Gene Expression: Transcription, Translation, and Mutations

Gene Expression: Transcription, Translation, and Mutations

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system: DNA is transcribed into RNA, which is then translated into protein. This process underlies how genetic information results in observable traits.

• DNA: The hereditary material containing genes.

• RNA: The intermediary molecule transcribed from DNA.

• Protein: The functional molecules that determine phenotype.

Structure and Function of DNA

DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a deoxyribose sugar, and a phosphate group. The sequence of bases encodes genetic information.

• Chargaff’s Rules: In any species, the amount of adenine (A) equals thymine (T), and the amount of guanine (G) equals cytosine (C).

• Double Helix: DNA consists of two antiparallel strands forming a helical structure.

Discovery of DNA Structure

X-ray crystallography, performed by Rosalind Franklin, provided critical evidence for the helical structure of DNA, which was interpreted by Watson and Crick to deduce the double helix model.

DNA as Genetic Material

Experiments by Frederick Griffith, and later by Hershey and Chase, demonstrated that DNA is the molecule responsible for heredity.

• Griffith’s Experiment: Showed that a "transforming principle" from dead pathogenic bacteria could make non-pathogenic bacteria virulent.

• Hershey-Chase Experiment: Used bacteriophages to show that DNA, not protein, is the genetic material transferred to bacteria during infection.

Transcription: DNA to RNA

Overview of Transcription

Transcription is the synthesis of RNA from a DNA template, catalyzed by RNA polymerase. In eukaryotes, this occurs in the nucleus; in prokaryotes, it occurs in the cytoplasm.

• Template Strand: The DNA strand used as a template for RNA synthesis.

• Coding Strand: The non-template DNA strand, which has the same sequence as the RNA (except T is replaced by U).

Stages of Transcription

• Initiation: RNA polymerase binds to the promoter region with the help of transcription factors (in eukaryotes).

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

• Termination: Transcription ends when RNA polymerase encounters a termination sequence.

Directionality of Nucleic Acids

Both DNA and RNA have directionality, defined by their 5' and 3' ends. RNA polymerase reads the DNA template from 3' to 5', synthesizing RNA from 5' to 3'.

Transcription in Prokaryotes vs. Eukaryotes

In prokaryotes, transcription and translation are coupled in the cytoplasm. In eukaryotes, transcription occurs in the nucleus, and the RNA transcript (pre-mRNA) undergoes processing before translation in the cytoplasm.

RNA Processing in Eukaryotes

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 noncoding introns and joining of exons by the spliceosome.

Translation: RNA to Protein

Overview of Translation

Translation is the process by which ribosomes synthesize proteins using the sequence of codons in mRNA. Each codon (a sequence of three nucleotides) specifies an amino acid.

• Ribosome: The molecular machine that facilitates translation.

• tRNA: Transfer RNA molecules bring amino acids to the ribosome, matching codons with their anticodons.

Genetic Code

The genetic code consists of 64 codons, 61 of which code for amino acids and 3 are stop signals. The code is redundant but not ambiguous.

• Start Codon: AUG (codes for methionine, Met)

• Stop Codons: UAA, UAG, UGA (signal termination of translation)

Codon	Amino Acid
AUG	Met (Start)
UUU, UUC	Phe
UAA, UAG, UGA	Stop

Translation Steps

• Initiation: Ribosome assembles at the start codon of mRNA.

• Elongation: tRNAs bring amino acids to the ribosome, which are joined to form a polypeptide.

• Termination: Translation ends at a stop codon; the completed polypeptide is released.

Mutations and Their Effects

Types of Mutations

Mutations are changes in the genetic material that can affect protein structure and function. They may arise spontaneously or be induced by mutagens.

• Point Mutation: Change in a single nucleotide pair.

• Substitution: One base pair is replaced by another.

• Insertion/Deletion: Addition or loss of nucleotide pairs.

• Frameshift Mutation: Insertions or deletions that alter the reading frame.

• Silent Mutation: No change in the amino acid sequence.

• Missense Mutation: Change in one amino acid.

• Nonsense Mutation: Change to a stop codon, leading to a truncated protein.

Examples of Mutation Effects

Different alleles of a gene can result from various mutations, leading to changes in the mRNA and protein product. The effect depends on the type and location of the mutation.

Allele	DNA Sequence Change	Protein Change	Mutation Type
B	One base pair changed	No change	Silent, substitution, point
C	One base pair changed	Premature stop codon	Nonsense, substitution, point
D	Insertion of two base pairs	Frameshift, many amino acids changed, early stop	Insertion, frameshift, nonsense

Summary Table: DNA Replication vs. Gene Expression

Process	Template	Product	Enzyme
DNA Replication	DNA	DNA	DNA Polymerase
Transcription	DNA	RNA	RNA Polymerase
Translation	mRNA	Protein	Ribosome

Additional info: The redundancy of the genetic code means that some mutations (silent mutations) do not alter the amino acid sequence of the protein. However, missense and nonsense mutations can have significant effects on protein function, potentially leading to disease.