of the original strands serves as a template for a new strand. Since the medium contains radioactive phosphate, the new strands will be radioactive (dashed lines). This generates two daughter cells, each with one original (solid) and one radioactive (dashed) DNA strand.

Step 2: DNA Replication in Daughter Cells

Both daughter cells still contain radioactive phosphate in the medium. Each of these two DNA molecules again replicates. Each original strand in the daughter cells serves as a template which leads to the creation of four DNA molecules. For the first daughter cell, one original solid strand pairs with a new radioactive strand forming one DNA molecule, and the radioactive strand pairs with a new radioactive strand to form the second DNA molecule. The same process happens for the second daughter cell.

Step 3: Division of Daughter Cells

Each daughter cell now divides, creating a total of four cells. Each of these new cells receives a DNA molecule. In total, two cells will have a combination of one original (solid) and one radioactive (dashed) DNA strand, and the other two cells will have both DNA strands as radioactive (dashed).

Key Concepts

DNA replication

DNA replication is a crucial process that occurs within the cell. It ensures that each new cell has an identical copy of the DNA. DNA replication is semi-conservative, meaning that each daughter DNA double helix consists of one strand from the parent and one newly synthesized strand. This process begins with the unwinding of the double helix, followed by the pairing of free nucleotides with the exposed bases on the original strands.
This creates two double-stranded DNA molecules from the original one. This accurate duplication is vital for the proper functioning of cells in growth, repair, and reproduction.

radioactive labeling

Radioactive labeling is a technique used in biological experiments to track and identify molecules. By incorporating radioactive elements, such as radioactive phosphate, scientists can observe and trace the replication of DNA. In the context of this exercise, when DNA replicates in a medium containing radioactive phosphate, the newly synthesized DNA strands incorporate the radioactive isotope.
This radioactive labeling allows researchers to differentiate between original and newly synthesized DNA strands. It also provides information on the genetic material's distribution during cell division and DNA replication cycles.

cell division

Cell division is a fundamental process in biology, involving the division of a parent cell into two or more daughter cells. This process includes both the replication of DNA and the division of cellular components. The main types of cell division are mitosis and meiosis.
In the scenario given, mitosis is the focus, where the cell first duplicates its DNA and then divides, ensuring that each daughter cell receives a complete set of chromosomes. This ensures consistency in genetic information across cells. Mitosis is crucial for growth, development, and tissue repair in multicellular organisms.

chromosomes

Chromosomes are long DNA molecules that contain genetic material. Each cell in an organism typically contains a fixed number of chromosomes, specific to that species. Chromosomes ensure DNA is accurately replicated and distributed during cell division. Humans, for example, have 46 chromosomes in a diploid cell.
Chromosomes are replicated during the S-phase of the cell cycle and consist of two sister chromatids, each becoming a chromosome in the daughter cells post-division. This organized structure helps protect DNA integrity and ensures proper genetic inheritance during cell division.