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🧬 Science · Biology · Lesson 4 of 7 · 6 min

DNA, RNA and protein synthesis

🧬 A single wrong letter in a gene can cause sickle cell disease. Here is how the cell reads DNA to build every protein you need.

💡 Key idea

DNA is transcribed to mRNA in the nucleus, and mRNA is translated to protein at the ribosome, one codon (3 bases) per amino acid.

🧠 The idea

DNA is a double helix of NUCLEOTIDES, each made of a deoxyribose sugar, a phosphate and a base. Bases pair by hydrogen bonds: adenine with thymine (A-T) and guanine with cytosine (G-C). RNA is single-stranded, uses the sugar ribose and replaces thymine with uracil (U). The central dogma is DNA to RNA to protein. In TRANSCRIPTION, inside the nucleus, RNA polymerase reads a DNA template and builds messenger RNA (mRNA). In TRANSLATION, at a ribosome in the cytoplasm, the mRNA is read three bases at a time; each three-base CODON names one amino acid. Transfer RNA (tRNA) brings the amino acid using its matching anticodon, and the ribosome links amino acids into a polypeptide. AUG is the start codon; UAA, UAG and UGA are stop codons. A MUTATION is a change in DNA sequence: a substitution changes one base, while an insertion or deletion can cause a frameshift.

✏️ Worked example

A DNA template strand reads 3'-TAC GGA-5'. Find the mRNA. Step 1: pair each base, using U instead of T: T→A, A→U, C→G, G→C, G→C, A→U. Step 2: the mRNA is 5'-AUG CCU-3'. Step 3: AUG is the start codon (methionine), and CCU codes for proline, so this message begins a protein Met-Pro.

📌 Takeaways

  • DNA: A-T and G-C. RNA: A-U and G-C, with ribose instead of deoxyribose.
  • Transcription (DNA to mRNA) happens in the nucleus; translation (mRNA to protein) at ribosomes.
  • Insertions and deletions shift the reading frame and usually do more damage than one substitution.

📖 Terms in this lesson

Transcription: The process of copying a gene's DNA into messenger RNA, which takes place in the nucleus; for example, the first step of protein synthesis.

Translation: The process in which a ribosome reads messenger RNA and links amino acids into a protein; for example, the second step of protein synthesis.

✅ Check yourself

Question 1 of 3

If one strand of DNA reads ATGC, what is the complementary DNA strand, paired base by base?

  1. CGTA
  2. ATGC
  3. TACG
  4. UACG
Show the answer and rationale

Answer: C

In DNA, A pairs with T and G pairs with C, so ATGC pairs with TACG. UACG uses uracil, which belongs in RNA, not DNA.

Question 2 of 3

Where does translation take place?

  1. In the nucleus
  2. In the lysosome
  3. At the ribosome
  4. In the Golgi apparatus
Show the answer and rationale

Answer: C

Translation happens at ribosomes, where mRNA codons are read and amino acids are joined. Transcription, not translation, happens in the nucleus, and the Golgi packages finished proteins.

Question 3 of 3

How many nucleotides make up one codon?

  1. One
  2. Three
  3. Four
  4. Two
Show the answer and rationale

Answer: B

A codon is three consecutive mRNA bases that specify one amino acid. One or two bases could not code for all 20 amino acids.

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