Gene Expression: From DNA to Protein

Gene Expression

DNA and RNA

Gene expression is the process by which information encoded in DNA is used to direct the synthesis of proteins, the functional molecules of the cell. Two main types of nucleic acids are involved: DNA and RNA.

• DNA (deoxyribonucleic acid):

  • Contains a 5-carbon sugar (deoxyribose).

  • Double-stranded helix structure.

  • Carries genetic information in the sequence of its bases: cytosine (C), guanine (G), adenine (A), and thymine (T).

• RNA (ribonucleic acid):

  • Contains a 5-carbon sugar (ribose).

  • Single-stranded molecule.

  • Carries protein-building information.

  • Bases: cytosine (C), guanine (G), adenine (A), and uracil (U) (uracil replaces thymine).

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system:

• DNA is transcribed into mRNA (messenger RNA).

• mRNA is translated into protein.

This can be summarized as:

• DNA → (transcription) → mRNA → (translation) → protein

The Genetic Code

The genetic code is the set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein.

• The four mRNA bases (A, U, G, C) are read in groups of three nucleotides called codons.

• There are 64 possible codons (43 combinations).

• Of these, 61 are sense codons that specify amino acids.

• AUG is the start codon (initiator codon) and codes for methionine.

• UAA, UAG, and UGA are stop codons; they do not code for amino acids and signal the end of translation.

• Most amino acids are specified by more than one codon (degeneracy of the code).

• Only methionine (AUG) and tryptophan (UGG) are specified by a single codon each.

• The genetic code is commaless (no punctuation between codons) and universal (shared by almost all organisms).

Table: Start and Stop Codons

Codon	Function	Amino Acid
AUG	Start codon	Methionine
UAA	Stop codon	None
UAG	Stop codon	None
UGA	Stop codon	None

Transcription

Transcription is the process by which the information in a DNA sequence is copied into a complementary RNA sequence. This process occurs in three main stages:

1. Initiation: Transcription factors and RNA polymerase assemble at the promoter region of a gene, and synthesis of RNA begins.

2. Elongation: RNA polymerase moves along the DNA template, synthesizing the RNA strand in the 5' to 3' direction.

3. Termination: Transcription ends, and the RNA transcript and RNA polymerase are released from the DNA.

RNA Modification after Transcription

In eukaryotes, the primary RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA:

• Facilitates export of mRNA from the nucleus to the cytoplasm.

• Protects mRNA from degradation by hydrolytic enzymes.

• Helps ribosomes attach to the 5' end for translation.

Split Genes and RNA Splicing

Eukaryotic genes are often interrupted by noncoding sequences:

• Introns: Noncoding regions ("junk DNA"), which make up about 98% of the human genome.

• Exons: Coding regions that are usually translated into amino acid sequences.

• RNA splicing: The process by which introns are removed and exons are joined together, resulting in an mRNA molecule with a continuous coding sequence.

Example: In human genes, the beta-globin gene contains three exons and two introns. During RNA processing, the introns are removed, and the exons are spliced together to form the mature mRNA.

Additional info: Alternative splicing can produce different proteins from the same gene by including or excluding certain exons, increasing protein diversity.