Genetic Code Study Guide

Introduction:

Before understanding the genetic code, we need to understand the term ‘gene’.

• A gene is a basic unit of heredity. It is made up of smaller parts called Deoxyribonucleic acid or DNA, which is made up of nitrogen-containing nucleobases called Adenine, Guanine, Thymine, and Cytosine.
• These are arranged as two polynucleotide chains that coil around each other to form a double helix that contains basic genetic information for the functioning of all living organisms.

What is the Genetic Code?

Genetic code is a set of instructions by which information present in molecules of heredity such as DNA or RNA sequences is translated into proteins by the ribosomes of cells of living organisms.

The part of the genome that codes for the amino acids in proteins or RNA (Ribonucleic acid) is called a gene.

Features of The Genetic Code

Source: Genetic code

• Decoding the information present in the nucleic acids, DNA or RNA is the basic function of genetic coding and is called gene expression.

• The genetic codon is a set of three-letter combinations of nucleotides specific for a particular amino acid or a stop signal. Francis Crick proposed the concept of codons in 1961.

• A total of 64 possible combinations of the three-letter codons can be built from the four nucleotides.

• From the 64 possible combinations, there are 61 codes for amino acids, and three of them act as stop signals.

• One codon is specific for one amino acid, and since there are twenty amino acids, one amino acid may be coded for by more than one codon. This is called degeneracy or redundancy of the genetic code.

Types of Genetic Code

• It can be expressed either as RNA codons or DNA codons.
• RNA codons are present in messenger RNA (mRNA) and are read or translated during the synthesis of polypeptides.
• Each molecule of RNA procures its nucleotide sequence by the process of transcription from the corresponding DNA.
• Except for the nucleotide Thymine present in the place of Uracil, DNA codons are translated similarly to RNA codons.

Types of Codons

1. Sense codons

These are the codons that code for amino acids. There are 61 sense codons that code for 20 amino acids

2. Signal codons

These codons code for signals during the synthesis of proteins. AUG, UAA, UAG, and UGA are the four codons that code for signals. These are of two types:

Start codons: They begin the process of translation, hence also known as initiation codons, as they start the synthesis of polypeptide chains. GUG codes for the amino acid Valine which is an example of a start codon. AUG is another example that codes for amino acid Methionine.

Stop codons: They give the signal for termination of polypeptide chains synthesis. Also known as termination codons, the code for the termination and release of the synthesized polypeptide chains. They do code for any particulate amino acid. Hence they are also called Non-sense Codons. Examples are UGA, UAG, and UAA.

Properties of the Genetic Code

a) Universal

Among most genes, whether in animals, plants, or microorganisms, the same codons are assigned to the same amino acids and the same start and stop signal with a few exceptions.

b) Triplet

Three RNA bases code for one amino acid in a triplet code. 64 codons code for 20 amino acids and the starting and stopping signals in the synthesis of polypeptide chains.

c) Non-overlapping

Three bases or nucleotides code for one amino acid. In a non-overlapping code, a single letter is read just once. Therefore in a non-overlapping code, six bases will code for two amino acids.

d) Commaless

There are no demarcation commas or lines between the codons, so they are continuous. If a single base is deleted in a comma less code, it changes the entire sequence of the following amino acids after the point of deletion.

e) Non-ambiguous

Of the 64 codons present in the genetic code, 61 codons code for 20 amino acids, but none codons code for more than one amino acid. Therefore it is ambiguous.

f) Redundant

In many cases, several codons code for the same amino acid. Only two amino acids, namely, methionine and tryptophan are coded by a single codon each. Isoleucine is coded by three codons, five amino acids by four codons each and three amino acids by six codons each.

The other nine amino acids are coded by two codons each. This redundant code system protects an organism against any dangerous mutation in case any single base gets mutated as other codons will code for the same amino acid, and there will be no changes in the polypeptide chain.

g) Polarity

This is the way of reading a message from a codon. The genetic code has a particular direction of reading that message. Reading from the opposite direction will code for some other amino acid. Since the reading is done from the five ends to the three end direction, the polarity of the genetic code is five ends to three ends.

Conclusion:

• The discovery of the double-helical structure of DNA by James Watson and Francis Crick in 1953 drastically changed the life sciences.
• The genetic code was then deciphered by Marshall Nirenberg, Har Khorana, and Severo Ochoa over many years and was one of the greatest discoveries of the twentieth century.
• Genetic code gave an insight into the basic constituents of living organisms.

FAQs:

1. What is a genetic code and its type?

The genetic code is a combination of rules which define how the four-letter code of DNA is translated into amino acids. The two types can be expressed as either RNA codons or DNA codons.

2. Why is genetic code important?

It provides evidence for the common origin of life on earth since it is similar in most living organisms.

3. What are the three principles of the genetic code?

The genetic code is always read in the 5’ to 3’ directionCodons are read as non-overlapping parts during translationThere are start codons and stop codons in the synthesis of amino acids.

4. How does the genetic code work?

It is the way the four bases of DNA, A, C, G, and T, are attached that the ribosome of the cell can translate the code and synthesize proteins.

5. What are the four main features of the genetic code?

The genetic code is a universal, non-overlapping, triplet, and redundant.

6. What are the four properties of the genetic code?

The genetic code is non-ambiguous, has a polarity, comma less, and is a triplet codon.

7. What is genetic code explained in detail?

• The genetic code is a collection of instructions used by all living organisms to translate coded information present within genetic materials called codons into amino acids.
• Living organisms need amino acids as basic building blocks.
• The ribosome then connects the translated amino acids in a way that is directed by the mRNA.
• The genetic code shows how codons or three chains of nucleotides specify which amino acid will be coded for and subsequently added during the next translation.

8. How many genes do humans have?

Humans have between 20,000 and 25,000 genes.

9. What does DNA stand for?

It stands for Deoxyribonucleic acid.

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Sources:

1. Genetic Code. https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/4.6/primary/lesson/genetic-code-bio/. Accessed on 1 Dec, 2021.
2. Genetic code. https://www.nature.com/scitable/definition/genetic-code-13/. Accessed on 1 Dec, 2021.
3. Genetic Code. https://www.genome.gov/genetics-glossary/Genetic-Code. Accessed on 1 Dec, 2021.

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