Ninth Grade · Science · week 21

DNA Structure and the Discovery of the Double Helix

Describe the structure of DNA, including complementary base pairing, and summarize the key experiments that revealed it.

The case that DNA is the molecule of heredity was built in pieces. In 1928, Frederick Griffith found that harmless bacteria could be permanently "transformed" into a dangerous strain just by exposure to remains of dead dangerous bacteria, showing some "transforming factor" could pass genetic information between organisms. In 1944, Avery, MacLeod, and McCarty identified that transforming factor specifically as DNA, not protein. Erwin Chargaff then discovered a striking pattern: in any organism's DNA, the amount of adenine always equals the amount of thymine, and the amount of guanine always equals the amount of cytosine. The final structural piece came from Rosalind Franklin's X-ray diffraction images of DNA fibers, especially the image known as Photo 51, which revealed a regular, measurable helical pattern; James Watson and Francis Crick combined Franklin's essential structural data with Chargaff's ratios to build the double-helix model in 1953.

DNA's monomer is the nucleotide: a deoxyribose sugar, a phosphate group, and one of four nitrogen bases (adenine, thymine, guanine, or cytosine). Two strands of nucleotides run in opposite directions (antiparallel), joined by a sugar-phosphate backbone on the outside, with bases paired in the middle by hydrogen bonds according to a strict complementary rule: adenine always pairs with thymine (two hydrogen bonds), and guanine always pairs with cytosine (three hydrogen bonds) -- exactly the mechanism behind Chargaff's ratios, now explained rather than just observed.

📋 For the teacher
Teach it (10–15 min):
  • A strawberry DNA extraction genuinely works at home: mash a strawberry in a zip-top bag, mix in a soapy salt-water solution (which breaks open cell and nuclear membranes and helps DNA clump), strain the mixture through a coffee filter into a clear cup, then slowly pour cold rubbing alcohol down the inside of the tilted cup to form a separate top layer -- DNA is insoluble in cold alcohol, so visible white, stringy clumps rise into that layer within a minute or two and can be spooled onto a wooden stick.
  • Have your student practice writing a complementary strand for a few different short sequences until the antiparallel, complementary-pairing logic is automatic -- it underlies everything in the next two weeks.
  • Discuss Rosalind Franklin's role directly and honestly: her X-ray diffraction data was essential evidence, and this is a good week to talk about how scientific credit and collaboration do not always get recognized evenly or immediately.
  • If your student enjoys history, the Griffith and Avery-MacLeod-McCarty experiments are worth a slower read together -- both are clean, clever examples of building a scientific case piece by piece, years before DNA's structure was even known.
Talk about it:
  • Why was determining that DNA, and not protein, was the genetic material such an important discovery on its own, even before anyone knew DNA's structure?
  • Why does the antiparallel, complementary structure of DNA make it such a reliable molecule for storing and copying information?
  • What does this week's history suggest about how scientific discoveries usually happen -- as one lone breakthrough, or as many contributing pieces of evidence?

Mastery looks like: They can build a complementary, antiparallel DNA strand from a given sequence, apply Chargaff's rule, and summarize the key experimental evidence behind the double-helix model.

Common stumbles: Pairing bases without reversing the strand direction (forgetting "antiparallel"); mixing up which base pairs with which.

Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
Records attendance + 15 min of Science for this learner.
← Back to todayAll coursesYear planner