Most hormones are proteins. The instructions for making proteins are held in our DNA. The process for making all proteins is the same, whether they are hormones, keratin, collagen or transport proteins.

Gene expression, as we’ve discussed several times already, involves making proteins using the genetic instructions coded in DNA. Now we will explore how that happens at a molecular level.

Why this is important…Our genetic instructions are the foundation for life, telling each cell what to make and how to function. Life would not be possible without the ability to read these instructions. This starts with making a copy of an individual gene. Molecular genetics is also playing an increasing role in medicine, whether through genetic testing, gene therapy, or development of therapeutic drugs.

Foundations

DNA is divided into functional units called genes. Each gene codes for a protein. Making a protein using information from a gene involves two complex processes – making an RNA copy of the gene, and then using that copy to assemble a protein.

From the very beginning of your life as a fetus, each cell in your body has been reading your genes to make proteins. Each cell makes a different mix of proteins to perform its specific role in the body. Hair follicle cells produce keratin to make hair, cells in the pancreas produce digestive enzymes and hormones, and nerve cells make neurotransmitters.

Making proteins starts with transcription – making a copy of the gene that can be taken out of the nucleus and used elsewhere. If you recall the rules of complimentary base pairing from DNA replication, you have most of the tools you need to understand transcription.

Key terms

Gene expression – the process of using the genetic code to make a protein; this involves both transcription and translation

Gene – a segment of DNA used to make a protein

Transcription – the process of making a strand of RNA using the code in DNA (verb: transcribe)

Promoter – a region of DNA before a gene that influences transcription of that gene

Messenger RNA (mRNA) – strand of RNA that carries the code from DNA to a ribosome

Details

Transcription results in RNA, which is slightly different than DNA. For one thing, RNA is usually single-stranded. RNA uses a different sugar – ribose instead of deoxyribose. RNA also has a much shorter lifespan than DNA. Other chemical differences are summarized in Figure 9-4.

RNA versus DNA. Nucleotides and polymer structures compared.

FIGURE 9-4. DNA versus RNA. ("DNA versus RNA" by Sponk is licensed under CC BY-SA 3.0)

Complimentary base pairing rules work almost like they did for DNA replication. The main difference is that RNA substitutes the nitrogen base of uracil (U) for thymine (T). Otherwise, it’s the same.

Transcription. Chromosome shown with some of DNA unwound. Part of the DNA, a gene, was being transcribed by RNA polymerase, which adds complimentary RNA nucleotides.

FIGURE 9-5. Transcription. RNA polymerase is shown matching complimentary bases to a gene.
("Transcription" by National Human Genome Research Institute is in the Public Domain)

VIDEO 9a: Transcription

After watching this video, you should be able to:

describe when and where transcription happens
explain the process of transcription
describe why and how mRNA matures
explain how transcription relates to gene expression

All the processes that we call molecular genetics (replication, transcription and translation) have roughly similar stages – initiation, elongation and termination. Breaking the process into stages highlights key parts of the process and helps us make sense of the complexity.

To start transcription RNA polymerase needs to find the right gene to copy (initiation phase). With 2 meters of DNA in the nucleus and 20,000 genes this is not an easy task. This means there are 20,000 locations (more technically called gene loci) where it might begin. To help get started, each gene has a promoter region right before the start of the gene. This part of the DNA binds proteins that guide RNA polymerase to the right location. Once it lands on the right gene, it splits the two strands of DNA apart so it can match complimentary bases.

Stop & Think #4: Why are promoters necessary?

Next, RNA polymerase constructs a strand of RNA using complimentary base pairing rules (elongation phase). RNA polymerase only needs to read one strand of DNA (the template strand). It moves along that strand in the 3’ to 5’ direction. If RNA polymerase reads G on the DNA strand, it will add a C onto the new RNA strand. If it reads an A on the DNA, it will add a U onto the RNA. Each new nucleotide is added to the growing chain. It repeats this thousands of times, constructing an RNA copy (or transcript) of the gene.

Stop & Think #5: Transcribe the following DNA sequence to construct an RNA molecule:
3’ -- T A A C A A G G C C A T T A C G G T – 5’

Once RNA polymerase reaches the end of the gene, transcription stops (termination phase). RNA polymerase knows it has reached the end of a gene because of another segment of DNA that tells it to stop – the termination site. Once it reaches this, RNA polymerase and the new RNA are released, and DNA seals up into a double helix again. Transcription is finished!

In eukaryotic cells, mRNA needs a few more modifications before it is ready to leave the nucleus. Prokaryotic cells do not have a nucleus and start translation even before transcription is done. But eukaryotic cells are more patient and modify the messenger RNA before it leaves the nucleus. These chemical modifications help RNA to be recognized by the ribosome and keeps the RNA from being broken down right away. Figure 9-6 and Video 9a go over the details of how mRNA matures.

Maturing of mRNA. Immature transcript

FIGURE 9-6. Maturing of mRNA. After transcription in eukaryotes, the mRNA must mature before it can leave the nucleus. Introns are spliced out. The segments between introns are called exons and they remain part of the mRNA. Stabilizing elements – 5’ cap and poly-A tail – are added to keep the mRNA from breaking down quickly. (“Maturing of mRNA” by J. Buegge is licensed under CC BY-SA 4.0)

Stop & Think #6: How is transcription similar to DNA replication? How is it different?

Learning objectives

Describe the process of transcription
Describe maturing of mRNA and what advantage it gives eukaryotic cells
Apply the principles of transcription to transcribe a sequence of DNA to make RNA

Stop & Think Questions Explained

4. Promoters make sure the right gene gets expressed. It also allows cells to increase or decrease the production of specific proteins.

5. If the DNA is: T A A C A A G G C C A T T A C G G T
Then RNA is: A U U G U U C C G G U A A U G C C A

6. Both replication and transcription happen in the same place using similar processes. However, RNA is different from DNA and only one strand is read in transcription. They also have different purposes and, therefore, occur at different times. What are the purposes of each