DNA and Gene Expression - TCE Biology (Tasmania)
“Protein synthesis: proteins are a diverse group of molecules, including enzymes; genes include coding (exons) and non-coding (introns and regulatory) components of DNA; transcription of a gene into messenger RNA in the nucleus; translation of mRNA into an amino acid sequence at the ribosome, using transfer RNA, codons and anticodons”
DNA is a double helix of nucleotides whose base sequence is a code. Transcription copies a gene into messenger RNA, and translation reads that mRNA in three-base codons at the ribosome to assemble a specific chain of amino acids, which folds into a protein.
What this dot point is asking
The structure of DNA
DNA (deoxyribonucleic acid) is a polymer made of repeating units called nucleotides. Each nucleotide has three parts: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases, adenine (A), thymine (T), cytosine (C), or guanine (G). Nucleotides join through sugar-phosphate bonds to form a strand with a sugar-phosphate backbone.
Two strands wind around each other into a double helix. The strands are held together by hydrogen bonds between bases following complementary base pairing: A pairs with T (two hydrogen bonds) and C pairs with G (three hydrogen bonds). The two strands run antiparallel, meaning one runs 5 prime to 3 prime and the other 3 prime to 5 prime. This complementary, antiparallel arrangement allows DNA to be copied accurately during replication.
A gene is a length of DNA that codes for a functional product, usually a polypeptide (protein) but sometimes a functional RNA molecule.
Transcription
Gene expression is the process of using the information in a gene to build a product. The first stage is transcription, which happens in the nucleus of eukaryotic cells.
- The enzyme RNA polymerase binds to a promoter region at the start of a gene.
- The DNA double helix unwinds and the two strands separate over the gene.
- RNA polymerase moves along the template (antisense) strand and assembles a complementary strand of messenger RNA (mRNA) using free RNA nucleotides.
- RNA uses the base uracil (U) in place of thymine, so an A on the template pairs with U in the mRNA.
- The completed mRNA detaches. In eukaryotes the pre-mRNA is processed: a cap and tail are added and non-coding introns are removed by splicing, leaving the coding exons.
The mRNA then leaves the nucleus through a nuclear pore and travels to a ribosome in the cytoplasm.
Translation
Translation reads the mRNA and builds a polypeptide. The genetic code is read in groups of three bases called codons. Each codon specifies one amino acid, and the code is described as degenerate because most amino acids are coded for by more than one codon. There is a start codon (AUG) and there are stop codons that end translation.
- The ribosome binds to the mRNA at the start codon.
- Transfer RNA (tRNA) molecules carry specific amino acids. Each tRNA has an anticodon, three bases that are complementary to a codon.
- A tRNA whose anticodon matches the codon binds, delivering its amino acid.
- Peptide bonds form between adjacent amino acids, building a chain.
- The ribosome moves along codon by codon until it reaches a stop codon, then the finished polypeptide is released.
The polypeptide folds into a three-dimensional shape, which determines the protein function. This flow of information, DNA to RNA to protein, is often called the central dogma of molecular biology.
A template DNA strand reads 3 prime TAC GGA ATC 5 prime. Transcription gives mRNA 5 prime AUG CCU UAG 3 prime. The codons are AUG (start, methionine), CCU (proline), then UAG (stop). The resulting peptide is methionine-proline before translation stops. This shows how a base change can shorten a protein if it creates a stop codon early.
Gene regulation
Cells do not express every gene all the time. Regulation controls which genes are switched on, when, and how strongly, which is why a nerve cell and a skin cell carry the same DNA but behave differently. This differential gene expression controls cell differentiation as tissues form, and the structural changes that happen during growth. The phenotype also depends on the products of other genes and on the environment.
In BIO315124 the only regulation mechanism you need in detail is the prokaryotic lac operon in E. coli: see gene regulation and the lac operon. Eukaryotic mechanisms such as transcription factors and DNA methylation are excluded from the external exam.
The genetic code is read in non-overlapping triplets and is nearly universal across all living things, which is why a human gene can be expressed in a bacterium. This shared code is strong evidence of common ancestry.
Students often swap the two terms. Transcription happens in the nucleus and produces mRNA from DNA. Translation happens at the ribosome and produces a polypeptide from mRNA. Also remember RNA uses uracil, not thymine, so never write thymine in an mRNA sequence.
Mutations
A mutation is a change in the DNA base sequence. A substitution swaps one base for another and may be silent (no change to the amino acid), missense (a different amino acid), or nonsense (creates an early stop codon). Insertions or deletions that are not multiples of three cause a frameshift, which shifts how every following codon is read and usually disrupts the whole protein. Mutations are the ultimate source of new alleles and therefore of variation. For the types of mutation, mutagens and their effects on proteins, see mutations.
Protein synthesis is part of criterion 5 (Section B of the exam). The external assessment specifications include exons and introns, transcription in the nucleus, translation at the ribosome with tRNA and anticodons, and reading the genetic code. They exclude the 5 prime to 3 prime direction of strands, so the antiparallel detail above is useful background rather than examined content.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original6 marksA short section of the template (antisense) DNA strand reads . Write the mRNA sequence transcribed from it, identify the anticodon of the tRNA that pairs with the first mRNA codon, and explain what the term degenerate means for the genetic code.Show worked answer →
A 6 mark answer transcribes correctly, gives the anticodon and defines degeneracy.
- mRNA sequence
- RNA is complementary to the template and uses U instead of T. Template gives mRNA .
- First codon and anticodon
- The first mRNA codon is (the start codon). The tRNA anticodon is complementary to it, read antiparallel: .
- Degenerate code
- The genetic code is degenerate because most amino acids are coded for by more than one codon. This means a change in the third base of a codon often still codes for the same amino acid, reducing the effect of some mutations.
Markers reward the correct mRNA (), the anticodon, and a correct definition of a degenerate code.
Original5 marksDistinguish between transcription and translation in terms of where each occurs, the template used and the product formed, and explain why gene regulation allows different cell types to have the same DNA but different structures.Show worked answer →
A 5 mark answer contrasts the two stages and explains differential gene expression.
- Transcription
- Occurs in the nucleus (in eukaryotes); uses a DNA template strand; produces mRNA.
- Translation
- Occurs at a ribosome in the cytoplasm; uses the mRNA as the template (read by tRNA); produces a polypeptide (protein).
- Gene regulation and cell types
- Every body cell contains the same DNA, but gene regulation switches different genes on or off in different cells. Each cell type expresses only the genes it needs, so it makes a different set of proteins, which gives it a different structure and function (for example a muscle cell versus a nerve cell).
Markers reward the location/template/product contrast for both stages and the explanation that selective gene expression from identical DNA produces different cell types.