DNA & Protein Synthesis

Why This Matters for Nursing: DNA contains the instructions for making proteins that run every body function. Understanding this helps you understand genetic diseases, how mutations cause problems, and how some medications (like antibiotics) work.

What You Need to Know

DNA Structure

DNA (Deoxyribonucleic Acid) is the molecule that stores genetic information.

Structure: Double helix (twisted ladder)

• Backbone: Sugar (deoxyribose) and phosphate
• Rungs: Base pairs

The Four Bases

Base Pairing Rules:

• A always pairs with T (A-T)
• G always pairs with C (G-C)

🧠 Memory Trick

"A-T, G-C" "Apples in Trees, Cars in Garages"

Or simply: "A and T are both tall letters"

DNA vs RNA: DNA has Thymine = "DNA stays home" (in the nucleus) RNA has Uracil = "RNA goes oUt" (leaves the nucleus)

DNA vs. RNA

Key: RNA has Uracil (U) instead of Thymine (T)

Protein Synthesis: The Central Dogma

DNA → RNA → Protein

"DNA makes RNA makes Protein"

Step 1: Transcription (DNA → RNA)

• Occurs in the nucleus
• DNA is "read" to make messenger RNA (mRNA)
• mRNA is a copy of the gene's instructions

Step 2: Translation (RNA → Protein)

• Occurs at ribosomes (in cytoplasm)
• mRNA is "translated" into a chain of amino acids
• Amino acid chain folds into a protein

Key Players

The Genetic Code

• The mRNA is read in groups of 3 bases called codons (the DNA sequence is first transcribed into mRNA)
• Each codon codes for one amino acid
• 64 codons total: 61 specify amino acids (about 20 amino acids, with redundancy) + 3 stop codons

Important Codons:

• AUG = Start codon (also codes for methionine)
• UAA, UAG, UGA = Stop codons (end translation)

✏️ Worked Examples

Example 1: Base Pairing

DNA strand: A-T-G-C-C-A Find the complementary strand.

Step 1 — Know the rules. DNA base pairing is non-negotiable: A always pairs with T. G always pairs with C. That's it. There are no other combinations.

Step 2 — Go base by base. Work through the strand one letter at a time:

• A → pairs with T
• T → pairs with A
• G → pairs with C
• C → pairs with G
• C → pairs with G
• A → pairs with T

Complementary strand: T-A-C-G-G-T

Memory check: A and T are "buddies." G and C are "buddies." They only pair with each other, never across groups.

Example 2: Transcription

DNA template strand: T-A-C-G-G-A What mRNA is produced?

Step 1 — Know the difference between DNA and RNA bases. DNA uses: A, T, G, C. RNA uses: A, U, G, C. The only change is that RNA has Uracil (U) where DNA has Thymine (T).

Step 2 — Apply the transcription rules. During transcription, RNA is built as the complement of the DNA template strand. The rules are the same as DNA base pairing — except wherever you'd write T in the new strand, write U instead (because this is RNA being made, not DNA).

Go base by base through the template:

• T → A (the RNA complement of T is A)
• A → U (the RNA complement of A is U — not T, because RNA uses U)
• C → G
• G → C
• G → C
• A → U

mRNA produced: A-U-G-C-C-U

Why does this matter? AUG is actually a start codon — it tells the ribosome "protein synthesis begins here." So this short sequence would kick off the translation process.

Example 3: Following the Process

Question: Where does transcription occur? Where does translation occur?

Step 1 — Think about what each process needs.

Transcription makes a copy of the DNA. Nuclear DNA generally stays in the nucleus (it's the master copy; a small, separate amount of DNA also resides in mitochondria). So transcription of nuclear genes happens in the nucleus, where that DNA is. The mRNA copy can then leave.

Translation turns that mRNA into protein. Translation happens at ribosomes. Ribosomes are out in the cytoplasm (or on the rough ER). So that's where translation happens.

Step 2 — Lock in the answers.

• Transcription: Nucleus (DNA → mRNA)
• Translation: Ribosome in cytoplasm (mRNA → Protein)

Answer: Transcription = Nucleus. Translation = Ribosome (cytoplasm).

Example 4: Clinical Connection

Question: Why do antibiotics that target bacterial ribosomes preferentially harm bacteria rather than the human cell's cytosolic protein-making machinery?

Step 1 — Understand the antibiotic's target. Some antibiotics (like streptomycin, erythromycin) work by attacking the ribosome — the machinery that builds proteins. No protein synthesis = bacteria can't survive.

Step 2 — Explain why we're not harmed. Bacterial ribosomes (called 70S) are structurally different from human ribosomes (80S). They're similar enough to do the same job, but different enough that the antibiotic can lock onto the bacterial version without fitting the human version.

Step 3 — Put it simply. It's like a key that fits the bacterial lock much better than the human cytosolic one, so the drug preferentially targets bacteria. The effect is selective, not absolute — side effects are still possible (human mitochondrial ribosomes resemble bacterial ones).

Answer: Bacterial ribosomes (70S) are structurally different from the human cytosolic ribosomes (80S) that make most of our proteins, so these antibiotics preferentially bind and disrupt the bacterial version. The effect is selective, not perfectly harmless — human mitochondrial ribosomes resemble bacterial ones, so side effects are still possible.

🏥 Nursing connection: This selectivity is why antibiotics can treat bacterial infections while largely sparing human cells — they exploit the structural differences — though adverse effects can still occur (partly because mitochondrial ribosomes are bacteria-like). Overuse or not completing therapy can promote resistant bacteria. Teach patients to take antibiotics exactly as prescribed.

Mutations

A mutation is a change in the DNA sequence.

Frameshift mutations are often more severe because they change every codon after the mutation. Note: adding or removing bases in a multiple of 3 (e.g., 3 or 6) does not shift the reading frame — it is an in-frame insertion/deletion instead.

💡 Pro Tips

• Nuclear DNA generally stays in the nucleus — that's why mRNA is needed as a messenger (a small amount of DNA also exists in mitochondria)
• A always pairs with T (or U in RNA), G always pairs with C
• Transcription = making a transcript (copy) → makes RNA
• Translation = translating language → RNA language to protein language
• On the TEAS: Focus on base pairing rules and where each process occurs

⚠️ Common Mistakes to Avoid

• Forgetting RNA uses U instead of T: DNA: A-T-G-C; RNA: A-U-G-C
• Confusing transcription and translation: Transcription = DNA→RNA; Translation = RNA→Protein
• Location errors: Transcription in nucleus; Translation at ribosomes
• Base pairing errors: A-T and G-C only (never A-G or T-C)

Quick Reference

Base Pairing

Protein Synthesis Summary

RNA Types

DNA and protein synthesis mastered! 💪 Next up: Biological Molecules