5. The number of polypeptide chains in a core nucleosome is ____.

Core Nucleosome Structure: Number of Polypeptide Chains in a Core Nucleosome

Introduction

Packaging nearly two meters of DNA into the microscopic nucleus of a eukaryotic cell is one of the greatest structural achievements in biology. This remarkable organization is made possible by chromatin, a highly ordered complex of DNA and histone proteins. The fundamental repeating unit of chromatin is the nucleosome, which compacts DNA while simultaneously regulating essential cellular processes such as transcription, DNA replication, recombination, DNA repair, and chromosome segregation.

The nucleosome consists of DNA wrapped around a protein core known as the histone octamer. This octamer is composed of eight individual histone polypeptide chains belonging to four different histone families.

Correct Answer

Correct Answer: 8

Detailed Explanation

The protein core of a nucleosome is known as the histone octamer. It is composed of eight histone polypeptide chains arranged in a highly symmetrical structure. Specifically, the octamer contains:

• Two molecules of Histone H2A
• Two molecules of Histone H2B
• Two molecules of Histone H3
• Two molecules of Histone H4

Since each histone molecule represents one polypeptide chain, the total number of histone polypeptide chains present in the core nucleosome is:

2 + 2 + 2 + 2 = 8

Approximately 146 base pairs of DNA wrap around this histone octamer nearly 1.65 times to form the nucleosome core particle.

Composition of the Core Nucleosome

Step-by-Step Organization of the Histone Octamer

The nucleosome core forms through an ordered assembly process. Initially, two H3 molecules and two H4 molecules associate to form a stable (H3-H4)₂ tetramer. Subsequently, two H2A-H2B dimers bind to opposite sides of this tetramer, completing the histone octamer. Finally, approximately 146 base pairs of DNA wrap around the octamer to produce the nucleosome core particle.

Difference Between Core Nucleosome and Complete Nucleosome

Role of Individual Histone Proteins

Structure of the Nucleosome

The nucleosome is composed of two major components. The first component is the histone octamer, which contains eight histone polypeptide chains. The second component is approximately 146 base pairs of DNA, which wrap around the histone core approximately 1.65 turns. Between adjacent nucleosomes lies linker DNA, which is associated with the linker histone H1 and contributes to higher-order chromatin folding.

Biological Significance

Nucleosomes are not merely structural units for DNA packaging; they also regulate gene expression by controlling DNA accessibility. Chemical modifications of histone proteins, including acetylation, methylation, phosphorylation, ubiquitination, and sumoylation, influence chromatin structure and determine whether genes remain transcriptionally active or inactive. Nucleosome positioning also affects DNA replication, DNA repair, recombination, chromosome condensation, and epigenetic inheritance.

Key Facts to Remember

Final Answer

A core nucleosome consists of a histone octamer containing:

2 × H2A + 2 × H2B + 2 × H3 + 2 × H4 = 8 polypeptide chains

Correct Answer: 8