Experimental evidence demonstrates that the nucleosomes present in a cell after the completion of S phase are composed of some 'old' histone dimers and some newly synthesized histone dimers. Describe the general design for an experiment that uses a protein label such as ³⁵S to show that nucleosomes are often a mixture of old and new histone dimers following DNA replication.

Verified step by step guidance

1. Design an experiment where cells are grown in a medium containing ³⁵S-labeled methionine during the S phase of the cell cycle. This will incorporate the radioactive label into newly synthesized histone proteins.
2. After the S phase, isolate the chromatin from the cells to analyze the nucleosomes.
3. Use a method such as SDS-PAGE followed by autoradiography to separate and visualize the histone proteins. This will allow you to detect the presence of ³⁵S-labeled histones.
4. Compare the labeled histones to unlabeled histones by using a control group of cells grown without ³⁵S-labeled methionine. This will help determine the proportion of new histones incorporated into nucleosomes.
5. Analyze the results to determine the presence of both labeled (new) and unlabeled (old) histone dimers in the nucleosomes, indicating a mixture of old and new histones after DNA replication.

Key Concepts

Nucleosome Structure

Nucleosomes are the fundamental units of chromatin, consisting of DNA wrapped around a core of histone proteins. Each nucleosome is made up of two copies of four different histone proteins (H2A, H2B, H3, and H4), forming an octamer. Understanding nucleosome structure is essential for grasping how histones can be old or newly synthesized after DNA replication.

DNA Replication and Histone Dynamics

During DNA replication, the original histones are distributed between the two daughter strands, while new histones are synthesized to fill in the gaps. This process results in nucleosomes that contain a mix of old and new histone dimers. Recognizing this dynamic is crucial for designing experiments that track histone incorporation into nucleosomes post-replication.

Protein Labeling Techniques

Protein labeling techniques, such as using radioactive isotopes like ³⁵S, allow researchers to trace the incorporation of specific proteins into cellular structures. By labeling newly synthesized histones, one can distinguish between old and new histone dimers in nucleosomes. This method is vital for demonstrating the mixture of histone types in nucleosomes after DNA replication.