Protein Structure, Gene-Protein Connection, and Mutations in Genetics

Protein Structure and Amino Acids

Amino Acids: Subunits of Proteins

Amino acids are the building blocks of proteins. Each amino acid contains a central carbon atom (the alpha carbon), an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R-group) that determines its chemical identity.

• R-groups: Responsible for the chemical properties and identity of each amino acid.

• Classification:

  • Nonpolar (hydrophobic): e.g., Glycine, Alanine, Valine

  • Polar (hydrophilic): e.g., Serine, Threonine

  • Positively charged (basic): e.g., Lysine, Arginine, Histidine

  • Negatively charged (acidic): e.g., Aspartic acid, Glutamic acid

• Peptide Bonds: Amino acids are linked by peptide bonds formed between the carboxyl group of one amino acid and the amino group of another.

Example: The sequence and chemical properties of amino acids determine protein structure and function.

Levels of Protein Structure

• Primary Structure: The linear sequence of amino acids in a polypeptide chain.

  • Denoted as: Primary level = amino acid sequence

• Secondary Structure: Local folding of amino acids within certain sections of the protein, stabilized by hydrogen bonds.

  • Common structures: Alpha helices and beta sheets

• Tertiary Structure: The overall 3D conformation of a single polypeptide chain.

• Quaternary Structure: The association of multiple polypeptide subunits, held together by noncovalent bonds.

Example: Hemoglobin is a protein with quaternary structure, consisting of multiple polypeptide chains.

Gene-Protein Connection

How Are Proteins Connected with Genes?

Genes encode the information for synthesizing proteins. The process involves transcription of DNA into mRNA and translation of mRNA into a polypeptide chain.

• One gene-one enzyme hypothesis: Each gene encodes a single enzyme (or protein), as demonstrated by Beadle and Tatum's experiments with mold (Neurospora).

• Phenylketonuria (PKU): An autosomal recessive disorder caused by mutations in the gene encoding phenylalanine hydroxylase, leading to intellectual disability if untreated.

Example: PKU illustrates how a single gene mutation can disrupt a metabolic pathway.

Gene Function Studies in Mold

• Prototrophs: Wild-type strains that can grow on minimal medium.

• Auxotrophs: Mutant strains that require supplementation with specific nutrients (e.g., amino acids) to grow.

• Beadle and Tatum's Findings:

  • Minimal medium supplemented with amino acids can restore growth in auxotrophs.

  • The supplement that restores growth identifies the metabolic block caused by the mutation.

Genetics and Biochemical Pathways

Genetic analysis can determine the order of biochemical steps in a pathway by studying mutants and their growth requirements.

Enzyme	Gene	Step in Pathway
A	arg1	Precursor → Ornithine
B	arg2	Ornithine → Citrulline
C	arg3	Citrulline → Arginine

Example: The arg1 mutant can be rescued by arginine, indicating its block is upstream in the pathway.

Mutations: Types and Effects

Types of Mutations

• Insertions and Deletions (Indels): Addition or removal of one or more nucleotides.

• Point Mutations (Substitutions): Change of a single nucleotide.

  • Transition: Purine for purine (A↔G) or pyrimidine for pyrimidine (C↔T)

  • Transversion: Purine for pyrimidine or vice versa

Outcomes of Point Mutations

• Nonsense Mutation: Changes a codon to a stop codon, terminating translation prematurely.

• Missense Mutation: Changes one amino acid to another.

  • Synonymous: Similar chemical properties

  • Non-synonymous: Different chemical properties

• Silent Mutation: Changes the nucleotide sequence without altering the amino acid (due to code redundancy).

Example: A silent mutation in the third position of a codon often does not change the encoded amino acid.

Mutation Effects on Genes

• Mutations can affect gene function by altering protein structure or expression.

• Frameshift mutations (indels not in multiples of three) can disrupt the reading frame, leading to nonfunctional proteins.

Classifying Mutations by Function

• Mutations are a source of genetic variation and the basis for natural selection.

• Some mutations cause disease.

• Mutations can be inherited if they occur in germline cells.

Mechanisms and Causes of Mutations

How Do Mutations Occur?

• Errors during DNA replication, transcription, or translation.

• Human error rates:

  • Translation: $1 \times 10^{-3}$ to $1 \times 10^{-4}$ amino acids

  • Transcription: $1 \times 10^{-4}$ to $1 \times 10^{-5}$ nucleotides

  • Replication: $1 \times 10^{-10}$ nucleotides (with proofreading)

  • ~3 x 109 nucleotides in the human genome

Spontaneous vs. Induced Mutations

• Spontaneous Mutations: Occur naturally due to errors in biological processes (e.g., replication errors, tautomeric shifts).

• Induced Mutations: Caused by external factors such as chemicals or radiation.

Mechanisms of Spontaneous Mutations

• Replication Errors: Wrong nucleotide inserted, slippage in repeat regions.

• Tautomeric Shifts: Cause anomalous base pairing (e.g., A pairs with C).

• Depurination: Loss of a purine base, leading to random nucleotide insertion.

• Deamination: Conversion of amino group to keto group (e.g., cytosine to uracil).

Transposable Elements

• Mobile DNA sequences that can insert into new locations, causing mutations.

Inheritance and Location of Mutations

• Somatic Mutations: Occur in non-reproductive cells; not inherited.

• Germline Mutations: Occur in reproductive cells; can be passed to offspring.

Additional info: These notes cover material relevant to Chapters 12, 13, and 14 of a college genetics course, including gene-protein relationships, protein structure, and mutation mechanisms.