What is DNA? Although the environment influences how an organism develops, the genetic information that is held in the molecules of DNA ultimately determines an organism’s traits. DNA achieves its control by determining the structure of proteins. Within the structure of DNA is the information for life—the complete instructions for manufacturing all the proteins for an organism.

The structure of nucleotides DNA is a polymer made of repeating subunits called nucleotides.(the monomer) Nucleotides have three parts: a simple sugar, a phosphate group, and a nitrogenous base. Nitrogenous base Phosphate group Sugar (deoxyribose)

The structure of nucleotides in DNA there are four possible nucleotides, each containing one of these four bases. The phosphate groups and deoxyribose molecules form the backbone of the chain, and the nitrogenous bases stick out like the teeth of a zipper. The Bases Bind the two sides of the chain together with hydrogen bonds Nitrogenous base (A, G, C, or T) Phosphate group Thymine (T) Nucleotide Sugar (deoxyribose) DNA nucleotide Sugar-phosphate backbone

The 4 Unique Nitrogenous Bases 2 Purines (larger): Adenine (A) Guanine (G) 2 Pyrimidines (smaller): Cytosine (C) Thymine (T)

Chargaff DNA composition: “Chargaff’s rules” varies from species to species all 4 bases not in equal quantity bases present in characteristic ratio Humans: A = 30.9% T = 29.4% G = 19.9% C = 19.8% A=T and C=G

Rosalind Franklin (1920-1958) A chemist by training, Franklin had made original and essential contributions to the understanding of the structure of graphite and other carbon compounds even before her appointment to King's College. Unfortunately, her reputation did not precede her. James Watson's unflattering portrayal of Franklin in his account of the discovery of DNA's structure, entitled "The Double Helix," depicts Franklin as an underling of Maurice Wilkins, when in fact Wilkins and Franklin were peers in the Randall laboratory. And it was Franklin alone whom Randall had given the task of elucidating DNA's structure. The technique with which Rosalind Franklin set out to do this is called X-ray crystallography. With this technique, the locations of atoms in any crystal can be precisely mapped by looking at the image of the crystal under an X-ray beam. By the early 1950s, scientists were just learning how to use this technique to study biological molecules. Rosalind Franklin applied her chemist's expertise to the unwieldy DNA molecule. After complicated analysis, she discovered (and was the first to state) that the sugar-phosphate backbone of DNA lies on the outside of the molecule. She also elucidated the basic helical structure of the molecule. After Randall presented Franklin's data and her unpublished conclusions at a routine seminar, her work was provided - without Randall's knowledge - to her competitors at Cambridge University, Watson and Crick. The scientists used her data and that of other scientists to build their ultimately correct and detailed description of DNA's structure in 1953. Franklin was not bitter, but pleased, and set out to publish a corroborating report of the Watson-Crick model. Her career was eventually cut short by illness. It is a tremendous shame that Franklin did not receive due credit for her essential role in this discovery, either during her lifetime or after her untimely death at age 37 due to cancer.

1953 | 1962 Structure of DNA James Watson and Francis Crick worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin and Maurice Wilkens Wilkins

Paired bases DNA structure Bases match together double helix 2 sides like a ladder Bases match together A pairs with T C pairs with G

DNA is a double-stranded helix Watson and Crick also proposed that DNA is shaped like a long zipper that is twisted into a coil like a spring. Because DNA is composed of two strands twisted together, its shape is called double helix.

The structure of DNA Hydrogen bond Base pair Partial chemical structure Ribbon model Computer model

The importance of nucleotide sequences The sequence of nucleotides forms the unique genetic information of an organism. The closer the relationship is between two organisms, the more similar their DNA nucleotide sequences will be. Scientists use nucleotide sequences to determine evolutionary relationships among organisms, to determine whether two people are related, and to identify bodies of crime victims. Chromosome

DNA Packing Metaphase chromosome DNA double helix (2-nm diameter Histones “Beads on a string” Nucleosome (10-nm diameter) Tight helical fiber (30-nm diameter) Supercoil (200-nm diameter) 700 nm Metaphase chromosome

Nucleosomes “Beads on a string” 1st level of DNA packing 8 histone molecules “Beads on a string” 1st level of DNA packing histone proteins 8 protein molecules positively charged amino acids bind tightly to negatively charged DNA

Replication of DNA Section objective: Summarize DNA replication

Replication of DNA Before a cell can divide by mitosis or meiosis, it must first make a copy of its chromosomes. The DNA in the chromosomes is copied in a process called DNA replication. Without DNA replication, new cells would have only half the DNA of their parents. DNA is copied during interphase prior to mitosis and meiosis. It is important that the new copies are exactly like the original molecules.

Replication of DNA DNA replication depends on specific base pairing In DNA replication, the strands are separated by an enzyme Enzymes then use each strand as a template to assemble the new strands Nucleotides Parental molecule of DNA Both parental strands serve as templates Two identical daughter molecules of DNA

Replication of DNA Semiconservative replication: Parental strands of DNA separate, serve as templates and produce 2 molecules of DNA that have one strand of parental DNA and one strand of new DNA

Copying DNA Matching bases allows DNA to be easily copied

DNA replication DNA Helicase: enzyme responsible for uncoiling the double helix and unzipping the weak hydrogen bonds between the base pairs

DNA replication Enzyme DNA polymerase adds new bases to the old strands DNA bases in nucleus DNA polymerase

Leading strand- elongates as DNA unwinds Lagging strand –elongates in opposite direction . Synthesized discontinuously into small segments called Okazaki fragments DNA ligase (an enzyme)links these sections

New copies of DNA Get 2 exact copies of DNA to split between new cells polymerase DNA polymerase