Gene Expression: Transcription and Translation – The Central Dogma of Biology

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Gene Expression and the Central Dogma of Biology

Overview of the Central Dogma

The central dogma of biology describes the flow of genetic information within a biological system. It explains how DNA codes for proteins through two main processes: transcription and translation. This concept is fundamental to understanding molecular biology and gene expression.

DNA serves as the information storage molecule.
RNA acts as the information carrier, specifically messenger RNA (mRNA).
Protein is the final product, functioning as structural and functional units in cells.
Gene expression occurs in two stages: transcription and translation.

Diagram of the central dogma: DNA to RNA to Protein Central dogma: DNA, RNA, and Protein flow

Transcription: DNA to RNA

Transcription is the process by which a segment of DNA is used as a template to synthesize a complementary RNA molecule. This step does not change the "language" of nucleic acids but simply copies the information.

Template strand: The DNA strand used by RNA polymerase to synthesize RNA.
Coding strand: The DNA strand whose sequence matches the RNA (except for T/U substitution).
RNA polymerase: The enzyme responsible for synthesizing RNA from the DNA template.
Transcription occurs in the 5' to 3' direction.
RNA is single-stranded, contains ribose sugar, and uses uracil (U) instead of thymine (T).

Transcription and translation overview RNA structure and nucleobases Transcription: DNA template to mRNA Transcription: coding and template strands

Anatomy of a Gene

Genes contain specific regions that regulate transcription and define the coding sequence.

Promoter: Site where RNA polymerase binds to initiate transcription.
Regulatory sequences: Sites for binding of transcription factors that influence transcription rate.
Terminator: Sequence signaling the end of transcription.

Gene anatomy: promoter, regulatory sequence, terminator

Stages of Transcription

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

Initiation: RNA polymerase binds to the promoter and unwinds DNA locally.
Elongation: RNA polymerase synthesizes RNA in the 5' to 3' direction, adding nucleotides complementary to the template strand.
Termination: RNA polymerase encounters a terminator sequence, releases the RNA transcript, and detaches from DNA.

Transcription initiation: RNA polymerase and promoter Transcription elongation: direction of synthesis Transcription termination: completed mRNA transcript

RNA Polymerase vs. DNA Polymerase

Both enzymes synthesize nucleic acids, but they differ in substrate, product, and requirements.

Similarities: Use nucleoside triphosphates as substrates and energy sources; base pairing rules apply.
Differences: RNA polymerase synthesizes single-stranded RNA, does not require a primer, and only copies one strand.

RNA polymerase mechanism

Eukaryotic RNA Processing

In eukaryotes, pre-mRNA undergoes several modifications before becoming mature mRNA. These steps are essential for stability, export, and translation.

5' capping: Addition of a modified guanine nucleotide to the 5' end.
Splicing: Removal of introns and joining of exons by the spliceosome.
Polyadenylation: Addition of a poly-A tail (50–250 adenine nucleotides) to the 3' end.
Transport: Mature mRNA is exported from the nucleus to the cytoplasm.

RNA splicing: introns and exons Spliceosome mechanism Polyadenylation: poly-A tail addition

Translation: RNA to Protein

The Genetic Code

Translation is the process by which the sequence of bases in mRNA is decoded to synthesize a polypeptide. The genetic code is read in triplets called codons.

Codon: Three consecutive nucleotides in mRNA that specify an amino acid.
The code is degenerate: most amino acids are specified by more than one codon.
The code is universal: nearly all organisms use the same code.
Start codon: AUG (methionine); Stop codons: UGA, UAA, UAG.

Transcription and translation: genetic code Genetic code: start and stop codons

Transfer RNA (tRNA) and Translation

tRNAs are adapter molecules that bring specific amino acids to the ribosome based on the codon sequence in mRNA.

Amino acid attachment site: 3' end of tRNA where the amino acid is covalently attached.
Anticodon: A triplet of bases that pairs with the complementary codon in mRNA.
Aminoacyl-tRNA synthetase: Enzyme that matches tRNA with its correct amino acid using ATP.

tRNA structure and function tRNA structure: amino acid attachment and anticodon Aminoacyl-tRNA synthetase: amino acid binding Aminoacyl-tRNA synthetase: ATP usage Aminoacyl-tRNA synthetase: tRNA binding Charged tRNA release

Ribosome Structure and Function

Ribosomes are the sites of protein synthesis, composed of rRNA and protein. They consist of two subunits and three functional sites.

Small subunit: Binds mRNA.
Large subunit: Binds tRNA and catalyzes peptide bond formation.
A site: Accepts incoming tRNA with amino acid.
P site: Holds tRNA with growing polypeptide chain.
E site: Exit site for discharged tRNA.

Ribosome structure: subunits and sites Ribosome A, P, and E sites

Stages of Translation

Translation occurs in three stages: initiation, elongation, and termination.

Initiation: Small ribosomal subunit binds mRNA, initiator tRNA binds start codon, large subunit joins.
Elongation: tRNA enters A site, peptide bond forms, ribosome translocates, tRNA exits via E site.
Termination: Stop codon encountered, release factor binds, polypeptide released, complex dissociates.

Translation initiation Translation elongation: codon recognition Translation elongation: peptide bond formation Translation elongation: translocation Translation termination

Types of RNA

Three main types of RNA are involved in translation, each with distinct roles.

mRNA (messenger RNA): Carries genetic information from DNA to ribosomes.
tRNA (transfer RNA): Brings amino acids to the ribosome based on codon-anticodon pairing.
rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

Mutations and Their Effects

Types of Mutations

Mutations are permanent changes in the DNA sequence. They can affect gene expression and protein function, and are a source of genetic variation.

Silent mutation: No change in amino acid sequence.
Missense mutation: Changes one amino acid in the sequence.
Nonsense mutation: Introduces a premature stop codon.
Frameshift mutation: Alters the reading frame by insertion or deletion.

Mutation Type	Effect
Silent	No change in protein
Missense	One amino acid changed
Nonsense	Protein truncated
Frameshift	Multiple amino acids changed

Summary Table: DNA, RNA, and Protein

Molecule	Function	Structure
DNA	Information storage	Double helix, deoxyribose, A/T/C/G
RNA	Information carrier	Single strand, ribose, A/U/C/G
Protein	Functional product	Polypeptide chain, amino acids

Key Equations and Concepts

Number of nucleotides required for a protein:
Base pairing rules:

Additional info: These notes expand on the lecture outline and provide context for each step of gene expression, including definitions, examples, and diagrams. Images included are directly relevant to the explanation of transcription, translation, and gene structure.