DNA, Protein Synthesis & Mutations Lesson 

Lesson Overview

• DNA Structure: The Blueprint of Life
• Transcription: DNA to RNA
• Translation: RNA to Protein
• RNA Types and Functions
• Prokaryotes vs. Eukaryotes: Genetic Differences
• Mutations: When the Code Changes
• Mutation Types and Effects
• Key Takeaway 

DNA holds the genetic code that directs all biological processes in living organisms. This lesson explores the structure of DNA, the mechanisms of protein synthesis (transcription and translation), and how mutations can impact these processes. Each section corresponds to key quiz questions, ensuring thorough preparation.

DNA Structure: The Blueprint of Life

Components of DNA:

• Nucleotide = Phosphate + Deoxyribose Sugar + Nitrogenous Base (A, T, G, C)
• Base Pairing Rules: A-T (2 hydrogen bonds), C-G (3 hydrogen bonds)

Key Terms:

• Purines: A, G
• Pyrimidines: C, T
• Backbone: Alternating sugar and phosphate molecules
• Hydrogen Bonds: Weak bonds holding complementary bases

Location: In eukaryotic cells, DNA is in the nucleus. In prokaryotes, DNA floats in the cytoplasm.

Chargaff's Rule: If Thymine = 19%, then Adenine = 19%, and Cytosine = Guanine = (100% - 38%)/2 = 31%

DNA Components and Functions

Transcription: DNA to RNA

Purpose: To create an mRNA copy of a gene for protein synthesis.

Steps:

• RNA polymerase binds to DNA at the promoter
• DNA unwinds; RNA polymerase reads the template strand (3'→5')
• Synthesizes mRNA (5'→3') using base pairing (A→U, T→A, C→G, G→C)

Product: Messenger RNA (mRNA)

Eukaryotic mRNA Processing:

• 5′ Cap: Ribosome binding site (needed for eukaryotic translation)
• Poly-A Tail: Stability and export from the nucleus
• Splicing: Removes introns, joins exons

Location: Transcription occurs in the nucleus of eukaryotes

Quiz Example: DNA: CCC GGG TTT ATA → mRNA: GGG CCC AAA UAU

Note: Transcription is not a type of RNA; it's a process. Types of RNA include mRNA, tRNA, rRNA.

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Translation: RNA to Protein

Purpose: Translate mRNA codons into a chain of amino acids (a protein)

Location: Occurs in the cytoplasm on ribosomes

Key Players:

• mRNA: Carries genetic code
• tRNA: Brings amino acids; has anticodon to match mRNA codon
• rRNA: Component of ribosomes

Process:

1. Ribosome binds to mRNA at the start codon (AUG)
2. tRNA brings Methionine to start the chain
3. Ribosome moves along mRNA, adding amino acids
4. Stop codon ends translation; polypeptide is released

Polyribosome: Multiple ribosomes translating one mRNA at once

Codons: Each 3-base sequence codes for an amino acid

• Start codon: AUG (Methionine)
• Stop codons: UAA, UAG, UGA

RNA Types and Functions

Translation Initiation:

• Eukaryotes: Requires 5′ cap
• Prokaryotes: Uses Shine-Dalgarno sequence

Prokaryotes vs. Eukaryotes: Genetic Differences

Both Prokaryotes and Eukaryotes: Use codons, anticodons, tRNA, mRNA, ribosomes, and DNA polymerase.

Mutations: When the Code Changes

Definition: Change in DNA sequence

Types of Mutations:

• Silent: Base changes but amino acid stays the same
• Missense: One amino acid is changed
• Nonsense: Creates a stop codon; shortens protein
• Frameshift: Insertion or deletion shifts reading frame

Frameshift vs. Point Mutation:

• Point (e.g., missense) = 1 amino acid affected
• Frameshift = changes entire downstream sequence

Quiz Insight: Deletions (frameshifts) are more damaging than substitutions because they change more of the protein

Mutation Types and Effects

Examples:

• "THE CAT ATE THE RAT" (original)
• Silent: "THE KAT ATE THE RAT" (still same meaning)
• Missense: "THE CAT ATE THE HAT" (different but logical)
• Nonsense: "THE CAT ATE" (incomplete)
• Frameshift: "HEC ATA TET HER AT..." (nonsense)

Key Takeaway 

• DNA contains genes that code for proteins via mRNA (transcription) and ribosomes (translation)
• Transcription occurs in the nucleus (eukaryotes), producing mRNA
• Translation occurs in the cytoplasm, producing proteins
• Prokaryotes can couple transcription and translation; eukaryotes cannot
• Mutations alter protein synthesis; frameshifts often have the greatest impact

By mastering these concepts, students can confidently approach questions on DNA, transcription, translation, and mutations.

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