Beginning within the nucleus, the first step leading to the synthesis of a polypeptide is _____.

The synthesis of a Polypeptide chain or Protein begins by the coding of the amino sequence, first, from the DNA sequence code into an intermediate mRNA sequence code. The mRNA sequence code is then converted into the amino acid sequence code.
The genetic material is comprised of genes, each of which encodes for a particular protein.
The DNA (deoxyribonucleic acid) sequence code of the gene is converted into an intermediate mRNA (messenger Ribonucleic Acid) by an enzyme called the DNA dependent RNA polymerase by utilising ribonucleotides and ATP (adenosine triphosphate as energy). This process is known as Transcription.
Transcription happens inside the cell's nucleus.
The resulting intermediate mRNA message is transported from the nucleus into the cytoplasm.
In the cytoplasm {or on Endoplasmic Reticulum (ER)}, the mRNA message is recognised by the Ribosomes (protein synthesising machinery) and are converted into polypeptide sequence by using amino-acyl tRNA synthase and ATP ( as energy). This process is known as Translation.
The polypeptide chain folds in the proper orientation to give rise to the functional proteins.