The Genetic Code Overview

This module will examine how information is encoded in DNA, and how that information is interpreted to bring about changes in cells and tissues.

Objectives :

  • Understand the triplet nature of the genetic code, and know the meaning of the term codon
  • Know that the code is degenerate, and what that means
  • Know that the code is unambiguous, and what that means
  • Know the identities of the start and stop codons, and understand how they work

The Genetic Code It has been mentioned in a variety of modules that DNA stores genetic information.

That much was clear from the experiments of Avery, Macleod, and McCarty and Hershey and Chase. However, these experiments did not explain how DNA stores genetic information. Elucidation of the structure of DNA by Watson and Crick did not offer an obvious explanation of how the information might be stored. DNA was constructed from nucleotides containing only four possible bases (A, G, C, and T). The big question was: how do you code for all of the traits of an organism using only a four letter alphabet? Recall the central dogma of molecular biology.

The information stored in DNA is ultimately transferred to protein, which is what gives cells and tissues their particular properties. Proteins are linear chains of amino acids, and there are 20 amino acids found in proteins. So the real question becomes: how does a four letter alphabet code for all possible combinations of 20 amino acids? By constructing multi-letter “words” out of the four letters in the alphabet, it is possible to code for all of the amino acids. Specifically, it is possible to make 64 different three letter words from just the four letters of the genetic alphabet, which covers the 20 amino acids easily.

This kind of reasoning led to the proposal of a triplet genetic code. Experiments involving in vitro translation of short synthetic RNAs eventually confirmed that the genetic code is indeed a triplet code. The three-letter “words” of the genetic code are known as codons. This experimental approach was also used to work out the relationship between individual codons and the various amino acids. After this “cracking” of the genetic code, several properties of the genetic code became apparent: The genetic code is composed of nucleotide triplets.

In other words, three nucleotides in mRNA (a codon) specify one amino acid in a protein. The code is non-overlapping. This means that successive triplets are read in order. Each nucleotide is part of only one triplet codon.  The genetic code is unambiguous. Each codon specifies a particular amino acid, and only one amino acid. In other words, the codon ACG codes for the amino acid threonine, and only threonine.  The genetic code is degenerate. In contrast, each amino acid can be specified by more than one codon.  The code is nearly universal.

Almost all organisms in nature (from bacteria to humans) use exactly the same genetic code. The rare exceptions include some changes in the code in mitochondria, and in a few protozoan species.  A Non-overlapping Code. The genetic code is read in groups (or “words”) of three nucleotides. After reading one triplet, the “reading frame” shifts over three letters, not just one or two. In the following example, the code would not be read GAC, ACU, CUG, UGA…  Rather, the code would be read GAC, UGA, CUG, ACU…  Degeneracy of the Genetic Code There are 64 different triplet codons, and only 20 amino acids. Unless some amino acids are specified by more than one codon, some codons would be completely meaningless. Therefore, some redundancy is built into the system: some amino acids are coded for by multiple codons. In some cases, the redundant codons are related to each other by sequence; for example, leucine is specified by the codons CUU, CUA, CUC, and CUG. Note how the codons are the same except for the third nucleotide position. This third position is known as the “wobble” position of the codon.

This is because in a number of cases, the identity of the base at the third position can wobble, and the same amino acid will still be specified. This property allows some protection against mutation – if a mutation occurs at the third position of a codon, there is a good chance that the amino acid specified in the encoded protein won’t change.  Reading Frames.If you think about it, because the genetic code is triplet based, there are three possible ways a particular message can be read, as shown in the following figure: Clearly, each of these would yield completely different results.

To illustrate the point using an analogy, consider the following set of letters:  the red fox at e the hot dog . If this string of letters is read three letters at a time, there is one reading frame that works: the red fox ate the hot dog  and two reading frames that produce nonsense:  t her edf oxa tet heh otd og  th ere dfo xat eth eho tdo g. Genetic messages work much the same way: there is one reading frame that makes sense, and two reading frames that are nonsense.  So how is the reading frame chosen for a particular mRNA? The answer is found in the genetic code itself.

The code contains signals for starting and stopping translation of the code. The start codon is AUG. AUG also codes for the amino acid methionine, but the first AUG encountered signals for translation to begin. The start codon sets the reading frame: AUG is the first triplet, and subsequent triplets are read in the same reading frame. Translation continues until a stop codon is encountered. There are three stop codons: UAA, UAG, and UGA. To be recognized as a stop codon, the triplet must be in the same reading frame as the start codon. A reading frame between a start codon and an in-frame stop codon is called an open reading frame.

Let’s see how a sequence would be translated by considering the following sequence. First, the code is read in a 5′ to 3′ direction. The first AUG read in that direction sets the reading frame, and subsequent codons are read in frame, until the stop codon, UAA, is encountered. Note that there are three nucleotides, UAG (indicated by asterisks) that would otherwise constitute a stop codon, except that the codon is out of frame and is not recognized as a stop. In this sequence, there are nucleotides at either end that are outside of the open reading frame.

Because they are outside of the open reading frame, these nucleotides are not used to code for amino acids. This is a common situation in mRNA molecules. The region at the 5′ end that is not translated is called the 5′ untranslated region, or 5′ UTR. The region at the 3′ end is called the 3′ UTR. These sequences, even though they do not encode any polypeptide sequence, are not wasted: in eukaryotes these regions typically contain regulatory sequences that can affect when a message gets translated, where in a cell an mRNA is localized, and how long an mRNA lasts in a cell before it is destroyed.

A detailed examination of these sequences is beyond the scope of this course.

The Genetic Code: Summary of Key Points

The genetic code is a triplet code, with codons of three bases coding for specific amino acids. Each triplet codon specifies only one amino acid, but an individual amino acid may be specified by more than one codon.  A start codon, AUG, sets the reading frame, and signals the start of translation of the genetic code. Translation continues in a non-overlapping fashion until a stop codon (UAA, UAG, or UGA) is encountered in frame. The nucleotides between the start and stop codons comprise an open reading frame.

Calculate the price
Make an order in advance and get the best price
Pages (550 words)
*Price with a welcome 15% discount applied.
Pro tip: If you want to save more money and pay the lowest price, you need to set a more extended deadline.
We know how difficult it is to be a student these days. That's why our prices are one of the most affordable on the market, and there are no hidden fees.

Instead, we offer bonuses, discounts, and free services to make your experience outstanding.
How it works
Receive a 100% original paper that will pass Turnitin from a top essay writing service
step 1
Upload your instructions
Fill out the order form and provide paper details. You can even attach screenshots or add additional instructions later. If something is not clear or missing, the writer will contact you for clarification.
Pro service tips
How to get the most out of your experience with MyHomeworkGeeks
One writer throughout the entire course
If you like the writer, you can hire them again. Just copy & paste their ID on the order form ("Preferred Writer's ID" field). This way, your vocabulary will be uniform, and the writer will be aware of your needs.
The same paper from different writers
You can order essay or any other work from two different writers to choose the best one or give another version to a friend. This can be done through the add-on "Same paper from another writer."
Copy of sources used by the writer
Our college essay writers work with ScienceDirect and other databases. They can send you articles or materials used in PDF or through screenshots. Just tick the "Copy of sources" field on the order form.
See why 20k+ students have chosen us as their sole writing assistance provider
Check out the latest reviews and opinions submitted by real customers worldwide and make an informed decision.
Business and administrative studies
excellent job
Customer 452773, March 12th, 2023
Business and administrative studies
excellent paper
Customer 452773, March 3rd, 2023
Business and administrative studies
Excellent work ,always done early
Customer 452773, February 21st, 2023
Customer 452591, March 18th, 2021
Thank you!!! I received my order in record timing.
Customer 452551, February 9th, 2021
Love this writer!!! Great work
Customer 452597, April 5th, 2021
Thank youuuu
Customer 452729, May 30th, 2021
Business and administrative studies
always perfect work and always completed early
Customer 452773, February 21st, 2023
English 101
IThank you
Customer 452631, April 6th, 2021
Business and administrative studies
Excellent job
Customer 452773, March 17th, 2023
I just need some minor alterations. Thanks.
Customer 452547, February 10th, 2021
Business and administrative studies
excellent, got a 100
Customer 452773, May 17th, 2023
Customer reviews in total
Current satisfaction rate
3 pages
Average paper length
Customers referred by a friend
15% OFF your first order
Use a coupon FIRST15 and enjoy expert help with any task at the most affordable price.
Claim my 15% OFF Order in Chat

Sometimes it is hard to do all the work on your own

Let us help you get a good grade on your paper. Get professional help and free up your time for more important courses. Let us handle your;

  • Dissertations and Thesis
  • Essays
  • All Assignments

  • Research papers
  • Terms Papers
  • Online Classes