Transcription requires a DNA sequence that signals the RNA polymerase where to begin transcribing a given gene. The promoter is the DNA sequence that allows the RNA polymerase to bind to the right spot on the DNA and begin transcription. This prevents transcription of partial proteins or protein fragments that would be non-functional and possibly harmful to the cell.

Transcription is the process of synthesizing RNA from a DNA template. In eukaryotic cells, chromosomal DNA is contained within the nucleus. Transcription requires access to this DNA, and therefore must occur in the nucleus. Resulting RNA molecules are then shuttled out of the nucleus to be used in other processes.

The DNA gets transcribed into RNA inside the nucleus. This is where DNA is housed; DNA never leaves the nucleus (except during mitosis, during which the nuclear envelope will briefly disappear so that two cells can be formed). After DNA gets transcribed into RNA, the RNA is modified and eventually transported out of the nucleus as mRNA, which is now ready for translation.

Translation occurs on ribosomes, which can either be bound to the rough endoplasmic reticulum or free-floating in the cytoplasm. The nucleolus is a structure within the nucleus where ribosomal RNA (rRNA) is produced and ribosomal subunits are assembled. Mitochondria are essential for cellular respiration and ATP synthesis. Lysosomes are responsible for digesting wastes and defective proteins.

When RNA polymerase binds to a template strand of DNA, it recruits complementary ribonucleotides to form a strand of RNA. This strand of RNA, however, is incomplete and must undergo post-transcriptional modification to become a mature mRNA product. The initial RNA transcript is known as heteronuclear RNA, or htRNA.

Introns are removed for the htRNA and a 5'cap and poly-A tail are added to convert it to mRNA.

Following DNA transcription, the resulting RNA molecule must be modified before leaving the nucleus. Proteins in the nucleus add a 5' cap to the 5' end of the RNA strand, and a poly-A tail to the 3' end. These additions help prevent degradation of the transcript by any hydrolytic enzymes in the cytosol. Protein complexes called spliceosomes interact with the transcript to remove segments of non-coding RNA called introns. The remaining transcript following the excision of introns is composed only of coding segments of RNA, known as exons.

Though introns are removed during post-transcriptional modification, exons are not inserted. Rather, they are simply the remaining RNA sequences after the introns have been spliced out.

RNA polymerase binds and transcribes the “non-coding strand." It acts as a template that is used to generate a RNA transcript that is complementary to the DNA strand that was transcribed. The term “coding strand” refers to the DNA strand with the identical sequence to the newly synthesized RNA. The coding strand can also be called the “non-template strand” or the “sense strand.” 

A coding sequence is the sequence of DNA that will code for a particular protein. In this process an messenger or mRNA molecule is transcribed from DNA and later translated into a protein by ribosomes. Messenger or mRNA is flanked by an upstream 5’ UTR untranslated leader sequence and a downstream 3’ UTR untranslated region that follows the termination codon of the synthesized protein.

The term “directionality” refers to the chemical orientation of a molecule. In DNA and RNA, directionality is described as 3’ (three prime) or 5’ (five prime). 3’ refers to the third carbon group in a sugar ring, which terminates in a hydroxyl group, and 5’ refers to the fifth carbon in the sugar ring. In transcription, RNA polymerase reads the template DNA strand in a 3’ to 5’ direction. Reading the template DNA strand in this direction allows transcription to proceed without primers and Okazaki fragments. This yields an RNA molecule that is formed in a mirrored 5’ to 3’ direction.

Termination is the final stage of transcription. During this stage, RNA polymerase reaches a terminator signal in the template DNA strand. This triggers the release of the transcriptional complex and the cleavage or release of the RNA transcript. Post cleavage, a sequence of adenines is added to the 3’ end of the new transcript through a process called polyadenylation. The resulting poly-A tail is important in translation, stability, and export of the transcript.

Promoter sequences are regions of DNA located upstream of transcription start sites. Transcription factors bind to the promoter sequence, which promotes the binding of RNA polymerase and initiation of transcription. Together, along with activators and repressors, these make up the pre-initiation complex. A well-characterized promoter sequence is called the TATA box, which is present within promoters of  of human genes.