DNA, RNA, and Protein Synthesis

Genes are functional regions of DNA

DNA stores biological instructions in the order of four bases: A, T, C, and G. A gene is a section of DNA used to make a functional RNA or protein. A chromosome is one long DNA molecule packaged with proteins.

Many proteins affect cell structure or chemical reactions. This creates a common path: DNA information leads to RNA, and RNA helps direct protein production.

A DNA nucleotide has three parts: deoxyribose sugar, phosphate, and one nitrogen base. Nucleotides join into a sugar-phosphate backbone. The bases point inward and pair A–T and C–G. This pairing lets either strand guide a matching copy.

Keep levels distinct. A chromosome is a packaged DNA molecule containing many genes. A gene is a DNA region whose expression produces a functional RNA or contributes to a protein product. A codon is only three bases in mRNA. One chromosome is not one gene, and one codon is not a whole protein instruction.

Replication copies DNA semiconservatively

Before a cell divides, replication copies its DNA. The two strands separate. Each old strand guides a new strand. A pairs with T, and C pairs with G.

Each finished DNA molecule has one old strand and one new strand. Enzymes proofread much of the new DNA, but an occasional change can remain.

Several enzymes cooperate during replication. Helicase separates the old strands. DNA polymerase adds matching DNA nucleotides and proofreads. Ligase joins short DNA pieces on the strand built in sections. Proofreading lowers errors but cannot remove every one.

Worked example. If an old strand reads A-C-G, the new DNA section reads T-G-C.

Transcription makes RNA

Transcription makes an RNA copy of a gene. RNA uses U instead of T, so DNA A pairs with RNA U. In eukaryotic cells, transcription occurs in the nucleus.

The finished messenger RNA, or mRNA, carries the instructions to a ribosome.

Worked example. DNA template TAC gives mRNA AUG because T pairs with A, A with U, and C with G.

Label the strand before converting. mRNA is complementary to the DNA template strand, but matches the coding strand except U replaces T. Template TAC gives AUG; coding ATG also corresponds to AUG when the sequences are shown in the stated directions.

Ribosomes translate codons

Translation happens at a ribosome. The ribosome reads mRNA three bases at a time. Each three-base word is a codon. Transfer RNA brings the matching amino acid.

The ribosome links amino acids into a chain. A start codon begins translation, and a stop codon ends it. The chain then folds into a working protein.

Each tRNA carries an amino acid and has an anticodon that pairs with an mRNA codon. If the anticodon cannot pair, the matching amino acid is not delivered efficiently. A stop codon does not name an amino acid; it tells a release factor to end the chain.

Worked example. For mRNA AUG-GCU-UAA, a codon chart gives methionine–alanine–stop.

Decode in a fixed sequence. Read mRNA from 5′ to 3′, separate it into codons from the stated start point, consult an mRNA codon chart, and stop at a stop codon. Anticodons belong to tRNA and pair complementarily with mRNA codons; a codon chart normally expects the mRNA codon, not the DNA triplet or tRNA anticodon.

Sequence changes can alter products

A mutation is a change in DNA. A substitution changes one base. An insertion adds a base, and a deletion removes one. Insertions and deletions can shift how later codons are read.

A mutation may change one amino acid, stop a protein early, or have no effect because more than one codon can name the same amino acid. Its effect depends on where the change occurs.

Changing one base is a substitution. It can be silent, change one amino acid, or create a stop. Adding or removing one base shifts the reading frame and can change many later codons. Adding or removing exactly three bases may add or remove one amino acid without shifting later codons.

Example: deleting one base near the start usually has a larger effect than a silent substitution near the end because more later codons can change.

Summary

• Genes are sections of DNA on chromosomes.
• Replication copies DNA before division.
• Transcription makes RNA from DNA.
• Translation reads mRNA codons at a ribosome.
• Mutations may change a protein or may have no effect.

DNA stores, RNA carries, ribosomes build. This is the usual flow, though some viruses can copy RNA back into DNA.

Sources

• OpenStax Anatomy and Physiology 2e
• National Human Genome Research Institute educational resources
• NIH National Institute of General Medical Sciences science education
• NHGRI — Transcription and Translation Lesson Plan
• NHGRI Genetics Glossary — Anticodon