From DNA to Protein: Gene Expression and Regulation

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Gene Expression: An Overview

Introduction to Gene Expression

Gene expression is the process by which genetic information encoded in DNA is used to direct the synthesis of proteins, which perform most cellular functions. This process involves two main steps: transcription (DNA to RNA) and translation (RNA to protein).

Gene Expression: The process by which information from a gene is used to synthesize a functional gene product, typically a protein.
Central Dogma: Describes the flow of genetic information in cells from DNA to RNA to protein.

From DNA to RNA: Transcription

Transcription in Eukaryotes and Prokaryotes

Transcription is the synthesis of RNA from a DNA template. In eukaryotes, this process includes additional steps such as mRNA processing, while in prokaryotes, mRNA is often ready for translation immediately after transcription.

mRNA Processing (Eukaryotes):
- 5' Capping: Addition of a modified guanine nucleotide to the 5' end of the mRNA.
- Polyadenylation: Addition of a poly(A) tail to the 3' end.
- Splicing: Removal of non-coding introns and joining of exons.
No mRNA Processing (Prokaryotes): Transcription and translation can occur simultaneously.

From RNA to Protein: Translation

Genetic Code and Codons

The genetic code is a set of rules by which information encoded in mRNA is translated into proteins. Codons are sequences of three nucleotides that correspond to specific amino acids or stop signals.

Codon: A sequence of three nucleotides in mRNA that specifies an amino acid or stop signal.
Start Codon: AUG (codes for methionine), signals the start of translation.
Stop Codons: UAA, UGA, UAG, signal the end of translation.

Genetic Codons Table:

Codon	Amino Acid	Function
AUG	Methionine	Start codon
UAA	None	Stop codon
UGA	None	Stop codon
UAG	None	Stop codon

Characteristics of the Genetic Code

Redundant: Most amino acids are encoded by more than one codon.
Nearly Universal: With few exceptions, the genetic code is the same in almost all organisms.

Reading Frames and Open Reading Frames (ORFs)

Reading Frame: The way nucleotides in mRNA are grouped into codons for translation.
There are three possible reading frames in any mRNA sequence, but only one encodes the correct protein (open reading frame).

Translation Machinery

Ribosome: The site of protein synthesis, composed of rRNA and proteins. Has large and small subunits.
tRNA (transfer RNA): Brings amino acids to the ribosome and matches them to the mRNA codons via its anticodon.
Aminoacyl-tRNA Synthetase: Enzyme that attaches the correct amino acid to its tRNA, with proofreading ability.

tRNA Structure and Function

Each tRNA has an anticodon that pairs with a codon in mRNA and a 3'-ACC end that attaches to an amino acid.
There are 30-40 different tRNAs, each specific for an amino acid.
Wobble base pairing allows some tRNAs to recognize more than one codon.

Ribosome Structure

Composed of a large and a small subunit.
Three binding sites for tRNA:
- A site: Aminoacyl-tRNA binding site
- P site: Peptidyl-tRNA binding site
- E site: Exit site for tRNA

Translation Process

Initiation: Assembly of the ribosome on the mRNA, with the initiator tRNA (carrying methionine) at the start codon.
Elongation: Sequential addition of amino acids to the growing polypeptide chain.
Termination: Occurs when a stop codon is reached; release factors promote the release of the polypeptide and disassembly of the ribosome.

Polyribosomes (Polysomes)

Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome or polysome.
This increases the efficiency of protein synthesis.

Summary of Translation

Translation proceeds from the N-terminus to the C-terminus of the protein, starting with methionine.
The ribosome reads the mRNA in the 5' to 3' direction.
Translation occurs in an ordered process: initiation, elongation, and termination.
Polysomes increase the efficiency of protein synthesis.

Control of Gene Expression

Overview of Gene Expression Regulation

Gene expression is regulated at multiple levels, allowing cells to respond to environmental conditions and differentiate into various cell types.

Regulation can occur at transcriptional, post-transcriptional, translational, and post-translational levels.
Small regulatory RNAs (miRNA, siRNA) play important roles in gene regulation.

Why Gene Regulation Makes Cells Different

All cells in an organism have the same DNA, but different gene expression patterns lead to different cell types and functions.
Gene regulation allows for specialization and adaptation.

Levels of Gene Expression Regulation

Transcriptional Regulation: Most important control point; involves transcription factors binding to regulatory DNA sequences.
Post-transcriptional Regulation: Includes mRNA processing, stability, and transport.
Translational and Post-translational Regulation: Control of protein synthesis and modification.

Regulatory Elements in Eukaryotic Genes

Transcription is controlled by transcription factors binding to regulatory DNA sequences (promoters, enhancers, silencers).
Regulatory sequences can be close (proximal) or distant (distal) from the promoter.

Element	Function
Promoter	Site where RNA polymerase binds to initiate transcription
Enhancer	Increases transcription levels; can be distant from the gene
Terminator	Signals the end of transcription

Key Terms and Concepts

Codon/Anticodon: Triplet nucleotide sequences in mRNA/tRNA involved in translation.
Polysome: A complex of multiple ribosomes translating a single mRNA.
Transcription/Translation: Processes of synthesizing RNA from DNA and protein from RNA, respectively.
Peptidyl Transferase: Enzyme activity of the ribosome that forms peptide bonds between amino acids.

Important Equations and Concepts

Number of possible codons:

Where 4 is the number of RNA bases (A, U, G, C) and 3 is the codon length.

Summary

Gene expression involves transcription and translation, regulated at multiple levels.
The genetic code is redundant and nearly universal, read in triplets (codons).
Translation is carried out by ribosomes, tRNAs, and various enzymes, proceeding from the N-terminus to the C-terminus of the protein.
Gene regulation allows cells with the same DNA to have different structures and functions.