Ch. 17 Bio 1201

Beadle and Tatum's Experiment

Exposed bread mold (Neurospora) to X-rays, creating mutants unable to survive on minimal media. Demonstrated that each gene dictates production of one specific enzyme, leading to the "one gene-one enzyme" hypothesis.

One gene-one enzyme vs. one gene-one polypeptide

The original hypothesis proposed each gene produces a single enzyme. It was later revised to "one gene-one polypeptide," since many proteins consist of multiple polypeptides, each encoded by a separate gene.

Difference between RNA and DNA

RNA acts as a bridge between genes and the proteins they code for through transcription; DNA stores genetic information.

Flow of genetic information

DNA → mRNA → Protein. DNA is transcribed into mRNA, which is then translated into a polypeptide chain (protein).

Transcription

The synthesis of RNA using a DNA template; produces mRNA.

Primary transcript

The initial RNA transcript from any gene before processing in eukaryotes.

Translation

The synthesis of a polypeptide using the information in mRNA; occurs on ribosomes.

Transcription and translation in prokaryotes

Occur simultaneously; translation can begin before transcription finishes since there is no nucleus.

Transcription and translation in eukaryotes

Transcription occurs in the nucleus; translation occurs in the cytoplasm. mRNA must be processed before translation.

Codon

A sequence of three nucleotides on mRNA that specifies a particular amino acid or stop signal during protein synthesis.

mRNA codon pairing rules

A pairs with U, and G pairs with C.

Relationship between codons and amino acids

The linear sequence of codons on mRNA determines the linear sequence of amino acids in a polypeptide.

Genetic code redundancy

More than one codon can specify the same amino acid.

Genetic code unambiguousness

Each codon specifies only one amino acid; there is no overlap or ambiguity.

Reading frame significance

Codons must be read in the correct grouping; a shift changes the entire amino acid sequence. AUG is the start codon; UAA, UAG, and UGA are stop codons.

Promoter

A DNA sequence where RNA polymerase binds to initiate transcription.

Terminator

A DNA sequence signaling the end of transcription.

Transcription factors

Proteins that help RNA polymerase bind to the promoter and initiate transcription.

Transcription initiation complex

The completed assembly of transcription factors and RNA polymerase bound to the promoter.

Termination of transcription in bacteria

RNA polymerase stops transcription at the terminator, and the mRNA is ready for translation without modification.

Termination of transcription in eukaryotes

RNA polymerase II transcribes the polyadenylation signal sequence; RNA is released 10-35 nucleotides downstream.

Three steps of transcription

Initiation: RNA polymerase binds promoter; Elongation: RNA nucleotides are added; Termination: RNA transcript is released.

RNA processing in eukaryotes

The 5′ end receives a 5′ cap; the 3′ end receives a poly-A tail.

Purpose of RNA modifications

Facilitate export to cytoplasm, protect mRNA from degradation, and help ribosomes attach to the 5′ end.

Ribozyme

Catalytic RNA molecule that acts as an enzyme, often removing introns or catalyzing splicing.

Introns

Noncoding sequences removed from pre-mRNA during RNA splicing.

Exons

Coding sequences that remain in mRNA and are expressed.

Evolutionary significance of introns

Allow exon shuffling, increasing the chances of beneficial combinations and protein diversity.

tRNA function

Transfers specific amino acids to the growing polypeptide chain on the ribosome.

Two-dimensional structure of tRNA

Cloverleaf shape with four base-paired regions and three loops; the 3′ end attaches to an amino acid; one loop contains the anticodon.

Three-dimensional structure of tRNA

L-shaped; anticodon and amino acid attachment site are on opposite ends; stabilized by hydrogen bonds.

Aminoacyl-tRNA synthetase

Enzyme that joins each tRNA to its correct amino acid using ATP energy.

Ribosome structure

Consists of a large and small subunit made of proteins and ribosomal RNAs (rRNA).

Ribosome function

Couples tRNA anticodons with mRNA codons during protein synthesis.

Steps of translation

Initiation: mRNA binds to ribosome; Elongation: tRNA adds amino acids; Termination: stop codon releases polypeptide.

Free vs. bound ribosomes

Free ribosomes synthesize proteins used in the cytoplasm; bound ribosomes (on RER) make proteins for secretion or membranes, determined by a signal sequence.

Protein synthesis in prokaryotes

Occurs in the cytoplasm; transcription and translation occur together.

Protein synthesis in eukaryotes

Transcription occurs in the nucleus; translation occurs in the cytoplasm; RNA must be processed first.

Point mutation

A change in a single nucleotide pair of a gene.

Missense mutation

Substitution that changes one amino acid to another; may or may not affect protein function.

Nonsense mutation

Changes a codon to a stop codon, causing premature termination of translation.

Frameshift mutation

Results from insertion or deletion of nucleotides that changes the reading frame, usually producing a nonfunctional protein.

Silent mutation

Changes a nucleotide but not the amino acid sequence due to redundancy in the genetic code.

Mutation impact ranking (greatest to least)

Frameshift → Nonsense → Missense → Silent; frameshift alters all downstream amino acids, nonsense truncates the protein, missense changes one amino acid, silent has no effect.