Gene Expression

Central Dogma of Molecular Biology

The central dogma of molecular biology 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 functional products.

• 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 full process by which genotype becomes expressed as phenotype, encompassing both transcription and translation.

Transcription

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 products such as proteins or functional RNAs.

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

• Terminator: DNA sequence where transcription ends.

• RNA Polymerase: Enzyme that synthesizes RNA from scratch, using DNA as a template (no primer needed).

• Upstream/Downstream: Terms describing direction relative to the transcription start site.

Overview of Transcription

DNA consists of two strands: the coding strand and the template strand. The RNA sequence produced during transcription is complementary to the template strand and nearly identical to the coding strand (except uracil replaces thymine).

• Base Pairing: A-U (in RNA), G-C.

• Directionality: RNA is synthesized in the 5' to 3' direction.

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 synthesized RNA.

Eukaryotic RNA Processing & Splicing

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

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

• Poly-A Tail: Addition of a string of adenine nucleotides to the 3' end.

• Functions: Facilitate export from nucleus, protect from degradation, and assist ribosome binding.

RNA Splicing

Genes in eukaryotes contain exons (coding regions) and introns (noncoding regions). Splicing removes introns and joins exons to produce mature mRNA. Alternative splicing allows a single gene to code for multiple proteins.

• Spliceosome: Complex responsible for removing introns.

• Alternative Splicing: Generates different mRNA transcripts from the same gene.

Types of RNA

Major Types of RNA

Cells use several types of RNA, each with distinct functions:

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

• 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

Structure and Use of the Genetic Code

The genetic code is a set of rules by which information encoded in mRNA is translated into proteins. It is read in triplets (codons), each specifying an amino acid or a stop signal.

• Redundancy: Multiple codons can code for the same amino acid.

• Start Codon: AUG (Methionine).

• Stop Codons: UAA, UAG, UGA.

Translation

Introduction to Translation

Translation is the process by which ribosomes synthesize proteins using the sequence of codons in mRNA. tRNA molecules bring amino acids to the ribosome, matching their anticodons to mRNA codons.

• Ribosome: Site of protein synthesis, composed of rRNA and proteins.

• tRNA: Carries specific amino acids and matches them to codons in mRNA via its anticodon.

• Charged tRNA: tRNA attached to an amino acid.

• Discharged tRNA: tRNA without an amino acid.

Ribosome Structure and tRNA Binding Sites

Ribosomes have two subunits and three tRNA binding sites:

• A (Aminoacyl) Site: Holds tRNA with the next amino acid.

• P (Peptidyl) Site: Holds tRNA with the growing polypeptide chain.

• E (Exit) Site: Where discharged tRNAs leave the ribosome.

• Prokaryotic Ribosomes: 70S (50S + 30S subunits).

• Eukaryotic Ribosomes: 80S (60S + 40S subunits).

Steps of Translation

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

• Initiation: Small ribosomal subunit binds mRNA and initiator tRNA (carrying methionine) at the start codon (AUG), followed by assembly of the large subunit.

• Elongation: Amino acids are added one by one to the growing chain; ribosome moves along mRNA 5' to 3'.

• Termination: When a stop codon is reached, release factors bind, and the completed polypeptide is released.

Post-Translational Modification

Types and Functions

After translation, proteins may undergo covalent modifications that regulate their activity, stability, or localization. These are called post-translational modifications (PTMs).

• Methylation

• Acetylation

• Ubiquitination

• Phosphorylation

• Glycosylation: Addition of carbohydrates to proteins.

Comparison: Transcription vs. Translation

Transcription and translation are distinct but related processes in gene expression.

Mutations

Definition and Types

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.

• Point Mutations: Single nucleotide changes (substitution, insertion, deletion).

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

• Missense Mutation: Changes one amino acid in the protein.

• Nonsense Mutation: Introduces a premature stop codon.

• Silent Mutation: Does not change the amino acid sequence.

Additional info: Mutations are a major source of genetic diversity and evolution. Some mutations are neutral, while others can be beneficial or harmful depending on their effect on protein function.