Gene Expression: How Cells Turn DNA Information into Proteins

Gene expression

The process by which information in DNA is used to produce a functional product, usually a protein (or sometimes a functional RNA).

Transcription

Synthesis of an RNA molecule using a DNA template strand, carried out by RNA polymerase.

RNA polymerase

Enzyme that builds an RNA strand by adding ribonucleotides to the 3' end, synthesizing RNA 5'→3' while reading the DNA template 3'→5'.

Template strand (DNA)

The DNA strand that RNA polymerase reads during transcription; the RNA produced is complementary to this strand.

Coding strand (DNA)

The DNA strand whose sequence matches the RNA transcript except that DNA has T where RNA has U (it is not the strand read by RNA polymerase).

Promoter

A DNA sequence where transcription begins; it is where RNA polymerase (and helpers) bind and it determines the direction of transcription (which strand is used as template).

Transcription factors

Proteins in eukaryotes that help RNA polymerase bind to the promoter and start transcription correctly; disrupting them can reduce or stop transcription.

pre-mRNA (primary transcript)

The initial RNA transcript made in eukaryotes that must be processed (capping, tailing, splicing) before becoming mature mRNA.

mRNA 5' cap & poly-A tail

Eukaryotic mRNA modifications: a 5' cap (modified guanine) and a 3' poly-A tail (adenine stretch) that increase stability, aid nuclear export, and help translation initiation/efficiency.

RNA splicing

Processing step in eukaryotes that removes introns from pre-mRNA and joins exons to produce a continuous coding sequence in mature mRNA.

Introns vs. exons

Introns are non-coding segments removed during RNA splicing; exons are expressed segments retained and joined in the final mRNA.

Spliceosome

A complex of proteins and small RNAs that performs RNA splicing by recognizing splice sites, removing introns, and ligating exons.

Alternative splicing

Splicing of the same pre-mRNA in different ways to include different combinations of exons, allowing one gene to produce multiple protein products.

Translation

The process of building a polypeptide by reading mRNA codons at a ribosome, using tRNAs to deliver amino acids.

Codon

A three-nucleotide sequence on mRNA that specifies an amino acid or a stop signal.

Reading frame

The grouping of mRNA nucleotides into consecutive, non-overlapping codons (sets of three); established by the start codon.

Start codon (AUG)

Codon that typically signals the start of translation and sets the reading frame; it codes for methionine (Met).

Stop codon (UAA, UAG, UGA)

Codons that do not code for an amino acid; they signal translation termination.

tRNA (transfer RNA)

Adapter molecule that brings specific amino acids to the ribosome; each tRNA carries an amino acid and contains an anticodon complementary to an mRNA codon.

Ribosome (A, P, E sites; rRNA catalysis)

rRNA-protein complex that translates mRNA; has A (incoming tRNA), P (tRNA holding growing chain), and E (exiting tRNA) sites, and rRNA catalyzes peptide bond formation.

Polyribosome (polysome)

A single mRNA being translated simultaneously by multiple ribosomes, increasing the rate of protein production from that transcript.

Frameshift mutation

Insertion or deletion of nucleotides not in multiples of three, shifting the reading frame and altering downstream codons, often causing a nonfunctional protein.

Nonsense mutation

A mutation that changes a codon for an amino acid into a stop codon, leading to premature termination and a shortened polypeptide.

Missense mutation

A mutation that changes a codon so a different amino acid is incorporated; effects depend on the role of the altered amino acid in the protein.

Silent mutation

A nucleotide change that does not alter the amino acid sequence (due to redundancy of the genetic code), often with little or no effect on protein function.