Biology Chapter 14: Gene to Protein

Gene Expression and Protein Synthesis

• Gene expression involves synthesizing proteins or functional RNAs from DNA information.
• Archibald Garrod linked "inborn errors of metabolism" to a person's inability to produce specific enzymes.
• Alkaptonuria is a condition illustrating Garrod's hypothesis, leading to black urine due to alkapton accumulation.

Beadle and Tatum's Research

• George Beadle hypothesized that eye color mutations in Drosophila inhibit pigment synthesis by blocking enzyme production.
• Beadle and Tatum used Neurospora to identify mutants with altered nutritional needs after bombarding it with X-rays.

Organisms and Mutations

• Wild-type Neurospora can grow on minimal media, allowing the identification of mutants unable to synthesize essential molecules.
• X-ray treatment increased mutation rates in Neurospora spores.
• Significant findings from Beadle and Tatum included connections between metabolic defects and defective genes, supporting the one gene-one enzyme hypothesis, later revised to one gene-one polypeptide due to protein structure complexities.

RNA vs. DNA

• RNA differs from DNA: ribose sugar vs. deoxyribose, uracil vs. thymine, and typically single-stranded vs. double-stranded.
• Monomers of nucleic acids are nucleotides; proteins are comprised of amino acids.

Transcription and Translation Processes

• Transcription creates RNA from a DNA template; translation synthesizes polypeptides from mRNA.
• Transcription occurs in the nucleus, while translation occurs in the cytoplasm.
• Pre-mRNA in eukaryotes is known as the primary transcript.
• The central dogma of genetics is DNA to RNA to protein.

Codons and Genetic Code

• Four nucleotide bases exist but code for 20 amino acids.
• It takes three nucleotides to code for each amino acid, resulting in 64 unique triplets (61 code for amino acids, 3 serve as stop signals).
• The start codon is AUG, while UAA, UAG, and UGA are stop codons.
• The genetic code is redundant yet unambiguous.

Ribosomes and Protein Synthesis

• Ribosomes consist of large and small subunits made of rRNA and proteins.
• Prokaryotic ribosomes are smaller and structurally different from eukaryotic, allowing antibiotics to target bacterial synthesis without affecting eukaryotic cells.
• The ribosome has A, P, and E sites for tRNA attachment: A site for new tRNA, P site for growing polypeptide, E site for discharged tRNA.

Stages of Translation

• Translation involves initiation, elongation, and termination.
• Initiation begins with the small ribosomal subunit binding to mRNA, with the initiator tRNA carrying methionine (Met) binding to the start codon AUG.
• Elongation involves tRNA base-pairing with mRNA codons, forming peptide bonds between amino acids.
• Termination occurs when a release factor binds to the stop codon, releasing the completed polypeptide.

Post-Translational Modifications

• Proteins undergo modifications post-translation, including folding, chemical modifications, and assembly into quaternary structures.
• Types of modifications involve chemical alterations, removal of initial amino acids, and subunit assembly for multi-chain proteins like hemoglobin.

Splicing and mRNA Processing

• RNA processing involves adding a 5' cap and a poly-A tail, promoting mRNA stability and ribosome attachment.
• Introns are noncoding sequences, while exons are coding sequences that are expressed.
• snRNPs and spliceosomes play crucial roles in recognizing splice sites and removing introns.
• Ribozymes are RNA molecules that can catalyze reactions, showcasing that not all biological catalysts are proteins.

Mutations

• A mutation is a change in nucleotide sequence in DNA or RNA.
• Point mutations involve a single nucleotide change, while frameshift mutations insert or delete nucleotides, altering the reading frame. ### Frameshift Mutation
• A frameshift mutation occurs due to insertion or deletion of nucleotides in a gene.
• The number of nucleotides involved is not a multiple of three.
• This mutation causes improper grouping of the remaining nucleotides into codons, impacting protein synthesis.

Nonsense vs. Missense Mutation

• Nonsense mutation converts an amino acid codon into a stop codon, leading to a truncated and often nonfunctional protein.
• Missense mutation results from a nucleotide-pair substitution, resulting in a codon that codes for a different amino acid.

Silent Mutation Mechanism

• Nucleotide-pair substitution can create a codon that translates to the same amino acid.
• Silent mutations do not produce any observable changes in phenotype.

Categories of Mutagens

• Mutagens are classified into two main types: physical and chemical.

Action of Chemical Mutagens

• Some chemical mutagens resemble normal DNA nucleotides but mispair during DNA replication.
• Certain chemicals integrate into DNA, distorting the double helix structure and hindering replication.
• Additional chemicals can chemically alter bases, changing their pairing properties and potentially leading to mutations.

Differences in Gene Expression: Bacteria vs. Eukaryotes

• Transcription termination varies between bacterial and eukaryotic cells.
• Bacteria can concurrently transcribe and translate genes due to the absence of a nucleus, allowing swift protein function.
• Eukaryotic gene expression is separated by the nuclear envelope, which segregates transcription from translation and facilitates extensive RNA processing, enhancing cellular regulation.

Definition of a Gene

• A gene is defined as a DNA region capable of being expressed to yield either a polypeptide or an RNA molecule, expanding on the previous notion that one gene codes solely for one polypeptide.

This quiz focuses on key concepts from Chapter 14 of biology regarding gene expression and its role in protein synthesis. Test your knowledge on important terms and theories presented in this chapter, including insights from Archibald Garrod on metabolism. Prepare to explore the mechanisms that bridge DNA and functional proteins.