Translation and Gene Regulation

Translation and Gene Regulation

Translation is the process by which the information present in the nucleotide sequence of mRNA is used to synthesise a specific sequence of amino acids, forming a protein.

सरल शब्दों में, mRNA में मौजूद genetic information को amino acid sequence में बदलकर protein बनाने की प्रक्रिया को translation कहते हैं।

DNA → Transcription → mRNA → Translation → Protein

Translation

Translation takes place on ribosomes. The ribosome reads the codons present on mRNA and joins the corresponding amino acids in the correct sequence.

Translation में mRNA के codons को ribosome एक-एक करके पढ़ता है और tRNA उन codons के अनुसार सही amino acids लेकर आता है। ये amino acids peptide bonds द्वारा जुड़कर polypeptide chain बनाते हैं।

Requirements for Translation

The major components required for protein synthesis are:

• mRNA
• Ribosomes
• tRNA
• Amino acids
• Enzymes
• ATP and other energy sources
• Various initiation, elongation and termination factors

Component	Role
mRNA	Carries codons specifying the amino acid sequence.
tRNA	Brings specific amino acids to the ribosome.
Ribosome	Provides the site for protein synthesis.
Amino acids	Building blocks of proteins.
Aminoacyl-tRNA synthetase	Helps attach the correct amino acid to its corresponding tRNA.
Energy molecules	Provide energy required for different steps of translation.

tRNA and Its Role

Transfer RNA (tRNA) acts as an adaptor molecule between the nucleotide sequence of mRNA and the amino acid sequence of a protein.

tRNA contains a specific anticodon that recognises the complementary codon on mRNA.

Each tRNA can carry a specific amino acid at its amino acid attachment site.

Charging of tRNA

The process of attaching an amino acid to its corresponding tRNA is called aminoacylation or charging of tRNA.

This reaction is catalysed by a specific enzyme called aminoacyl-tRNA synthetase.

अर्थात् translation शुरू होने से पहले tRNA को उसके correct amino acid से जोड़ना आवश्यक है। इस charged tRNA को aminoacyl-tRNA कहा जाता है।

Ribosome

The ribosome is the cellular machinery responsible for protein synthesis.

Ribosomes are made up of ribosomal RNA (rRNA) and proteins.

In prokaryotes, the ribosome is 70S, consisting of:

• 50S large subunit
• 30S small subunit

In eukaryotes, the ribosome is 80S, consisting of:

• 60S large subunit
• 40S small subunit

Organism	Ribosome	Subunits
Prokaryotes	70S	50S + 30S
Eukaryotes	80S	60S + 40S

Note: S stands for Svedberg unit, which represents sedimentation coefficient and is not simply an additive unit. इसलिए 50S + 30S = 70S और 60S + 40S = 80S होते हैं।

Stages of Translation

Translation can be divided into three major stages:

1. Initiation
2. Elongation
3. Termination

1. Initiation

Translation begins when the ribosome assembles on the mRNA near the start codon.

The start codon is generally AUG, which specifies methionine.

The small ribosomal subunit first associates with mRNA. The initiator tRNA carrying methionine recognises the AUG start codon through its complementary anticodon.

The large ribosomal subunit then joins to form the complete ribosome.

2. Elongation

During elongation, amino acids are added one after another to the growing polypeptide chain.

The ribosome moves along the mRNA in the 5′ → 3′ direction.

Each incoming tRNA recognises the appropriate codon through its anticodon and brings the corresponding amino acid.

Peptide Bond Formation

The amino acids are joined by peptide bonds.

The ribosome catalyses peptide bond formation through its catalytic rRNA activity. The growing polypeptide chain is transferred from one tRNA to the amino acid attached to the next tRNA.

इस प्रकार amino acids एक निश्चित क्रम में जुड़ते जाते हैं और polypeptide chain लगातार लंबी होती जाती है।

3. Termination

Translation terminates when a stop codon enters the appropriate site of the ribosome.

The three stop codons are:

UAA, UAG and UGA

These codons do not code for any amino acid.

When a stop codon is encountered, a release factor helps release the newly synthesised polypeptide chain from the ribosome.

Polysomes

A single mRNA molecule can be translated simultaneously by several ribosomes.

A group of ribosomes attached to the same mRNA molecule is called a polysome or polyribosome.

Polysomes allow many copies of the same protein to be produced simultaneously from a single mRNA molecule.

Central Dogma

The central dogma describes the basic flow of genetic information:

DNA → RNA → Protein

DNA information is first transcribed into RNA, and the information in mRNA is then translated into protein.

However, some biological systems also show exceptions, such as RNA → DNA during reverse transcription.

Gene Regulation

Gene regulation refers to the control of when, where and to what extent a gene is expressed.

हर gene हर समय active नहीं रहता। Cell अपनी आवश्यकता के अनुसार कुछ genes को switch on और कुछ genes को switch off रखती है। इसी control को gene regulation कहते हैं।

Gene regulation allows cells to:

• respond to environmental changes,
• save energy and resources,
• produce proteins only when required,
• perform specialised functions.

Regulation of Gene Expression in Bacteria

In prokaryotes, gene expression is commonly regulated at the level of transcription.

A classic example is the lac operon of Escherichia coli.

Lac Operon

The lac operon controls the utilisation of lactose by E. coli.

It was proposed by Jacob and Monod.

The lac operon is an example of an inducible operon.

Components of Lac Operon

The lac operon consists of structural genes and regulatory regions.

Component	Function
i gene	Produces the repressor protein.
Promoter (P)	Site where RNA polymerase binds.
Operator (O)	Regulatory site where the repressor can bind.
z gene	Codes for β-galactosidase.
y gene	Codes for permease.
a gene	Codes for transacetylase.

Role of Structural Genes

Gene	Enzyme / Protein	Function
z	β-galactosidase	Breaks lactose into glucose and galactose.
y	Permease	Facilitates entry of lactose into the bacterial cell.
a	Transacetylase	Participates in lactose metabolism-related processes.

Lac Operon in the Absence of Lactose

When lactose is absent, the lac repressor remains active.

1. The repressor protein binds to the operator.
2. RNA polymerase cannot efficiently proceed through the structural genes.
3. The structural genes remain largely untranscribed.
4. Proteins required for lactose utilisation are therefore not produced in significant amounts.

इस स्थिति में cell उन enzymes को बनाने में energy खर्च नहीं करती जिनकी आवश्यकता नहीं है।

Lac Operon in the Presence of Lactose

When lactose is available, a derivative of lactose called allolactose acts as the inducer in the lac operon.

1. The inducer interacts with the repressor.
2. The repressor is unable to bind effectively to the operator.
3. The operator becomes available for transcription.
4. RNA polymerase transcribes the structural genes.
5. The required enzymes are produced.

इस प्रकार lactose की presence में lac operon active हो जाता है और lactose utilisation से जुड़े proteins बनने लगते हैं।

Negative Regulation

The lac operon is primarily an example of negative regulation because the repressor protein prevents transcription when it is bound to the operator.

The inducer does not directly switch on the structural genes; it removes the repressor-mediated block.

Positive Regulation by Glucose

The lac operon is also influenced by the availability of glucose.

When glucose levels are low, the concentration of cAMP increases. cAMP binds with CAP (catabolite activator protein), and the cAMP-CAP complex helps RNA polymerase bind efficiently to the promoter.

Therefore, maximum expression of the lac operon occurs when:

Lactose is present + Glucose is low

Lactose	Glucose	Lac Operon Expression
Absent	Present/Low	Off or strongly repressed
Present	High	Low expression
Present	Low	Maximum expression

Gene Regulation in Eukaryotes

In eukaryotes, gene expression can be regulated at several levels:

• Chromatin structure
• Transcription
• RNA processing
• RNA transport
• Translation
• Post-translational modification

Eukaryotic gene regulation is therefore more complex than the simple bacterial operon model.

Operon Concept

An operon is a functional unit of DNA containing a cluster of genes controlled by a common promoter and regulatory region, allowing coordinated expression of related genes.

In an operon, several structural genes may be transcribed together as a single polycistronic mRNA.

Important Differences: Prokaryotic and Eukaryotic Gene Regulation

Feature	Prokaryotes	Eukaryotes
Operons	Common	Generally not organised as typical operons
Regulation	Often strongly regulated at transcription level	Can occur at multiple levels
RNA processing	Limited compared with eukaryotes	Extensive RNA processing occurs
Cellular organisation	No membrane-bound nucleus	Membrane-bound nucleus present

How to Understand Translation

Translation can be understood as a three-step process:

How to Understand Lac Operon

Think of the lac operon as a controlled system for producing lactose-utilising enzymes.

Lactose absent: Repressor → Operator → Transcription blocked.

Lactose present: Inducer interacts with repressor → Operator becomes available → Structural genes are transcribed.

Lactose present + low glucose: cAMP-CAP further enhances transcription → Maximum expression.

Board Important Questions

Very Short Answer Questions

1. What is translation?
2. Where does translation occur?
3. What is the role of mRNA in translation?
4. What is the role of tRNA?
5. What is an anticodon?
6. What is aminoacylation?
7. Name the enzyme responsible for charging tRNA.
8. What is the start codon?
9. Name the three stop codons.
10. What is a polysome?
11. What is gene regulation?
12. What is an operon?
13. Who proposed the lac operon model?
14. Name the structural genes of the lac operon.
15. What is the function of the lac repressor?

Short Answer Questions

1. Explain the role of tRNA in translation.
2. Describe the three stages of translation.
3. Explain the process of initiation of translation.
4. What happens during elongation of a polypeptide chain?
5. Explain termination of translation.
6. What are polysomes? State their significance.
7. Explain the concept of gene regulation.
8. What is an operon? Explain its significance.
9. Explain the role of promoter and operator in the lac operon.

Long Answer Questions

1. Describe the complete process of translation from initiation to termination.
2. Explain the role of mRNA, tRNA and ribosomes in protein synthesis.
3. Explain the lac operon model of gene regulation.
4. Describe the lac operon in the presence and absence of lactose.
5. Explain how glucose affects the expression of the lac operon.

Multiple Choice Questions

1. Translation is the process of:
   (A) DNA synthesis
   (B) RNA synthesis
   (C) Protein synthesis using mRNA information
   (D) DNA degradation
   Answer: (C) Protein synthesis using mRNA information
2. Translation occurs on:
   (A) Lysosomes
   (B) Ribosomes
   (C) Golgi bodies
   (D) Centrosomes
   Answer: (B) Ribosomes
3. The molecule that carries amino acids to the ribosome is:
   (A) mRNA
   (B) rRNA
   (C) tRNA
   (D) DNA
   Answer: (C) tRNA
4. The start codon is:
   (A) UAA
   (B) UAG
   (C) AUG
   (D) UGA
   Answer: (C) AUG
5. Which of the following is a stop codon?
   (A) AUG
   (B) UAA
   (C) UGG
   (D) GCU
   Answer: (B) UAA
6. The bond between two amino acids is called:
   (A) Glycosidic bond
   (B) Hydrogen bond
   (C) Peptide bond
   (D) Phosphodiester bond
   Answer: (C) Peptide bond
7. A group of ribosomes attached to one mRNA is called:
   (A) Operon
   (B) Polysome
   (C) Nucleosome
   (D) Codon
   Answer: (B) Polysome
8. The lac operon was proposed by:
   (A) Watson and Crick
   (B) Jacob and Monod
   (C) Meselson and Stahl
   (D) Mendel
   Answer: (B) Jacob and Monod
9. The inducer associated with the lac operon is:
   (A) Glucose
   (B) Allolactose
   (C) Sucrose
   (D) Starch
   Answer: (B) Allolactose
10. The lac operon structural gene z codes for:
    (A) Permease
    (B) Transacetylase
    (C) β-galactosidase
    (D) Repressor
    Answer: (C) β-galactosidase
11. The operator of the lac operon is the site where:
    (A) Ribosome binds
    (B) Repressor binds
    (C) tRNA binds
    (D) Amino acid binds
    Answer: (B) Repressor binds
12. Maximum expression of the lac operon occurs when:
    (A) Lactose absent and glucose high
    (B) Lactose absent and glucose low
    (C) Lactose present and glucose low
    (D) Lactose present and glucose high
    Answer: (C) Lactose present and glucose low
13. The ribosome of bacteria is:
    (A) 80S
    (B) 70S
    (C) 60S
    (D) 90S
    Answer: (B) 70S
14. The ribosome of eukaryotic cytoplasm is:
    (A) 70S
    (B) 80S
    (C) 50S
    (D) 30S
    Answer: (B) 80S
15. The enzyme responsible for charging tRNA is:
    (A) DNA polymerase
    (B) RNA polymerase
    (C) Aminoacyl-tRNA synthetase
    (D) DNA ligase
    Answer: (C) Aminoacyl-tRNA synthetase

Fill in the Blanks

1. Protein synthesis from mRNA is called ______.
2. Translation takes place on ______.
3. The molecule that carries amino acids is ______.
4. The start codon is ______.
5. The three stop codons are ______, ______ and ______.
6. A group of ribosomes attached to one mRNA is called a ______.
7. The lac operon was proposed by ______ and ______.
8. The inducer of the lac operon is ______.
9. The lac operon structural gene z codes for ______.
10. The operator is bound by the ______ protein.

Match the Following

Column A	Column B
1. mRNA	a. Carries amino acid
2. tRNA	b. Carries codons
3. AUG	c. β-galactosidase
4. lac z gene	d. Start codon

Answers: 1-b, 2-a, 3-d, 4-c

Quick Revision

Translation converts the information in mRNA into a protein, while gene regulation controls when and how strongly genes are expressed.