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.
Understanding The Genetic Code
A gene is not copied all the time. Cells first control whether it is needed. Proteins can bind near a gene and help RNA polymerase begin copying, or block it.
RNA polymerase opens a small section of the DNA double helix and moves along one DNA strand. This strand is called the template strand. The other strand is called the coding strand because its base sequence closely matches the RNA message, except RNA has U where DNA has T.
In plants, animals, and fungi, the first RNA copy is usually edited before it leaves the nucleus. Some sections are removed by splicing, while the remaining sections are joined. This allows one gene to sometimes produce more than one protein form.
Translation depends on transfer RNA, or tRNA. Each tRNA carries a particular amino acid and has an anticodon that pairs with an mRNA codon. Inside a ribosome, tRNAs enter one at a time.
The ribosome checks whether the anticodon matches the message. If it matches, the ribosome joins the new amino acid to the growing chain by making a peptide bond. The ribosome then moves forward by one codon.
The start codon does more than add methionine. It sets the reading frame, meaning the exact groups of three bases that will be read.
A stop codon does not bring an amino acid. It brings proteins that release the finished chain from the ribosome.
Several codons can specify the same amino acid. This feature gives the code some protection from small changes. A change in one base may produce the same amino acid, so the protein is unchanged.
It may produce a different amino acid, which can have a small effect or a serious one depending on its location. A changed codon can even become a stop signal, making a shortened protein. Insertions and deletions are often more disruptive when they involve a number of bases not divisible by three.
They shift the reading frame, changing every codon after the mutation. Some inherited disorders result from these changes. Sickle cell disease, for example, comes from a single amino acid change in hemoglobin.
Students often lose marks by mixing up DNA strands or reading a sequence in the wrong direction. Start by identifying whether a sequence is the template strand, coding strand, or mRNA. Build the mRNA carefully from the template using base pairing rules.
Then separate the mRNA into groups of three from the start position. Do not group the original DNA first unless the task clearly gives the coding strand. It helps to write each codon below its matching tRNA anticodon and then list the amino acids in order.
The final amino acid sequence is only the beginning. A protein must fold into a specific shape, and many proteins are later modified or sent to particular parts of the cell. Its shape determines what it can do.
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.