Gene Expression: From Gene to Protein (Campbell Biology, Ch. 17) – Study Notes

Gene Expression: From Gene to Protein

Introduction

Gene expression is the process by which the information encoded in DNA directs the synthesis of proteins, which are essential for cellular structure and function. This process involves two main stages: transcription and translation. Proteins serve as the link between genotype and phenotype, determining the traits of an organism.

Basic Principles of Transcription and Translation

Overview of Gene Expression

• Transcription: The synthesis of RNA using DNA as a template. This process produces messenger RNA (mRNA).

• Translation: The synthesis of a polypeptide (protein) using the information in mRNA. This occurs at the ribosome.

• RNA: Acts as the bridge between genes (DNA) and protein synthesis.

In prokaryotes, translation can begin before transcription is finished. In eukaryotes, the nuclear envelope separates transcription (in the nucleus) from translation (in the cytoplasm), and RNA transcripts are processed before becoming mature mRNA.

The Central Dogma of Molecular Biology

• The central dogma describes the flow of genetic information: DNA → RNA → Protein.

• This concept highlights the cellular chain of command in gene expression.

Codons: Triplets of Nucleotides

The Genetic Code

• The genetic code is based on a triplet code: three-nucleotide sequences (codons) in mRNA specify amino acids.

• Each codon is read in the 5′ → 3′ direction during translation.

• The coding strand of DNA has the same sequence as the mRNA (except T is replaced by U in RNA).

• There are 64 codons: 61 code for amino acids, and 3 are stop signals.

• The code is redundant (multiple codons for one amino acid) but not ambiguous (each codon specifies only one amino acid).

• Codons must be read in the correct reading frame to produce the correct polypeptide.

Universality and Evolution of the Genetic Code

• The genetic code is nearly universal among all organisms, indicating a common evolutionary origin.

• Genes can be expressed after being transferred between species.

Transcription: DNA-Directed Synthesis of RNA

Molecular Components of Transcription

• RNA polymerase: Enzyme that synthesizes RNA by joining RNA nucleotides complementary to the DNA template strand. No primer is needed.

• Promoter: DNA sequence where RNA polymerase attaches to initiate transcription.

• Terminator (in bacteria): Sequence signaling the end of transcription.

• Transcription unit: The stretch of DNA that is transcribed into RNA.

Stages of Transcription

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

• Elongation: RNA nucleotides are added to the growing RNA strand.

• Termination: RNA synthesis ends when the polymerase reaches the terminator sequence (in bacteria).

RNA Processing in Eukaryotes

Modification of mRNA Ends

• The 5′ end of pre-mRNA receives a 5′ cap (modified nucleotide).

• The 3′ end receives a poly-A tail (a string of adenine nucleotides).

• Functions of these modifications:

  • Facilitate export of mRNA from the nucleus

  • Protect mRNA from degradation

  • Help ribosomes attach to the 5′ end

RNA Splicing

• Most eukaryotic genes contain introns (noncoding regions) and exons (coding regions).

• RNA splicing removes introns and joins exons to produce a continuous coding sequence.

• Splicing is carried out by spliceosomes, complexes of proteins and small RNAs that recognize splice sites and catalyze the reaction.

Functional and Evolutionary Importance of Introns

• Some introns regulate gene expression or affect gene products.

• Alternative RNA splicing allows a single gene to code for multiple proteins, increasing protein diversity.

Translation: RNA-Directed Synthesis of a Polypeptide

Molecular Components of Translation

• Transfer RNA (tRNA): Brings amino acids to the ribosome and matches them to the mRNA codon via its anticodon.

• Each tRNA carries a specific amino acid and has a complementary anticodon to the mRNA codon.

• tRNA molecules are about 80 nucleotides long, fold into a cloverleaf structure, and have an L-shaped 3D conformation.

Structure and Function of Ribosomes

• Ribosomes are made of proteins and ribosomal RNAs (rRNAs).

• They facilitate the coupling of tRNA anticodons with mRNA codons during protein synthesis.

• Eukaryotic ribosomes are larger and differ from bacterial ribosomes, which is the basis for some antibiotics.

• Ribosomes have three binding sites for tRNA:

  • P site: Holds the tRNA with the growing polypeptide chain.

  • A site: Holds the tRNA with the next amino acid.

  • E site: Exit site for discharged tRNAs.

Stages of Translation

• Initiation: The small ribosomal subunit binds to mRNA and a special initiator tRNA (carrying methionine). The complex assembles at the start codon (AUG).

• Elongation: Amino acids are added one by one to the growing chain. Steps include codon recognition, peptide bond formation, and translocation. Elongation factors and energy are required.

• Termination: When a stop codon is reached, a release factor binds, causing the addition of a water molecule and releasing the polypeptide.

Polyribosomes

• Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome).

• This allows cells to produce many copies of a protein rapidly.

Summary Table: Key Steps in Gene Expression

Step	Location (Eukaryotes)	Main Enzyme/Structure	Product
Transcription	Nucleus	RNA polymerase	Pre-mRNA
RNA Processing	Nucleus	Spliceosome, enzymes	Mature mRNA
Translation	Cytoplasm (ribosome)	Ribosome, tRNA	Polypeptide

Key Equations and Concepts

• Central Dogma:

  • $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• Codon-Anticodon Pairing:

  • For example, if the mRNA codon is 5'-AUG-3', the tRNA anticodon is 3'-UAC-5'.

Additional info:

• Alternative splicing is a major mechanism for increasing protein diversity in eukaryotes.

• Some antibiotics target bacterial ribosomes, exploiting differences between prokaryotic and eukaryotic translation machinery.