Transcription

Describe the process of transcription in biology

The process of transcription takes place in three steps: initiation, elongation, and termination. We will discuss each of these steps in the following section.

Initiation

Initiation begins with the enzyme RNA polymerase binding to a specific sequence on the DNA double-strand known as the promoter, which signifies the start of the gene. This step forms the RNA polymerase-promoter closed complex.

The DNA then unwinds at the promoter region, creating what is called an open complex. The portion of the DNA double-strand that unwound then forms a transcription bubble.

The RNA polymerase binds to a region called the transcription start site in the transcription bubble. The RNA polymerase is then ready to “read” the bases in the sequence of the unwound DNA strand and produce RNA with a complementary base sequence.

Elongation

The RNA polymerase “reads” the bases by traveling along the DNA strand from 3′ → 5’. As it travels through the strand, it “copies” the strand by adding complementary base pairs from 5′ → 3′.

Recall that the nucleotide bases in DNA pair up as follows:

• Adenine (A) pairs with thymine (T)

• Cytosine (C) always pairs with guanine (G)

For example, a guanine (G) in DNA would indicate the addition of a cytosine (C) into the RNA. Similarly, a thymine (T) in DNA will be copied into an adenine (A) in RNA.

We cannot emphasize this enough: it is important to note that during the elongation process, an adenine (A) would be copied into uracil (U) instead of thymine (T) in RNA. For example, a DNA sequence CGATGG would be copied into GCUACC in RNA.

Additionally, the RNA polymerase forms the sugar-phosphate backbone of the resulting RNA. However, unlike DNA, in which the sugar is deoxyribose, the RNA will have ribose as its sugar component.

Termination

Once the RNA polymerase crosses a termination sequence in the gene, it signals the end of transcription (a stage called termination). During termination, the hydrogen bonds that join the RNA and DNA helices during transcription break. This releases the newly formed RNA molecule.

While in prokaryotic cells, the transcription process ends here, in eukaryotic cells, the transcript undergoes additional steps: capping, polyadenylation, and splicing. We will discuss this further later under eukaryotic transcription.

Transcription in prokaryotes vs. eukaryotes

While the basic process is the same, there are some key differences in how transcription takes place in prokaryotic and eukaryotic cells. We will discuss some of these differences in the following section.

Prokaryotic transcription

Prokaryotes are organisms that do not have a membrane-bound nucleus. In prokaryotic cells, transcription occurs in the cytoplasm (the semifluid substance that fills the cell). Prokaryotes have only one type of polymerase.

Another distinguishing characteristic of transcription in prokaryotic cells is the presence of operators, repressors, and activator proteins. At the start of many genes in prokaryotic cells are sequences called operators that instruct proteins called repressors to bind to the DNA ahead of the transcription start site and to prevent the RNA polymerase from accessing the DNA. By physically blocking the RNA polymerase, the transcription of the gene is prevented.

Repressors may be released from this function when other molecules (such as activator proteins) in the cell send signals indicating the need for gene expression.

Eukaryotic transcription

Eukaryotes are organisms that have a membrane-bound nucleus and other membrane-bound organelles. In eukaryotic cells, transcription occurs in the nucleus.

Another distinguishing characteristic of the transcription process in eukaryotes is that the RNA polymerase in eukaryotes is more complex than those of prokaryotes. Three RNA polymerases (polymerase I, II, and III) are involved in the process.

In eukaryotic cells, there are also additional steps that the newly transcribed mRNAs must undergo before they can be moved from the nucleus to the cytoplasm and then translated into a protein. These additional steps give the eukaryotic mRNA a longer half-life compared to prokaryotic mRNA. For example, a eukaryotic mRNA can last for several hours, while an E. coli mRNA typically lasts only up to five seconds.

The initial product of transcription called pre-mRNA must undergo three additional steps: the addition of a cap to the 5’ end of the molecule, the addition of a poly-A tail to the 3’ end of the molecule, and pre-mRNA splicing.

Capping

The 5’ end of the pre-mRNA transcript will be covered with proteins that will stabilize it, preventing it from breaking down while it is being processed and transported out of the nucleus. This step occurs while the pre-mRNA is synthesized during elongation.

Polyadenylation

After elongation, a poly-A tail (a chain of around 200 adenine residues) will attach to the pre-mRNA. The poly-A tail will provide added protection and will signal the need for the pre-mRNA to be transported to the cytoplasm.

Pre-mRNA splicing

Eukaryotic genes contain protein-coding sequences (called exons) and intervening sequences (called introns). Introns do not encode functional proteins; hence, it is important that they are removed from the pre-mRNA prior to protein synthesis, as this ensures that the exons are joined correctly for the encoding of amino acids. The process of removing introns from pre-mRNA and then joining the exons is called splicing.

Should there be an error in this process, the exons would be misaligned, and the protein would become nonfunctional. Such errors are thought to lead to cancer and other diseases.

Transcription factors

Eukaryotic cells also have accessory proteins called transcription factors. Transcription factors are molecules that regulate the activity of a gene by signifying when transcription is needed. While the RNA polymerase initiates the transcription process, transcription factors determine the efficiency of RNA polymerase.

There are many types of transcription factors, but they tend to work together in protein complexes to carry out their functions. These functions include the following:

• By binding to promoter regions, they can activate or repress gene transcription.

• They can determine what happens to individual cells. For example, homeotic genes regulate the development of the body. Homeotic proteins can activate or repress genes so that the different parts of the body develop in the correct order.

• They can also respond to signals from other cells and other environmental stimuli.

• They can also control the genes that transcribe them.

Difference between transcription and translation in biology

Transcription and translation are different steps in gene expression and protein synthesis. Specifically, transcription precedes translation. Whereas transcription is the process of copying a DNA sequence into mRNA, translation is the process of “reading” the information contained in the mRNA and converting it into an amino acid sequence. The by-product of transcription is RNA, while the by-product of translation is protein.