The DNA in the nucleus of a cell is the information source for making proteins. If the information is changed, different proteins will be made. The basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA. This creates two copies of the DNA in a reproducing cell and they need to get separated from each other. DNA copying is accompanied by the creation of an additional cellular apparatus, and then the DNA copies separate, each with its own cellular apparatus.

Introduction:
Proteins are essential macromolecules that play a crucial role in various cellular processes. They are involved in structural support, enzymatic reactions, transportation, signaling, and many other functions. The process of protein synthesis begins with the information encoded in the DNA, which is then transcribed into RNA and translated into proteins. Therefore, DNA acts as the primary source of information for making proteins in a cell.

Explanation:
DNA:
- Deoxyribonucleic Acid (DNA) is a molecule that carries the genetic information in most organisms.
- It is composed of two strands twisted into a double helix structure, with each strand made up of nucleotides.
- The sequence of nucleotides in DNA determines the genetic code and provides instructions for the synthesis of proteins.
- The genetic code is a set of instructions that specifies the order of amino acids in a protein.
- The DNA sequence is transcribed into RNA in a process called transcription, which occurs in the nucleus of the cell.

RNA:
- Ribonucleic Acid (RNA) is a single-stranded molecule that plays a vital role in protein synthesis.
- There are three main types of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).
- mRNA carries the genetic information from DNA to the ribosomes, where protein synthesis occurs.
- tRNA helps in the translation of mRNA into a sequence of amino acids during protein synthesis.
- rRNA forms the structural and functional components of ribosomes, the cellular machinery responsible for protein synthesis.

Protein Synthesis:
- The process of protein synthesis involves two main steps: transcription and translation.
- During transcription, an enzyme called RNA polymerase binds to the DNA and synthesizes a complementary RNA strand using the DNA template.
- This newly synthesized RNA molecule, called mRNA, carries the genetic information from the nucleus to the cytoplasm.
- In the cytoplasm, translation takes place, where the mRNA is read by ribosomes, and the sequence of nucleotides is converted into a sequence of amino acids.
- tRNA molecules bring the appropriate amino acids to the ribosomes, guided by the codons (three-nucleotide sequences) on the mRNA.
- The amino acids are joined together in the correct order to form a polypeptide chain, which then folds into a functional protein.

Conclusion:
DNA acts as the primary source of information for making proteins in a cell. It carries the genetic code that specifies the sequence of amino acids in a protein. This information is transcribed into RNA, which is then translated into proteins. Therefore, the correct answer is option 'A' - DNA.

But mRNA transfer the coding of the protein. Then why It happens ?