Transcription and translation explain how information in DNA becomes a working protein. This cheat sheet helps students connect the flow of genetic information with the cell structures that carry it out. It is useful for reviewing gene expression, reading diagrams, and understanding how mutations can affect proteins.
The visual sequence is DNA to RNA to protein.
Key Facts
- The central dogma is DNA -> RNA -> protein, showing how genetic information is copied and used to build proteins.
- Transcription occurs in the nucleus of eukaryotic cells and uses one DNA template strand to make a complementary pre-mRNA strand.
- RNA base-pairing rules during transcription are A pairs with U and C pairs with G.
- RNA processing in eukaryotes adds a 5' cap, adds a poly-A tail, and removes introns while joining exons together.
- Translation occurs at ribosomes, where mRNA codons are read in groups of three nucleotides.
- The start codon is AUG, which codes for methionine and signals the ribosome to begin translation.
- Stop codons are UAA, UAG, and UGA, and they signal the end of translation without adding an amino acid.
- A polypeptide has one amino acid for each translated codon, except stop codons do not code for amino acids.
Vocabulary
- Transcription
- Transcription is the process of copying a gene from DNA into a complementary RNA molecule.
- mRNA
- Messenger RNA is the RNA copy of a gene that carries instructions from DNA to a ribosome.
- RNA Processing
- RNA processing modifies pre-mRNA by adding protective ends and removing noncoding introns.
- Codon
- A codon is a sequence of three mRNA bases that codes for one amino acid or a stop signal.
- tRNA
- Transfer RNA carries a specific amino acid and has an anticodon that pairs with an mRNA codon.
- Ribosome
- A ribosome is the cell structure that reads mRNA and links amino acids into a polypeptide.
Common Mistakes to Avoid
- Using T in RNA sequences is wrong because RNA contains uracil, U, instead of thymine, T.
- Confusing the coding strand with the template strand is wrong because RNA is built complementary to the template strand, not copied directly from it.
- Reading codons one base at a time is wrong because codons must be read as non-overlapping groups of three bases.
- Counting a stop codon as an amino acid is wrong because UAA, UAG, and UGA end translation and do not add to the polypeptide.
- Skipping RNA processing in eukaryotes is wrong because pre-mRNA must be capped, tailed, and spliced before it is usually translated.
Practice Questions
- 1 A DNA template strand reads TAC GGA CTT. What mRNA sequence is made during transcription?
- 2 An mRNA sequence is AUG GCU UUU UGA. How many amino acids are added to the polypeptide before translation stops?
- 3 If a processed mRNA has 900 nucleotides in its coding region before the stop codon, how many amino acids will be in the polypeptide?
- 4 Explain why a mutation that changes one base in DNA might change a protein, but might also have no effect on the amino acid sequence.