DNA Jessica Gutierrez Madison Arterburn

Structure of DNA Nucleic Acid (DNA) is an organic compound Made up of repeating sub-units called nucleotides each DNA molecule consist of two long chains of nucleotides

DNA Nucleotide A DNA nucleotide has 3 parts A sugar molecule called deoxyribose A phosphate group that consists of Phosphorus (p), surrounded by Oxygen (O) and a molecule, nitrogen-containing base that contains Nitrogen (N) Deoxyribose sugar & Phosphate group are identical in all DNA nucleotides, but the nitrogen containing base may be any of these 4 kinds Adenine (A) Guanine (G) Cytosine (C) Thymine (T)

Adenine, Guanine, Cytosine, Thymine Adenine & Guanine have 2 rings of carbon & nitrogen atoms Cytosine & Thymine have only 1 ring of carbon & nitrogen atoms Bases that have 2 rings of Carbon & Nitrogen are called Purines, so that applies to Adenine and Guanine Bases with only 1 ring of Carbon & Nitrogen are called Pyrimidines, and that applies to Cytosine and Thymine

Double Helix A Double Helix is 2 nucleotic chains that wrap around each other (spiral Staircase) Individual nucleotides are connected by covalent bonds between the deoxyribose sugar and phosphate molecules. The alternating deoxyribose sugar & phosphate molecules form a “backbone” to which the nitrogen-containing base attach. The Nitrogen-containing base also faces toward the center of the helix and are perpendicular to the sugar phosphate backbone By facing towards the center, the bases on 1 chain of DNA face the bases of the other chain of DNA, with which they form bonds called hydrogen bonds

Hydrogen Bonds A Hydrogen Bond is a relatively weak bond that usually forms between molecules They form when 2 atoms share a hydrogen nucleus- one proton The hydrogen bonds that form between the bases in DNA form between hydrogen atoms and either an oxygen or a nitrogen atom The base pairs are a uniform length because in each case one base is a double-ringed purine and the other is a single-ringed pyrimindine

Complementary base pairing Complementary base pairs: Cytosine pairs with GuanineBase-pairing rules Adenine pairs with Thymine Complementary base pairs are connected to each other by hydrogen bonds C & G form 3 hydrogen bonds A & T form 2 hydrogen bonds The nucleotide sequence in 1 nucleotide chain of DNA is an exact complement of the nucleotide sequence in the other chain Led to suggestions of how DNA might copy itself The ability of DNA to make exact copies of itself is important because cells that divide must pass exact copies of their DNA to offspring cells

Replication of DNA The process of DNA is called replication Process The 2 nucleotide chains separate by unwinding and each chain serves as a template for a new nucleotide chain The point at which the 2 chains are separated is called helicase As the helicase enzymes move along the DNA molecule, they break hydrogen bonds between the complementary bases, and the chains separate.

DNA polymerases They bind to the separated chains of DNA. As DNA polymerases move along the separated chains, new chains of DNA are assembled using nucleotides in the surrounding medium that are complementary to the existing DNA chains. Nucleotides are joined to the new chains by covalent bonds between deoxyribose sugars and phosphate groups. They are joined to the original nucleotide chain by hydrogen bonds. The complementary nature of the two chains of DNA is the foundation for accurate DNA replication. DNA replication does not begin at one end of the molecule and proceed to the other, rather, the DNA polymerases begin replication simultaneously at many points along the separated nucleotide chains Replication is initiated simultaneously at about 6,000 sites in fruit fly DNA

Accuracy and Repair Replication of DNA happens with total accuracy about one error in every 10,000 paired nucleotides A change in the nucleotide sequence at even one point is called mutation. Errors and mutations in DNA replication is reduced as enzymes proofread DNA and repair errors in the complementary base paired structured of DNA If non-complementary bases are paired, the abnormal DNA structure can be recognized and repaired by specific enzymes. Even though proofreading prevents replication errors, errors still do occur In addition DNA can be damaged by a variety of agents, including chemicals and ultraviolet radiation from the sun Cells are continuously proofreading and repairing their DNA, the number of mutations is reduced