This cheat sheet covers how genetic information is stored in DNA, copied into RNA, and used to build proteins. Students need these ideas to understand inheritance, gene expression, mutations, and many topics in modern biology. It connects molecular structure to the central dogma, which is the flow of information from DNA to RNA to protein.
The most important concepts are complementary base pairing, transcription, translation, and the genetic code. DNA uses A-T and C-G base pairs, while RNA uses A-U and C-G base pairs. During transcription, RNA polymerase builds mRNA from a DNA template strand.
During translation, ribosomes read mRNA codons so tRNA can bring amino acids in the correct order.
Key Facts
- The central dogma is DNA -> RNA -> protein.
- In DNA, adenine pairs with thymine and cytosine pairs with guanine, so A = T and C = G in double-stranded DNA.
- In RNA, adenine pairs with uracil and cytosine pairs with guanine, so A pairs with U and C pairs with G.
- A DNA nucleotide contains deoxyribose sugar, a phosphate group, and one nitrogen base: A, T, C, or G.
- An RNA nucleotide contains ribose sugar, a phosphate group, and one nitrogen base: A, U, C, or G.
- During transcription, the mRNA sequence is complementary to the DNA template strand and matches the DNA coding strand except U replaces T.
- A codon is a group of 3 mRNA bases, and each codon usually codes for one amino acid.
- Translation begins at the start codon AUG and ends at a stop codon: UAA, UAG, or UGA.
Vocabulary
- DNA
- DNA is the double-stranded molecule that stores genetic instructions in the sequence of its bases.
- RNA
- RNA is a usually single-stranded nucleic acid that helps copy and use genetic information to make proteins.
- Transcription
- Transcription is the process of making an RNA copy from a DNA template.
- Translation
- Translation is the process in which a ribosome reads mRNA codons to build a protein.
- Codon
- A codon is a sequence of three mRNA bases that specifies an amino acid or a stop signal.
- Anticodon
- An anticodon is a three-base sequence on tRNA that pairs with a matching mRNA codon.
Common Mistakes to Avoid
- Using T in RNA sequences is wrong because RNA contains uracil instead of thymine, so A pairs with U during RNA base pairing.
- Confusing the coding strand with the template strand is wrong because mRNA is complementary to the template strand but nearly identical to the coding strand except U replaces T.
- Reading codons from the wrong direction is wrong because mRNA is read in groups of three from the 5' end to the 3' end during translation.
- Assuming every mutation changes a protein is wrong because some mutations are silent or occur outside a coding region.
- Forgetting the start codon is wrong because translation normally begins at AUG, which sets the reading frame for the rest of the protein.
Practice Questions
- 1 A DNA template strand is TAC GGA CTT. What mRNA sequence is made during transcription?
- 2 An mRNA sequence is AUG UUU GGC UAA. How many amino acids are added before translation stops?
- 3 A double-stranded DNA sample has 24 percent adenine. What percent of the sample is thymine, cytosine, and guanine?
- 4 Explain why a substitution mutation in DNA might not change the amino acid sequence of a protein.
Understanding DNA, RNA & Protein Synthesis
DNA is more than a list of bases. Its two strands run in opposite directions, called five-prime to three-prime and three-prime to five-prime. This direction matters because enzymes can add new nucleotides only to one end of a growing strand.
The bases hold the strands together through weak hydrogen bonds. Each bond is weak alone, yet millions of them make the molecule stable. The bonds can still separate when a cell needs to copy DNA or read a gene.
In cells, DNA is wrapped around proteins and packed into chromosomes. A gene must be accessible before it can be used. Cells control access to genes, which is one reason nerve cells and muscle cells can have the same DNA but make very different proteins.
Transcription starts when proteins recognize a control region near a gene, often called a promoter. RNA polymerase attaches there, opens a short section of DNA, then moves along one strand. Only one DNA strand serves as the template for a particular gene.
This is a common source of sequence errors in schoolwork. Always identify whether a question gives the template strand or the coding strand before writing an RNA sequence. In eukaryotic cells, the first RNA copy is usually edited before leaving the nucleus.
Sections called introns are removed, while exons are joined together. Different exon combinations can sometimes be joined, allowing one gene to produce several related proteins.
Translation depends on careful reading of the message. A ribosome moves along mRNA in groups of three bases without skipping or overlapping groups. The starting point sets the reading frame.
If one base is inserted or deleted, every codon after that point may change. This is called a frameshift mutation, and it often has a serious effect on the protein. Transfer RNA molecules carry particular amino acids.
Each one has an anticodon that fits a codon on the mRNA. Inside the ribosome, amino acids are linked by peptide bonds to form a chain.
The chain folds into a specific three-dimensional shape. Its shape helps determine whether it works as an enzyme, hormone, receptor, antibody, or structural material.
Mutations do not always cause obvious harm. Some change a codon but still result in the same amino acid because the genetic code has repeated codons. Some alter one amino acid with little effect.
Others prevent a protein from forming properly. These differences help explain inherited conditions, cancer-causing changes in cells, and the appearance of new traits in populations. Protein synthesis is used in real life when scientists study genetic tests, viruses, medicines, and biotechnology.
When learning this topic, track every sequence one base at a time. Keep DNA, mRNA, tRNA anticodons, codons, and amino acid names clearly separated. Check the reading frame, locate the start signal, then stop only at a stop codon.