DNA Structure and History Notes

What is DNA?

  • DNA is a big molecule called a nucleic acid.

  • It's made of smaller parts called nucleotides.

  • Each nucleotide has three things: a phosphate group, a 5-carbon sugar (deoxyribose), and a nitrogenous base.

  • A DNA molecule has billions of these nucleotides.

  • There are only four types of nucleotides, which are different because of their nitrogenous bases.

Nucleotides in DNA

  • There are four nitrogenous bases:

    • Adenine (A)

    • Guanine (G)

    • Cytosine (C)

    • Thymine (T)

Nucleotides

  • Purines are big bases with a double-ring shape:

    • Guanine

    • Adenine

  • Pyrimidines are smaller bases with a single-ring shape:

    • Cytosine

    • Thymine

Nitrogenous Bases

  • In 1950, Erwin Chargaff learned some important things about DNA.

  • He looked at DNA from different living things and saw that they all had the same four nitrogenous bases.

  • But the amounts of these bases were different in different creatures.

  • Chargaff noticed that the amount of adenine (A) was about the same as the amount of thymine (T). Also, the amount of cytosine (C) was about the same as the amount of guanine (G).

Chargaff’s Rule

  • Chargaff’s Rule says that in any cell, the amount of pyrimidine bases is the same as the amount of purine bases, making a 1:1 ratio.

  • Specifically, Guanine (G) equals Cytosine (C), and Adenine (A) equals Thymine (T).

  • Complementary Base Pairs: Nucleotides that are connected by a hydrogen bond on opposite sides of DNA.

Let's Think

  • Example: If a DNA strand is 12% Adenine:

    • Thymine would also be 12% (because A = T).

    • Remaining percentage: 100−12−12=76100 - 12 - 12 = 76

    • Since Cytosine and Guanine are in equal amounts: 76/2=3876 / 2 = 38

    • So, Cytosine = 38% and Guanine = 38%.

Base Pairs

  • Adenine (A) always pairs with Thymine (T).

  • Guanine (G) always pairs with Cytosine (C).

Discovery of DNA Structure

  • Early 1950s: James Watson and Francis Crick.

  • They were studying proteins but were interested in the spiral shape of some proteins.

  • They thought that DNA might also be a spiral.

DNA Structure

  • Rosalind Franklin used X-rays to study DNA.

  • She shot X-rays at DNA, which made patterns on film.

  • Franklin’s X-ray pictures helped Watson and Crick a lot.

  • The patterns and angles showed that DNA is a helix made of two strands with a regular width.

Watson and Crick

  • They said that DNA is a Double Helix: two strands of DNA wind around each other like a twisted ladder.

  • The strands are complementary, which means they fit together and are opposite of each other.

  • Example: If one strand is ACACAC, the other strand is TGTGTG.

DNA Structure

  • DNA nucleotides in one strand are connected by strong bonds between the sugar of one nucleotide and the phosphate of the next.

  • The DNA double helix is held together by weak bonds between the bases in