First Step of Protein Synthesis

Instructor: Dominic Corsini
How are proteins made? How is the message coded for in DNA actually used? This lesson introduces the process of transcription, which is the first step in protein synthesis. A brief summary and quiz are included.

What Is Protein?

To begin, let's start with a question: what are your muscles made of? Or, more specifically, what food should you eat in order to build more muscle? Any ideas? The answer to these questions is protein. Protein is a large molecule made from chains of amino acids. Amino acids are the building blocks of protein. They're analogous to links in a chain - the links are the amino acids and the chain itself represents the protein.

To create a protein, your cells undergo several steps. For the purposes of this lesson, we'll focus on just one of these: transcription, the first step in protein synthesis. Now let's explore this step more closely.


Before getting into the process of transcription, we need to review some basic biology. First, understand that cells contain your DNA and that DNA is housed inside the cell's nucleus. The nucleus is the cellular organelle that contains genetic material and controls cellular function. DNA does not leave the nucleus, but instead remains protected inside. However, protein synthesis occurs outside the nucleus and is based off the code held within DNA. Thus there's a problem: how does the cell get the message contained within DNA out of the nucleus without removing the DNA itself?

The first step in the process of protein synthesis provides our answer. As we noted earlier, this is known as transcription and it refers to the creation of a new molecule called messenger RNA, or mRNA for short. mRNA is a single-stranded molecule that is capable of leaving the nucleus. This mRNA is built using a DNA template, so the message it contains is the same as the message of DNA. Sound confusing? Have a look at our example below.

DNA is made from four different molecules (adenine, thymine, cytosine, and guanine) that are represented by the letters A, T, C, and G. Suppose this is one side of a DNA strand that contains the following code:


This DNA cannot leave the nucleus. But, fortunately for us, DNA and mRNA each follow certain rules. Most pertinent to this discussion are two facts:

  1. Adenine (A) always bonds with thymine (T), and cytosine (C) always bonds with guanine (G).
  2. In mRNA, uracil (U) replaces thymine (T).

Based on these rules, we can use our DNA code to produce an mRNA molecule. So here's our mRNA created from the above DNA code: UAAGCGUUAGCCGAU

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