Loading
Unit 6
12–16% of examThis unit follows genetic information from DNA to protein. You'll learn how DNA is copied, how genes are transcribed into RNA and translated into proteins, how cells switch genes on and off, and how mutations and biotechnology change or read DNA. It ties your genes directly to your traits.
Longer videos that cover the whole unit. Good for a first pass or a final review.
DNA (and in some viruses, RNA) stores hereditary information: prokaryotes usually have one circular chromosome, eukaryotes have several linear chromosomes wound around histone proteins, and both can carry small DNA circles called plasmids. Bases always pair the same way, a double-ringed purine (A or G) with a single-ringed pyrimidine (T, C, or U in RNA).
Key terms
DNA is copied semiconservatively, so each new double helix keeps one old strand and gets one new one. Helicase unzips the helix, topoisomerase relieves the twisting ahead of it, DNA polymerase builds new DNA in the 5′ to 3′ direction starting from RNA primers (smoothly on the leading strand, in pieces on the lagging strand), and ligase joins the pieces.
Key terms
In transcription, RNA polymerase reads the template strand of DNA and builds a complementary RNA in the 5′ to 3′ direction; mRNA carries the message, tRNA brings amino acids and rRNA forms ribosomes. In eukaryotes, the new mRNA is processed: a poly-A tail makes it more stable, a cap on its 5′ end (called the GTP cap in AP materials) helps the ribosome recognize it, and introns are cut out while exons can be joined in different combinations (alternative splicing).
Key terms
Ribosomes read mRNA three bases (one codon) at a time, starting at AUG, while tRNAs bring the matching amino acids until a stop codon ends the chain. Almost all life uses the same genetic code, which is evidence of common ancestry, and retroviruses like HIV run the flow backward by copying their RNA into DNA with reverse transcriptase.
Key terms
Cells switch genes on and off using regulatory DNA sequences and the proteins that bind them. In bacteria, related genes sit together in operons: an inducible operon (like the lac operon) stays off until it's needed, and a repressible operon (like the trp operon) stays on until its product builds up. Epigenetic changes to DNA or histones can also turn genes up or down without changing the DNA sequence.
Key terms
Nearly every cell in your body has the same DNA, but different cells switch on different genes. Proteins called transcription factors attach to DNA regions (promoters and enhancers) and help RNA polymerase start transcribing a gene, repressor proteins can block it, and small RNA molecules can silence a gene's mRNA after it is made.
Key terms
A mutation is a change in DNA sequence that can be helpful, harmful or neutral depending on its effect and the environment; substitutions, insertions and deletions are common types, and insertions or deletions can shift the whole reading frame. Errors in meiosis can change chromosome number, and bacteria pick up new genes from other cells through transformation, transduction and conjugation.
Key terms
Scientists have tools to read and change DNA. PCR makes millions of copies of a DNA segment, gel electrophoresis sorts DNA fragments by size, bacterial transformation puts new genes into bacteria, and DNA sequencing reads the exact order of bases, which is used in forensics, medicine and studying evolution.
Key terms