Gene Expression and Regulation: From DNA to Protein

Gene Expression and Regulation

Central Dogma of Molecular Biology

The central dogma describes the unidirectional flow of genetic information from DNA to RNA to protein. This process is fundamental to all living organisms and explains how genetic information is expressed as cellular function.

• Transcription: The process of synthesizing RNA from a DNA template.

• Translation: The process of synthesizing proteins using the information encoded in mRNA.

• Gene Expression: The combined process of transcription and translation, resulting in the manifestation of genotype as phenotype.

Key Point: DNA is replicated, RNA can be reverse-transcribed into DNA, but information transfer from nucleic acid to protein is irreversible.

Transcription: From DNA to RNA

Introduction to Transcription

Transcription is the process by which an RNA molecule is synthesized from a DNA template within a gene. Genes are specific sequences of DNA that encode functional products, typically proteins.

• Promoter: DNA sequence where transcription begins; site of RNA polymerase attachment.

• Terminator: DNA sequence where transcription ends.

• RNA Polymerase: Enzyme that synthesizes RNA from the DNA template without the need for a primer.

• Upstream/Downstream: Upstream refers to sequences before (5' to) the gene; downstream refers to sequences after (3' to) the gene.

DNA Strands in Transcription

Genes are located on double-stranded DNA, but only one strand (the template strand) is used for transcription. The other strand is the coding strand, which has the same sequence as the RNA (except T is replaced by U in RNA).

• Coding Strand: Non-template DNA strand; matches the RNA sequence (with U for T).

• Template Strand: DNA strand used by RNA polymerase to synthesize RNA.

• Base Pairing: A pairs with U (in RNA), T with A, C with G, and G with C.

Steps of Transcription

Transcription occurs in three main steps: initiation, elongation, and termination.

• Initiation: RNA polymerase binds to the promoter and unwinds the DNA.

• Elongation: RNA polymerase synthesizes the RNA strand by adding nucleotides complementary to the DNA template.

• Termination: RNA polymerase reaches the terminator sequence and releases the newly formed RNA molecule.

Note: In eukaryotes, transcription factors are required for RNA polymerase to bind to the promoter.

Eukaryotic RNA Processing & Splicing

In eukaryotes, the initial RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA ready for translation.

• 5' Cap: Modified guanine nucleotide added to the 5' end for protection and ribosome recognition.

• Poly-A Tail: Sequence of adenine nucleotides added to the 3' end for stability and export from the nucleus.

• RNA Splicing: Removal of noncoding regions (introns) and joining of coding regions (exons) by the spliceosome.

• Alternative Splicing: Allows a single gene to code for multiple proteins by varying exon combinations.

Translation: From RNA to Protein

Types of RNA

Three main types of RNA are involved in translation:

• Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes; contains codons (triplets of nucleotides).

• Ribosomal RNA (rRNA): Structural and catalytic component of ribosomes.

• Transfer RNA (tRNA): Brings amino acids to the ribosome; contains anticodons complementary to mRNA codons.

The Genetic Code

The genetic code is a set of rules by which the sequence of nucleotides in mRNA is translated into the sequence of amino acids in a protein. It is nearly universal and redundant (more than one codon can specify the same amino acid).

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

• Start Codon: AUG (methionine) signals the start of translation.

• Stop Codons: UAA, UAG, UGA signal the end of translation.

Translation Process

Translation is the process by which ribosomes synthesize proteins using the sequence of codons in mRNA. It occurs in three main steps: initiation, elongation, and termination.

• Initiation: The small ribosomal subunit binds to mRNA and the initiator tRNA (carrying methionine) binds to the start codon. The large subunit then joins.

• Elongation: Amino acids are added one by one to the growing polypeptide chain as the ribosome moves along the mRNA.

• Termination: When a stop codon is reached, release factors bind, causing the polypeptide to be released and the ribosome to dissociate.

Post-Translational Modification

After translation, proteins may undergo post-translational modifications (PTMs) that alter their function, activity, or location. Common PTMs include methylation, acetylation, ubiquitination, phosphorylation, and glycosylation.

• Glycosylation: Addition of carbohydrates to proteins.

• Phosphorylation: Addition of phosphate groups, often reversible and regulates protein activity.

Comparison of Transcription and Translation

	Transcription	Translation
Product Formed	RNA Molecule	Polypeptide (Protein)
Macromolecule Change?	Nucleic Acid → Nucleic Acid	Nucleic Acid → Protein
Major Enzyme/Structure	RNA Polymerase	Ribosome
Location	Nucleus (Eukaryotes)	Cytoplasm
Direction of Synthesis	5' → 3'	N-terminus → C-terminus

Mutations and Gene Regulation

Mutations are permanent changes in the DNA sequence. They can affect gene expression and protein function, and may be spontaneous or induced by environmental factors (mutagens). Types include point mutations, insertions, deletions, and frameshifts.

Gene Regulation ensures that genes are expressed only when needed. In prokaryotes, gene expression is often regulated at the transcriptional level using operons (e.g., lac and trp operons). In eukaryotes, regulation occurs at multiple levels, including chromatin modification, transcriptional, post-transcriptional, translational, and post-translational control.

Additional info: For a comprehensive understanding, students should also review the mechanisms of operon regulation (inducible vs. repressible), the role of transcription factors, and the impact of epigenetic modifications such as DNA methylation and histone acetylation.