Transcription, Genetic Code & Translation | Biology Class 12 - NEET PDF Download

Transcription of DNA

Transcription of DNA is a cellular process where the genetic information encoded in DNA is converted into RNA. It initiates with RNA polymerase binding to the DNA at a specific promoter region. Then, the enzyme unwinds the DNA and synthesizes a complementary RNA strand by following the DNA template. This process continues until a termination signal is reached, leading to the release of the newly formed RNA molecule, which carries the genetic code for protein synthesis.

Transcription of DNA is a cellular process where the genetic information encoded in DNA is converted into RNA. It initiates with RNA polymerase binding to the DNA at a specific promoter region. Then, the enzyme unwinds the DNA and synthesizes a complementary RNA strand by following the DNA template. This process continues until a termination signal is reached, leading to the release of the newly formed RNA molecule, which carries the genetic code for protein synthesis.

Transcription of DNA Definition

• Transcription is the process of copying genetic information from DNA to RNA. It involves RNA polymerase creating a complementary RNA strand using a DNA template.

What is Transcription?

• Transcription of DNA is a fundamental cellular process that converts the genetic information stored in DNA into RNA. Both DNA and RNA are nucleic acids. While DNA stores genetic information, RNA mostly helps in the transfer and expression of information. DNA, being chemically and structurally more stable, is a better genetic material. The process of transcription is governed by the principle of complementarity. In transcription, only a segment of DNA and only one of the strands is copied into RNA. This process occurs in three main stages: initiation, elongation, and termination.

RNA Polymerase

RNA polymerase is a crucial enzyme involved in gene expression and the transcription of genetic material. Its primary function lies in transcribing DNA sequences into RNA molecules. During transcription, RNA polymerase identifies specific promoter regions on the DNA, indicating where transcription should begin. As it progresses along the DNA template, RNA polymerase separates the double helix and generates a complementary RNA strand. The resulting RNA, primarily messenger RNA (mRNA), contains the instructions for protein synthesis. There are multiple types of RNA polymerases, with RNA polymerase II being the main enzyme responsible for transcribing protein-coding genes in eukaryotic cells. By regulating gene expression, RNA polymerase plays a fundamental role in the development, maintenance, and regulation of various cellular processes essential for living organisms.

Stages of Transcription

Initiation: This phase signifies the beginning of transcription. RNA polymerase, in conjunction with other transcription factors, identifies and attaches to the promoter region on the DNA. The DNA double helix unwinds, exposing the template strand for transcription.

Elongation: Subsequently, with the RNA polymerase bound and the DNA unwound, the enzyme progresses along the DNA template strand. It constructs a complementary RNA strand by integrating ribonucleotides based on the DNA template. This phase continues until a termination signal is encountered. 

Termination: The transcription process culminates when the RNA polymerase identifies a specific termination signal on the DNA. This signal prompts the polymerase to release the newly formed RNA molecule. The DNA double helix reverts to its original state, and the RNA product is released.

RNA Processing

RNA processing is a crucial step in gene expression, notably in eukaryotes, where the initial RNA transcript (pre-mRNA) undergoes various modifications to mature into mRNA.

• Capping: A modified guanosine cap is added to the 5' end of the pre-mRNA, safeguarding the mRNA and facilitating ribosome binding during translation.
• Splicing: In this process, introns (non-coding regions) are excised from the pre-mRNA, while exons (coding regions) are spliced together. This step forms the mature mRNA sequence carrying genetic information for protein synthesis.
• Polyadenylation: A poly-A tail consisting of adenine nucleotides is appended to the 3' end of the mRNA. This tail enhances mRNA stability and aids in its transport out of the nucleus.

The outcome of RNA processing is mature mRNA, which exits the nucleus and acts as a blueprint for protein synthesis at ribosomes during translation. These processing stages ensure precise transcription of genetic information, preparing it for efficient protein production.

Inhibitors of Transcription

Inhibitors of transcription are substances that disrupt the normal process of transcription, impeding the synthesis of RNA from DNA. These inhibitors can act in various ways, such as inhibiting RNA polymerase activity or interfering with the binding of transcription factors to DNA.

• Streptolydigin:Streptolydigin functions by binding to the polymerase, thus preventing the elongation of nucleic acid chains. This action effectively halts the activity of RNA polymerase within the cell.
• Rifampicin (rifamycin):Rifampicin, a medication used to combat tuberculosis, hinders mitochondrial RNA polymerase by attaching to the beta subunit of bacterial RNA polymerase.
• Alpha amanitin:Alpha amanitin, an inhibitor found in Amanita phalloides, obstructs both the initiation and elongation phases of RNA II polymerase.
• Cordycepin:Cordycepin works by lacking a hydroxyl moiety at the 3' position, which leads to the prevention of RNA synthesis and transcription elongation.
• Actinomycin D:Actinomycin D, an antibiotic with antibacterial and anticancer properties, impedes rRNA transcription.

Transcription Termination

• Rho-Dependent Termination: In this process, a protein known as Rho factor attaches to the newly formed RNA strand and progresses along it until it catches up with RNA polymerase. This interaction between Rho factor and RNA polymerase leads to the termination of transcription.
  
• Rho-Independent (Intrinsic) Termination: In this method, a specific termination signal on the DNA template strand triggers the formation of a hairpin loop in the newly synthesized RNA. This loop disrupts the RNA-DNA hybrid, causing RNA polymerase to momentarily pause and then release the RNA molecule.
  

RNA Translation

The Central Dogma states that once information has been translated into protein, it cannot be reversed. Specifically, while transferring information from nucleic acids to other nucleic acids or from nucleic acids to proteins is feasible, the reverse transfer, from protein to protein or from protein to nucleic acid, is not possible. In this context, "information" pertains to the precise sequence identification, whether it's the sequence of bases in nucleic acids or amino acid residues in proteins.

There are three main types of biopolymers: DNA, RNA (both nucleic acids), and proteins. Between these three types, there are nine possible direct exchanges of information. According to the central dogma, these exchanges are grouped into three categories: three general transfers, two special transfers, and four unknown transfers.

General transfers include DNA replication, transcription (where DNA information is transcribed into mRNA), and translation (where proteins are synthesized using mRNA as a template).

Translation is the process of decoding mRNA and using its information to build a polypeptide, which essentially forms a protein. This process occurs in ribosomes, where tRNA molecules bind to mRNA codons, delivering specific amino acids that are linked together to form a polypeptide chain.

In prokaryotes, translation occurs in the cytoplasm, while in eukaryotes, it can happen in the cytoplasm or across the endoplasmic reticulum membrane through co-translational translocation. After translation, the newly formed polypeptide may be retained in the endoplasmic reticulum for further processing or immediately secreted.

Several types of RNA, such as transfer RNA, ribosomal RNA, and small nuclear RNA, do not participate in translation into proteins. Many antibiotics target translation by affecting ribosomes, exploiting differences between prokaryotic and eukaryotic ribosomal structures.

DNA, or deoxyribonucleic acid, is a double-stranded polymer consisting of two polynucleotide chains wound into a helical structure. It carries genetic instructions for the genesis, functioning, growth, and reproduction of organisms and viruses. DNA is composed of nucleotides, each containing a nitrogenous base (cytosine, guanine, adenine, or thymine), deoxyribose sugar, and a phosphate group. The nitrogenous bases pair up according to specific rules (A with T and C with G), forming the double helix structure. This structure ensures that identical biological information is stored in both strands of DNA.

Ribosome

Ribosomes are essential cellular structures found in all living organisms. Their main function is to synthesize proteins through a process known as mRNA translation. Ribosomes work by linking amino acids together in the sequence specified by messenger RNA (mRNA) codons, forming polypeptide chains. There are two main parts of ribosomes: the small and large ribosomal subunits, each containing ribosomal RNA (rRNA) molecules and ribosomal proteins (RPs or r-proteins). Together with other components, ribosomes form what is often referred to as the translational machinery.

When a protein's amino acid sequence encoded in DNA needs to be produced, it is transcribed into messenger RNA. Ribosomes bind to this mRNA and, using its sequence, assemble the correct amino acids in the proper order to create the specific protein. Transfer RNA (tRNA) molecules play a crucial role in this process by selecting and carrying amino acids to the ribosome, where they bind to the mRNA through an anti-codon stem-loop structure. Each tRNA corresponds to a specific codon in the mRNA and carries the correct amino acid for incorporating into the growing polypeptide chain.

A ribosome is a complex made up of RNA and proteins. The 30S subunit primarily acts as a decoder and is associated with mRNA, while the 50S subunit functions as a catalytic enzyme and interacts with aminoacylated tRNAs.

Protein Synthesis Process:

• Initiation: Protein synthesis begins with the start codon AUG on the mRNA.
• Elongation: Amino acids are added to the growing polypeptide chain.
• Termination: The process stops when a stop codon (UAA, UAG, or UGA) is reached.
• Recycling: Components are released and the ribosome is ready for a new cycle.

Ribosomes are considered ribozymes because the ribosomal RNA within them facilitates the peptidyl transferase activity, which is responsible for connecting amino acids during protein synthesis.

• Proteins are created through a series of four stages: initiation, elongation, termination, and recycling.
• The sequence AUG serves as the start codon in all mRNA molecules.
• The stop codon, UAA, UAG, or UGA, signals the end of translation, as tRNA molecules do not recognize these codons.
• Ribosomes act as ribozymes, utilizing ribosomal RNA to catalyze peptidyl transferase activity that links amino acids.

RNA

In cells, various types of RNA play distinct roles. The key RNA types involved in translation are messenger RNA (mRNA) and transfer RNA (tRNA). mRNA, serving as the messenger between DNA and proteins, consists of the four amino acids CGAU (Cytosine, Guanine, Adenine, Uracil) in each mRNA strand.

Transfer RNA (tRNA) bridges mRNA and amino acids. On one end of tRNA, an amino acid is situated, while the other end contains an anticodon that matches a codon on mRNA. Consequently, each codon on mRNA corresponds to an anticodon on tRNA, representing a specific amino acid.

Ribosomal RNA (rRNA)

Ribosomal RNA, or rRNA, is a crucial component found in ribosomes, the cellular structures responsible for protein synthesis. The three-dimensional structure of ribosomes is heavily influenced by the core structure of rRNA. This structure is maintained by ribosomal proteins, which interact with the core. Within the nucleus, nucleoli are specialized structures where ribosomal RNA is translated. The production of ribosomes relies on the retention of ribosomal proteins within nucleoli.

Functions

• The unique three-dimensional structure of rRNA, characterized by internal helices and loops, plays a key role in creating specific sites (A, P, and E sites) within the ribosome.
• Various ribosomal proteins can bind to specific residues on rRNA after thorough examination of both RNA and proteins.
• Recent discoveries have identified binding sites for antibiotics like tetracycline and streptomycin on bacterial rRNA.
• Preribosomal RNA, a precursor to rRNA, has been associated with the production of microRNA, a regulator of inflammation and heart disease triggered by mechanical stress, offering a new perspective on rRNA's function.

Structure

Ribonucleic acid shares some similarities with DNA, containing nitrogen bases like adenine, guanine, and cytosine. In RNA, adenine pairs with uracil, forming a base pair held together by two hydrogen bonds. RNA molecules exhibit a hairpin structure, with nucleotides arranged akin to DNA. Phosphate groups, known as nucleosides, are instrumental in nucleotide synthesis in DNA.

Transfer RNA (tRNA)

The transfer RNA plays a crucial role in selecting the appropriate proteins or amino acids needed by an organism, thereby aiding the ribosomes. It is situated at the ends of each amino acid. Commonly referred to as soluble RNA, it acts as a connector between the messenger RNA and the amino acid.

• Transfer RNAs are relatively small molecules, typically around 70-90 nucleotides (5 nm) in length, and are produced from a variety of genes.
• The structure of a tRNA, including components like the D-arm and T-arm, accounts for its high specificity and efficiency. Despite the chemical similarities among many amino acids, the error rate in connecting an amino acid to a tRNA is remarkably low, at just 1 in 10,000.
• Similar to other biological nucleic acids, transfer RNAs possess a sugar-phosphate backbone. The orientation of the ribose sugar determines the molecule's directionality.
• 
  
  • The T-arm significantly influences tRNA's function during translation. Besides thymidine, which is common in DNA, tRNA molecules contain numerous modified bases. The T-arm facilitates the interaction between tRNA and the ribosome. Additionally, the variable arm, which is less than 20 nucleotides long, separates the anticodon loop and the T-arm. While essential for AATS to recognize tRNA, this arm may not be present in all species.

The T-arm and tRNA

• The T-arm plays a crucial role in modulating tRNA functionality throughout translation processes.
• tRNA molecules are distinguished by their inclusion of several modified bases beyond thymidine, commonly found in DNA.
• The T-arm facilitates the interaction of tRNA with ribosomes.
• A variable arm, shorter than 20 nucleotides, separates the anticodon loop from the T-arm.
• AATS is responsible for recognizing tRNA, though its presence may vary among different species.

Messenger RNA (mRNA)

• mRNA acts as an intermediary, carrying genetic information from DNA to guide protein synthesis.
• Regulatory regions within mRNA influence the timing and speed of translation, ensuring orderly protein production by facilitating ribosome, tRNA, and protein binding sites.
• Proteins produced by cells serve various functions like structural support, enzymatic activities, and cellular movement.

Structural Complexity of mRNA

Eukaryotic mRNA molecules, prevalent in organisms with defined nuclei, exhibit unique features such as a cap structure formed by 5′-triphosphate residue esterification. They also possess a poly(A) tail at the 3′ end post-transcription. Eukaryotic mRNA processing involves cleavage, reconnection, and the presence of introns and exons. In contrast, prokaryotic mRNAs lack stability and degrade faster due to the absence of poly(A) tails and cap structures.

Amino acid Activation

The process of attaching an amino acid to its corresponding transfer RNA is termed amino acid activation, also known as aminoacylation or tRNA charging. Initially, the AMP-amino acid complex binds to a tRNA molecule via the action of aminoacyl tRNA synthetase, releasing AMP and joining the amino acid to the tRNA. This results in the formation of charged aminoacyl-tRNA, essential for initiating translation and protein synthesis. Activating amino acids through covalent coupling to tRNA molecules is necessary because the synthesis of peptide bonds is energetically unfavorable. The energy stored in the tRNA-aminoacyl bond is utilized to drive peptide bond formation, thus enhancing the reactivity of the amino acid and facilitating peptide bond formation. This activation step primes the aminoacylated tRNA for the initiation phase of translation, where mRNA and aminoacyl-tRNA bind to the ribosome.

To form a 5′ aminoacyl adenylate intermediate, the carboxyl group of the amino acid initially forms a covalent bond with the -phosphate of ATP, yielding inorganic pyrophosphate (PPi) (aa-AMP):

• aa + ATP ⟶ aa-AMP + PPi

Subsequently, a nucleophilic attack on the aminoacyl adenylate intermediate by the 3′-OH of the tRNA results in the attachment of the aminoacyl group to the tRNA, liberating an AMP molecule:

• aa-AMP + tRNA ⟶ aa-tRNA + AMP

Aminoacyl tRNA synthetases are categorized into Class I and Class II enzymes. Class I enzymes transfer the aminoacyl group to the 2′-OH of the tRNA via a transesterification process before transferring it to the 3′-OH of the tRNA. In contrast, Class II enzymes catalyze the transfer of the aminoacyl group directly from the 3′-OH of the tRNA to the aminoacyl group in a single step:

• aa + ATP + tRNA ⟶ aa-tRNA + AMP + PPi

RNA Translation

Translation or protein synthesis involves 3 steps i.e., Initiation, Elongation, and Termination.

Initiation

Before translation begins, several key components are required:

• Ribosomes exist in two subunits, large and small.
• An initiator tRNA carrying the initial amino acid, typically methionine.
• An mRNA containing instructions for the protein being synthesized.

These elements must align precisely during initiation to form the initiation complex, a crucial framework for protein production.

In eukaryotic cells, translation initiation unfolds as follows: the methionine-carrying tRNA binds to the small ribosomal subunit. Together, they identify the mRNA's 5' GTP cap and associate with it. Upon encountering the start codon, typically AUG, they halt their progression along the mRNA strand in the 3' direction.

In contrast, bacterial translation initiation varies. Here, the small ribosomal subunit does not traverse the mRNA from 5' to 3'. Instead, it directly binds to specific mRNA sequences, like Shine-Dalgarno sequences, preceding the start codon to guide the ribosome.

• Amino Acid: During initiation, the amino acid, tRNA, and mRNA assemble within the ribosome. The mRNA remains intact, with AUG serving as the start codon, initiating the protein sequence.
• Initiation Factors: Proteins known as initiation factors interact with the small ribosomal subunit to facilitate translation initiation. These factors, including IFs in bacteria and eIFs in eukaryotes, can aid, hinder, or expedite translation processes.

Elongation

Before any amino acids are linked to form a chain, following the establishment of the initiation complex, the process of elongation begins. The ribosome's P site, located at the center, hosts the initial tRNA carrying methionine. Simultaneously, a new codon is exposed at the adjacent A site. The subsequent tRNA, featuring an anticodon perfectly complementary to the exposed codon, occupies the A site. Subsequently, a peptide bond is formed between the amino acid of the second tRNA at the A site and the methionine from the first tRNA, resulting in a small polypeptide comprising two amino acids.

The elongation process involves the continuous addition of amino acids to extend the polypeptide chain. Following the peptide bond formation, the mRNA is translocated through the ribosome by one codon. This movement allows the initial tRNA to exit through the E site and reveals a new codon in the A site, facilitating the ongoing cycle. This elongation process repeats numerous times, potentially up to 33,000 repetitions. For instance, titin, a lengthy polypeptide found in muscles, can consist of up to 33,000 amino acids.

Elongation Factor

• Elongation factors are a group of proteins that accelerate translational elongation during protein synthesis at the ribosome.
• In prokaryotes, prevalent elongation factors such as EF-Tu, EF-Ts, and EF-G aid in the speedy formation of peptide bonds in developing polypeptides.
• While both bacteria and eukaryotes employ elongation factors, they exhibit structural differences and have distinct research nomenclatures.
• Elongation is the fastest phase of translation, with bacteria adding 15 to 20 amino acids per second (approximately 45-60 nucleotides per second), while eukaryotes add around two amino acids per second (roughly 6 nucleotides per second).

Termination

Inevitably, polypeptides must conclude, just as all good things do. The termination phase marks the conclusion of translation. When a stop codon (UAA, UAG, or UGA) on the mRNA enters the A site, the process halts. Release factors, distinct from tRNAs but fitting snugly into the P site, are proteins that recognize stop codons. By catalyzing the addition of a water molecule to the final amino acid in the chain, release factors disrupt the enzyme responsible for peptide bond formation. This action results in the separation of the chain from the tRNA.

• Termination factor: A protein in molecular biology that orchestrates the cessation of RNA transcription by pinpointing a transcription terminator and triggering the release of freshly synthesized mRNA. This mechanism governs RNA transcription to uphold gene expression integrity, observed in both eukaryotes and prokaryotes, with bacterial processes being well-documented.
• Rho (ρ) factor: The Rho protein, a prominent transcriptional termination factor, identifies a cytosine-rich segment of the elongating mRNA. Although specifics regarding the recognized sequences and cleavage process remain unclear, Rho moves along the mRNA as a ring-shaped hexamer, consuming ATP in the direction of RNA polymerase (from 5' to 3' relative to mRNA). Transcription halts upon Rho protein reaching the RNA polymerase complex, leading to the separation of RNA polymerase from DNA. The structural and functional resemblance between the Rho protein and the F1 subunit of ATP synthase supports the notion of an evolutionary link between the two.

Regulations of RNA

RNA interference by miRNAs

• RNA interference, facilitated by miRNAs, regulates the expression levels of many genes post-transcriptionally. This process involves specific small RNA molecules, called miRNAs, binding to particular sections of mRNA, marking them for degradation.
• In order to bind to a target mRNA section, the RNA must undergo processing through an antisense-based mechanism. Once binding occurs, additional proteins instruct nucleases to cleave the mRNA.

Long non-coding RNAs

• Long non-coding RNAs, such as Xist, play a crucial role in X chromosomal inactivation regulation. Researchers, including Jeannie T. Lee, found that these RNAs recruit the Polycomb complex to silence chromatin regions, thereby preventing mRNA transcription.
• Other long non-coding RNAs (lncRNAs), characterized by their length (more than 200 base pairs) and lack of coding capability, are associated with controlling stem cell pluripotency and cell division.

Enhancer RNAs

• Enhancer RNAs represent a significant class of regulatory RNAs. It is uncertain if they form a distinct RNA type of varying lengths or if they are a subset of lncRNAs. These RNAs originate from enhancers, DNA regions near controlled genes.
• Enhancer RNAs upregulate the transcription of the genes they control, which are adjacent to the enhancer region.

Prokaryotic Regulatory RNA

• Initially, regulatory RNA was thought to be exclusive to eukaryotes, contributing to the apparent higher transcription levels in more complex organisms.
• Research revealed the presence of potential RNA regulators in bacteria, termed short RNA (sRNA).
• Bacterial short RNAs often interact with mRNA through antisense pairing to hinder translation by affecting stability or cis-binding capacity.
• Riboswitches, which are cis-acting regulatory RNA sequences, have also been identified. They undergo structural changes upon binding metabolites, altering their ability to bind chromatin and regulate gene expression.

Regulatory RNA in Archaea

• Archaea also possess regulatory RNA systems.
• The CRISPR system, employed for in situ DNA manipulation, defends archaea and bacteria against viral intrusion by utilizing regulatory RNAs.

Processing of RNA

• Various RNAs participate in RNA modification processes.
• Spliceosomes, containing multiple short nuclear RNAs (snRNA) or ribozymes, remove introns from pre-mRNA.
• RNA modifications include altering nucleotides beyond A, C, G, and U.
• Small nucleolar RNAs (snoRNA) located in the nucleolus and Cajal bodies guide RNA nucleotide modifications in eukaryotes by binding to enzymes and directing them to specific RNA locations for nucleotide alteration.
• rRNAs, tRNAs, snRNAs, and mRNAs undergo significant modifications, including methylation of RNA molecules.

RNA Modification

• Various RNAs play roles in RNA modification processes.
• Spliceosomes, containing short nuclear RNAs (snRNA) or ribozymes, remove introns from pre-mRNA.
• RNA alterations can occur by changing nucleotides to bases other than A, C, G, and U.
• Small nucleolar RNAs (snoRNA) found in the nucleolus and Cajal bodies guide nucleotide changes in eukaryotes.
• snoRNAs bind to enzymes, directing them to specific RNA locations via base-pairing for nucleotide modifications.
• Enzymes modify nucleotides on rRNAs, tRNAs, snRNAs, and mRNAs, including methylation processes.

Genomes of RNA

• RNA, akin to DNA, can carry genetic information.
• RNA viruses possess genomes consisting of RNA that encodes multiple proteins.
• These proteins aid in replicating the viral genome and safeguarding it during host cell transfer.
• Viroids, a pathogenic RNA type, lack protein encoding and replicate using the host plant cell's polymerase.

RNA with Double-Strand

• Double-stranded RNA (dsRNA) comprises two complementary RNA strands, resembling cellular DNA with uracil instead of thymine.
• dsRNA is found in some viruses, known as double-stranded RNA viruses.
• In eukaryotes, dsRNA like viral RNA or siRNA can trigger RNA interference and in vertebrates, an interferon response.
• dsRNA involvement activates the innate immune system against viral infections in eukaryotes.

Importance of RNA Translation

• Translational control plays a crucial role in the growth and survival of cancer.
• Cancer cells often need to regulate the translation process of gene expression, even though the reason for prioritizing translation over transcription remains unclear.
• While genetic alterations in translation factors are common in cancer cells, they are more inclined to adjust the levels of existing translation factors.
• Cancer cells adjust translation to cope with cellular stress effectively.
• Under stressful conditions, cells translate specific mRNAs that aid in cellular adaptation and survival.
• Future cancer treatments might involve disrupting the translation mechanisms of cancer cells to counteract the consequences of cancer.

What is Genetic Code

• Genetic code is the sequence of nucleotides on DNA or RNA that gets translated into sequences of amino acids forming proteins.
• During replication and transcription, nucleic acids are copied to form other nucleic acids. Translation involves transferring genetic information from nucleotide polymers to amino acid polymers.
• Changes in nucleic acids are responsible for altering the amino acid sequences in proteins, leading to the establishment of the genetic code that dictates the amino acid sequence during protein synthesis.
• Physicist George Gamow proposed that with only 4 bases coding for 20 amino acids, the code must consist of combinations of bases. He suggested that to code for all 20 amino acids, the code should comprise 3 nucleotides due to the permutation combination 4^3 (4*4*4).

Salient Features of the Genetic Code

• Some amino acids are encoded by multiple codons, making the genetic code degenerate.
• There are 61 codons that specify amino acids, while 3 codons serve as stop codons and do not code for any amino acids, hence the codons are in triplets.
• The codon "AUG" has a dual function, coding for Methionine (Met).
• The stop terminator codons are "UAA," "UGA," and "UAG."
• The genetic code is nearly universal across organisms.
• Codons are read on mRNA continuously without punctuation marks.

What is Genetic Table?

The genetic code refers to the relationships between amino acids and codons. When this information is organized into a table, it is termed as a Genetic Code Table. In this table, each amino acid is represented by more than one codon. For instance, leucine can be written in six different ways in the language of mRNA.

Properties of Genetic Code

• Nonoverlapping Code: This concept involves reading the code in sets of three, where each set is distinct and does not overlap with the next set, ensuring clarity in translation.
• Degenerate Code: With the exception of tryptophan and methionine, most amino acids can be coded by multiple codons. This redundancy is known as the degeneracy of the genetic code.
• Triplet Code: Scientific evidence confirms that sequences of three nucleotides encode a single amino acid in a protein, demonstrating the triplet nature of the genetic code.
• Universal Nature or Non-ambiguous: The genetic codes are consistent across all organisms, showcasing their universal characteristics. Additionally, each codon unambiguously codes for a specific amino acid.
• Comma less Code: In the genetic code, there are no interruptions or punctuation between codons. Each codon is directly adjacent to the preceding one without any nucleotides in-between.
• Start and Stop Codons: The initiation codon is AUG, marking the start of translation, while the termination codons include UAG, UGA, and UAA, indicating the end of translation.
• Polarity: Codons are read directionally from 5' to 3'. The initial base is at the 5' end, followed by the middle and last bases, maintaining a fixed polarity in the genetic code.

Exceptions of Genetic Code

Due to similar codons being seen in all organisms, the genetic code is universal along with the same START and STOP signals in the genes of microorganisms and plants. Therefore there are few exceptions which include one or two STOP codons of an amino acid.

Though GUG is meant for valine both the codons GUG and AUG code for methionine as starting codons which breaks the property of non-ambiguousness. Therefore, few codes are different from universal code or non-ambiguous code.