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.
Understanding Transcription & Translation Visual
A gene is not copied from its first DNA letter to its last without control. Proteins called transcription factors bind near a gene and help RNA polymerase begin at the correct place. This nearby control region is often called a promoter.
Different cell types use different sets of transcription factors. That is why a nerve cell can make proteins needed for signaling while a muscle cell makes proteins needed for contraction, even though both cells contain nearly the same DNA.
Signals from outside the cell, such as hormones or stress, can change which genes are transcribed. Gene expression is therefore controlled, not automatic.
One common source of confusion is the two DNA strands. RNA polymerase reads the template strand, then builds RNA with complementary bases. The other DNA strand is called the coding strand because its sequence matches the RNA sequence except that DNA has thymine where RNA has uracil.
RNA polymerase can build only in one direction. Diagrams may show DNA running in opposite directions, so students should first identify the template strand and the direction of polymerase movement. This prevents many base pairing mistakes.
In eukaryotes, processing can produce different mature messenger RNAs from the same original transcript. By joining exons in different patterns, a cell may produce related protein forms from one gene.
At a ribosome, transfer RNAs act as adaptors. Each transfer RNA carries a particular amino acid and has an anticodon that pairs with a messenger RNA codon. A separate enzyme attaches the correct amino acid to each transfer RNA before translation begins.
This loading step is important because the ribosome mainly checks codon and anticodon pairing. Ribosomes move along messenger RNA one codon at a time. They form peptide bonds between amino acids, creating a growing chain.
The chain later folds into a specific shape. Its shape helps determine its job, such as speeding up a chemical reaction, carrying oxygen, or receiving a cell signal.
Changes in DNA do not all have the same result. A base change may leave the amino acid sequence unchanged because more than one codon can specify the same amino acid. Another change can replace one amino acid, which may have a small effect or seriously alter folding.
An insertion or deletion that is not a multiple of three can shift the reading frame. This changes every codon after the mutation and often produces an early stop signal. Students meet these ideas when studying inherited disorders, cancer, viruses, and biotechnology.
When reading a sequence problem, find the reading frame first, group bases into threes, locate the start signal, and stop when a stop codon appears. Keep track of whether the question gives DNA template, DNA coding, or messenger RNA before using a codon chart.