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Genetic Code: Introduction, Structure of DNA and RNA

Genetic Code: Translation of a language is analogous to the process of synthesizing a protein from DNA information with the assistance of RNA. Check this article to know more.
authorImageJasdeep Bhatia13 Jun, 2024
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Genetic Code

Genetic Code: Have you ever sent one of your siblings a secret letter? If so, you might have used a code to conceal the message from your parents. For instance, you put together the alphabet differently by adhering to specific guidelines.

Your sibling on the other end would need to be aware of the code and use the same guidelines in reverse to decipher what you had written. Likewise, a cellular language is written as genetic codes in the DNA in the cell's nucleus. This article will help you understand how this cellular language works.

Gene Flow

Introduction

The process of synthesizing a protein from DNA information with the help of RNA is comparable to translating a language. A cellular language is converted during protein synthesis into 20 amino acids. RNA nucleotides are used to "rewrite" the DNA sequence of a gene. The nucleotide sequence in the mRNA is then "translated" into an amino acid sequence in a protein. A genetic code is a cellular language that decodes the information and specifies the order of amino acids that make up proteins. Codons, which are groupings of three nucleotides, are read by cells to decode DNA first and then mRNA. Genes contain the instructions for a living thing's various traits. Thus, genetic information is transmitted as code from generation to generation.

Genetic Code Codons Amino Acids

Structure of DNA and RNA

DNA and RNA stands for deoxyribonucleic acid and ribonucleic acid. DNA is a very long double helical molecule constructed from various combinations of four different nucleotide subunits, adenine (A), cytosine (C), guanine (G), and thymine (T). Each nucleotide comprises a nitrogenous base, five-carbon deoxyribose sugar, and a phosphate group. The sugar-phosphate portion of nucleotides also forms the backbone of double-stranded DNA and RNA. In contrast, RNA is a very long single-stranded molecule constructed from various combinations of four different nucleotide subunits, adenine (A), cytosine (C), guanine (G), and uracil (U). Each nucleotide comprises a nitrogenous base, five-carbon ribose sugar, and a phosphate group. The nitrogenous bases in DNA are complementary to each other—for example, adenine pairs with thymine, and cytosine pairs with guanine. The complementary strand of mRNA in DNA is called the sense strand. It is decoded by mRNA. mRNA stands for messenger ribonucleic acid—the adenine of DNA pairs with the uracil of mRNA. The bases of DNA and mRNA make up the codons, and the cellular language that decodes is known as genetic code.

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How was Genetic Code Discovered?

Each nucleotide in a DNA or RNA molecule may be mapped to a single amino acid in a polypeptide in one of the most basic codes. Since proteins typically include 20 amino acids and DNA or RNA only has four nucleotide bases, this code cannot function. As a result, scientists knew that the code could not simply be a one-to-one correspondence between nucleotides and amino acids. A physicist, George Gamow, postulated in the middle of the 1950s that a gene might code for one amino acid in a polypeptide by a series of three sequential nucleotides. Gamow's triplet theory was generally accepted because it made sense. With work from the American scientist Marshall Nirenberg, the genetic code was first cracked in 1961. Nirenberg and his coworkers recognized individual nucleotide triplets that matched particular amino acids for the first time.

Ganongs Potometer

Genetic Code

The genetic code is the entire set of rules or cellular language encoded between codons and amino acids within DNA, the genetic material. Codons and nucleotide sequences are read consecutively in three groups to decode DNA and mRNA.

Codons

A codon is a grouping of three nucleotides in a DNA or RNA molecule that functions as a unit of genetic code. The codons code for specific amino acids, while some unique codons do not code for any amino acid. The standard DNA and RNA codon tables are given below.

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Standard DNA Codon Table

Amino-acid biochemical properties Nonpolar Polar Basic Acidic ↓ Termination: stop codon * Initiation: possible start codon
1st base 2nd base 3rd base
T C A G
T TTT (Phe/F) Phenylalanine TCT (Ser/S) Serine TAT (Tyr/Y) Tyrosine TGT (Cys/C) Cysteine T
TTC TCC TAC TGC C
TTA (Leu/L) Leucine TCA TAA Stop ( Ochre ) * TGA Stop ( Opal ) * A
TTG TCG TAG Stop ( Amber ) * TGG (Trp/W) Tryptophan G
C CTT CCT (Pro/P) Proline CAT (His/H) Histidine CGT (Arg/R) Arginine T
CTC CCC CAC CGC C
CTA CCA CAA (Gln/Q) Glutamine CGA A
CTG CCG CAG CGG G
A ATT (Ile/I) Isoleucine ACT (Thr/T) Threonine AAT (Asn/N) Asparagine AGT (Ser/S) Serine T
ATC ACC AAC AGC C
ATA ACA AAA (Lys/K) Lysine AGA (Arg/R) Arginine A
ATG (Met/M) Methionine ACG AAG AGG G
G GTT (Val/V) Valine GCT (Ala/A) Alanine GAT (Asp/D) Aspartic acid ↓ GGT (Gly/G) Glycine T
GTC GCC GAC GGC C
GTA GCA GAA (Glu/E) Glutamic acid ↓ GGA A
GTG GCG GAG GGG G

Standard RNA Codon Table

Amino-acid biochemical properties Nonpolar Polar Basic Acidic ↓ Termination: stop codon * Initiation: possible start codon
1st base 2nd base 3rd base
U C A G
U UUU (Phe/F) Phenylalanine UCU (Ser/S) Serine UAU (Tyr/Y) Tyrosine UGU (Cys/C) Cysteine U
UUC UCC UAC UGC C
UUA (Leu/L) Leucine UCA UAA Stop ( Ochre ) * UGA Stop ( Opal ) * A
UUG UCG UAG Stop ( Amber ) * UGG (Trp/W) Tryptophan G
C CUU CCU (Pro/P) Proline CAU (His/H) Histidine CGU (Arg/R) Arginine U
CUC CCC CAC CGC C
CUA CCA CAA (Gln/Q) Glutamine CGA A
CUG CCG CAG CGG G
A AUU (Ile/I) Isoleucine ACU (Thr/T) Threonine AAU (Asn/N) Asparagine AGU (Ser/S) Serine U
AUC ACC AAC AGC C
AUA ACA AAA (Lys/K) Lysine AGA (Arg/R) Arginine A
AUG (Met/M) Methionine ACG AAG AGG G

Types of Codons

There are three types of codons, and they are
  • Start codon: The start codon sequence is AUG, which plays a specific role by acting as the codon at which translation starts. In most species, it encodes for the amino acid methionine (Met).
  • Normal codon: In the genetic code, most codons encode amino acids, known as normal codons. For example, UUU, UCU, and UAU codes for phenylalanine.
  • Stop codon: The are three stop codons in the genetic code, UAA, UAG, and UGA. The stop codons stop the translation of proteins.

Difference Between DNA and RNA

Properties of Genetic Code

The properties of genetic code are listed below
  • Triplet Code: A codon is described as a set of nucleotides that specify an amino acid. Substantial evidence indicates that a sequence of three nucleotides (triplet) codes for an amino acid in the protein.
  • Non-ambiguous and Universal: The genetic code is unambiguous. Each codon will only ever code for one single amino acid. It has also been determined that all species share the same genetic code, making them universal.
  • Degenerate Code: Every amino acid, except tryptophan (UGG) and methionine (AUG), is encoded by a different codon; this characteristic is called the degeneracy of the genetic code. For instance, in yeast mitochondria, UGA codes for tryptophan.
  • Nonoverlapping Code: A nucleotide that is a part of one triplet never becomes a part of the following triplet when the code is read in groups of three sequentially.
  • Punctuation: There is no space for punctuation, indicating that each codon immediately follows the one before it without any nucleotides in between.
  • Start and Stop Codons: The AUG codon is the initiating or start codon. In  Eukaryotes, it codes for methionine, and in prokaryotes may be the first component of the polypeptide chain is N- formylmethionine.
UAG, UAA, and UGA are termination codons or stop codons. In contrast, no tRNA molecule can read them, and they never encode any amino acids.

Genetic Drift

  • Polarity: The message is read in a  5’ → 3’  direction by the genetic code. Reading in the opposite direction will code for a different amino acid. It means that the polarity of the codons is fixed.

Genetic Code <span style=

Q1. Name the four nitrogenous bases present in DNA.

Answer: The four nitrogenous bases present in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T).

Q2. Name the four nitrogenous bases present in RNA.

Answer: The four nitrogenous bases present in RNA are adenine (A), cytosine (C), guanine (G), and uracil (U).

Q3. What is the sequence for the start codon, and which amino acid it encodes?

Answer: The sequence for the start codon is AUG, and it encodes for Methionine (Met) amino acid in most organisms.

Q4. How many stop codons are there in the genetic code? Write down their sequence as well.

Answer: The are three stop codons in the genetic code, UAA, UAG, and UGA.

Q5. What do you mean by normal codon?

Answer: In the genetic code, most codons encode amino acids, known as normal codons. For example, UUU, UCU, and UAU codes for phenylalanine.
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