Click image to open full size
The flow of genetic information from DNA to protein is one of the central ideas in biology. DNA stores the instructions for building molecules that help cells function, grow, and respond to their environment. Those instructions are not used directly in most cases, so cells first copy a gene into RNA and then use that RNA to build a protein. Understanding this pathway helps explain inheritance, gene expression, and many diseases caused by mutations.
Transcription is the process that uses a DNA template to make messenger RNA, or mRNA. Translation is the process in which a ribosome reads the mRNA in groups of three bases called codons and links amino acids together in the correct order. Transfer RNA, or tRNA, helps match each codon with its amino acid using a complementary anticodon. The final amino acid chain folds into a protein whose shape determines its function.
Key Facts
- Central dogma: DNA -> RNA -> Protein
- Transcription makes mRNA from a DNA template strand using complementary base pairing.
- In RNA, uracil (U) replaces thymine (T), so DNA A pairs with RNA U and DNA C pairs with RNA G.
- Translation reads mRNA in codons, where 1 codon = 3 nucleotides = 1 amino acid or stop signal.
- Start codon: AUG, which codes for methionine and usually begins translation.
- A gene with n codons for amino acids produces a polypeptide with n amino acids; stop codons do not add an amino acid.
Vocabulary
- Transcription
- Transcription is the process of copying a gene's DNA sequence into a complementary mRNA sequence.
- Translation
- Translation is the process in which a ribosome reads mRNA codons and builds a polypeptide.
- Codon
- A codon is a sequence of three nucleotides in mRNA that specifies an amino acid or a stop signal.
- Ribosome
- A ribosome is a cell structure made of rRNA and proteins that carries out translation.
- tRNA
- tRNA is an RNA molecule that brings a specific amino acid to the ribosome and matches its anticodon to an mRNA codon.
Common Mistakes to Avoid
- Using the coding DNA strand instead of the template strand during transcription, which gives the wrong mRNA sequence because RNA is complementary to the template strand.
- Forgetting that RNA uses U instead of T, which leads to incorrect mRNA codons and wrong amino acids during translation.
- Reading codons in overlapping groups or starting at the wrong base, which shifts the reading frame and changes every codon after that point.
- Assuming every codon adds an amino acid, which is wrong because stop codons signal the end of translation and do not code for an amino acid.
Practice Questions
- 1 A DNA template strand has the sequence TAC GGA CTT AAA. What is the mRNA sequence transcribed from it?
- 2 An mRNA sequence begins AUG UUU GGC UGA. Using the codon meanings AUG = Met, UUU = Phe, GGC = Gly, and UGA = Stop, what amino acid sequence is produced?
- 3 A mutation changes one mRNA codon from UAU to UAA. Explain how this could affect the protein and why the effect may be large.