What Is the Central Dogma? / DNA Replication (Semiconservative Replication) / Importance of the Central Dogma

 

Introduction to the Central Dogma of Molecular Biology

In molecular biology, the central dogma explains how genetic information flows within a cell. Simply put, it describes how the information stored in DNA is used to produce proteins, which perform most of the functions in a living organism.

The flow of information follows a clear and orderly pathway:

DNA → RNA → Protein

DNA contains the instructions needed to build all proteins. However, DNA does not directly produce proteins. Instead, RNA acts as a messenger, carrying the genetic instructions from DNA to the ribosomes, where proteins are made.

Central Dogma Definition

The central dogma is the process by which genetic information flows from DNA to RNA and finally to protein, resulting in the formation of a functional product.

What Is the Central Dogma?

The central dogma illustrates three essential biological processes:

1. DNA replication

2. Transcription (DNA to RNA)

3. Translation (RNA to protein)

Together, these processes explain how genetic information is stored, copied, and expressed inside a cell.

Biologically important molecules such as DNA, RNA, and proteins are known as biopolymers. The transfer of genetic information among these molecules can be classified into:

• General transfers – occur in all living cells (transcription and translation)

• Special transfers – occur only in exceptional laboratory conditions

• Unknown transfers – do not occur in nature

DNA Replication (Semiconservative Replication)

Before a cell divides, its DNA must be copied. This process is called DNA replication.

Replication is described as semiconservative, meaning:

• Each new DNA molecule contains one original (parent) strand

• The other strand is newly synthesized

This ensures accurate transmission of genetic information to daughter cells.

Steps of the Central Dogma

The central dogma operates through two major steps:

1. Transcription

Transcription is the process by which genetic information from DNA is copied into RNA.

Key Points:

• The enzyme RNA polymerase carries out transcription.

• The DNA segment involved contains three regions:

  • Promoter – start site

  • Structural gene – coding region

  • Terminator – stop signal

Only one DNA strand acts as a template strand, while the other is called the coding strand.

Process:

• RNA polymerase binds to the promoter.

• It synthesizes RNA in the 5′ to 3′ direction using the DNA template.

• When the enzyme reaches the terminator, transcription stops.

• The newly formed RNA is released and undergoes post-transcriptional modifications.

2. Translation

Translation is the process by which the RNA sequence is used to synthesize a protein.

Key Components:

• mRNA (messenger RNA) – carries genetic instructions

• tRNA (transfer RNA) – brings amino acids

• Ribosomes – site of protein synthesis

Process:

• Ribosomes consist of a small subunit and a large subunit.

• mRNA binds to the small subunit.

• tRNA molecules recognize specific codons on the mRNA.

• The large subunit positions tRNAs close enough for peptide bond formation.

• With the help of energy, amino acids are linked together to form a polypeptide chain.

This process continues until a stop codon is reached.

Genetic Code

The genetic code is the set of rules by which nucleotide sequences in RNA are translated into amino acids.

Important Features:

• Each codon consists of three nucleotides

• Four nitrogenous bases form 64 possible codons (4 × 4 × 4)

• Only 20 amino acids are naturally occurring

Characteristics of the Genetic Code:

• Degenerate – more than one codon may code for the same amino acid

• Unambiguous – each codon codes for only one amino acid

• Universal – nearly the same in all organisms

Special Codons:

• AUG – initiator codon; codes for Methionine

• UAA, UAG, UGA – stop codons that terminate translation

Importance of the Central Dogma

The central dogma explains how:

• Genetic information is expressed

• Proteins are synthesized

• Cells grow, function, and reproduce

Multiple Choice Questions (MCQs)

1. The central dogma of molecular biology describes the flow of information from:

A. RNA → DNA → Protein
B. Protein → RNA → DNA
C. DNA → RNA → Protein
D. DNA → Protein → RNA

Correct Answer: C

2. Who proposed the concept of the central dogma?

A. Gregor Mendel
B. James Watson
C. Francis Crick
D. Charles Darwin

Correct Answer: C

3. The process of copying genetic information from DNA to RNA is called:

A. Replication
B. Translation
C. Transcription
D. Mutation

Correct Answer: C

4. Which enzyme is responsible for transcription?

A. DNA polymerase
B. RNA polymerase
C. Ligase
D. Helicase

Correct Answer: B

5. The DNA strand used for RNA synthesis is called:

A. Coding strand
B. Sense strand
C. Template strand
D. Antisense protein

Correct Answer: C

6. Translation occurs at the level of:

A. Nucleus
B. Mitochondria
C. Ribosomes
D. Golgi apparatus

Correct Answer: C

7. Which molecule carries amino acids during translation?

A. mRNA
B. rRNA
C. DNA
D. tRNA

Correct Answer: D

8. The start codon for protein synthesis is:

A. UAA
B. UAG
C. UGA
D. AUG

Correct Answer: D

9. How many stop codons are present in the genetic code?

A. One
B. Two
C. Three
D. Four

Correct Answer: C

10. The nature of DNA replication is described as:

A. Conservative
B. Dispersive
C. Semiconservative
D. Random

Correct Answer: C

11. Which statement about the genetic code is correct?

A. Each codon codes for multiple amino acids
B. It is different in every organism
C. It is degenerate
D. It contains only 20 codons

Correct Answer: C

12. The degeneracy of the genetic code means:

A. Codons change during translation
B. One amino acid may be coded by multiple codons
C. Codons code for multiple proteins
D. Amino acids are unstable

Correct Answer: B

Frequently Asked Questions (FAQs)

Q1. What is the central dogma of molecular biology?

The central dogma explains how genetic information flows from DNA to RNA and then to protein.

Q2. Why is RNA necessary in the central dogma?

RNA acts as a messenger that carries genetic information from DNA to ribosomes, where proteins are synthesized.

Q3. What is transcription?

Transcription is the process by which RNA is synthesized from a DNA template using the enzyme RNA polymerase.

Q4. What is translation?

Translation is the process by which the information in mRNA is used to assemble amino acids into a protein.

Q5. What are ribosomes?

Ribosomes are cellular structures made of rRNA and proteins where translation and protein synthesis occur.

Q6. What is a codon?

A codon is a sequence of three nucleotides on mRNA that codes for a specific amino acid.

Q7. Why is the genetic code called universal?

Because nearly all living organisms use the same codons to code for the same amino acids.

Q8. What is the role of tRNA?

tRNA brings specific amino acids to the ribosome during protein synthesis.

Q9. What is semiconservative DNA replication?

It is a type of replication in which each new DNA molecule contains one original strand and one newly synthesized strand.

Q10. Why is the central dogma important?

It explains how genes control cell structure, function, growth, and development.

Understanding this concept is fundamental to genetics, biotechnology, medicine, and molecular biology.

To explore more simplified, exam-oriented explanations of molecular biology, keep visiting HN Series.