Gene Expression: From DNA to Protein

Overview of Gene Expression

Gene expression is the process by which information encoded in DNA is used to produce functional products, such as proteins, that determine traits. This process involves two main stages: transcription and translation. The flow of genetic information follows the central dogma: DNA → RNA → Protein.

• DNA contains genes, which are sequences of nucleotide bases (A, T, C, G).

• Transcription converts DNA information into messenger RNA (mRNA).

• Translation uses mRNA to build a protein, which expresses a trait.

• Proteins are the final products that carry out cellular functions and determine phenotypes.

Transcription: DNA to mRNA

Transcription is the first step in gene expression, where a gene's DNA sequence is copied into mRNA. This process occurs in the nucleus and involves three main stages:

• Initiation: RNA polymerase binds to a region of DNA called the promoter, which signals where transcription should begin and which strand to use as a template.

• Elongation: RNA polymerase moves along the DNA template strand (3' to 5' direction), synthesizing a complementary mRNA strand (5' to 3' direction) by pairing bases: C-G, A-U, T-A, G-C.

• Termination: When RNA polymerase reaches a terminator sequence, it stops transcription and releases the newly formed mRNA molecule.

Example: If the DNA template sequence is CATG, the mRNA sequence will be GUAC.

Key Enzyme: RNA Polymerase

• RNA polymerase is responsible for unzipping DNA and assembling the mRNA strand.

Processing of Pre-mRNA

Before mRNA leaves the nucleus, it undergoes several processing steps to become mature mRNA:

• 5' Cap: A modified guanine nucleotide is added to the 5' end, helping ribosomes recognize where translation begins.

• Poly-A Tail: A sequence of 150-200 adenines is added to the 3' end, facilitating mRNA export from the nucleus and protecting it from degradation by hydrolytic enzymes.

• Splicing: Non-coding regions called introns are removed, and coding regions called exons are joined together by a complex called the spliceosome.

Definitions:

• Introns: DNA sequences that do not code for proteins and are removed during splicing.

• Exons: DNA sequences that code for proteins and are retained in the mature mRNA.

Translation: mRNA to Protein

Translation is the process by which the mRNA sequence is decoded to build a polypeptide (protein). This occurs in the cytoplasm at the ribosome and involves three stages:

• Initiation: The small ribosomal subunit, mRNA, initiator tRNA (with anticodon UAC and amino acid methionine), and initiation factors assemble. The start codon (AUG) signals where translation begins.

• Elongation: tRNA molecules bring amino acids to the ribosome. The ribosome reads mRNA codons, and tRNA anticodons pair with them. Amino acids are linked together, forming a growing polypeptide chain.

• Termination: When the ribosome encounters a stop codon (UAA, UAG, or UGA), translation ends. The polypeptide is released, and the ribosome dissociates.

Ribosome Structure and Function

• Ribosomes are composed of ribosomal RNA (rRNA) and proteins, forming large and small subunits.

• There are three tRNA binding sites in the ribosome:

Example: The start codon AUG codes for methionine, which is always the first amino acid in a new polypeptide chain.

Translation Steps

• tRNA enters the A site with its amino acid.

• tRNA moves to the P site, where its amino acid is added to the polypeptide chain.

• tRNA moves to the E site and exits the ribosome.

• The ribosome reads the next codon, and the process repeats.

Polypeptide Processing and Protein Targeting

After translation, the polypeptide may undergo further modifications and folding to become a functional protein. The first few amino acids often act as a signal, determining whether the protein stays in the cell or is secreted.

• Some proteins remain in the cell for cellular functions.

• Others are processed in the rough endoplasmic reticulum (ER) and transported via vesicles for secretion (exocytosis).

• Proteins may be modified by the addition of sugars, phosphates, or lipids.

Summary Table: Transcription vs. Translation

Key Terms and Definitions

• Gene Expression: The process by which a gene's information is used to produce a functional product.

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of a polypeptide from an mRNA template.

• Codon: Three-nucleotide sequence in mRNA that specifies an amino acid.

• Anticodon: Three-nucleotide sequence in tRNA complementary to the mRNA codon.

• Ribosome: Cellular structure where translation occurs.

• Exocytosis: Process by which proteins are secreted from the cell.

Relevant Equations and Concepts

• Central Dogma of Molecular Biology:

• Base Pairing in Transcription:

• Start Codon: (codes for methionine)

• Stop Codons: , , (do not code for amino acids)

Example: A gene coding for a protein that forms ion channels in the cell membrane may result in normal or abnormal traits, such as cystic fibrosis if mutated.

Additional info: The notes reference figures from a textbook (e.g., Figure 12.12, 12.14, 12.16, 12.17, 12.19, 12.20) for visual summaries of these processes. Students are encouraged to review these figures for further clarification.