DNA Structure and Composition

Study Notes

DNA stands for Deoxyribonucleic Acid, a molecule containing instructions for an organism's development, life, and reproduction.
DNA is a nucleic acid and a major macromolecule essential for all life.
Discovered by James Watson and Francis Crick in 1953, building upon Rosalind Franklin's work.
DNA's structure is a double helix, resembling a twisted ladder.
The ladder sides are made of alternating sugar (deoxyribose) and phosphate molecules.
The ladder steps are made of pairs of nitrogen bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).
Bases pair specifically: Adenine always pairs with Thymine (A-T), and Guanine always pairs with Cytosine (C-G).
These pairs are held together by hydrogen bonds.

DNA is a double-stranded helix with two biopolymer strands coiling around each other.
Each strand, called a polynucleotide, is made of monomer units called nucleotides.
Each strand has a 5' end (with a phosphate group) and a 3' end (with a hydroxyl group).
The strands are antiparallel, running in opposite directions (one 5' to 3', the other 3' to 5').
The strands are held together by hydrogen bonds and are complementary to each other.
DNA is composed of deoxyribonucleotides linked by 3' - 5' phosphodiester bonds.
The nitrogenous bases in deoxyribonucleotides are adenine, cytosine, thymine, and guanine.
The complementary nature of strands arises from the specific base pairings (A-T and C-G).
The helix shape is stabilized by hydrogen bonding and hydrophobic interactions between bases.
The double helix diameter is 2nm, with a repeat interval of 3.4nm, corresponding to ten base pairs.

The double helical nature of DNA creates two asymmetric grooves: a major groove and a minor groove.
This asymmetry results from the bond angles between phosphate, sugar, and base groups.
The major groove is where the backbones are further apart, while the minor groove is where they are closer.
These grooves expose the edges of the bases, allowing proteins to recognize specific DNA sequences.
This recognition is crucial for various cellular functions.

DNA helices can be right-handed or left-handed, but the B conformation (right-handed) is most stable.
On heating, the two DNA strands separate (denature), and upon cooling, they re-associate (hybridize).
The temperature at which strands completely separate is called the melting temperature (Tm), specific to each DNA sequence.
A higher melting temperature indicates a higher C-G content, due to the three hydrogen bonds between C and G.
The DNA base sequence encodes for the amino acid sequence of all proteins in organisms.

Eukaryotes (animals, plants, fungi) store most of their DNA in the nucleus, with some in organelles like mitochondria.
DNA can be classified based on its location:

Located within the nucleus of eukaryotic cells.
Usually has two copies per cell.
Structure is linear with open ends, comprising 46 chromosomes containing 3 billion nucleotides.
Nuclear DNA is diploid, inheriting from both parents.
Mutation rate is relatively low, less than 0.3%.