BIOB 160 Principles of Living Systems · Unit 4: Molecular genetics · on Final exam
4.1 DNA: the genetic material and its structure
Study notes for DNA: the genetic material and its structure in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, key formulas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in StudyCat.
Big ideas
- Griffith (1928): harmless bacteria were transformed by heat-killed virulent bacteria. Avery, MacLeod and McCarty identified the transforming substance as DNA.
- Hershey and Chase (1952) labeled phage DNA with ³²P and protein with ³⁵S; only the ³²P entered the bacteria, showing that DNA is the genetic material.
- Chargaff’s rules: base composition varies between species, and in DNA %A = %T and %G = %C.
- Rosalind Franklin’s X-ray images showed a helix; Watson and Crick built the double-helix model: two antiparallel sugar-phosphate backbones outside, paired bases inside, about 2 nm wide, with a full turn every 3.4 nm (10 base pairs).
- A purine always pairs with a pyrimidine (A–T, G–C), keeping the width uniform. The sequence of bases carries the information.
Key formulas
Chargaff
$$\%A = \%T, \quad \%G = \%C, \quad \%A + \%G = 50\%$$
Worked example
A DNA sample is 22% adenine. What are the percentages of T, G and C?
Show the solution
T = A = 22%. A + T = 44%, leaving 56% for G + C, so G = C = 28% each.
Common mistakes
- Applying Chargaff’s rules to single-stranded RNA. They hold for double-stranded DNA.
- Mixing up which label marked what in Hershey–Chase: ³²P marked DNA, ³⁵S marked protein.
- Thinking bases form the outside of the helix. The backbone is outside.
Exam tipReading: Campbell ch. 16 (pp. 314–320). Know each experiment by its question, method and conclusion.
Keep going in StudyCat
StudyCat is free for Montana State students: an endless quizzer with worked explanations, one-question lessons, flashcards, exam prep checklists, the class calendar with every deadline, a grade calculator and a study board for BIOB 160.