# DNA

DNA (abbreviated form of deoxyribonucleic acid) are nucleotides that serve as the "building blocks" of life as we know it.

## History of DNA

* History of DNA
* 1868
* Friedrich Miescher
* Chemical makeup of cell nuclei: half protein and half "something else". Later in 1890, scientists figure out that the "something else" from Miescher was DNA.
* 1902
* Walter Sutton
* Heredity in chromosomes.
* 1928
* Frederick Griffith
* Transformation - One strain of bacteria takes in the DNA of a different bacteria and makes the proteins from that DNA code.
* 1944
* Avery, MacLeod, McCarty
* Discovered that genes are composed of DNA.
* 1952
* Chase and Hershey
* Radioactive markers--finding out that viruses have DNA.
* Chargaff
* Discovered that the percent of each nitrogen base was the same for a variety of organisms and developed the base-pairing rule for DNA.
* Rossalin Franklin and Wilkins
* DNA
* Used X-Ray diffraction to examine the structure of DNA.
* 1953
* Watson and Crick
* Developed the double-helix model of the structure of DNA.
* 2000
* Human Genome Project
* Mapped off the DNA on all 46 human chromosomes.

## Components

Nucleotides are individual units that make up the strands of DNA. Two strands of nucleotides make up a DNA molecule; Sugar and phosphate groups make up the backbone (two sides) of a DNA molecule (sugar-phosphate backbone). These two nucleotide strands are connected by a hydrogen bond between the nitrogen bases.
The three components of a DNA nucleotide include a phosphate group, a sugar (deoxyribose), and a nitrogen base. The nitrogen base could be either Adenine (A), Thymine (T), Cytosine (C), or Guanine (G). All of these nucleobases are represented by the letters G-C-A-T. Chargaff's law states that Thymine combines with Adenine (T-A or A-T) and Cytosine combines with Guanine (C-G or G-C). "Base pairing" is the same as Chargaff's law.
The rungs of the DNA ladder are made up of one purine bonded to one pyrimidine.

## Forms

| | Prokaryotes (bacteria) | Eukaryotes |
| Where is DNA located? | In the cytoplasm (no nucleus) | In a nucleus |
| Amount of DNA | Single Nucleoid (circle of DNA: Plasmid) | Multiple Chromosomes |

### Chromosomes

    See also w:Chromosomes
* Condensed Chromatin

#### Chromosome Structure

Histones: The proteins that DNA wraps around.
Chromatin: Strands of DNA wrapped around histones.

#### Gene

    See also w:Gene
* Specific pieces of DNA that code for specific traits.
* Exons: The parts of genes that code for proteins.
* Introns: The parts of genes that do not code for proteins.

## DNA Replication

DNA Replication: The process of creating two new identical copies of a DNA molecule. This occurs during the S-phase (synthesis) in the Interphase of the cell cycle.

### Steps

* The enzyme, helicase, unzips the original DNA molecule.
* The enzyme, DNA polymerase, joins the new nucleotides to the old nucleotides.
* The enzyme, ligase, zips back up. New 2 DNA.
In the end of DNA replication, each old strand of DNA has been copied to produce two new strands. Therefore, DNA replication is a semi-conservative process.

### Origins of Replication

Replication of DNA begins at special sites called origins of replication. In prokaryotes, bacteria have a single origin of replication. On the other hand, eukaryotes have 100s or 1000s of origins of replication to help speed up the process of replication. Helicase (enzyme) helps to unwind DNA at "replication forks". DNA are antiparallel to each other... which means that the sugar-phosphate backbones run in opposite directions. This is how the new nucleotides are added to the old strands.
DNA polymerase (enzyme) can only add new nucleotides to the 3' end of a DNA strand. The new DNA strand created using the 3' to 5' strand of old DNA is called the leading strand.
The new DNA strand creating using the 5' to 3' strand of old DNA is called the lagging strand. The lagging strand creates DNA fragments called Okazaki fragments. The fragments are later joined together by ligase (enzyme).

## See Also

* RNA