Gene Expression—Gene to Protein

Gene: A DNA sequence that is transcribed to form a functional product, either RNA or a polypeptide.
Gene function: Genes specify proteins via two main processes:
- Transcription: The DNA-directed synthesis of RNA.
- Translation: The RNA-directed synthesis of a polypeptide.
Mutation: Changes in DNA sequences that can affect protein structure and function.

Understanding the processes of transcription and translation is crucial for comprehending gene expression.
Central Dogma of Molecular Biology: Describes the flow of genetic information from DNA to RNA to protein.
- Notable exception: Reverse transcription (the process by which certain viruses convert RNA back into DNA).

Process: Involves several key steps:
- Initiation
- Elongation
- Termination
Transcription in Eukaryotic Cells: After transcription, RNA undergoes further processing (capping, polyadenylation, and splicing).
Transcription in Prokaryotic Cells: Occurs without RNA processing and begins even before transcription finishes.
Promoter and terminator sequences: Recognized by RNA polymerase, essential for initiating and terminating transcription, respectively.
RNA Polymerases:
- Eukaryotic cells contain three types of RNA polymerases (I, II, III) that perform specific roles in transcribing different types of genes.
- Prokaryotes only have one RNA polymerase type.
TATA box: A conserved sequence found in the promoter region that is recognized by RNA polymerases during transcription initiation.
Consensus sequences: Are sequences that are generally similar in different genes and help in regulating gene expression.

Pre-mRNA undergoes extensive processing before becoming mature mRNA:
- 5’ cap: Added to the beginning to protect mRNA and assist in ribosome binding.
- 3’ poly-A tail: A long adenine tail added to enhance stability and aid in export from the nucleus.
- Splicing: Removal of non-coding regions (introns) and joining of coding regions (exons).
Alternative RNA splicing: Allows a single mRNA to be processed in multiple ways, producing different protein variants from one gene.

Objective: To synthesize polypeptides from mRNA transcripts.
Components Required:
- mRNA transcript
- Amino acids
- Ribosomes
- Energy (usually in the form of ATP)
- Various protein factors (e.g., initiation factors, elongation factors).
Ribosome Structure: Composed of a large and a small subunit, each made up of rRNA and proteins.
tRNA (Transfer RNA):
- The molecule that carries amino acids to ribosomes during protein synthesis.
- Approximately 30-50 unique tRNA molecules exist, each matching to a specific amino acid via an enzyme known as aminoacyl tRNA synthetase.
Translation Phases:
- Initiation: The ribosomal subunits bind to mRNA to begin translation.
- Elongation: The ribosome passes along the mRNA, adding amino acids to the growing polypeptide chain.
- Termination: Occurs when a stop codon is reached; a release factor binds and releases the newly formed polypeptide.

Types of mutations:
- Silent mutations: Base changes that do not alter the amino acid due to redundancy in the genetic code.
- Missense mutations: Base changes that result in a different amino acid being incorporated into the protein.
- Nonsense mutations: Introduce a premature stop codon, often resulting in incomplete and nonfunctional proteins.
- Frameshift mutations: The insertion or deletion of nucleotides changes the reading frame of the mRNA, potentially producing entirely different polypeptides.
- Insertions/deletions of triplet nucleotides can cause specific amino acid insertion or deletion without shifting the frame.

Biotechnology and Gene Expression: The universal nature of DNA allows for manipulation and expression of foreign genes in various organisms.
Transgenic Organisms: Usage of constructs like Green Fluorescent Protein (GFP) in research and medical applications.

Cytoplasmic protein synthesis: Ribosomes in the cytoplasm produce proteins that function within the cell.
Secretory protein synthesis: Involves ribosomes bound to the endoplasmic reticulum (ER). The process includes recognition of a signal peptide that directs the nascent polypeptide into the ER for further processing and eventual secretion.

Understanding gene expression involves the interconnected processes of transcription and translation, each regulated by various sequences and factors. Mutations can significantly affect protein synthesis, adding a layer of complexity to genetic functioning, while practical applications in biotechnology demonstrate the utility of these concepts in real-world scenarios.

Gene: A DNA sequence transcribed to form a functional product (RNA or polypeptide).
Gene function: Specifies proteins through:
- Transcription: DNA-directed synthesis of RNA.
- Translation: RNA-directed synthesis of a polypeptide.
Mutation: Changes in DNA sequences affecting protein structure/function.

Process: Initiation, Elongation, Termination.
Eukaryotes: RNA processing (capping, polyadenylation, splicing) occurs after transcription.
Prokaryotes: Transcription and translation are coupled; no RNA processing.
Key elements: Promoter and terminator sequences, RNA Polymerases (three types in eukaryotes, one in prokaryotes), TATA box for initiation.

Pre-mRNA undergoes:
- 5’ cap: Protection and ribosome binding.
- 3’ poly-A tail: Stability and nuclear export.
- Splicing: Intron removal, exon joining.
- Alternative RNA splicing: Generates multiple protein variants from one gene.

Objective: Synthesize polypeptides from mRNA.
Components: mRNA, amino acids, ribosomes, tRNA, energy, protein factors.
Ribosomes: Composed of rRNA and proteins.
tRNA: Carries amino acids to ribosomes; recognized by aminoacyl tRNA synthetase.
Phases: Initiation, Elongation, Termination (at stop codon).

Silent mutations: No amino acid change.
Missense mutations: Different amino acid incorporated.
Nonsense mutations: Premature stop codon; often nonfunctional proteins.
Frameshift mutations: Insertion/deletion of nucleotides (not multiples of three) altering the reading frame.

Cytoplasmic: Ribosomes synthesize proteins for internal cell function.
Secretory: Ribosomes bound to ER synthesize proteins for secretion or membranes; involves a signal peptide.