How a gene in DNA becomes a protein
TEAS 7 outline: Relationship between genetic material and protein structure
Also covers HESI A2 Biology (DNA)
- DNA pairs A–T and G–C; RNA uses U in place of T.
- Replication is semiconservative: each new DNA keeps one original strand.
- Transcription makes mRNA in the nucleus; translation builds protein at ribosomes.
- A codon is three bases; AUG starts, and UAA, UAG, UGA stop.
- A frameshift usually does more harm than a single base swap.
Swipe the picture sideways to see all of it.
A gene is a stretch of DNA. In the nucleus, RNA polymerase binds the promoter, a starting signal at the beginning of the gene.
Key facts
- A nucleotide has three parts: a sugar, a phosphate group, and a nitrogenous base; DNA uses deoxyribose and RNA uses ribose. A and G are double-ring purines; C, T, and U are single-ring pyrimidines.
- In DNA, A pairs with T (two hydrogen bonds) and G pairs with C (three hydrogen bonds); in RNA, uracil (U) replaces thymine and pairs with A.
- DNA is a double-stranded helix with antiparallel strands and a sugar-phosphate backbone; RNA is usually single-stranded.
Show all 7 key factsShow fewer
- Replication is semiconservative: each new DNA molecule has one original strand and one new strand.
- Transcription (DNA to mRNA) happens in the nucleus; the mRNA is complementary to the template strand and matches the coding strand, with U in place of T. Translation (mRNA to protein) happens at ribosomes in the cytoplasm.
- A codon is three mRNA bases; 61 of the 64 codons code for amino acids, AUG is the start codon (methionine), and UAA, UAG, and UGA are stop codons.
- A substitution can be silent, missense, or nonsense; inserting or deleting a base (not in a multiple of three) causes a frameshift that usually ruins the protein.
Quick check1 question · 2 more at the end
Which feature is found in RNA but not in DNA?
Correct answer: B.
DNA uses the bases A, T, G, and C, while RNA uses A, U, G, and C: uracil takes the place of thymine and pairs with adenine. RNA also has ribose instead of deoxyribose and is usually single-stranded. Both nucleic acids have a backbone of sugars linked by phosphate groups.
A Both DNA and RNA have a sugar-phosphate backbone, so phosphate groups are not unique to RNA.
B Correct: uracil is found in RNA, where it replaces DNA's thymine.
C Cytosine is found in both DNA and RNA; uracil replaces thymine, not cytosine.
D Deoxyribose is DNA's sugar; RNA contains ribose, so this reverses the two molecules.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: A gene's DNA sequence is transcribed by RNA polymerase into messenger RNA in the nucleus, with uracil in place of thymine. The mRNA travels to a ribosome in the cytoplasm, where tRNAs read it three bases (one codon) at a time and link the matching amino acids into a protein. A change in the DNA can change that protein.
1 What are DNA and RNA made of?Each nucleotide has three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. 1 min
Both are chains of nucleotides. Each nucleotide has three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. The sugars and phosphates link up to form the backbone. In DNA, the bases of the two strands pair up in the middle like the rungs of a ladder.
DNA has four bases: adenine (A), guanine (G), cytosine (C), and thymine (T). A and G are purines, with a double ring; C and T (and RNA's U) are pyrimidines, with a single ring. Each pair joins one purine to one pyrimidine.
The two strands are held together by hydrogen bonds between paired bases. The pairing is strict: A always with T (two hydrogen bonds), G always with C (three hydrogen bonds). So if you know one strand, you know the other.
DNA's two strands twist into a double helix and run in opposite directions, which is called antiparallel.
| DNA | RNA | |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Strands | Two (double helix) | Usually one |
| Bases | A, T, G, C | A, U, G, C |
| Main job | Stores genetic information | Helps build proteins |
RNA comes in three main working types:
- mRNA carries the gene's message to the ribosome.
- tRNA brings amino acids.
- rRNA is a main building block of the ribosome.
2 How does DNA copy itself?A cell copies its DNA before it divides. 1 min
A cell copies its DNA before it divides. Enzymes including helicase untwist the helix and separate the two strands. DNA polymerase then adds free nucleotides that pair with the bases on each old strand (A with T, G with C). This builds a new partner strand for each one.
Because each old strand serves as a template, every new DNA molecule ends up with one original strand and one new strand. That is called semiconservative replication.
3 What happens in transcription?Transcription copies one gene from DNA into messenger RNA (mRNA). 1 min
Transcription copies one gene from DNA into messenger RNA (mRNA). In eukaryotes it happens in the nucleus.
- RNA polymerase binds a starting signal on the DNA called the promoter.
- It reads one DNA strand, the template strand, and builds a complementary RNA strand. Wherever the template has A, the RNA gets U instead of T. The finished RNA therefore matches the other DNA strand, the coding strand, except that it has U where the coding strand has T.
- At the end of the gene, a terminator sequence ends transcription.
In eukaryotes, the first transcript is then edited. Non-coding pieces called introns are cut out, and the coding pieces (exons) are joined. The finished mRNA travels out of the nucleus into the cytoplasm.
4 What happens in translation?Translation turns the mRNA's base sequence into a chain of amino acids at a ribosome in the cytoplasm (free, or on the rough ER). 1 min
Translation turns the mRNA's base sequence into a chain of amino acids at a ribosome in the cytoplasm (free, or on the rough ER).
The mRNA is read in groups of three bases called codons. There are 64 codons: 61 stand for amino acids and 3 (UAA, UAG, UGA) are stop codons. AUG is the start codon and codes for methionine. The code is redundant: several codons can mean the same amino acid.
Each tRNA carries one specific amino acid and has a three-base anticodon that pairs with a matching codon. Step by step:
- The small ribosome subunit binds the mRNA, and the start tRNA pairs with AUG.
- The large subunit joins.
- tRNAs arrive one by one, and the ribosome links their amino acids with peptide bonds.
- At a stop codon, a release factor frees the finished chain, and the ribosome comes apart.
5 What happens when the DNA changes?A mutation is a change in the DNA sequence. 1 min
A mutation is a change in the DNA sequence. Its effect depends on what it does to the codons.
| Mutation | What changes | Effect on the protein |
|---|---|---|
| Silent | One base swapped; codon still means the same amino acid | No change |
| Missense | One base swapped; codon now means a different amino acid | One amino acid differs; may or may not matter |
| Nonsense | One base swapped; codon becomes a stop codon | Protein ends early |
| Frameshift | A base inserted or deleted | Every codon after it is misread; usually nonfunctional |
Frameshifts are usually the most damaging because the ribosome keeps reading in threes, so one missing base scrambles every group downstream.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Uracil replaces thymine in RNA, not cytosine; RNA still has A, G, and C.
Watch out: Transcription makes RNA from DNA and translation makes protein from RNA, not the other way around.
Watch out: The mRNA has the same sequence as the coding strand (with U for T), not the template strand it was copied from; the anticodon is on the tRNA, not the mRNA.
Watch out: A single-base deletion usually does more damage than a single-base substitution, because it shifts every codon after it.
Watch out: Semiconservative means each new double helix keeps one old strand, not that half the molecules are old and half are new.
How to remember it
Memory hook: "Transcribe the script, then translate the language." Transcription stays in the same language (nucleic acid to nucleic acid); translation switches languages (nucleic acid to protein).
Self-check:
- Write the RNA partner for the DNA bases A, T, G, C.
- Say where transcription and translation each happen in a human cell.
- Explain why deleting one base does more harm than swapping one base.