Gene Expression: From Gene to Protein

Central Dogma of Molecular Biology

The central dogma of biology describes the flow of genetic information within a biological system. It states that information flows from DNA to RNA through transcription, and from RNA to protein through translation. This process is fundamental to all living organisms and underlies gene expression.

• Transcription: The synthesis of RNA using DNA as a template.

• Translation: The synthesis of proteins using the information encoded in mRNA.

• Gene Expression: The process by which genetic instructions are used to synthesize gene products (proteins or functional RNAs).

• Irreversibility: While DNA can be transcribed to RNA and RNA can be reverse-transcribed to DNA, the flow from nucleic acid to protein is not reversible.

Genes Specify Proteins via Transcription & Translation

Genes contain the instructions for building proteins. The process involves two main steps: transcription (DNA to RNA) and translation (RNA to protein). RNA acts as the intermediary between DNA and protein synthesis.

• RNA vs. DNA: RNA contains ribose sugar and uracil (U) instead of deoxyribose and thymine (T). RNA is usually single-stranded.

• Types of RNA: Messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA) each play distinct roles in gene expression.

Prokaryotic vs. Eukaryotic Gene Expression

Gene expression differs between prokaryotes and eukaryotes, primarily due to cellular compartmentalization.

• Prokaryotes: Transcription and translation occur simultaneously in the cytoplasm, as there is no nucleus.

• Eukaryotes: Transcription occurs in the nucleus, and the resulting pre-mRNA undergoes processing before being exported to the cytoplasm for translation.

Transcription: DNA to RNA

Initiation of Transcription

Transcription begins at specific DNA sequences called promoters, where RNA polymerase binds and unwinds the DNA. In eukaryotes, transcription factors are required for RNA polymerase to bind to the promoter.

• Promoter: DNA sequence where transcription starts and RNA polymerase attaches.

• RNA Polymerase: Enzyme that synthesizes RNA from the DNA template without the need for a primer.

• Terminator: DNA sequence signaling the end of transcription.

Elongation of Transcription

During elongation, RNA polymerase moves along the DNA template strand, synthesizing a complementary RNA strand in the 5’ to 3’ direction. The coding strand of DNA has the same sequence as the RNA (except T is replaced by U).

• Direction: RNA is synthesized from 5’ to 3’.

• Multiple Polymerases: Several RNA polymerases can transcribe a gene simultaneously.

Termination of Transcription

Transcription ends when RNA polymerase reaches a terminator sequence. In prokaryotes, this results in the release of the RNA transcript. In eukaryotes, the pre-mRNA is released and requires further processing.

• Prokaryotes: RNA polymerase detaches and releases the RNA transcript directly.

• Eukaryotes: Pre-mRNA is released and undergoes RNA processing before translation.

RNA Processing in Eukaryotes

RNA Processing and Splicing

In eukaryotes, the initial RNA transcript (pre-mRNA) undergoes processing to become mature mRNA. This includes the addition of a 5’ cap, a poly-A tail, and the removal of introns through splicing.

• 5’ Cap: Modified guanine nucleotide added to the 5’ end for stability and ribosome binding.

• Poly-A Tail: String of adenine nucleotides added to the 3’ end for stability and export from the nucleus.

• RNA Splicing: Removal of noncoding introns and joining of coding exons by the spliceosome.

• Alternative Splicing: Allows a single gene to code for multiple proteins by varying exon combinations.

Translation: RNA to Protein

The Genetic Code

The genetic code is a set of rules by which the sequence of nucleotides in mRNA is translated into the sequence of amino acids in a protein. It is redundant (multiple codons for one amino acid) but not ambiguous (each codon specifies only one amino acid).

• Codon: Sequence of three mRNA nucleotides that codes for a specific amino acid.

• Start Codon: AUG (methionine) signals the start of translation.

• Stop Codons: UAA, UAG, UGA signal the end of translation.

Ribosomes and tRNA

Translation occurs at the ribosome, which is composed of rRNA and proteins. tRNA molecules bring amino acids to the ribosome, matching their anticodon with codons on the mRNA.

• Ribosome Structure: Composed of large and small subunits; prokaryotic (70S) and eukaryotic (80S) ribosomes differ in size.

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

• Binding Sites: A (aminoacyl), P (peptidyl), and E (exit) sites on the ribosome facilitate translation.

Steps of Translation

Translation consists of initiation, elongation, and termination.

• Initiation: Small ribosomal subunit binds mRNA and initiator tRNA; large subunit joins to form the complete ribosome.

• Elongation: Amino acids are added one by one to the growing polypeptide chain as the ribosome moves along the mRNA.

• Termination: When a stop codon is reached, release factors promote the release of the polypeptide and disassembly of the translation complex.

Mutations and Their Effects on Proteins

Types of Mutations

Mutations are permanent changes in the DNA sequence. They can affect gene expression and protein function, leading to various outcomes.

• Point Mutations: Change in a single nucleotide pair. Can be silent (no effect), missense (change in amino acid), or nonsense (introduces a stop codon).

• Insertions and Deletions: Addition or loss of nucleotide pairs, often causing frameshift mutations that alter the reading frame and usually have severe effects on the protein product.

• Causes: Mutations can be spontaneous or induced by mutagens (chemical or physical agents).

Example: Sickle-cell disease is caused by a single nucleotide substitution in the hemoglobin gene, resulting in abnormal protein structure and function.

Summary Table: Key Differences in Gene Expression

Key Terms and Concepts

• Gene Expression: The process by which information from a gene is used to synthesize a functional gene product.

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of a polypeptide using the information in mRNA.

• mRNA, tRNA, rRNA: Types of RNA involved in protein synthesis.

• Mutation: Permanent change in the DNA sequence.

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

• Introns/Exons: Noncoding/coding regions in eukaryotic genes.