Gene Expression: From DNA to Protein (Central Dogma and the Genetic Code)

Gene Expression and the Flow of Genetic Information

Overview of Genetic Information Flow

The DNA inherited by an organism determines its traits by directing the synthesis of proteins. Proteins serve as the link between genotype (genetic makeup) and phenotype (observable traits). The process by which DNA directs protein synthesis is called gene expression, which includes two main stages: transcription and translation.

• Gene expression: The process of translating the information in DNA into functioning molecules within the cell.

• Proteins: Molecules that determine the structure and function of cells, thus influencing traits.

• Gene function can be studied at two levels:

  • Molecular function: The role of the protein product.

  • Organism's trait: The phenotype conferred by the gene.

The Central Dogma of Molecular Biology

DNA → RNA → Protein

The central dogma, proposed by Francis Crick, describes the flow of genetic information in cells:

• DNA codes for RNA through transcription.

• RNA codes for protein through translation.

• Protein determines the function of the cell.

• In prokaryotes, translation can begin before transcription is finished. In eukaryotes, the nuclear envelope separates transcription (in the nucleus) from translation (in the cytoplasm).

Replication is the process by which DNA is copied before cell division.

RNA: The Intermediary Between Genes and Proteins

Messenger RNA (mRNA)

• mRNA carries genetic information from DNA to the site of protein synthesis (ribosomes).

• The enzyme RNA polymerase synthesizes RNA using DNA as a template.

• In eukaryotes, RNA processing modifies the primary transcript (pre-mRNA) into mature mRNA before translation.

Summary of steps: DNA → pre-mRNA → mRNA → Protein

Genotype and Phenotype

Relationship Between DNA, mRNA, and Proteins

• Genotype: The sequence of DNA nucleotides in an organism.

• Phenotype: The observable characteristics determined by the sequence of amino acids in proteins.

• mRNA is complementary to the DNA template strand and carries the code for protein synthesis.

The Gene and DNA Strands

Template and Coding Strands

• DNA is double-stranded, but only one strand serves as the template for mRNA synthesis.

• Template strand (3' to 5'): Also called the non-coding, antisense, or template strand.

• Coding strand (5' to 3'): Also called the sense or non-template strand; its sequence matches the mRNA (except T is replaced by U).

Transcription

Process and Enzyme Involved

• Gene: An organized unit of DNA sequences that can be transcribed into RNA and ultimately results in a functional product.

• Transcription: The DNA-directed synthesis of RNA by RNA polymerase.

• RNA polymerase binds to the promoter, unwinds DNA, and synthesizes mRNA from 5' to 3' using the template strand.

Stages of Transcription

1. Initiation: RNA polymerase recognizes the promoter and forms an open complex.

2. Elongation: RNA polymerase synthesizes RNA by adding complementary nucleotides.

3. Termination: RNA polymerase reaches a termination sequence and releases the RNA transcript.

The Genetic Code

Triplet Code and Codons

• The genetic code specifies the relationship between nucleotide sequences in DNA/RNA and amino acid sequences in proteins.

• Each group of three nucleotides in mRNA is called a codon.

• There are 64 possible codons, coding for 20 amino acids and stop signals.

• The code is degenerate (redundant): multiple codons can specify the same amino acid.

• The code is not ambiguous: each codon specifies only one amino acid.

Start and Stop Codons

• Start codon (AUG): Codes for methionine and defines the reading frame.

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

Reading Frame

• The reading frame is set by the start codon and determines how the sequence is divided into codons.

• Shifting the reading frame (by insertion or deletion) changes the resulting amino acid sequence.

Using the Genetic Code

Predicting Sequences

• To predict the mRNA sequence from a gene, copy the coding strand (5' to 3') and replace T with U.

• To predict the amino acid sequence:

  • Find the start codon (AUG).

  • Use the genetic code table to translate each codon into an amino acid.

Example Table: The Genetic Code

Codon	Amino Acid
AUG	Methionine (Start)
UUU, UUC	Phenylalanine
UAA, UAG, UGA	Stop
GGC	Glycine
CAA	Glutamine
Additional info: See a full codon table for all 64 codons and their corresponding amino acids.

Practice Example

• Given the DNA coding strand: 5'-ATG GGC CAA TGA CTT TCA ATA A-3'

• Transcribe to mRNA: 5'-AUG GGC CAA UGA CUU UCA AUA A-3'

• Translate to amino acids: Met-Gly-Gln-Stop

Summary Table: Key Terms

Term	Definition
Gene expression	Process by which information from a gene is used to synthesize a functional gene product (protein or RNA)
Transcription	Synthesis of RNA from a DNA template
Translation	Synthesis of a polypeptide using the information in mRNA
Codon	Three-nucleotide sequence in mRNA that codes for an amino acid
Reading frame	The way nucleotides in mRNA are grouped into codons for translation

Key Equations and Concepts

• Transcription: $\text{DNA (template strand)} \xrightarrow{\text{RNA polymerase}} \text{mRNA}$

• Translation: $\text{mRNA} \xrightarrow{\text{ribosome}} \text{Protein (polypeptide)}$

Additional info: The "one gene-one enzyme" hypothesis was revised to "one gene-one polypeptide" as not all proteins are enzymes and some proteins are made of multiple polypeptides.