DNA

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

The structure of DNA

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.

HB = Hydrogen Bond. A sidenote: Covalent Bonds are stronger than Hydrogen Bonds

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

1. The enzyme, helicase, unzips the original DNA molecule.
2. The enzyme, DNA polymerase, joins the new nucleotides to the old nucleotides.
3. 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.

DNA replication begins at specific regions on the DNA molecule known as origins of replication. From each origin, replication proceeds in both directions, forming structures called replication forks. As helicase separates the DNA strands, single-strand binding proteins stabilize the exposed strands to prevent them from rejoining. At the same time, enzymes called topoisomerases reduce twisting and tension ahead of the replication fork, preventing the DNA from becoming overwound.

DNA polymerase can only synthesize DNA in the 5′ to 3′ direction, meaning the two new strands are produced differently due to the antiparallel nature of DNA. The leading strand is synthesized continuously toward the replication fork, while the lagging strand is synthesized discontinuously away from the fork in short segments known as Okazaki fragments. Replication cannot begin without a starting point, so an enzyme called primase synthesizes a short RNA primer that provides a free 3′-OH group for DNA polymerase to extend. After synthesis, the RNA primers are removed and replaced with DNA nucleotides, and DNA ligase joins the fragments together by sealing gaps in the sugar-phosphate backbone.

DNA replication is generally highly accurate due to the proofreading activity of DNA polymerase, which helps correct mismatched nucleotides and reduces the likelihood of mutations.

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.

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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.

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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).

DNA Diagram

See Also

Search for DNA on Wikipedia.

• RNA