Unit 8: Genetics
Content Outline: The DNA History (8.1)
I.
Erwin Chargaff (1950)
A. A chemist who developed Chargaff’s rules.
B. He stated that the bases of DNA were paired together in a specific order.
1. Adenine (A) pairs with Thymine (T): A=T, T=A
2. Cytosine (C) pairs with Guanine (G): C=G, G=C
3. If you know the amount of one of the bases, you can figure out all four using Chargaff’s Rules.
a. Example: If you know that Adenine is 16% of a sample, then there is 16%
Thymine (A=T). Together, Adenine and Thymine make 32% of the total sample.
According to Chargaff’s rules, knowing that 32% of the sample is Adenine and
Thymine, we know how much Cytosine and Guanine is there by simply
subtracting 100 – 32 and we are left with 68% of both remaining bases. By
dividing (68 ÷ 2) we figure out that there is 34% Cytosine and 34% Guanine.
II.
Rosalind Franklin (1951)
A. A British female scientist who made a huge contribution to the study of DNA by establishing the density
of DNA and that it existed in a spiral three –dimensional shape with the use of X-ray diffraction.
B. This discovery help lay the foundation for Watson and Crick to create the first DNA model.
III.
James Watson and Francis Crick (1953)
A. They created the first accurate DNA model with the help of Chargaff and Franklin’s research.
B. The structure was a double helix (long twisted ladder) with the sides consisting of subunits called
nucleotides (a sugar - deoxyribose, a phosphate and a nitrogen base).
C. The sides of the double helix are the alternating sugar and phosphate.
D. The center or steps of the “ladder” are the paired nitrogen bases.
1. Adenine always pairs with Thymine.
2. Cytosine always pairs with Guanine.
3. This is supported by Chargaff’s Rule.
E. Hydrogen Bonds hold the two sides together and it is twisted into the Double Helix shape.
1. Hydrogen bonds are weak bonds. This is very helpful when DNA replication occurs.
F. They were awarded the Nobel Prize in Science in 1962 for their DNA model.
IV.
A Closer Look at the Structures of DNA
A. DNA (deoxyribonucleic acid) is a nucleic acid that is present in all living cells. This molecule contains
all of the instructions that determine the traits a living thing needs to survive. It acts as a “blue print” for
living organisms.
1. Almost every cell in the human body has 2 meters of DNA.
2. DNA gives each organism its unique characteristics, but it functions in the same manner.
3. DNA is often coiled around proteins and coiled into strands that are found in the nucleus of
Eukaryotes called chromosomes.
a. Genes are segments of nucleotides located on the chromosomes that give specific
instructions for traits.
4. DNA chromosomes in prokaryotes forms loose loops within the cell’s cytoplasm.