In this explainer, we will learn how to describe the process of transcription and to outline the roles of DNA, mRNA, and RNA polymerase.
Do you like mushrooms? When picking wild mushrooms, you need to be careful about which ones you select to eat. This one in particular needs to be avoided at all costs.
In the picture above, we see the so-called “death cap” mushroom, which can lead to death within a few days of ingestion. The poison inside the mushroom, called -amanitin, is able to inhibit transcription by interacting with the enzyme RNA polymerase. Without transcription, cells cannot produce the proteins necessary for their survival, and they die.
Definition: Transcription
Transcription is the process of converting a DNA sequence into mRNA.
Transcription is the process that converts a gene in DNA to messenger RNA (mRNA), which can then go on to be translated to a protein during translation (see Figure 2). The “central dogma of molecular biology” begins with transcription that takes place in the nucleus of eukaryotic cells or the cytoplasm in prokaryotic cells.
Definition: DNA (deoxyribonucleic acid)
DNA is the molecule that carries the genetic instructions for life. It is composed of two strands that coil around each other to form a double helix.
Definition: mRNA (messenger RNA)
mRNA is a message that is transcribed from the DNA of a gene and can be translated to make the corresponding protein.
Key term: Eukaryotic Cell
A eukaryotic cell is a cell that contains a membrane-bound nucleus and other membrane-bound organelles.
Example 1: Understanding Where Transcription Takes Place in the Cell
In what organelle of a eukaryotic cell does transcription take place?
Answer
The DNA in our cells contains the instructions for building all the proteins that make up our body. These instructions are in the form of genes. In order for a gene in DNA to be converted into protein, messenger RNA (mRNA) must first be formed in a process called transcription. mRNA is produced in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. Once formed, it can then be translated to give the corresponding protein for that gene.
Therefore, transcription takes place inside the nucleus of eukaryotic cells.
Before we get into the details of the steps of transcription, let’s first talk about how RNA polymerase, the major enzyme involved in transcription, uses double-stranded DNA as a template to make single-stranded mRNA.
Key term: RNA Polymerase
RNA polymerase is the enzyme that catalyzes transcription by synthesizing RNA from a DNA template.
DNA is a double stranded molecule wrapped tightly in a helical shape. RNA polymerase can bind to DNA and unwind the helix to separate the two strands and expose the individual nucleotides. It can then use one of the two strands of DNA as a template to synthesize a complementary sequence of mRNA as shown in Figure 3.
Key term: Nucleotide
A nucleotide is a monomer of a nucleic acid polymer. Nucleotides consist of a pentose sugar, a phosphate group, and a nitrogen-containing base.
Key term: Template
The template strand is the strand of DNA (or RNA) that is used by an enzyme (like DNA or RNA polymerase) to attach complementary nucleotides.
Since there are two strands in DNA, which strand is actually being used as the template?
One strand is called the sense strand and the other is called the antisense strand. The sense strand is read in the to direction and corresponds to the mRNA that is translated. The antisense strand is the to strand that is complementary to this and is actually the strand used as a template for transcription. You can see this in Figure 4.
As you may have noticed from Figure 4, when RNA is transcribed, thymine (T) is replaced by uracil (U). This indicates that the DNA base thymine is replaced by the RNA base uracil.
Key Term: Sense Strand
The sense strand is one of the two strands of DNA (the to strand) that has the same sequence of the mRNA that is translated. It is complementary to the antisense strand.
Key Term: Antisense Strand (Template Strand)
The antisense strand is one of the two strands of DNA (the to strand) that is used as a template during transcription. It is complementary to the sense strand.
Example 2: Converting a DNA Sequence to mRNA
A single strand of DNA undergoing transcription reads -AATCCGATCG-. Reading -, what will the sequence on the complementary strand of mRNA be?
- GGAUUCGAUC
- UUAGGCUAGC
- AATCCGATCG
- TTAGGCTAGC
Answer
In order for a cell to produce a protein from a gene, the DNA of the gene must first be transcribed to mRNA that is then translated into the corresponding protein. mRNA is produced in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells.
RNA polymerase is the enzyme that synthesizes mRNA from DNA. It uses one strand of the DNA as a template to form the complementary nucleotide sequence of the mRNA (with thymine in DNA being replaced by uracil in RNA). The template strand is sometimes called the antisense strand (which goes in the to direction), while the other strand is called the sense strand (which goes in the to direction). mRNA is synthesized in the to direction with new nucleotides being added to the end of the mRNA. You can see this below.
The question is asking us to form the complementary sequence for the to antisense strand -AATCCGATCG-.
By following the standard complementary base pairing rules (and substituting thymine for uracil in RNA), this becomes -UUAGGCUAGC-.
Therefore, the correct answer is option B, UUAGGCUAGC.
Now that we understand how RNA polymerase uses DNA as a template to form mRNA, let’s go through the steps of transcription.
The first step of transcription is called initiation and involves RNA polymerase binding to a specific sequence in the DNA called the promoter. This is where RNA polymerase initiates transcription.
Once bound, the next step involves RNA polymerase unwinding the helical structure of DNA and breaking the hydrogen bonds between the complementary nucleotides on the two strands of DNA, as shown in Figure 5.
The next stage of transcription is called elongation. Now that the two DNA strands are separated, the exposed DNA bases on the antisense strand can now hydrogen bond with complementary RNA nucleotides. RNA polymerase joins adjacent mRNA nucleotides by forming phosphodiester bonds. Nucleotides are added to the end of the mRNA strand, which grows in the to direction. RNA polymerase moves along in this direction and continues to unzip the DNA while the mRNA is synthesized. You can see this in Figure 6.
The final stage of transcription is termination. Transcription ends once a terminator sequence is reached. This is a specialized sequence that causes the RNA polymerase to dissociate from the DNA and to release the fully synthesized mRNA strand.
Example 3: Understanding the Stages of Transcription
The diagram provided outlines the main stages of transcription in an incorrect order. Use the letters to state the correct order.
Answer
In order for a cell to produce a protein from a gene, the DNA of that gene must first be transcribed to mRNA that is then translated into the corresponding protein. mRNA is produced in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells.
RNA polymerase is the enzyme that synthesizes mRNA from DNA. It uses one strand of the DNA as a template to form the complementary nucleotide sequence of mRNA (with thymine in DNA being replaced by uracil in RNA).
There are several stages of transcription. The first step is where RNA polymerase binds to a specific sequence in the DNA called the promoter. Once bound, it is able to unzip the DNA by separating the hydrogen bonds of the complementary nucleotides on the two strands of DNA. Then, these exposed bases can accept new nucleotides that are complementary and mRNA synthesis can begin. RNA polymerase forms a phosphodiester bond between the newly accepted nucleotides and elongates the strand, while also unzipping the helix as it moves while transcribing the DNA. Transcription stops once a terminator sequence is reached, and the RNA polymerase dissociates from the DNA and mRNA.
Therefore, the correct answer is W Y Z X.
Transcription of a single DNA template can occur simultaneously by numerous RNA polymerases. If you were to view the transcription of a strand of DNA under a microscope, you might see multiple mRNAs branching out from the DNA molecule, which get longer as they are synthesized. It does not take very long for an RNA polymerase to synthesize mRNA from its DNA template—about 10 minutes for a 10 000 nucleotide gene, which is about 1 000 nucleotides/minute!
If you recall the beginning of this explainer about the death cap mushroom, -amanitin is such a deadly poison because it can bind to RNA polymerase very tightly in such a way to constrain its motion. This reduces the mobility of RNA polymerase and slows down mRNA synthesis dramatically, going from thousands of nucleotides per minute to just a few. Without efficient transcription of mRNA, proteins that are essential for life cannot be synthesized and our cells will begin to die.
When mRNA is first made, it is actually called pre-mRNA because it needs to go through a few more steps of post-transcriptional processing. One of these steps is called polyadenylation. Here, a string of adenine nucleotides, or a poly(A) tail, is added to the end of an mRNA. Another step involves adding a modified nucleotide called a cap to one end of the pre-mRNA. Both of these alterations help to stabilize the mRNA, otherwise, it would degrade more quickly.
Key Term: Pre-mRNA (Primary Transcript, Precursor mRNA)
Pre-mRNA is the unprocessed mRNA that still includes introns and can be processed to form mRNA.
Another post-transcriptional event is called RNA splicing. You will recall that only a small portion of the genome contains coding DNA, the kind of DNA that actually codes for proteins. The remainder of the genome contains noncoding DNA, and some of this can be included in transcripts. With RNA splicing, noncoding sections (called “introns”) can be removed. Introns need to be removed so the coding sequences (called “exons”) can be stitched together to form the complete sequence for the gene.
Key Term: RNA Splicing
RNA splicing is a process where pre-mRNA is processed to remove introns and join exons to make mature mRNA.
You can see an overview of post-transcriptional processing in Figure 7.
Example 4: Identifying Post-Transcriptional Processing Events for mRNA
In eukaryotes, the process of transcription produces pre-mRNA. Which of the following processes is involved in converting this pre-mRNA into mRNA, ready for translation?
- Methylation
- Splicing
- Mutation
- Duplication
- Mitosis
Answer
In order for a cell to produce a protein from a gene, the gene must first be transcribed to mRNA that is then translated into the corresponding protein. mRNA is produced in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. In eukaryotes, mRNA must be processed before it can be translated. Pre-mRNA undergoes different post-transcriptional events like polyadenylation to improve stability (where a string of adenines is added to the end of the pre-mRNA) or RNA splicing.
In the genome there are significant noncoding DNA. Splicing is a way to remove the noncoding DNA so the remaining coding DNA can be stitched together to form the coding sequence needed for the protein.
Let’s look at the different options to see which one best describes a process of converting pre-mRNA into mRNA.
Option A, methylation, is incorrect. Although methylation can be used in regulating mRNA expression levels, it is not involved in converting pre-mRNA to mRNA for translation.
Option B, splicing, is correct. RNA splicing is a post-transcriptional process that converts pre-mRNA to mRNA.
Option C, mutation, is incorrect. Mutations change nucleotides in DNA or RNA, and this has nothing to do with converting pre-mRNA to mRNA.
Option D, duplication, is incorrect. Duplication refers to whole chromosomes, or parts of chromosomes, being duplicated.
Option E, mitosis, is incorrect. Mitosis is a type of cell division.
Therefore, the answer is option B. Splicing is involved in converting pre-mRNA into mRNA.
Once the pre-mRNA is fully processed in eukaryotic cells, it is now mature and can exit the nucleus through the nuclear pores to be translated into a protein by the ribosome. In prokaryotic cells, which do not have a nucleus, the mRNA is ready for translation immediately after transcription.
Example 5: Identifying How mRNA Leaves the Nucleus
In a eukaryotic cell, what happens to a strand of mRNA once it is formed?
- It is broken down into its component nucleotides.
- It leaves the cell through the cell membrane.
- It binds to other mRNA strands and forms a complex protein molecule.
- It is packaged in vesicles and transported to the Golgi apparatus.
- It leaves the nucleus through the nuclear pores.
Answer
In order for a cell to produce a protein from a gene, the gene must first be transcribed to mRNA that is then translated into the corresponding protein. mRNA is produced in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. In eukaryotes, before the transcript is released to the cytoplasm, it must first be processed. This pre-mRNA can undergo different post-transcriptional events, including polyadenylation (where a string of adenines is added to the end of the pre-mRNA), or RNA splicing. Once post-transcriptional processing is complete, the mRNA can leave the nucleus through the nuclear pores.
Now let’s look at the possible options to see which one best describes what happens to mRNA after it is formed.
Option A, it is broken down into its component nucleotides, is not accurate. mRNA will be broken down over time, which can happen naturally or by targeted processes, but initially this is not what happens.
Option B, it leaves the cell through the cell membrane, is incorrect. mRNA is synthesized in the nucleus and exits through its pores into the cytoplasm, where it can be translated to protein. It does not leave the cell membrane.
Option C, it binds to other mRNA strands and forms a complex protein molecule, is incorrect. mRNA is not a protein, so a complex of mRNA cannot be referred to as protein. Aside from this, this description is not something that mRNA does after being formed.
Option D, it is packaged in vesicles and transported to the Golgi apparatus, is incorrect. The Golgi apparatus transports or packages proteins, not mRNA.
Option E, it leaves the nucleus through the nuclear pores, is correct. After it is synthesized and processed, mRNA exits the nucleus through the nuclear pores to enter the cytoplasm, where it can be translated to protein.
Therefore, the correct answer is option E. Once a strand of mRNA is formed, it leaves the nucleus through the nuclear pore.
Let’s recap some of the key points we have covered in this explainer.
Key Points
- Transcription is the formation of mRNA from DNA, and it takes place in the nucleus in eukaryotic cells or in the cytoplasm in prokaryotic cells.
- Transcription is carried out by the enzyme RNA polymerase.
- RNA polymerase uses the antisense strand of DNA as a template to make mRNA.
- Pre-mRNA is unprocessed and can be post-transcriptionally modified by processes such as polyadenylation and RNA splicing.
