Which of the following best describes a nucleosome?
A. DNA wrapped around a core of 4 histone proteins B. RNA wrapped around a core of 8 histone proteins C. DNA wrapped around a core of 8 histone proteins D. RNA wrapped around a core of 4 histone proteins
The Correct Answer and Explanation is :
The correct answer is C. DNA wrapped around a core of 8 histone proteins.
Explanation:
A nucleosome is the fundamental structural unit of chromatin in eukaryotic cells, playing a crucial role in organizing and compacting DNA within the nucleus. It is composed of DNA wrapped around a core of histone proteins. Here’s a breakdown of the structure:
- DNA and Histone Proteins: The nucleosome consists of about 146 base pairs of DNA wrapped around a core of eight histone proteins. The histone core is made up of two copies each of four different histones: H2A, H2B, H3, and H4. These proteins help package the DNA into a more compact structure, making it fit inside the cell’s nucleus.
- The Structure of the Nucleosome: The histone proteins form an octamer, which is a complex of eight proteins. The DNA wraps around this histone octamer approximately 1.65 times, forming a structure that looks like beads on a string. This arrangement significantly reduces the length of DNA, helping to fit it into the limited space of the nucleus.
- Role of Histones: Histones are positively charged because they have many amino acids with basic side chains. This positive charge allows them to interact with the negatively charged phosphate backbone of the DNA, helping to stabilize the structure. The histones themselves are highly conserved proteins, meaning they have a similar structure across many eukaryotic species, emphasizing their essential role in cellular functions.
- Chromatin Structure: The nucleosome is the basic unit of chromatin, which is further organized into higher-order structures that facilitate processes like gene regulation, DNA repair, and replication. Chromatin can be tightly packed into a structure called a chromosome during cell division.
Thus, nucleosomes are vital for packaging DNA into a manageable form, ensuring it is both protected and accessible for various cellular processes.