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The Genetic Code
From DNA to Protein
Related Worksheets
The genetic code is the rulebook cells use to turn information stored in DNA into proteins. DNA contains four bases, A, T, G, and C, arranged in sequences that carry instructions for building living things. Proteins do much of the work in cells, including forming structures, speeding up reactions, and sending signals. Understanding DNA to protein explains how traits are inherited and how mutations can change cell function.
The process begins with transcription, where a gene in DNA is copied into messenger RNA, or mRNA. In mRNA, the base U replaces T, so RNA uses A, U, G, and C. During translation, a ribosome reads the mRNA three bases at a time as codons, and each codon specifies an amino acid or a stop signal. The amino acids are linked in order to form a polypeptide chain that folds into a working protein.
Key Facts
- Central dogma: DNA -> mRNA -> Protein
- DNA bases are A, T, G, and C, while RNA bases are A, U, G, and C.
- Base pairing in transcription: DNA A pairs with RNA U, DNA T pairs with RNA A, DNA G pairs with RNA C, and DNA C pairs with RNA G.
- A codon is a sequence of 3 mRNA bases that codes for one amino acid or a stop signal.
- Start codon: AUG codes for methionine and usually begins translation.
- There are 64 possible codons because 4^3 = 64.
Vocabulary
- Gene
- A gene is a segment of DNA that contains instructions for making a functional product, often a protein.
- Transcription
- Transcription is the process of copying a DNA sequence into a complementary mRNA sequence.
- Codon
- A codon is a three base sequence on mRNA that corresponds to an amino acid or a stop signal.
- Ribosome
- A ribosome is a cellular structure that reads mRNA codons and links amino acids together to build a protein.
- Mutation
- A mutation is a change in a DNA sequence that can alter mRNA codons and may affect the protein produced.
Common Mistakes to Avoid
- Using T in an mRNA sequence instead of U, which is wrong because RNA contains uracil rather than thymine.
- Reading codons from the wrong starting point, which is wrong because shifting the reading frame changes every codon after the shift.
- Assuming every DNA change changes the protein, which is wrong because some mutations are silent or occur outside coding regions.
- Thinking one amino acid has only one codon, which is wrong because the genetic code is redundant and many amino acids are coded by multiple codons.
Practice Questions
- 1 A DNA template strand has the sequence TAC GGA CTT ACT. Write the complementary mRNA sequence and divide it into codons.
- 2 An mRNA sequence contains 27 bases from the start codon to the last codon before a stop codon. How many amino acids are added to the growing protein chain?
- 3 A mutation changes the mRNA codon UUU to UUC, and both codons code for phenylalanine. Explain why this mutation may not change the protein.