Protein Synthesis and Mutations Lecture

Location and Initial Process: Protein synthesis begins in the nucleus of the cell with the creation of mRNA from a DNA template. This specific phase is identified as transcription.

Step 1: Unwinding and Unzipping: The DNA double helix must first unwind and the two strands must separate or "zip" open.
Step 2: Complementary Base Pairing: Nucleotides are added to the template strand following the rules of base pairing.
Step 3: Backbone Formation (Ligase): The sugar-phosphate backbone of the new strand is sealed, often involving the enzyme ligase.
Step 4: Splicing: The resulting molecule is processed or "cut" to produce the final messenger RNA ( $mRNA$ ).
Final Output: The process converts DNA into mRNA, which then exits the nucleus to be read by the ribosomes in groups of three.

Structure of mRNA: The mRNA is read in sets of three nucleotides, each set known as a codon.
Step 1: Initiation: The process begins when the ribosome identifies a start codon. The specific sequence sought is $AUG$ .
Step 2: Elongation:
- Transfer RNA ( $tRNA$ ) molecules bring amino acids to the ribosome.
- The $tRNA$ molecule contains a sequence at the bottom called an anticodon, which is the complementary base pair to the mRNA codon. For example, if the mRNA codon is $AUG$ , the matching $tRNA$ anticodon is $UAC$ .
- Peptide Bonds: As amino acids are brought in, they are linked together by peptide bonds to form a polypeptide chain.
Step 3: Termination: The process continues until the ribosome reaches a stop codon. At this point, the polypeptide chain and the ribosomal complex break apart.

Transcription (Language to Self): This is compared to an English teacher asking a student to transcribe Shakespeare. While the style might change (Old English to Modern English), the language remains English. In biological terms, this is going from a nucleotide (DNA) to a nucleotide (mRNA).
Translation (Language to Language): This is compared to translating a paragraph from French to English. It involves moving to a completely different "language." In biology, this is the transition from a nucleotide sequence (mRNA) to an amino acid sequence (protein).

Definition: A mutation is a permanent change in the genes or the DNA sequence.
Implications for Structure: The sequence of DNA determines the sequence of amino acids. Because the amino acid sequence determines the shape of the protein, a change in DNA can fundamentally alter the protein's function.
Inheritance (Somatic vs. Germline):
- Somatic Body Cells: These include eye, skin, heart, and muscle cells. Mutations in these cells affect the individual but are not passed on to offspring.
- Germline Cells (Gametes): These are sperm or egg cells. Mutations occurring here can be inherited by future generations.

Replication Errors: Errors occur when DNA is being copied. While rare ( $1 \text{ in every billion}$ copies) due to the proofreading function of DNA polymerase, they do happen.
Mutagens: These are environmental factors that cause mutations. Examples include:
- Radiation (UV light, X-rays).
- Chemicals (Pesticides, food additives, certain chips/processed foods).
- Enzymes in the body attempt to repair this damage, making mutagen-induced mutations relatively rare compared to other types.
Transposons ("Jumping Genes"): Considered the most common cause of mutations. These are DNA sequences that spontaneously move from one location in the genome to another, altering the sequence permanently. The exact cause of this "jumping" is unknown.

Point Mutation (Substitution): This involve a single nucleotide being replaced by another.
- Silent Mutation: A change in the nucleotide that does not result in a change to the amino acid sequence.
- Nonsense Mutation: A change that causes the sequence to hit a stop codon prematurely.
- Missense Mutation: A change that results in the substitution of one amino acid for another, completely changing the sequence.
Frameshift Mutation: This occurs when a nucleotide is added (insertion) or removed (deletion). Because the ribosome reads DNA in "frames" of three (codons), adding or removing a base shifts the entire reading frame for all subsequent codons. Frameshift mutations generally cause more significant changes than point mutations.

Mechanism: A point mutation changes a single amino acid in hemoglobin. Specifically, a polar Glutamate is replaced by a non-polar Valine/Lysine variant.
Result: This single change in polarity causes the protein to hide inside itself to avoid water, changing the red blood cell from a "donut" shape to a "sickle" shape. This drastically affects the cell's ability to carry oxygen.

Scenario: A casino robbery in 2025 involving three murders. The perpetrator left a blood sample on a shard of glass.
Objective: Use DNA sequencing logic to identify characteristics of the perpetrator.
Biological Markers:
- Male Sequence: Methionine, Leucine, Proline.
- Female Sequence: Methionine, Leucine, Leucine.
Task Steps:
1. Transcribe the DNA sample into mRNA.
2. Translate the mRNA into an amino acid sequence.
3. Highlight the start and stop codons.
4. Use the resulting phenotypic traits (e.g., eye color, hair color, sex) to draw a portrait of the suspect.

Question (Student): What is the monomer for mRNA?
Response (Instructor): It is still a nucleotide. Whether it is DNA or mRNA, the monomer is a nucleotide.
Discussion on X-rays: The instructor noted cultural differences in safety standards. In Australia, patients are sometimes given a protective cloth for the pelvic region during dental X-rays, whereas in other regions, a full lead vest is standard to block mutagens.
Discussion on Skin Cancer: In Australia, the thin ozone layer leads to high UV exposure. Skin cancer is so common there that residents often treat its removal as a routine, non-jarring medical procedure.