Transcription and Translation
Protein Synthesis Overview
How exactly do all those letters in DNA code for making a living thing? The short answer is that the code of letters in DNA is a recipe to make proteins. Proteins form structures, enzymes, or other chemicals to create the building blocks of organisms and maintain homeostasis. When a cell needs to make a protein, say the enzyme to digest lactose (the sugar in milk), it needs to use/read the DNA code to make proteins. We call this process protein synthesis.
Protein synthesis requires the use of DNA and all three forms of RNA to read the code and make the desired protein. This occurs in two steps:
- Transcription
- Translation
Watch the following video to preview the events in Protein Synthesis:
Types of RNA
Before we look at the two stages of protein synthesis, familiarize yourself with the three types of RNA. Each shares the chemical and structural features of RNA discussed in the first lesson, but differ in their three-dimensional form and their role in the cell.
All forms of RNA are made from DNA in the nucleus by Transcription. Their fate is determined by their sequence of letters, or nucleotides. Remember that RNA contains the nucleotides: Adenine (A), Uracil (U), Guanine (G), Cytosine (C). When RNA pairs up, A will pair with U. C still pairs with G.
There are three types of RNA:
| Type of RNA | Description | Role | Picture |
|---|---|---|---|
| Messenger RNA (mRNA) | a single uncoiled strand | carries code for a protein from DNA to the ribosomes; contains a series of codons (sequence of three bases) |   |
| Tranfer RNA (tRNA) | a single folded strand | carries a specific amino acid to the ribosome; contains one anticodon (three bases) |  |
| Ribosomal RNA (rRNA) | multiple strands of RNA; globular form becomes ribosome | reads the mRNA codons and matches them to tRNA codons to create a chain of amino acids (protein) |  |
Transcription: DNA → RNA
Transcription is the first step in protein synthesis. It is the process of forming a short strand of mRNA from one gene on a long DNA strand.
The mRNA strand serves as a “disposable photocopy” of the master DNA code for a gene locked in the “vault” (the nucleus). We wouldn’t want to use our master code (DNA) to make our protein recipes, right? We use a photocopy (mRNA) instead!
When you think of transcription, think of the word “script” and associate it with letters. In this stage we are changing the script of one gene from DNA letters into RNA letters. Take a look at the diagram below showing Transcription. Study it and then read the steps below to understand it better.
- Initiation: The DNA double helix is separated by RNA polymerase to make a transcription bubble at the start of the gene desired. RNA polymerase begins matching complementary base pairs of RNA nucleotides to the DNA. Note that it matches U with A; RNA does not have Thymine (T).
- Elongation: The mRNA sequence elongates as RNA polymerase moves along the DNA. This occurs at a rate of 40 nucleotides per second!
- Termination: RNA polymerase reaches a termination sequence and detaches from the DNA. The completed mRNA molecule is released.
The resulting strand of mRNA codes for the making of a protein. It begins with a start codon and ends with a stop codon. A codon is a sequence of three letters on mRNA or DNA that codes for a specific amino acid. mRNA will now travel out of the nucleus and into the cytoplasm to find a ribosome to transcribe it’s code.
Translation: RNA → Protein
Translation is the assembling of protein molecules from information encoded in mRNA. It occurs at the ribosome. All three types of RNA participate in translation. When you think of “translation” think of languages – all three types of RNA help to “translate” from the base pair language in the mRNA into the amino acid language that makes up a protein.
Steps of Translation
- Initiation: mRNA is transported to the cytoplasm and attaches to a ribosome so that the start codon is read first. A tRNA with an anti-codon that is complementary to the start codon arrives at the ribosome, donating it’s amino acid, methionine. The tRNA is recycled and is free to pick up another methionine.
- Elongation: The ribosome moves along the mRNA strand, reading the next codon of the mRNA in order. A new tRNA with an anti-codon that is complementary to the next codon arrives at the ribosome, donating it’s amino acid. (The tRNA is carrying an amino acid that is specific to that codon.) This process repeats for each mRNA codon. The chain of amino acids at the ribosomes grows as more codons are read.
- Termination: When the stop codon is reached, translation stops. The mRNA leaves the ribosome. The polypeptide chain will fold into its three-dimensional shape, be taken to the Golgi apparatus where it will be processed, perhaps joined with other polypeptide chains, and packaged to be delivered as a final protein product to wherever it is needed.
As a review of both Transcription and Translation, click through the tutorial to see them in action.
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