RBSE Solutions Class 11 Biology Chapter 12 Molecular Biology

RBSE Class 11 Biology Chapter 12 Multiple Choice Objective Questions

 

Question 1. Scientists who propounded double helical model of DNA-
(a) Khorana & Nirenberg
(b) Watson & Crick
(c) Beedle & Tautum
(d) Morgan & Bridges
Answer: (b) Watson & Crick
In simple words: The scientists Watson and Crick are famous for proposing the double helix shape of DNA. This model explains how DNA carries genetic information.

🎯 Exam Tip: Remember key scientists and their major contributions, especially famous discoveries like the DNA structure, as these are often tested.

 

Question 2. Single standard DNA is found in-
(a) Bacteria
(b) TMV
(c) Ф Х 174 bacteriophage
(d) Salmonella
Answer: (c) Ф Х 174 bacteriophage
In simple words: Most living things have DNA with two strands, but some viruses, like the Ф X 174 bacteriophage, have DNA with only one strand. This is a unique feature for certain viruses.

🎯 Exam Tip: Pay attention to exceptions in biological rules, such as organisms with single-stranded DNA or RNA, as these are common areas for questions.

 

Question 4. Codon is
(a) Sequence of two nucleotides in mRNA
(b) Sequence of three nucleotides in rRNA
(c) Sequence of three nucleotides in mRNA
(d) Sequence of two nucleotides in tRNA
Answer: (c) Sequence of three nucleotides in mRNA
In simple words: A codon is a set of three building blocks (nucleotides) found in mRNA. Each codon tells the cell which amino acid to add next when making a protein.

🎯 Exam Tip: Clearly distinguish between codons, anticodons, and the different types of RNA (mRNA, tRNA, rRNA) in terms of their structure and function.

 

Question 5. Cistron is a................... unit of gene.
(a) Functional
(b) Recombination
(c) Complementary
(d) Mutation
Answer: (a) Functional
In simple words: A cistron is the part of a gene that carries out a specific job. It is considered the functional unit of a gene because it codes for one polypeptide chain.

🎯 Exam Tip: Understand the different units of a gene (cistron, recon, muton) and their specific roles in genetics to answer definition-based questions accurately.

 

Question 6. Function of RNA polymerase is –
(a) Synthesis of rRNA
(b) Synthesis of Hn-RNA
(c) Synthesis of tRNA
(d) All of the options
Answer: (d) All of the options
In simple words: RNA polymerase is an enzyme that helps make different types of RNA, like rRNA, Hn-RNA, and tRNA. It creates RNA from a DNA template.

🎯 Exam Tip: Remember that in eukaryotes, there are different types of RNA polymerases, each responsible for synthesizing specific types of RNA, but generally, RNA polymerase is crucial for all RNA synthesis.

RBSE Class 11 Biology Chapter 12 Very Short Answer Questions

 

Question 1. Where DNA is found in organisms?
Answer: DNA is primarily found in the nucleus of eukaryotic cells. In prokaryotic cells, it is found in the cytoplasm within a region called the nucleoid. Small amounts of DNA can also be found in mitochondria and chloroplasts, which are organelles within eukaryotic cells.
In simple words: DNA is mainly found in the nucleus of cells, but also in parts like mitochondria. In simpler organisms, it is in the main cell fluid.

🎯 Exam Tip: Specify the locations for both prokaryotic and eukaryotic cells, and mention the organelles containing DNA for a complete answer.

 

Question 3. Name any two types of DNA.
Answer: Two common types of DNA are A-DNA and Z-DNA. These forms differ in their helical structure, with B-DNA being the most common type found in living cells. A-DNA is a right-handed helix, while Z-DNA is a left-handed helix.
In simple words: Two kinds of DNA are A-DNA and Z-DNA. They have slightly different spiral shapes.

🎯 Exam Tip: While B-DNA is the most prevalent, knowing about A-DNA and Z-DNA demonstrates a deeper understanding of DNA's structural diversity.

 

Question 4. Which nitrogen base is found in RNA in place of thymine.
Answer: In RNA, the nitrogen base uracil (U) is found in place of thymine (T). This is a key difference between the chemical composition of DNA and RNA. Uracil pairs with adenine in RNA.
In simple words: Instead of thymine, RNA uses a different chemical building block called uracil.

🎯 Exam Tip: Highlight the key differences between DNA and RNA (sugar, nitrogen bases, and strand number) to show a clear understanding.

 

Question 5. What is recon?
Answer: A recon is the smallest unit of a gene that can undergo recombination. It can be as tiny as a single nucleotide pair. Recombination is the process where genetic material is exchanged between different molecules.
In simple words: Recon is the smallest part of a gene that can be swapped or rearranged during genetic mixing.

🎯 Exam Tip: Define recon precisely as the smallest unit of recombination, and briefly mention its size to show a detailed understanding.

 

Question 6. What is triplet codon ?
Answer: A triplet codon is a sequence of three nucleotides that codes for a specific amino acid. This sequence helps in building proteins, with each unique three-letter combination acting as a genetic instruction. For example, AUG is a start codon.
In simple words: A triplet codon is a group of three building blocks (nucleotides) in DNA or RNA that tell the cell which amino acid to use.

🎯 Exam Tip: Emphasize that a codon is always a triplet (three nucleotides) and specify its role in coding for amino acids during protein synthesis.

 

Question 7. Where RNA polymerase is found in eukaryotic cell ?
Answer: In eukaryotic cells, RNA polymerase is primarily found in the nucleoplasm, which is the substance inside the cell nucleus. Different types of RNA polymerase are localized in various parts of the nucleus and cytoplasm to synthesize different RNA molecules.
In simple words: In cells with a nucleus, RNA polymerase is found inside the nucleus, specifically in the nucleoplasm.

🎯 Exam Tip: Differentiate between the locations of RNA polymerase in prokaryotes (cytoplasm) and eukaryotes (nucleoplasm) for accuracy.

 

Question 8. Which is the initiation codon in protein synthesis.
Answer: The initiation codon in protein synthesis is AUG. This specific codon signals the start of protein translation and also codes for the amino acid methionine (or formylmethionine in prokaryotes). It acts as a starting signal for the cellular machinery.
In simple words: The AUG codon tells the cell where to start making a protein.

🎯 Exam Tip: Always specify AUG as the start codon and mention that it codes for methionine, which is crucial for protein initiation.

RBSE Class 11 Biology Chapter 12 Short Answer Questions

 

Question 1. Describe the chemical components of DNA.
Answer: DNA is made up of three main chemical components: a phosphoric acid group, a pentose sugar (deoxyribose), and nitrogen bases. The phosphoric acid group connects the sugar molecules, forming the backbone. The deoxyribose sugar is a five-carbon sugar with one less oxygen atom at carbon number 2, which helps distinguish DNA from RNA. The nitrogen bases are divided into two categories: purines (adenine (A) and guanine (G)), which have a double-ring structure, and pyrimidines (cytosine (C) and thymine (T)), which have a single-ring structure. These components link together to form a nucleotide, which is the basic building block of DNA.
In simple words: DNA is built from three main parts: a phosphate, a special sugar called deoxyribose, and nitrogen bases. The bases are A, T, C, and G, which carry genetic information.

🎯 Exam Tip: List all three components (phosphate, deoxyribose sugar, and nitrogenous bases) and briefly describe their role and structure, including the two types of nitrogen bases.

 

Question 2. Give an account of Z- DNA.
Answer: Z-DNA is a unique type of DNA that forms a left-handed helix, unlike the more common right-handed B-DNA. It has a zig-zag backbone structure, which gives it its name, "Z" for zig-zag. Z-DNA typically consists of alternating purine and pyrimidine bases. Here's a comparison of its attributes with other DNA forms:

Z-DNA is believed to play roles in gene regulation, particularly in controlling gene expression. Its formation can be influenced by certain sequences of DNA and environmental conditions. Understanding Z-DNA helps scientists appreciate the diverse structures DNA can adopt and their potential biological functions.
In simple words: Z-DNA is a special type of DNA that twists to the left, making a zig-zag shape. It is different from the usual DNA and might help control how genes work.

🎯 Exam Tip: When describing different DNA forms, always mention their helix sense (right or left-handed) and provide quantitative features like base pairs per turn or diameter for a detailed answer.

 

Question 3. Define gene.
Answer: A gene is the fundamental unit of heredity in living organisms. It is a linear sequence of nucleotides within DNA that carries the genetic information to determine specific traits. Genes store and express genetic information, which is then passed on to future generations. Each gene occupies a specific location on a chromosome, known as its locus. The term "gene" was introduced by Johanson in 1909. Understanding genes is crucial for studying how traits are inherited. Scientists have further divided genes into functional units like:
1. **Cistron:** This is the largest functional part of a gene (DNA) that can synthesize one polypeptide chain.
2. **Recon:** This is a small part of a gene, often as small as a single nucleotide pair, that can undergo recombination (crossing over).
3. **Muton:** This is the smallest unit of a gene that can change or mutate.
4. **Operon:** This is a group of genes that work together, including an operator gene, structural genes, and other regulatory genes, all functioning as a unit.
5. **Replicon:** This is the smallest part of DNA that can replicate from a single starting point, including any genetic element that controls its replication.
In simple words: A gene is a basic unit of heredity in DNA that holds instructions for a specific trait. It is like a blueprint for living things.

🎯 Exam Tip: Provide a clear definition of a gene, mention its location, and briefly explain its sub-units (cistron, recon, muton) to show a comprehensive understanding.

 

Question 4. What are complementary genes?
Answer: Complementary genes are two or more genes that, when present together, interact to produce a trait that is different from what any single gene could produce on its own. Neither gene can express the trait alone; they need each other to work. This interaction results in a qualitatively new effect. For instance, in sweet peas, two genes are needed for purple flower color, where each gene produces no color alone but purple when together.
In simple words: Complementary genes are genes that must work together to create a specific trait. If only one is present, the trait will not show up.

🎯 Exam Tip: Define complementary genes by emphasizing their interdependent nature and the qualitative change in phenotype when both are present, perhaps with a simple example.

 

Question 5. Define genetic codon.
Answer: A genetic codon is a sequence of three nucleotides (bases) in messenger RNA (mRNA) that specifies a particular amino acid or signals the start or stop of protein synthesis. This genetic message is originally encoded in DNA and is transcribed into mRNA. The sequence of nucleotides in mRNA that codes for the 20 amino acids is known as the genetic code. Early theories considered singlet (one base per amino acid) and doublet (two bases per amino acid) codes, but these were insufficient to code for all 20 amino acids. The triplet codon concept, where three nucleotides code for one amino acid, was proven correct by Crick, Brenner, and others, yielding 64 possible genetic codes (4x4x4=64), which is more than enough. Scientists like Nirenberg and Mathaei first revealed the nature of a codon. Holley, Khorana, and Nirenberg later studied all 64 codes, earning a Nobel Prize for their work.
In simple words: A genetic codon is a group of three nucleotide building blocks in mRNA that tells the cell which amino acid to use when making proteins. It's like a three-letter instruction.

🎯 Exam Tip: Explain the genetic codon as a triplet nucleotide sequence in mRNA, its role in specifying amino acids, and briefly mention the historical context of its discovery.

 

Question 6. What is unidirectional flow of information?
Answer: The unidirectional flow of information refers to the concept that genetic information generally flows in one direction: from DNA to RNA, and then from RNA to protein. This idea, known as the Central Dogma of Molecular Biology, was proposed by Francis Crick in 1958. It describes how the instructions for building and operating an organism are carried out. This means DNA is copied into mRNA (transcription), and mRNA is then used to make proteins (translation).
In simple words: Unidirectional flow of information means genetic messages go from DNA to RNA, and then from RNA to make proteins, but not usually in reverse.

🎯 Exam Tip: State Crick's Central Dogma clearly, explaining the sequence of information transfer: DNA \( \rightarrow \) RNA \( \rightarrow \) Protein, and the processes involved (transcription and translation).

 

Question 7. Give an account of polymerases found in eukaryotes.
Answer: In eukaryotes, the nucleus contains three main types of RNA polymerases, which synthesize different kinds of RNA. This was identified by R. Roeder and W. Rutter in 1969. These polymerases ensure that all necessary RNA molecules are produced for cell function.
1. **RNA Polymerase I:** This enzyme synthesizes three large ribosomal RNAs (rRNAs): 28S rRNA, 18S rRNA, and 5.8S rRNA. These rRNAs are important components of ribosomes, the cell's protein-making machinery.
2. **RNA Polymerase II:** This polymerase is responsible for synthesizing heterogeneous nuclear RNA (hnRNA), which is the precursor to messenger RNA (mRNA). mRNA carries genetic information from DNA to the ribosomes.
3. **RNA Polymerase III:** This enzyme synthesizes small RNA molecules, including transfer RNA (tRNA), 5S rRNA, and small nuclear RNA (snRNA). tRNA molecules are crucial for carrying amino acids to the ribosome during protein synthesis.
Each RNA polymerase has a specific role, highlighting the complexity and organization of gene expression in eukaryotic cells. In contrast, prokaryotes typically have only one type of RNA polymerase.
In simple words: Eukaryotic cells have three main types of RNA polymerases (Polymerase I, II, and III). Each one makes a different kind of RNA, like rRNA, mRNA, and tRNA, which are all needed for the cell to work.

🎯 Exam Tip: Clearly list and describe the three types of RNA polymerases in eukaryotes, stating which specific RNA molecules each one synthesizes.

RBSE Class 11 Biology Chapter 12 Essay Type Questions

 

Question 1. Explain that DNA is a hereditary material.
Answer: Scientists in molecular biology have shown through experiments that DNA is the material that carries hereditary information. This means DNA passes traits from parents to offspring. Here are two important pieces of evidence:
**(A) Griffith's Experiment (1928):** Frederick Griffith discovered "transformation" using *Diplococcus pneumoniae* bacteria and mice.
* He used two strains: Type II-R (rough, non-virulent, harmless) and Type III-S (smooth, virulent, causes disease).
* When Type III-S bacteria were injected, the mice died.
* When Type II-R bacteria were injected, the mice lived.
* When heat-killed Type III-S bacteria were injected, the mice lived.
* However, when heat-killed Type III-S bacteria were mixed with live Type II-R bacteria and injected, the mice died. Live Type III-S bacteria were found in the dead mice.
* Griffith concluded that some "transforming principle" from the dead Type III-S bacteria changed the harmless Type II-R bacteria into deadly Type III-S bacteria. This principle was later identified as DNA.
**(B) Hershey-Chase Experiments (1952):** Alfred Hershey and Martha Chase provided definitive proof that DNA is the genetic material, using bacteriophages (viruses that infect bacteria).
* They used radioactive isotopes to label specific molecules: \( P^{32} \) labeled DNA and \( S^{35} \) labeled proteins.
* Bacteriophages were grown in mediums containing either \( P^{32} \) or \( S^{35} \).
* These labeled phages were allowed to infect bacteria.
* After infection, the mixture was blended to separate the phages from the bacterial cells and then centrifuged.
* They found that \( P^{32} \) (DNA) entered the bacterial cells, while \( S^{35} \) (protein) mostly remained outside.
* The bacteria that contained \( P^{32} \) then produced new phages.
* This experiment showed that DNA, not protein, carries the genetic instructions for making new viruses. Therefore, DNA is the hereditary material.
Both experiments together strongly supported the idea that DNA is the molecule responsible for carrying and transmitting genetic traits.
In simple words: DNA is the material that carries traits from parents to children. Experiments by Griffith with bacteria and Hershey-Chase with viruses showed that DNA is indeed the blueprint of life, carrying all the genetic information.

🎯 Exam Tip: For this essay question, describe both Griffith's transformation experiment and the Hershey-Chase experiment, clearly stating their procedures, results, and conclusions about DNA being the hereditary material.

 

Question 2. Describe Watson & Crick model of DNA.
Answer: The Watson & Crick model, proposed in 1953, describes DNA as a double helix structure. This groundbreaking model earned J.D. Watson and F.H.C. Crick a Nobel Prize in 1962. DNA is found in plants, animals, prokaryotes, and viruses, primarily in the nucleus of eukaryotes. It is typically double-stranded (dsDNA), though some viruses have single-stranded DNA (ssDNA). The key features of the Watson & Crick model are:
1. **Double Polynucleotide Chains:** DNA consists of two long chains of nucleotides that twist around each other to form a spiral staircase-like structure called a double helix.
2. **Antiparallel Strands:** The two strands run in opposite directions. One strand runs from 5' to 3' and the other runs from 3' to 5'. This antiparallel arrangement is crucial for DNA replication and transcription.
3. **Right-Handed Coiling:** The DNA molecule typically exhibits a right-handed coiling pattern, meaning the helix turns clockwise.
4. **Dimensions of the Helix:** Each full turn of the helix is approximately \( 34Å^{\circ} \) long and contains about 10 pairs of nucleotides. This means that each nucleotide pair is separated by a distance of \( 3.4Å^{\circ} \). The diameter of the helix is about \( 20Å^{\circ} \).
5. **Sugar-Phosphate Backbone:** The backbone of each strand is made of alternating sugar (deoxyribose) and phosphate groups, which are located on the outside of the helix.
6. **Nitrogenous Bases Inside:** The nitrogenous bases (Adenine, Thymine, Guanine, Cytosine) are stacked horizontally inside the helix, protected from the aqueous environment.
7. **Specific Base Pairing (Chargaff's Rule):** Adenine (A) always pairs with Thymine (T) via two hydrogen bonds, and Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds. This complementary base pairing is essential for DNA's ability to store and replicate genetic information. According to Chargaff's rule, the amount of A equals T, and the amount of G equals C. Also, the ratio of purines (A+G) to pyrimidines (C+T) is usually 1:1.
8. **Base Tilt:** The angle of nucleotides from the axis (base tilt normal to the helix axis) is typically around \( 6^{\circ} \).
9. **B-DNA:** The most common form of nuclear DNA is B-DNA, which the Watson-Crick model primarily describes.
This model provided the framework for understanding how genetic information is stored, replicated, and expressed, becoming a cornerstone of molecular biology.
In simple words: The Watson & Crick model shows DNA as a double helix, like a twisted ladder. The sides are made of sugar and phosphate, and the rungs are made of pairs of A with T, and G with C. The two strands run in opposite directions.

🎯 Exam Tip: Clearly list and describe at least 5-7 key features of the Watson & Crick model, including its double helix structure, antiparallel strands, base pairing rules, and dimensions, to score full marks.

 

Question 3. Give an account of replication of DNA.
Answer: DNA replication is the process by which a DNA molecule makes an exact copy of itself. This process is essential for cell division and the inheritance of genetic information. Arthur Kornberg first described the complete process of replication.
(i) **Semiconservative Method:** Replication occurs via a semiconservative method, as proposed by Watson and Crick (1953) and later proven by Meselson and Stahl (1958) using radioisotope techniques (N14 & N15 isotopes). In this method, the two polynucleotide chains of the parent DNA separate, and each old strand serves as a template for synthesizing a new, complementary strand. Thus, each new DNA molecule consists of one old strand and one new strand.
(ii) **Origin of Replication:** Replication begins at specific points on the DNA molecule called origins of replication. There can be one or multiple such points depending on the organism.
(iii) **RNA Primer Requirement:** To start the replication process, a short piece of RNA, known as an RNA primer, is required. These primers are typically 50-100 nucleotides long in eukaryotes and 1000-2000 nucleotides long in prokaryotes.
(iv) **Direction of Synthesis:** DNA synthesis always proceeds from the 5' end to the 3' end (5'\( \rightarrow \)3' end). This means new chains are always formed starting at the 5' end.
(v) **Discontinuous Replication:** The replication process is discontinuous, meaning it occurs in small segments called Okazaki fragments. Okazaki and Coworkers (1968) discovered these fragments. They are smaller in eukaryotes (100-150 nucleotides) than in prokaryotes (1000-2000 nucleotides).
(vi) **Unidirectional or Bidirectional:** Replication can be either unidirectional or bidirectional. Normally, it is bidirectional, meaning it proceeds in both directions from the origin.
(vii) **Enzymatic Machinery (Replisome):** The replication process involves about 20 different enzymes and proteins collectively known as the replisome. Key enzymes include:
* **DNA Helicase:** This enzyme separates (unwinds) the two strands of the DNA molecule at the replication fork, acting as an unwinding enzyme.
* **Restriction Endonuclease:** These enzymes cut the DNA molecule at specific recognition nucleotide sequences (restriction sites), acting as "molecular scissors."
* **DNA Polymerase I and III:** These enzymes are crucial for synthesizing new polynucleotide chains. Polymerase III is the main enzyme for adding new nucleotides, while Polymerase I helps remove RNA primers and replace them with DNA.
* **DNA Ligase:** This enzyme joins the Okazaki fragments together to form a continuous DNA strand. In circular DNA, it links the two ends of the DNA chain.
DNA replication is a highly coordinated and accurate process, ensuring that genetic information is faithfully passed on during cell division.
In simple words: DNA replication is how DNA makes copies of itself. It uses one old strand to build a new one. This process needs special enzymes and starts at specific points, happening in small steps to ensure accurate copying of genetic information.

🎯 Exam Tip: Structure your answer by explaining the semiconservative nature, the steps (initiation, elongation, termination), and the main enzymes involved (helicase, polymerases, ligase) to cover all aspects of DNA replication.

 

Question 5. Describe various types of RNA.
Answer: RNA (Ribonucleic Acid) is a vital molecule with many different functions in living cells. Unlike DNA, RNA often exists as a single strand, but it can fold into complex 3D shapes. \[ \frac { A + U }{ C + G } \ne 1 \text{ or Purines} \ne \text{Pyrimidines} \] RNA can be broadly classified into two main types based on its function: Genetic RNA and Non-genetic RNA. This distinction helps us understand the diverse roles RNA plays in biology.
(A) Genetic RNA:
In some viruses, RNA acts as the genetic material, similar to how DNA functions in other organisms. These viruses use RNA to store and transmit their genetic information.
1. Plant viruses: Examples include the Rice dwarf virus, which has double-stranded RNA (dS RNA).
2. Animal viruses: These include the Rheo virus (dS RNA), wound tumour virus (dS RNA), Influenza virus (single-stranded RNA or SS RNA), and Polio virus (SS RNA).
3. Bacteriophages: Such as MS2 (SS RNA) and S-17 coliphage (SS RNA).
(B) Non-genetic RNA:
In organisms where DNA is the genetic material, RNA plays a crucial role in protein synthesis and gene expression. This non-genetic RNA comes in three main types:
(i) Messenger RNA (mRNA): mRNA molecules carry genetic instructions from DNA in the nucleus to the ribosomes in the cytoplasm. These instructions are like a recipe, telling the cell which proteins to make and in what order. The information is coded as a sequence of nucleotides, which are read in groups of three.
(ii) Ribosomal RNA (rRNA): rRNA is a major component of ribosomes, which are the cell's protein-making factories. It is the most abundant type of RNA, making up about 80% of all cellular RNA. rRNA helps to form the structure of the ribosome and also acts as an enzyme (ribozyme) to catalyse the formation of peptide bonds during protein synthesis. It helps align the messenger RNA and transfer RNA correctly.
(iii) Transfer RNA (tRNA): tRNA molecules act as transporters, bringing specific amino acids to the ribosome during protein synthesis. There are about 60 different types of tRNA, each designed to carry a particular amino acid. The structure of tRNA, often described by the "Clover leaf model" proposed by Robert Holley, has a special anticodon region that matches the codons on the mRNA, ensuring the correct amino acid is added to the growing protein chain. tRNA helps activate amino acids and ensures they are delivered to the right place for protein assembly.
In simple words: RNA is like a helper molecule for DNA, but in some viruses, it carries genetic information itself. There are three main types: mRNA carries DNA instructions, rRNA helps build proteins in ribosomes, and tRNA brings the right building blocks (amino acids) to make those proteins. Different types of RNA ensure all steps of protein creation happen correctly.

🎯 Exam Tip: When describing RNA types, remember to clearly state the primary function and location of each type (mRNA, rRNA, tRNA) as this is what examiners look for.

 

Question 6. Write an essay on genetic code.
Answer: The genetic code is a set of rules that cells use to translate the information encoded within messenger RNA (mRNA) into proteins. This code is essentially a language that dictates how a sequence of nucleotides (bases) in mRNA specifies a sequence of amino acids in a protein. The information stored in DNA is first copied into mRNA through a process called transcription. Then, at the ribosomes, this mRNA message is translated into a protein.

Early ideas about the genetic code explored whether one base (singlet codon) or two bases (doublet codon) could code for an amino acid. However, with only 4 types of nucleotides (Adenine, Uracil, Guanine, Cytosine), a singlet code would give only 4 possible genetic codes (4¹ = 4), which is far too few to code for the 20 common amino acids. A doublet code would provide 16 possible codes (4² = 16), which is still not enough. It was later established that the genetic code is a triplet code, meaning three nucleotides (a codon) are required to specify one amino acid. This gives 64 possible codons (4³ = 64), which is more than enough for the 20 amino acids. This triplet codon concept was first demonstrated by Crick, Brenner, and their colleagues. Marshall Nirenberg and Heinrich J. Mathaei were pioneers in deciphering the nature of these codons.

The detailed study of all 64 genetic codes, including their specific amino acid assignments, was completed by Holley, Khorana, and Nirenberg, who received the Nobel Prize in 1968 for their groundbreaking work. Nirenberg, with Leder, identified 54 codons, though Leder was not awarded the prize.

Here is a basic representation of how some codons map:

Salient Features of Triplet Genetic Code:
1. The genetic code is collinear: This means that the nucleotides in mRNA are arranged in a continuous, linear sequence. This property is known as colinearity, ensuring that the genetic message is read in an unbroken fashion.
2. The genetic code is universal: With very few exceptions, the genetic code is the same across all organisms, from bacteria to humans, and even in viruses. This universality highlights the shared ancestry of all life. However, some minor differences exist, for example, in mammalian mitochondria.
3. The genetic code is non-overlapping: Each nucleotide is part of only one codon, and codons are read sequentially without any overlap. Once three nucleotides are read as a codon, the next three nucleotides form a new codon, ensuring accuracy.
4. The genetic code is non-ambiguous: Each codon specifies only one particular amino acid. This clarity prevents confusion in protein synthesis, making sure the correct protein is always produced.
5. The genetic code is commaless: There are no "commas" or spaces between codons in the mRNA sequence. The ribosomes read the codons continuously, one after another, without skipping any nucleotides or adding extra bases.
6. The genetic code always begins with AUG: The codon AUG serves as the starting signal for protein synthesis and also codes for the amino acid methionine. It is therefore known as the starting or initiating codon. There are also specific "stop codons" that do not code for any amino acid but signal the end of protein synthesis. These include UAA, UAG, and UGA, which are sometimes called non-sense codons because they don't produce an amino acid. No tRNAs exist to bind to these stop codons.
8. The genetic code exhibits degeneracy: This means that most amino acids are specified by more than one codon. For example, several different codons might all code for the same amino acid. This redundancy provides some protection against mutations, as a change in a single nucleotide might still result in the production of the same amino acid, thereby maintaining protein function. Eighteen out of the 20 amino acids are expressed by more than one genetic code.

Here is a comprehensive genetic code table showing all codon-amino acid mappings:

In simple words: The genetic code is like a secret language that tells cells how to build proteins from DNA instructions. It uses groups of three letters (codons) to specify which amino acid comes next. This code is almost the same for all living things and has special start and stop signals. Since many codons can code for the same amino acid, it provides a backup system, making the code robust against small changes.

🎯 Exam Tip: When writing about the genetic code, ensure you cover its key features: triplet nature, universality, non-overlapping, non-ambiguous, commaless, and degeneracy. These are the main points an examiner will look for.

 

Question 7. Explain translation in protein synthesis.
Answer: Translation is the process where the genetic information carried by messenger RNA (mRNA) is converted into a specific sequence of amino acids to form a protein. This crucial process takes place in the ribosomes, which act as cellular factories for protein production. Accuracy in this process is vital because even a small error can lead to a non-functional protein.

Process of Translation:
Translation involves three main stages: initiation, elongation, and termination.

Initiation of polypeptide chain: This is the starting phase where the ribosome assembles around the mRNA and the first amino acid is brought in.

• In prokaryotes: Initiation begins when the small ribosomal subunit (30S) binds to the 5' end of the mRNA at a specific sequence, along with an initiator tRNA carrying formylated methionine (f-Met). This forms an initiation complex. Then, the large ribosomal subunit (50S) joins, completing the 70S ribosome, and translation starts. Initiation factors (IF1, IF2, IF3) and GTP are necessary for this process.
• In eukaryotes: The initiation process is similar to prokaryotes, but an ordinary methionine amino acid, not formylated methionine, is used. The initiator tRNA carrying methionine binds to the small ribosomal subunit, which then scans the mRNA from the 5' end to find the start codon (AUG). Once found, the large ribosomal subunit joins to form the complete ribosome.

(B) Elongation: This stage involves the sequential addition of amino acids to the growing polypeptide chain. Once the ribosome is fully assembled, the polypeptide chain starts growing from the N-terminal (amino end) towards the C-terminal (carboxyl end). Ribosomes move along the mRNA in a 5' → 3' direction, reading each codon.

• The large ribosomal subunit has two main binding sites for tRNA: the P-site (Peptidyl site) and the A-site (Aminoacyl site). The initiator tRNA (carrying f-Met or Met) sits in the P-site. The second codon on the mRNA is now exposed in the A-site. An aminoacyl-tRNA (a tRNA carrying the next specific amino acid) binds to this A-site, matching its anticodon to the mRNA codon. This binding requires elongation factors (like EF-Tp) and GTP.

Next, a peptide bond is formed between the amino acid in the P-site and the amino acid in the A-site. This reaction is catalysed by peptidyl transferase, an enzyme present in the large ribosomal subunit. The amino acid from the P-site is transferred to the amino acid on the A-site tRNA. The tRNA in the P-site, now empty (decylated), leaves the ribosome. The ribosome then moves along the mRNA by one codon (translocation), a process that also requires GTP and elongation factors (like EF-G). This shifts the dipeptidyl-tRNA from the A-site to the P-site, making the A-site available for the next incoming aminoacyl-tRNA. This cycle repeats, adding one amino acid at a time to the growing polypeptide chain.

As a single mRNA molecule is typically long, multiple ribosomes can attach to it simultaneously, forming a "polysome" or "polyribosome." This allows many copies of the same protein to be made from a single mRNA template, improving efficiency.

Termination: Elongation continues until the ribosome encounters a stop codon (UAA, UAG, or UGA) on the mRNA. There are no tRNAs that correspond to these stop codons. Instead, release factors bind to the stop codon in the A-site. These release factors cause the completed polypeptide chain to be released from the tRNA in the P-site and from the ribosome. The ribosome then disassembles into its subunits, and the mRNA is released, ready for degradation or another round of translation. If a stop codon appears prematurely in the mRNA, it can lead to early termination of protein synthesis, resulting in a truncated and often non-functional protein.

Modification of released polypeptide: The newly synthesised polypeptide chain is linear and in its primary structure. It often undergoes further modifications to become a functional protein. For example, the formyl group from f-Met (in prokaryotes) is removed by an enzyme called deformylase. Other amino acids might be removed from either end (N-terminal or C-terminal) by exopeptidases. The polypeptide then folds into its characteristic secondary, tertiary, and sometimes quaternary structures, guided by chaperone proteins, to become biologically active.
In simple words: Translation is how a cell builds a protein by reading the instructions on an mRNA molecule. It happens in three steps: first, the ribosome starts at a specific spot on the mRNA (initiation). Second, it adds amino acids one by one, like building a chain (elongation). Third, it stops when it reaches a special signal (termination), releasing the finished protein. This protein might then be changed a bit to work correctly.

🎯 Exam Tip: To score well on this question, clearly explain the three main stages of translation (initiation, elongation, termination) and mention the key components involved like mRNA, tRNA, ribosomes, and the role of codons/anticodons.