RNA synthesis starts differently in prokaryotes and eukaryotes. It always occurs in 5′ to 3′ direction. And it always requires the enzyme RNA polymerase. The first step in transcription is recognition of the ‘Start’; that point on the gene where the gene is starting. After recognition, it binds to a short sequence upstream the “start” site. The site where it binds is known as promoter site. The enzyme is quite large. It covers not only the promoter site, but also the starting part of the gene.
After proper binding of the enzyme with the promoter, the enzyme splits the DNA double helix over a short distance to form a transcription bubble. Remember, transcription will never occur without the formation of transcription bubble. Transcription bubble is formed so that a single template strand is available for transcription. Transcription process occurs only one strand of DNA. This strand is also known as non-coding or template strand. It is in 3’ to 5’ direction. Transcription attaches complementary nucleotides to this non-coding strand in 5’ to 3’ direction.
So, if you say the following statement about transcription,
Part a) “does it start from the start codon ‘TAC’ to ‘AUG’
then it is incorrect. Start codon plays role in starting translation, not transcription.
Part b) “does the mRNA synthesis start at the +1 site of the template part of the DNA?”
– is also slightly incorrect. Slight 5’ UTR is left in the gene before transcription starts. I wrote slightly incorrect because this 5’UTR does not affect protein synthesis.