Gene Expression: From Gene to Protein

Gene expression is the precise biological process through which deoxyribonucleic acid ($DNA$) directs the synthesis of proteins. It serves as the critical mechanism bridging the gap between an organism's genotype (genetic makeup) and its phenotype (observable traits). The fundamental principle of biology, known as The Central Dogma, dictates the flow of genetic information in the sequence: DNA ightarrow RNA ightarrow ext{Protein}.

Gene expression involves two distinct stages:

1. Transcription: The synthesis of messenger $RNA$ ($mRNA$) using a $DNA$ template. This occurs within the nucleus in eukaryotic cells.

2. Translation: The biochemical process where the $mRNA$ sequence is decoded to synthesize a polypeptide chain (protein), occurring at the ribosomes within the cytosol.

The Mechanisms of Gene Transcription

Transcription is the first step of gene expression where a segment of $DNA$ is copied into $RNA$. This process relies on a specific template strand of $DNA$, which provides the pattern for ordering complementary nucleotides in the $RNA$ transcript. While the template strand remains constant for any individual gene, the opposite strand may serve as the template for different genes elsewhere on the chromosome. The non-template strand is referred to as the coding strand because its sequence mirrors the $mRNA$.

The Three Stages of Transcription

1. Initiation   - All genes possess a specific promoter sequence called the TATA box.   - Transcription factors (TFs) bind directly to the TATA box.   - These TFs recruit RNA Polymerase II to the designated start site.   - The combination of TFs and $RNA ext{ Polymerase II}$ is known as the Transcription Initiation Complex.   - The $RNA ext{ Polymerase II}$ unwinds the $DNA$ helix to begin transcription.

2. Elongation   - $RNA ext{ Polymerase II}$ transcribes the $DNA$ by successively adding complementary nucleotides to the $3'$ end of the growing $RNA$ transcript.   - Base-pairing rules are adhered to, with the exception that Uracil (U) replaces thymine (T) in $RNA$.   - Correspondence: $A$ pairs with $U$, and $C$ pairs with $G$.

3. Termination   - Transcription continues until the $RNA ext{ polymerase}$ encounters a polyadenylation sequence.   - The initial resultant transcript is known as Pre-mRNA, which contains both coding regions (exons) and non-coding sequences (introns).

Eukaryotic RNA Editing and Processing

Before leaving the nucleus, the pre-mRNA undergoes several modifications to become mature $mRNA$:

• $5'$ Cap: A modified nucleotide is added to the $5'$ end.

• Poly-A Tail: A stretch of adenine nucleotides is added to the $3'$ end.

These modifications serve three primary functions:

1. Facilitate the export of $mRNA$ from the nucleus to the cytoplasm.

2. Protect the $mRNA$ from degradation by hydrolytic enzymes.

3. Assist ribosomes in attaching to the $5'$ end for translation.

RNA Splicing

RNA splicing removes introns (intervening nucleotides) and joins exons (expressed nucleotides). This editing is performed by spliceosomes within the nucleus. Only after this process occurs can the mature $mRNA$ enter the cytosol for translation.

The Genetic Code and Components of Translation

Translation involves transferring the nucleotide message into an amino acid sequence. This code uses codons, which are triplets (3 nucleotides) that encode for a specific amino acid.

• Redundancy: The genetic code is redundant (multiple codons can code for the same amino acid), which helps mitigate the effects of mutations.

• Stop Codons: Include $UAA$, $UAG$, and $UGA$.

• Start Codon: $AUG$ (codes for methionine).

Key Cellular Components

• Transfer RNAs (tRNAs): Decode the $mRNA$ by using an anticodon to bind to the $mRNA$ codon in an antiparallel fashion. They transfer specific amino acids to the ribosome.

• Ribosomes: Composed of large and small subunits and ribosomal $RNA$ ($rRNA$). They contain three binding sites for $tRNA$:
    - P site: Holds the $tRNA$ carrying the growing polypeptide.
    - A site: Holds the $tRNA$ carrying the next amino acid.
    - E site: The exit site where discharged $tRNAs$ leave.

• Aminoacyl-tRNA synthetases: Enzymes that catalyze the attachment of amino acids to their corresponding $tRNAs$ using $ATP$.

The Three Stages of Translation

Translation requires protein factors and energy in the form of $GTP$.

1. Initiation

• The small ribosomal subunit binds $mRNA$ and an initiator $tRNA$ (carrying methionine).

• The small subunit scans the $mRNA$ until it reaches the $AUG$ start codon.

• Initiation factors and $GTP$ facilitate the binding of the larger ribosomal subunit.

1. Elongation

• Amino acids are added sequentially to the C-terminus via elongation factors in three steps:
    - Codon recognition (requires energy).   - Peptide bond formation.   - Translocation (requires energy): the ribosome moves along the $mRNA$ strand.

1. Termination

• Occurs when a stop codon reaches the A site.

• A release factor (protein) binds and promotes hydrolysis (addition of a water molecule).

• The completed polypeptide is released, and the ribosomal assembly dissociates using 2 \, GTP ightarrow 2 \, GDP + 2 \, P_i.

Protein Targeting to the Endoplasmic Reticulum (ER)

Polypeptides destined for the endomembrane system or secretion follow a specific pathway:

1. Synthesis begins on a free ribosome in the cytosol.

2. A signal-recognition particle (SRP) binds to the emerging signal peptide, halting synthesis.

3. The SRP binds to a receptor on the $ER$ membrane.

4. The SRP detaches, and synthesis resumes as the polypeptide translocates across the membrane into the $ER$ lumen.

5. An enzyme cleaves the signal peptide; the final protein folds in the lumen.

Bacterial vs. Eukaryotic Gene Expression

Genetic Mutations

Mutations are changes in the genetic material of a cell. Point mutations involve changes in a single nucleotide pair.

Nucleotide-Pair Substitutions

• Silent Mutations: No change in the amino acid sequence due to code redundancy.

• Missense Mutations: One amino acid is substituted for another.

• Nonsense Mutations: A codon is changed into a stop codon, usually resulting in a non-functional protein.

Insertions and Deletions

These mutations often have disastrous effects because they may alter the reading frame of the genetic message, leading to a frameshift mutation.

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