Unit 7: DNA, RNA, and Protein Synthesis
How the code stored in DNA gets copied, read, and built into the proteins that do a cell's actual work.
7 topics · 24 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
DNA Structure and the Double Helix
- How it worksHow do complementary base pairs A-T and G-C hold the two strands of the double helix together?
- Common mistakeWhy isn't DNA a single strand instead of two strands wound around each other?
- Going furtherWhat happens to a DNA strand's structure when it gets tightly coiled around histone proteins?
Your turn1 of 2
A single strand of human DNA, stretched out, would measure about six feet long, yet it fits inside a nucleus only a few microns across. How does that much DNA fit into such a tiny space?DNA Replication
- How it worksHow does DNA polymerase use each original strand as a template to build a matching new strand?
- How it worksWalk me through why one new DNA strand is built continuously while the other is built in short fragments.
- Common mistakeWhy doesn't DNA replication build both new strands from scratch with no connection to the original?
- Going furtherWhat happens to the two DNA molecules that result from replicating one original double helix?
Your turn1 of 2
Before a cell divides, it must copy its entire genome with extremely few errors, since any mistake gets passed on to both daughter cells. Why does a cell need to make a perfect copy of all its DNA before it divides?From DNA to RNA: Transcription
- How it worksHow does RNA polymerase read a DNA template strand to build a matching messenger RNA strand?
- Common mistakeWhy does messenger RNA use uracil in place of the thymine that DNA uses?
- Going furtherWhat happens to a strand of messenger RNA after it leaves the nucleus and enters the cytoplasm?
Your turn1 of 2
A cell's DNA stays inside the nucleus at all times, yet ribosomes building proteins sit out in the cytoplasm. Why does a cell copy a gene into RNA instead of sending the original DNA out to the ribosome?The Genetic Code and Codons
- How it worksHow does a codon chart translate a three-letter sequence like AUG into a specific amino acid?
- Worked exampleFind the amino acid sequence coded by the messenger RNA strand AUG-UUU-GGC-UAA.
- Common mistakeWhy doesn't every different codon code for a completely different amino acid?
Your turn1 of 2
A finished protein contains 354 amino acids. Not counting the stop codon at the very end, how many nucleotides long is the mRNA coding sequence that specifies this protein?Translation: Building a Protein
- How it worksWalk me through how a ribosome, messenger RNA, and transfer RNA work together like an assembly line to build a protein.
- How it worksHow does a stop codon signal the ribosome to release the finished protein chain?
- Common mistakeWhy doesn't translation happen directly from DNA without messenger RNA as a go-between?
- Worked exampleFind the number of amino acids in a protein translated from an mRNA strand 303 nucleotides long including the stop codon.
Your turn1 of 2
A protein contains 563 amino acids. Including the stop codon at the very end of the coding sequence, how many nucleotides long is the mRNA strand that was translated to build it?Types of Mutations
- How it worksHow does a point mutation swap one DNA base for another during replication?
- How it worksHow does a frameshift mutation caused by an insertion or deletion scramble every codon after it?
- Worked exampleFind how a single base insertion changes the reading frame of the sequence AUG-CGA-UUU into new codons.
- Common mistakeWhy doesn't every mutation in a gene actually change the resulting protein?
Your turn1 of 2
A frameshift mutation from a single inserted or deleted base scrambles every codon that comes after it, while a point mutation that swaps one base for another usually only affects a single codon. Why does an insertion or deletion cause so much more disruption than a substitution?How Mutations Affect Proteins and Traits
- How it worksHow does a mutation that changes a protein's shape end up changing an organism's trait?
- Common mistakeWhy doesn't a mutation you get from sunburn or a chemical get passed on to your children?
- Ties togetherHow does a mutation in a body cell differ from a mutation in a sex cell in what it can pass on?
Your turn1 of 2
A sunburn or exposure to a chemical can damage and mutate DNA in your skin cells, yet your children never inherit that particular mutation. Why doesn't a mutation you get from sunburn or a chemical get passed on to your children?