Ch 17

The Connection between Genes and Proteins

• Archibald Garrod's Contribution
    - Reasoned that genes dictate phenotypes through enzymes.
    - Suggested that a malfunction in a gene could lead to a metabolic disorder, exemplified by alkaptonuria.

• Beadle and Tatum's Experiments
    - Utilized the fungus Neurospora crassa to demonstrate how genes control metabolic pathways.
    - Showed that specific mutations resulted in the inability to synthesize certain compounds, supporting the idea that genes control individual steps in metabolism.

• The One Gene Hypotheses
    - One Gene-One Enzyme Hypothesis: Proposed by Beadle and Tatum, stating that each gene corresponds to a specific enzyme.
    - Later revised to One Gene-One Polypeptide Hypothesis to reflect that not all proteins are enzymes, and genes may also dictate polypeptides that are not enzymes.

• Differences between RNA and DNA
    - RNA contains ribose sugar, while DNA contains deoxyribose.
    - RNA is usually single-stranded, whereas DNA is double-stranded.
    - RNA has uracil (U) instead of thymine (T), which is present in DNA.

• Information Flow from Gene to Protein
    - The central dogma describes the flow: DNA → RNA → Protein.
    - This process involves transcription (DNA to RNA) followed by translation (RNA to protein).
    - There are exceptions and cases where the central dogma is violated, such as in retroviruses (e.g., HIV) where RNA can form DNA through reverse transcription.

• Transcription vs. Translation
    - Transcription: The synthesis of RNA from a DNA template.
    - Translation: The synthesis of proteins from an mRNA template.

• Location of Processes
    - In bacteria, both transcription and translation occur in the cytoplasm simultaneously.
    - In eukaryotes, transcription occurs in the nucleus while translation occurs in the cytoplasm.

Codons and the Genetic Code

• Definition of a Codon
    - A codon is a sequence of three nucleotides in mRNA that specifies a particular amino acid.

• Codon-Amino Acid Relationship
    - The linear sequence of codons on mRNA directly corresponds to the linear sequence of amino acids in a polypeptide during translation.

• Techniques for Identifying Amino Acids
    - Early experiments by Nirenberg and Matthaei used synthetic mRNA to identify which codons corresponded to which amino acids. For example, the codon UUU was determined to specify phenylalanine.

• Initiation with Methionine
    - Polypeptides begin with methionine (AUG codon) as it is the start codon recognized by the ribosome during translation initiation.

• Genetic Code Properties
    - The genetic code is redundant, meaning multiple codons can specify the same amino acid.
    - It is unambiguous; each codon corresponds to only one amino acid.

• Significance of Reading Frame
    - The reading frame during translation is critical since codons must be read in groups of three for accurate protein synthesis. Changing the reading frame via insertions or deletions can drastically alter the resulting protein product.

• Evolutionary Significance
    - A nearly universal genetic code illustrates common ancestry and facilitates gene transfer among diverse organisms.

RNA Synthesis and Processing

• Roles of RNA Polymerase and Transcription Regulation
    - RNA polymerase binds to the promoter region of a gene to initiate transcription.
    - The terminator signals the end of transcription.
    - The region between the promoter and terminator is known as the transcription unit.

• Transcription Process
    - The process of transcription involves three major steps:
      1. Initiation: RNA polymerase binds to the promoter and unwinds the DNA.
      2. Elongation: RNA polymerase synthesizes RNA, adding nucleotides complementary to the DNA template strand.
      3. Termination: Transcription ends when RNA polymerase reaches the terminator region.

• Post-Transcriptional Modifications in Eukaryotes
    - Eukaryotic pre-mRNA undergoes modifications:
      - A 5' cap is added to the beginning of mRNA for stability and ribosome recognition.
      - A poly-A tail is added to the 3' end, enhancing stability and facilitating export from the nucleus.

• Role of Ribozymes
    - Ribozymes are RNA molecules that catalyze biochemical reactions, including RNA splicing.
    - Properties that enable ribozymes to function:
      - Ability to form three-dimensional structures through self-pairing.
      - Certain bases within RNA have functional groups that enhance catalytic activity.
      - Capable of forming hydrogen bonds with other nucleic acid strands.

• Introns and Exons
    - Introns are non-coding regions that are removed during RNA splicing to produce a mature mRNA.
    - Exons are coding regions that are joined together to create a continuous coding sequence.
    - Alternative splicing allows a single gene to code for multiple proteins, increasing protein diversity beyond the total number of genes.

The Synthesis of Protein

• Transfer RNA (tRNA)
    - tRNA carries amino acids to ribosomes, where protein synthesis occurs.
    - Each tRNA has a specific anticodon that base-pairs with the corresponding codon on mRNA.
    - The structure of tRNA looks like a cloverleaf in a two-dimensional view and folds into an L-shaped three-dimensional molecule.

• Wobble Hypothesis
    - Refers to the flexibility in base-pairing at the third position of a codon, allowing some tRNAs to pair with multiple codons.

• tRNA Amino Acid Activation
    - Each amino acid is linked to its corresponding tRNA by an enzyme called aminoacyl-tRNA synthetase, ensuring accurate translation.

• Ribosome Structure and Function
    - Composed of two subunits (large and small) comprised of ribosomal RNA (rRNA) and proteins.
    - Has three binding sites for tRNA:
      - P site: holds tRNA with the growing polypeptide chain.
      - A site: holds tRNA with the next amino acid to be added.
      - E site: exit site where discharged tRNAs leave the ribosome.

• Translation Process
    - Initiation: The small ribosomal subunit binds to mRNA and the initiator tRNA (with methionine) before the large ribosomal subunit joins, forming the initiation complex.
    - Elongation: tRNAs carry amino acids to ribosomes, creating polypeptide chains through peptide bonds.
       - Steps in elongation include codon recognition, peptide bond formation, and translocation facilitated by elongation factors.
    - Termination: Occurs when a release factor binds to the A site, triggering the release of the completed polypeptide chain.

• Polyribosomes
    - Many ribosomes can translate a single mRNA simultaneously, enhancing protein synthesis efficiency.

• Protein Folding and Post-Translational Modifications
    - Newly synthesized polypeptides undergo folding into functional 3D shapes, often assisted by chaperones.
    - Post-translational modifications include cleavage, addition of chemical groups, or assembly into functional complexes.

• Ribosome Location
    - Free ribosomes synthesize proteins that function in the cytosol, while bound ribosomes synthesize proteins destined for the endomembrane system or secretion. Ribosomes can switch between free and bound states based on the protein being synthesized.
    - Proteins meant for the endoplasmic reticulum (ER) are marked by a signal peptide, recognized by a signal-recognition particle (SRP) that guides the ribosome to the ER.

Genetic Mutations and Their Effects

• Definition and Types of Point Mutations
    - A mutation is a change in the DNA sequence.
    - Point mutations involve changes in a single nucleotide, potentially leading to altered proteins.
    - Two major types include:
       - Base-pair substitutions: one nucleotide and its partner are replaced (e.g., G to A).
       - Base-pair insertions or deletions: additional or fewer nucleotides are added or removed, which can cause frameshift mutations.

• Consequences of Mutations
    - Missense mutations result in a different amino acid, potentially affecting protein function.
    - Nonsense mutations convert an amino acid codon to a stop codon, leading to truncated and often nonfunctional proteins.
    - Insertions and deletions typically have broader and more deleterious effects than substitutions due to frameshifting.

• Examples of Mutations
    - Various examples demonstrate silent mutations, missense mutations, and nonsense mutations, showing how they affect protein synthesis.

• Mutagens and Spontaneous Mutations
    - Mutagens are physical or chemical agents that induce mutations.
    - Spontaneous mutations arise from errors during DNA replication or repair processes.