DNA Structure Lesson 1

DNA stands for Deoxyribonucleic acid.
DNA is a molecule that serves as the "book of code" that tells your cells what to do.

Location: DNA is primarily found within the nucleus of eukaryotic cells.
Quantity: Each of your body's cells (with the exception of red blood cells) contains approximately 2 meters of DNA.
Organization: This 2 meters of DNA is intricately coiled and packed into structures called chromosomes inside the nucleus.
Astounding Scale: If all the DNA in your body were stretched out end-to-end, it would extend from the Earth to the Sun hundreds of times, illustrating its immense length and compact packaging within cells.

DNA is characteristically shaped like a double helix, looks like a twisted ladder.

The fundamental building blocks (monomers) of the DNA molecule are called nucleotides.
Each nucleotide is composed of three main components:
- A phosphate group.
- A deoxyribose sugar molecule.
- One of four different nitrogenous bases.

There are four types of nitrogenous bases found in DNA:
- Adenine (A)
- Guanine (G)
- Thymine (T)
- Cytosine (C)
These bases are categorized based on their carbon-nitrogen ring structure:
- Purines: Adenine (A) and Guanine (G) are purines because they possess two carbon-nitrogen ring structures. (Mnemonic: Pure As Gold - Purine, Adenine, Guanine).
- Pyrimidines: Thymine (T) and Cytosine (C) are pyrimidines because they possess one carbon-nitrogen ring structure. (Mnemonic: CUT the Py - Cytosine, Uracil (in RNA), Thymine, Pyrimidine).

The phosphate and sugar groups are like the “backbone” of the DNA molecule.
In between the “backbone” sides, there are pairs of nitrogenous bases held together by two or three hydrogen bonds.
The sequence of the base pairs is THE GENETIC CODE.
One side of the ladder is called the 5’ (five prime) and the other is called the 3’ (three prime) side. This indicates how the phosphate and the sugar are linked, the two sides of the backbone are flip-flopped compared to one another.
- One side is referred to as the 5' (five prime) end.
- The other side is referred to as the 3' (three prime) end.
- This designation (5' and 3') indicates the chemical orientation of the sugar-phosphate links along the strand.

The nitrogenous bases pair specifically with each other, forming the "rungs" of the DNA ladder. These pairings are crucial for DNA's structure and function.
Adenine (A) always pairs with Thymine (T). This pairing is stabilized by two hydrogen bonds.
Cytosine (C) always pairs with Guanine (G). This pairing is stabilized by three hydrogen bonds.
Thymine and Cytosine have one carbon-nitrogen ring structure and are called Pyrimidines.
Structural Requirement: For the bases to fit correctly between the two sides of the DNA backbone, a purine must always bond with a pyrimidine. This ensures a consistent width for the DNA Helix.
- For example: Adenine (purine) pairs with Thymine (pyrimidine); Guanine (purine) pairs with Cytosine (pyrimidine).
Example of Complementary Strand Pairing:
- Given DNA sequence: 5' - \text{A} \text{T} \text{G} \text{C} \text{G} \text{C} \text{T} \text{A} \text{T} \text{T} \text{G} - 3'
- Complementary strand: 3' - \text{T} \text{A} \text{C} \text{G} \text{C} \text{G} \text{A} \text{T} \text{A} \text{A} \text{C} - 5'

The specific sequence of these base pairs along the DNA molecule constitutes THE GENETIC CODE. This code carries all the instructions for building and maintaining an organism.

Hydrogen Bond Count:
- "Two Angry Tigers" refers to Adenine-Thymine (A-T) pairing with 2 hydrogen bonds.
- "Three Calm Giraffes" refers to Cytosine-Guanine (C-G) pairing with 3 hydrogen bonds.
Letter Visuals:
- "The two Circle letters (C and G) pair together."
- "The two Stick letters (A and T) pair together."