2.2.2: Protein Synthesis

Genes in DNA encode proteins that do the work of the cell.

 Master this section and you'll be able to

• Explain how the genetic code stored within DNA determines the protein that will form.
• Describe the processes of transcription and translation.
• Discuss the function of ribosomes.

Blueprints and Builders: DNA, Genes, and Proteins

Earlier, we mentioned that DNA acts like a “blueprint” for the cell. That means it holds the instructions to build one of the most important molecules in your body: proteins. Proteins help form the structure of cells, and almost everything a cell does depends on proteins in some way.

One especially important group of proteins is called enzymes. Enzymes speed up the chemical reactions that cells need to survive. These reactions might involve building larger molecules (like during DNA replication or making microtubules) or breaking down larger molecules (like when your cells extract energy from food).

In short, if your cells are doing something, proteins are probably involved.

Just like the genome is all the DNA in a cell, the proteome is the full set of proteins the cell can make. The instructions for making these proteins come from genes — specific segments of DNA that carry the code to build a particular protein. This process is called gene expression. It is how cells turn genetic instructions into working proteins, which then shape the cell’s structure and control what it can do. 

 

How Do Genes Build Proteins?

Let's break it down: As you have learned, proteins are made of long chains of smaller units called amino acids — kind of like beads on a necklace. The order of those amino acids determines what kind of protein it is and what job it will do in your body.

So how does your body know which amino acids to put together, and in what order? That is where your DNA comes in.

DNA is made of four building blocks called nucleotides, represented by the letters A, T, C, and G. These letters form "words" in your genetic code. Every three-letter combination (called a triplet) gives instructions to add a specific amino acid. For example, the triplet CAC tells your cell to add the amino acid valine.

Think of it like this: just like the letters d-o-g create the word “dog,” the DNA letters C-A-C create a signal for “valine.”

A gene is a long string of these triplets, each one contributing an amino acid to a growing protein chain. When the full sequence is read, your body knows how to build a complete protein with the correct structure and function.

This process of going from DNA to protein happens in two main steps (see image on the left):

1. Transcription: The DNA’s instructions are copied into a molecule called RNA (kind of like making a working copy of a recipe).

2. Translation: That RNA copy travels to a ribosome (your cell’s protein-building station), where it is read and used to assemble a chain of amino acids—the final protein.

The image on the right offers another artistic rendition of these two key biological steps that shows how a cell reads genetic instructions to build a protein:
Starting at the top: 

The DNA sequence contains the genetic code. Since DNA is double-stranded, only one — the template strand — is used to guide the process of transcription. During transcription the cell makes a copy of the template strand code in the form of messenger RNA (mRNA). This is like writing down a recipe to take with you.

The RNA uses bases A, U, C, and G (note: U replaces T in RNA).

For the process of translation, each set of three RNA bases (called a codon) is translated into one amino acid. For example, the first codon AUG codes for the amino acid methionine (Met), which is always the starting amino acid.

The result is a protein, a chain of amino acids folded into a working molecule that does jobs in your body, such as building muscle, carrying oxygen (hemoglobin), or speeding up reactions in the form of enzymes.

 

....And a little more detail:

Step 1: From DNA to RNA — How the Message Gets Out

Your DNA is stored safely inside the nucleus of the cell, but proteins, the workers of the cell, are built in the cytoplasm. So, how do the instructions get from one place to the other?

That job goes to a special messenger called messenger RNA (mRNA). It is a single-stranded copy of a gene’s instructions, and it travels out of the nucleus into the cytoplasm, where it helps build proteins.
 

     Meet RNA: DNA’s Close Cousin

There are different types of RNA in the cell, but the one we are focusing on is mRNA—the one that carries the recipe for making a protein.

RNA is very similar to DNA, but with a few important differences:

• RNA is single-stranded, not double-stranded like DNA.
• The sugar in RNA is ribose, which has one more oxygen atom than the sugar in DNA (called deoxyribose).
• RNA uses a base called uracil (U) instead of thymine (T). So, whenever DNA has an A, RNA will match it with a U

     Transcription: Copying the Code

The first step in making a protein is called transcription. In this step, your cell makes an RNA copy of the gene it needs. That is why it is called “transcription”—just like copying notes from the board in class.

Here is how it works:

1. The section of DNA that contains the needed gene unwinds.

2. The two strands of DNA separate just in that region.

3. Using one of the DNA strands as a guide (= the template strand), the cell builds an mRNA strand with the matching bases.

The DNA is read in triplets, and each triplet gets copied into a matching three-base unit on the mRNA, called a codon. Each codon will eventually tell the cell which amino acid to add to the protein chain.

In the figure on the left, you can see how a gene on the DNA molecule is transcribed by an enzyme called RNA polymerase into a complementary mRNA molecule:

 

Polishing the Message: What Happens Before mRNA Leaves the Nucleus

Before the mRNA can go out into the cytoplasm and help build a protein, it needs to go through a little editing. At this point, it is still called pre-mRNA because it is not quite ready yet.

One important editing step is called splicing (see image on the right). Your DNA (and the mRNA copied from it) contains some stretches of code that do not actually code for proteins. These parts are called non-coding regions, or introns, and scientists are still figuring out exactly what they do. But for now, the cell snips them out.

A special structure in the cell, called a spliceosome, does the cutting. It removes the introns, the parts of the RNA that are not needed, and stitches together the exons, which are the useful coding parts.

🔹 Introns = the “in-between” parts that get removed
🔹 Exons = the parts that stay and get used to build the protein

Here is something cool: Sometimes, even coding regions (exons) can be spliced in different ways. This is called alternative splicing. By mixing and matching different exons, your cells can make many different versions of a protein from just one gene. That means more variety in what your body can build, even with a limited number of genes!

Once the mRNA is cleaned up and ready, it leaves the nucleus and heads into the cytoplasm, where the next step (translation) will turn that message into a real, working protein.

 

Step 2: Translation — From RNA to Protein (a.k.a. Turning the Genetic Recipe into Dinner)

Think of this step like translating a recipe written in one language (RNA) into another (protein). Each three-letter “word” on the mRNA strand is called a codon, and each codon stands for a specific amino acid, the building blocks of proteins. The goal of translation is to string these amino acids together to make a polypeptide, which will eventually fold into a fully functioning protein.

To pull this off, the cell needs two main tools:

1. A translator: someone (or something) who can read the RNA and know which amino acid goes where.
2. A workspace: like a kitchen counter or desk where this translation happens.

Lucky for us, the cell already has both. The translator job goes to tRNA, and the workspace is the ribosome, a protein-building machine made partly of another type of RNA called rRNA (ribosomal RNA). See image on the left.

You might remember from an earlier biology course that many ribosomes hang out on the endoplasmic reticulum (ER), making it the rough ER. Each ribosome is made of two parts, a large and a small subunit. When it is time to start protein synthesis, these two parts snap together on an mRNA strand and get to work, aligning everything just right for accurate decoding. If you have never heard of that, no worries, we will cover it in some details in Module 3.

---

 

    tRNA: The Protein Builder’s Uber

Now let’s zoom in on our translator: tRNA, short for transfer RNA. These clever little molecules do the legwork (literally!) of bringing the right amino acid to the ribosome. They cruise around the cytoplasm, pick up their assigned amino acid, and deliver it to the assembly line.

But here’s the cool part: each tRNA has two key features that make this possible:

• One end has a spot to grab a specific amino acid (like glycine, for example).
• The other end carries an anticodon, a three-letter sequence that matches up with the codon on the mRNA (like a plug fitting into a socket).

Let’s say the mRNA strand shows the codon CGA, which encodes the amino acid arginine. A tRNA with the anticodon GCU will recognize and bind to it, bringing arginine along for the ride. The ribosome helps this happen smoothly, joining the new amino acid to the growing chain one-by-one until the full protein is complete. But how does the cell know the protein is complete?  When the ribosome stops translating mRNA when it reaches a special “stop” codon. The stop codon signals the end of translation. At that point, the finished protein is released — ready to fold into its final shape and go to work. Thus, a gene within the DNA molecule is transcribed into mRNA, which is then translated into a protein product.

In the top part of the figure on the right, you can see a large ribosomal subunit coming into contact with the mRNA that already has the small ribosomal subunit attached. A tRNA and an anticodon are in proximity. In the second panel, the tRNA also binds to the same site as the ribosomal subunits. In the bottom panel, a polypeptide chain is shown emerging from the complex.

 

In summary: a gene on DNA is first transcribed into mRNA, and that mRNA is then translated into a functional protein

 

Interactive Link

Watch this video (opens in new window) to learn about ribosomes. The ribosome binds to the mRNA molecule to start translation of its code into a protein. What happens to the small and large ribosomal subunits at the end of translation?