Gene Expression: From Gene to Protein (Central Dogma and Molecular Mechanisms)

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Gene Expression and the Central Dogma

Genes, Alleles, and Phenotype

Genes are segments of DNA that code for specific proteins, which in turn determine an organism's traits. Alleles are different versions of a gene, and variation in alleles leads to observable differences in phenotype. For example, a gene may code for an enzyme required for pigment synthesis in deer; a functional allele results in brown pigmentation, while a faulty allele (as in albino deer) leads to a lack of pigment due to a nonfunctional enzyme.

Gene: DNA sequence coding for a functional product (protein or RNA).
Allele: Variant form of a gene.
Phenotype: Observable traits resulting from gene expression.
Genotype: Genetic makeup of an organism.
Example: The albino phenotype in deer is caused by a mutation in the gene encoding a pigment-producing enzyme.

Brown and albino deer illustrating genetic variation in pigmentation

The 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 principle, proposed by Francis Crick in 1956, establishes that information flows from nucleic acids to proteins, but not in reverse.

Central Dogma: DNA → RNA → Protein
Transcription: Synthesis of RNA from a DNA template.
Translation: Synthesis of protein from an mRNA template.
Key Point: Proteins are the link between genotype and phenotype.

Diagram of the Central Dogma from Crick's original notes

Transcription and Translation: Cellular Context

Prokaryotic vs. Eukaryotic Gene Expression

In prokaryotes (e.g., bacteria), transcription and translation occur in the cytoplasm, often simultaneously. In eukaryotes, transcription occurs in the nucleus, and the resulting pre-mRNA undergoes processing before being exported to the cytoplasm for translation.

Prokaryotes: No nucleus; mRNA is translated as soon as it is transcribed.
Eukaryotes: Transcription in nucleus; pre-mRNA processed to mature mRNA before translation in cytoplasm.

Transcription and translation in a bacterial cell Transcription, RNA processing, and translation in a eukaryotic cell

DNA and RNA: Structure and Function

Nucleic Acid Structure

DNA and RNA are polymers of nucleotides, each consisting of a phosphate group, a five-carbon sugar, and a nitrogenous base. DNA contains deoxyribose and the bases A, T, G, and C; RNA contains ribose and the bases A, U, G, and C.

DNA: Deoxyribose sugar; bases A, T, G, C.
RNA: Ribose sugar; bases A, U, G, C.
Function: DNA stores genetic information; RNA acts as a messenger and functional molecule in protein synthesis.

Structure of nucleic acids, nucleotides, and nitrogenous bases

The Genetic Code

Codons and the Triplet Code

The genetic code is read in sets of three nucleotides (codons), each specifying an amino acid. With four nucleotides, there are 64 possible codons, more than enough to code for the 20 amino acids. The code is redundant (more than one codon per amino acid) but not ambiguous (each codon specifies only one amino acid).

Codon: Sequence of three mRNA bases that codes for an amino acid.
Start Codon: AUG (codes for methionine; signals start of translation).
Stop Codons: UAA, UAG, UGA (signal termination of translation).
Redundancy: Multiple codons can code for the same amino acid.
No ambiguity: Each codon codes for only one amino acid.

Genetic code table showing codons and corresponding amino acids

Transcription: From DNA to RNA

Stages of Transcription

Transcription involves three main stages: initiation, elongation, and termination. RNA polymerase binds to the promoter, unwinds the DNA, and synthesizes an RNA transcript complementary to the DNA template strand.

Initiation: RNA polymerase binds to promoter; DNA unwinds.
Elongation: RNA polymerase synthesizes RNA in the 5' to 3' direction.
Termination: RNA polymerase releases the completed RNA transcript.

Initiation of transcription: RNA polymerase binds to promoter Elongation of transcription: RNA polymerase synthesizes RNA Termination of transcription: RNA polymerase releases RNA transcript

Promoters and the TATA Box

In eukaryotes, promoters often contain a TATA box, a conserved DNA sequence that helps position RNA polymerase II for transcription initiation. Transcription factors are required for the assembly of the transcription initiation complex.

Promoter: DNA sequence where RNA polymerase binds to initiate transcription.
TATA Box: Conserved sequence (TATAAA) about 25 bases upstream of the start site.
Transcription Factors: Proteins that help RNA polymerase bind and initiate transcription.

Promoter region with TATA box and start point Transcription factors binding to promoter Transcription initiation complex with RNA polymerase II

RNA Processing in Eukaryotes

Before mRNA can be translated, eukaryotic pre-mRNA undergoes processing: addition of a 5' cap, a poly-A tail, and splicing to remove introns. These modifications stabilize mRNA, facilitate export from the nucleus, and aid in translation.

5' Cap: Modified guanine nucleotide added to the 5' end.
Poly-A Tail: String of adenine nucleotides added to the 3' end.
Splicing: Removal of non-coding introns; joining of coding exons.
UTRs: Untranslated regions at 5' and 3' ends; important for regulation.

Structure of a mature mRNA with cap, UTRs, coding region, and poly-A tail Pre-mRNA and mature mRNA showing exons and introns RNA splicing by spliceosome

Translation: From mRNA to Protein

tRNA and the Ribosome

Translation is the process by which ribosomes synthesize proteins using mRNA as a template. Transfer RNAs (tRNAs) bring amino acids to the ribosome, matching their anticodon with codons on the mRNA. Ribosomes have three binding sites for tRNA: A (aminoacyl), P (peptidyl), and E (exit).

tRNA: Adapter molecule with an anticodon and an amino acid attachment site.
Anticodon: Three-nucleotide sequence on tRNA complementary to mRNA codon.
Ribosome: Molecular machine composed of rRNA and proteins; catalyzes peptide bond formation.

Translation: tRNAs bring amino acids to ribosome Secondary structure of tRNA Tertiary structure of tRNA Ribosome structure with tRNA binding sites Ribosome showing E, P, and A sites

Translation: Initiation, Elongation, and Termination

Translation proceeds in three stages:

Initiation: Small ribosomal subunit binds mRNA; initiator tRNA pairs with start codon; large subunit completes the initiation complex.
Elongation: tRNAs bring amino acids to the ribosome; peptide bonds form; ribosome moves along mRNA.
Termination: Stop codon is reached; release factor binds; polypeptide is released.

Polyribosomes: Multiple ribosomes can translate a single mRNA simultaneously, increasing efficiency.

Mutations and Their Effects

Types of Mutations

Mutations are changes in the DNA sequence that can affect protein structure and function. Point mutations involve a single nucleotide change, while insertions and deletions can cause frameshift mutations, altering the reading frame and potentially leading to nonfunctional proteins.

Point Mutation: Change in a single nucleotide pair.
Base-pair Substitution: Replacement of one nucleotide and its partner.
Insertion/Deletion: Addition or loss of nucleotides; may cause frameshift.
Frameshift Mutation: Insertion/deletion not in multiples of three; shifts reading frame.
Silent Mutation: No effect on amino acid sequence.
Missense Mutation: Changes one amino acid.
Nonsense Mutation: Creates a premature stop codon.

Summary Table: Key Terms and Concepts

Term	Definition
Gene	DNA sequence coding for a functional product
Allele	Variant form of a gene
Transcription	Synthesis of RNA from DNA
Translation	Synthesis of protein from mRNA
Codon	Three-nucleotide sequence on mRNA
Anticodon	Three-nucleotide sequence on tRNA
Mutation	Change in DNA sequence
Introns	Non-coding regions removed from pre-mRNA
Exons	Coding regions joined in mature mRNA

Additional info: This guide covers the molecular mechanisms of gene expression, including the central dogma, transcription, RNA processing, translation, and the impact of mutations. It is suitable for exam preparation in introductory college biology courses.