Transcription

The first part of protein synthesis is called transcription. In general, transcription is the formation of mRNA from the template DNA strand to be used to build proteins. The template strand provides the sequence, and the complementary strand is its mirror; for example, if the template strand reads AGCCGA, the complementary strand will read UCGGCU. In order for transcription to happen, the enzyme RNA polymerase (a protein) must be present, along with RNA nucleotides. Transcription happens in three main steps: initiation, elongation, and termination.

The first step of transcription is called initiation and involves a promoter. A promoter is the sequence of DNA to which RNA polymerase binds to initiate the process of transcription. Promoters are very important to the process because they determine where the transcription of the DNA will start and which of the two DNA strands will be used for the process. The promoters are aligned in a certain direction, which helps point the RNA polymerase the right way. The initiation site of the promoter is where transcription will begin. Specialized proteins in eukaryotes that aid in transcription, called transcription factors, bind to various regions of the DNA and influence which genes are expressed at any given time in the cell.

After the RNA polymerase is bound to a promoter site, it can begin the second step of protein synthesis. Elongation is the step in transcription that results in the addition of nucleotides to the growing mRNA transcript. The RNA polymerase unwinds the DNA and reads it from the 3′ to the 5′ end. This convention describes the orientation of the strand, with the 3′ representing the third carbon in the sugar (with a hydrogen attached) and the 5′ representing the fifth carbon in the sugar (with the phosphate group attached). RNA nucleotides bind to the unwound DNA following base pairing rules (cytosine with guanine and uracil with adenine). The polymerase catalyzes the formation of bonds between nucleotides, resulting in an mRNA sequence complementary to the DNA sequence of the gene. Unlike DNA, RNA is not proofread (checked for errors) while, or after, it is formed. Therefore, there is a much greater chance of errors happening. However, RNA does not last very long, so the mutations it may possess do not have as great an impact as those found in DNA. The end of transcription is called termination. Here, a specific base sequence codes for how and when the transcription of the DNA should end and the mRNA transcript is released.

Once the mRNA has been transcribed in the nucleus, it undergoes several modifications before it is ready to leave. One modification is the addition of a cap to the 5′ end. This cap is a modified form of GTP (guanosine triphosphate) and helps the mRNA bind with the ribosome. It also protects the mRNA from being destroyed too quickly by enzymes that break down RNA. Another modification is the addition of a poly-A tail to the 3′ end of the mRNA. This is a collection of adenine nucleotides that help stabilize the new mRNA molecule as it moves from the nucleus out into the cytoplasm. The last step in the preparation of the mRNA is RNA splicing. In eukaryotes, genes contain both coding and non-coding regions of sequence. The DNA sequence within a gene sequence that codes for a specific protein that must be combined during mRNA processing is called an exon. The DNA sequence within a gene sequence that does not code for a specific protein and must be removed from the mRNA during processing is called an intron. These contain regulatory sequences and, if left in place, would produce non-functional protein chains. A large molecule made of RNA and proteins that removes introns and joins the adjacent exon ends to form the mature strand of mRNA is called a spliceosome. It attaches to intron sequences, releasing the intron. The spliceosome then joins the adjacent exon ends to form the mature strand of mRNA, ready to leave the nucleus.