1.2.7Inquiry Process
Chargaff’s Data — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 4 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Chargaff's Findings
Erwin Chargaff measured the proportions of the four bases in DNA taken from many different species.
Base ratios differ
- Base ratios differ The percentage of each base varies from one species to another, so DNA is not a repeating unit of identical composition.
A equals T
- A equals T In every sample the amount of adenine was equal to the amount of thymine.
G equals C
- G equals C In every sample the amount of guanine was equal to the amount of cytosine.
Purines and pyrimidines
- Purines and pyrimidines The total number of purine bases always equalled the total number of pyrimidine bases.
Base pairing
- Base pairing A purine pairs with a pyrimidine, so the double helix keeps a constant width along its length.
| Pair | Bases | Hydrogen bonds |
|---|---|---|
| A–T | Purine with pyrimidine | 2 |
| G–C | Purine with pyrimidine | 3 |
Section 2 of 5Falsifying the Tetranucleotide Hypothesis
Earlier ideas about DNA were overturned by new evidence.
1
Phoebus Levene identified the sugars in DNA and RNA and proposed that nucleic acid was a simple repeating block of four different nucleotides.
2
This tetranucleotide hypothesis predicted that all four bases occur in equal amounts in every kind of DNA.
3
Chargaff's analyses showed unequal base proportions between species, which contradicted this prediction.
4
A single reliable result that disagrees with a hypothesis is enough to show that the hypothesis is false.
5
The pairing regularities he found then guided the building of the double-helix model.
Section 3 of 5Induction and Falsification
Induction and Falsification
Inductive method
- Inductive method Scientists collect specific observations and use them to propose a general rule or hypothesis.
Problem of induction
- Problem of induction No number of supporting observations can prove a general rule true, because a future result might contradict it.
Falsification
- Falsification Karl Popper argued that science advances by testing hypotheses so that they can be shown to be false.
Tentative theories
- Tentative theories Because of this, scientific theories are always regarded as the best current explanation rather than final truth.
Section 4 of 5Worked Example
Question
- A sample of DNA from one species contains 30.3% adenine.
- Calculate the percentages of the other three bases.
- Chargaff's rule says A equals T, so thymine is also 30.3%.
- Adenine plus thymine = 30.3 + 30.3 = 60.6%.
- Guanine plus cytosine = 100 − 60.6 = 39.4%.
- G equals C, so each is 39.4 ÷ 2 = 19.7%.
Answer T = 30.3%, G = 19.7%, C = 19.7%
Section 5 of 5Exam Checklist & Tips
1.2.7 – Key Points for the Exam
- Purine A base with a double ring structure: adenine or guanine.
- Pyrimidine A base with a single ring structure: cytosine, thymine or uracil.
- Complementary base pairing The specific pairing of A with T and G with C through hydrogen bonds.
- Falsification Showing by evidence that a hypothesis is incorrect.
Common Mistakes
- Assuming that A + T always equals G + C; the ratio varies between species even though A = T and G = C.
- Saying that A pairs with G because both are purines; only a purine with a pyrimidine fits the helix width.
- Believing that repeated agreement with data proves a hypothesis for certain.
- Mixing up the number of hydrogen bonds: A–T has two and G–C has three.
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