S phase.. since DNA replication takes place during s... and the function of histone protein is to help in packaging of the DNA helix into structural units called Nucleosome..

Introduction:
Histone proteins are essential components of chromatin, the complex of DNA and proteins that make up chromosomes. They play a crucial role in packaging DNA and regulating gene expression. Synthesis of histone proteins is tightly regulated during the cell cycle to ensure proper chromatin organization and DNA replication.

Synthesis of Histone Proteins:
The synthesis of histone proteins occurs primarily during the S phase of the cell cycle.

S phase:
- The S phase is the phase of the cell cycle during which DNA replication takes place.
- It is the period when DNA is duplicated to produce two identical copies.
- During this phase, the cell's DNA polymerase enzyme synthesizes a new complementary strand of DNA for each original strand.

Role of Histone Proteins in DNA Replication:
- Histone proteins are responsible for packaging and organizing DNA into a compact structure called chromatin.
- They play a crucial role in DNA replication by helping to maintain the integrity and accessibility of the DNA molecule.
- During DNA replication, the histone proteins are temporarily removed from the DNA strand to allow the replication machinery to access the DNA.

Importance of Synthesis in S phase:
- Since synthesis of histone proteins is necessary for proper DNA replication and chromatin organization, it occurs during the S phase.
- The newly synthesized histone proteins are incorporated into the newly replicated DNA strands, ensuring that the chromatin structure is properly restored.

Other Cell Cycle Phases:
- The G1 phase is the period before DNA replication occurs. During this phase, cells grow and prepare for DNA synthesis.
- The G0 phase represents a non-dividing state where cells are neither actively dividing nor preparing for division.
- The G2 phase is the period after DNA replication has taken place but before cell division occurs.

Conclusion:
In summary, synthesis of histone proteins occurs during the S phase of the cell cycle. This is because histone proteins play a crucial role in DNA replication and chromatin organization, and therefore, need to be present during this phase to ensure proper DNA synthesis and packaging.