Transcription and the Genetic Code

Transcription and the Genetic Code

Gene expression is the process by which the information stored in a gene is used to produce a functional product, such as an RNA molecule or a protein.

For protein-coding genes, the genetic information generally flows from DNA → RNA → Protein. This is called the central dogma of molecular biology.

DNA → Transcription → RNA → Translation → Protein

Transcription

Transcription is the process of copying genetic information from one strand of DNA into an RNA molecule.

In simple terms, DNA की information को RNA के form में copy करने की प्रक्रिया को transcription कहते हैं।

Unlike DNA replication, transcription involves only a small segment of DNA corresponding to a particular gene, and generally only one of the two DNA strands acts as the template.

Why is Transcription Necessary?

DNA is the stable storage molecule of genetic information, while RNA can carry this information to the sites where proteins are synthesised. Therefore, transcription provides an RNA copy of the information present in DNA.

Transcription Unit

A typical transcription unit in DNA consists of three main regions:

1. Promoter
2. Structural gene
3. Terminator

Region	Function
Promoter	Region where RNA polymerase binds and transcription begins.
Structural gene	DNA region that is transcribed to produce RNA.
Terminator	Region that signals the termination of transcription.

Template Strand and Coding Strand

Only one of the two DNA strands is transcribed. The two strands are distinguished as the template strand and the coding strand.

Feature	Template Strand	Coding Strand
Other name	Antisense strand	Sense strand
Role	Acts as template for RNA synthesis	Does not act as template
Direction	3′ → 5′	5′ → 3′
Relation with RNA	Complementary to RNA	Same sequence as RNA except T is replaced by U

Structure of a Transcription Unit

Promoter → Structural Gene → Terminator

The promoter is located upstream of the structural gene and provides the site for binding of RNA polymerase. The terminator marks the end of transcription.

Mechanism of Transcription

In prokaryotes, transcription can be broadly understood in three stages:

1. Initiation
2. Elongation
3. Termination

1. Initiation

RNA polymerase binds to the promoter region of DNA with the help of associated factors.

The DNA strands locally separate, exposing the template strand. RNA synthesis then begins using ribonucleotides.

2. Elongation

RNA polymerase moves along the template strand and adds complementary ribonucleotides to the growing RNA chain.

RNA synthesis occurs in the 5′ → 3′ direction.

DNA Template Base	RNA Base Added
A	U
T	A
G	C
C	G

3. Termination

When RNA polymerase reaches the termination region, transcription stops and the newly formed RNA molecule is released.

RNA Polymerase

RNA polymerase is the enzyme responsible for synthesising RNA during transcription.

In bacteria, a single RNA polymerase is responsible for transcription of different types of RNA.

RNA Polymerases in Eukaryotes

RNA Polymerase	Main Function
RNA polymerase I	Transcribes major rRNAs such as 28S, 18S and 5.8S rRNA.
RNA polymerase II	Transcribes precursor mRNA (hnRNA) and some other RNAs.
RNA polymerase III	Transcribes tRNA, 5S rRNA and some other small RNAs.

Transcription in Eukaryotes

Eukaryotic transcription is more complex than prokaryotic transcription because the primary RNA transcript must undergo processing before it becomes a mature functional RNA.

The primary transcript produced by RNA polymerase II is called hnRNA (heterogeneous nuclear RNA) in the context of mRNA formation.

RNA Processing

The hnRNA undergoes three major processing steps:

1. 5′ capping
2. 3′ polyadenylation
3. Splicing

5′ Capping

A modified guanine nucleotide called 7-methylguanosine (m⁷G) is added to the 5′ end of the RNA.

This cap helps in RNA stability and other functions related to RNA processing and translation.

3′ Polyadenylation

A stretch of adenine nucleotides called the poly-A tail is added at the 3′ end of the RNA.

Splicing

The primary transcript contains both exons and introns.

• Exons are expressed sequences that are retained in mature RNA.
• Introns are intervening sequences that are removed during RNA processing.

During splicing, introns are removed and exons are joined together to form mature mRNA.

hnRNA → Capping + Polyadenylation + Splicing → Mature mRNA

Split Genes

Genes in eukaryotes are often described as split genes because their coding sequences are interrupted by introns.

RNA splicing removes the introns and joins the exons.

Genetic Code

The genetic code is the set of rules by which the nucleotide sequence of mRNA specifies the amino acid sequence of a protein.

In simple words, mRNA के nucleotide sequence को amino acid sequence में translate करने के लिए जो rules होते हैं, उन्हें genetic code कहते हैं।

Codon

A codon is a sequence of three nucleotides in mRNA that specifies an amino acid or provides a signal for termination of translation.

Since a codon consists of three bases, it is called a triplet code.

There are four types of bases in mRNA: A, U, G and C.

The number of possible triplet codons is:

4³ = 64 codons

Out of these 64 codons:

• 61 codons specify amino acids.
• 3 codons are stop codons.

Stop Codons

The three stop codons are:

UAA, UAG and UGA

These codons do not specify any amino acid and signal termination of translation.

Start Codon

AUG is the start codon in the standard genetic code.

It generally codes for methionine and also acts as the initiation signal for translation.

Properties of Genetic Code

1. Triplet

The genetic code is a triplet code. Each codon contains three nucleotides.

2. Degenerate

The genetic code is degenerate because most amino acids are specified by more than one codon.

For example, several different codons can specify the same amino acid.

3. Unambiguous

The genetic code is unambiguous because a particular codon specifies only one amino acid.

4. Nearly Universal

The genetic code is nearly universal, meaning that the same codons generally specify the same amino acids in different organisms.

5. Commaless

The genetic code is commaless. There are no punctuation marks or gaps between successive codons.

6. Non-overlapping

The genetic code is generally described as non-overlapping, meaning one nucleotide is not normally shared between two consecutive codons in the same reading frame.

Important Genetic Code Facts

Feature	Fact
Total codons	64
Amino-acid specifying codons	61
Stop codons	3: UAA, UAG, UGA
Start codon	AUG
Start codon amino acid	Methionine
Nature	Triplet, degenerate, unambiguous, nearly universal

Reading the Genetic Code

The sequence of bases in mRNA is read in groups of three nucleotides called codons.

For example:

5′-AUG-GCU-AAA-UGA-3′

Here:

• AUG → Methionine
• GCU → Alanine
• AAA → Lysine
• UGA → Stop

Thus, the corresponding amino acid sequence begins with methionine and ends when a stop codon is reached.

mRNA, tRNA and rRNA

RNA	Full Form	Main Function
mRNA	Messenger RNA	Carries genetic information from DNA to ribosome.
tRNA	Transfer RNA	Carries amino acids to the ribosome and recognises codons through anticodon.
rRNA	Ribosomal RNA	Forms an important structural and functional component of ribosomes.

Codon and Anticodon

A codon is present on mRNA, whereas an anticodon is a complementary sequence present on tRNA.

Codon	Anticodon	Location
Codon	—	mRNA
—	Anticodon	tRNA

mRNA Codon ↔ tRNA Anticodon

Transcription vs Replication

Feature	Replication	Transcription
Purpose	Copies DNA	Produces RNA from DNA
Template	Both DNA strands participate as templates	Generally one DNA strand acts as template for a given gene
Main enzyme	DNA polymerase	RNA polymerase
Product	DNA	RNA
Base used	Thymine	Uracil

How to Solve a Transcription Question

When a DNA template sequence is given and the question asks for RNA sequence, follow these steps:

1. Identify whether the given strand is the template strand.
2. Write the complementary RNA bases.
3. Use U instead of T because the product is RNA.
4. Remember that RNA is synthesised in the 5′ → 3′ direction.

Example

Suppose the DNA template strand is:

3′-TAC GGA CTT-5′

The RNA transcript will be:

5′-AUG CCU GAA-3′

यहाँ DNA template और RNA complementary हैं, और RNA में thymine की जगह uracil आता है।

Board Important Questions

Very Short Answer Questions

1. What is transcription?
2. What is a transcription unit?
3. Name the three regions of a transcription unit.
4. What is a promoter?
5. What is a terminator?
6. What is the template strand?
7. What is the coding strand?
8. Name the enzyme responsible for transcription.
9. What is hnRNA?
10. What is splicing?
11. What is a codon?
12. How many codons are present in the genetic code?
13. Name the three stop codons.
14. What is the start codon?
15. What is an anticodon?

Short Answer Questions

1. Explain the structure of a transcription unit.
2. Differentiate between template and coding strands.
3. Describe the three stages of transcription.
4. Explain RNA processing in eukaryotes.
5. What are exons and introns?
6. Explain the major properties of the genetic code.
7. Differentiate between codon and anticodon.
8. Explain why the genetic code is called degenerate.

Long Answer Questions

1. Explain the mechanism of transcription in prokaryotes.
2. Describe transcription in eukaryotes, including RNA processing.
3. Explain the genetic code and discuss its important properties.
4. Differentiate between DNA replication and transcription.

Multiple Choice Questions

1. The process of formation of RNA from DNA is called:
   (A) Translation
   (B) Replication
   (C) Transcription
   (D) Mutation
   Answer: (C) Transcription
2. The enzyme involved in transcription is:
   (A) DNA polymerase
   (B) RNA polymerase
   (C) DNA ligase
   (D) Helicase
   Answer: (B) RNA polymerase
3. The promoter is the site where:
   (A) Ribosome binds
   (B) DNA polymerase binds
   (C) RNA polymerase binds
   (D) tRNA binds
   Answer: (C) RNA polymerase binds
4. The template strand is also called:
   (A) Sense strand
   (B) Antisense strand
   (C) Coding strand
   (D) Non-functional strand
   Answer: (B) Antisense strand
5. The coding strand has the same sequence as RNA except that:
   (A) A is replaced by G
   (B) G is replaced by C
   (C) T is replaced by U
   (D) C is replaced by U
   Answer: (C) T is replaced by U
6. The primary transcript in eukaryotic cells is called:
   (A) tRNA
   (B) hnRNA
   (C) rRNA
   (D) DNA
   Answer: (B) hnRNA
7. Removal of introns from hnRNA is called:
   (A) Translation
   (B) Replication
   (C) Splicing
   (D) Capping
   Answer: (C) Splicing
8. The genetic code consists of:
   (A) Doublets
   (B) Triplets
   (C) Quartets
   (D) Single bases
   Answer: (B) Triplets
9. The total number of possible codons is:
   (A) 16
   (B) 20
   (C) 32
   (D) 64
   Answer: (D) 64
10. The start codon is:
    (A) UAA
    (B) UAG
    (C) AUG
    (D) UGA
    Answer: (C) AUG
11. Which of the following is a stop codon?
    (A) AUG
    (B) UGG
    (C) UAA
    (D) GCU
    Answer: (C) UAA
12. The genetic code is called degenerate because:
    (A) One codon specifies many amino acids
    (B) Many codons can specify the same amino acid
    (C) Codons contain four bases
    (D) Codons overlap
    Answer: (B) Many codons can specify the same amino acid
13. Anticodon is present on:
    (A) mRNA
    (B) DNA
    (C) tRNA
    (D) rRNA
    Answer: (C) tRNA
14. RNA contains which pyrimidine instead of thymine?
    (A) Adenine
    (B) Guanine
    (C) Cytosine
    (D) Uracil
    Answer: (D) Uracil
15. Which RNA polymerase transcribes precursor mRNA in eukaryotes?
    (A) RNA polymerase I
    (B) RNA polymerase II
    (C) RNA polymerase III
    (D) DNA polymerase
    Answer: (B) RNA polymerase II

Fill in the Blanks

1. The process of formation of RNA from DNA is called ______.
2. The enzyme responsible for transcription is ______.
3. The three regions of a transcription unit are promoter, structural gene and ______.
4. The template strand is also called the ______ strand.
5. The coding strand is also called the ______ strand.
6. The primary transcript in eukaryotes is called ______.
7. Removal of introns is called ______.
8. The genetic code is a ______ code.
9. The start codon is ______.
10. The three stop codons are ______, ______ and ______.

Match the Following

Column A	Column B
1. Promoter	a. Removes introns
2. RNA polymerase	b. Start codon
3. Splicing	c. Binding site for RNA polymerase
4. AUG	d. RNA synthesis

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

Quick Revision

DNA stores the information, transcription converts it into RNA, and the genetic code determines how nucleotide information specifies amino acids.