Transcription, Translation, and Mutations: The Flow of Genetic Information

Structure and Function of DNA II: Transcription & Mutations

Flow of Genetic Information: DNA to RNA to Protein

The central dogma of molecular biology describes the flow of genetic information within a biological system. It explains how DNA is transcribed into RNA, which is then translated into protein. This process is fundamental to gene expression in all living organisms.

• DNA stores genetic information in the form of nucleotide sequences.

• Transcription is the process by which a segment of DNA is copied into messenger RNA (mRNA).

• Translation is the process by which the sequence of the mRNA is decoded to build a polypeptide (protein).

• Proteins are the functional molecules that perform most cellular activities.

Example: The gene for hemoglobin is transcribed into mRNA, which is then translated to produce the hemoglobin protein in red blood cells.

Chemical Composition and Structure of DNA and RNA

DNA and RNA are nucleic acids with distinct chemical and structural features.

	DNA	RNA
Location	Nucleus (eukaryotes), cytoplasm (prokaryotes)	Nucleus, cytoplasm
Type of Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Number of Strands	Double-stranded	Single-stranded (usually)

The Genetic Code

The genetic code is the set of rules by which information encoded in mRNA sequences is translated into proteins by living cells.

• Codon: A sequence of three nucleotides in mRNA that specifies a particular amino acid.

• There are 64 possible codons (43 combinations of A, U, C, G), but only 20 amino acids, making the code redundant (more than one codon can specify the same amino acid).

• Start codon: AUG (codes for methionine and signals the start of translation).

• Stop codons: UAA, UAG, UGA (signal the end of translation).

Formula:

$\text{Number of codons} = 4^3 = 64$

Example: The codon UUU codes for the amino acid phenylalanine.

Transcription: From DNA to RNA

Transcription is the process by which the information in a DNA sequence is copied into a complementary RNA sequence.

• Template: One strand of DNA serves as the template for RNA synthesis.

• Enzyme: RNA polymerase catalyzes the synthesis of RNA from the DNA template.

• Steps:

  1. Initiation: RNA polymerase binds to the promoter region of the gene.

  2. Elongation: RNA polymerase moves along the DNA, synthesizing RNA in the 5' to 3' direction.

  3. Termination: RNA polymerase reaches a terminator sequence and releases the newly made RNA.

Example: In prokaryotes, the mRNA produced can be immediately translated. In eukaryotes, the primary transcript (pre-mRNA) undergoes processing before translation.

RNA Processing in Eukaryotes

In eukaryotic cells, the primary RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA.

• 5' Capping: Addition of a modified guanine nucleotide to the 5' end.

• Polyadenylation: Addition of a poly-A tail to the 3' end.

• Splicing: Removal of non-coding sequences (introns) and joining of coding sequences (exons).

Example: The human beta-globin gene contains introns that are removed during RNA processing.

Translation: From mRNA to Protein

Translation is the process by which ribosomes synthesize proteins using the sequence of codons in mRNA.

• Ribosome: The molecular machine that reads mRNA and assembles amino acids into a polypeptide chain.

• tRNA: Transfer RNA molecules bring amino acids to the ribosome, matching their anticodon to the mRNA codon.

• Steps:

  1. Initiation: Ribosome assembles at the start codon (AUG).

  2. Elongation: Amino acids are added one by one to the growing chain.

  3. Termination: Ribosome reaches a stop codon and releases the completed polypeptide.

Formula:

$\text{Number of nucleotides required for a polypeptide of } n \text{ amino acids} = (n + 1) \times 3$

Example: For a polypeptide of 100 amino acids, 303 nucleotides are needed (including the stop codon).

Mutations: Types and Effects

Mutations are changes in the nucleotide sequence of DNA. They can affect gene function and phenotype in various ways.

• General Categories:

  1. Substitution mutations: One base is replaced by another.

  2. Insertion/Deletion (indel) mutations: One or more bases are added or removed.

• Types of Substitution Mutations:

  1. Silent: No change in amino acid sequence.

  2. Missense: Changes one amino acid in the protein.

  3. Nonsense: Creates a premature stop codon.

• Frameshift Mutation: Insertion or deletion that alters the reading frame, often resulting in a nonfunctional protein.

• Mutagen: An agent that causes mutations (e.g., UV light, chemicals).

Definition	Example
Mutagen	UV radiation, benzene

Example: Sickle-cell disease is caused by a missense mutation in the beta-globin gene, resulting in abnormal hemoglobin.

Summary Table: DNA vs. RNA

	DNA	RNA
Location	Nucleus (eukaryotes), cytoplasm (prokaryotes)	Nucleus, cytoplasm
Type of Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Number of Strands	Double-stranded	Single-stranded (usually)

Additional info: In eukaryotes, RNA processing includes capping, polyadenylation, and splicing. The redundancy of the genetic code is also called 'degeneracy.' Mutations can be beneficial, neutral, or harmful depending on their effect on protein function.