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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 Phosphate group Nitrogenous base (A, G, C, or T) Thymine (T) Nucleotide Sugar (deoxyribose) Sugar-phosphate backbone DNA nucleotide The 4 Unique Nitrogenous Bases 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) 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 – 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 Chromosome 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. DNA Packing DNA double helix (2-nm diamete r 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 – histone proteins • 8 protein molecules • positively charged amino acids • bind tightly to negatively charged DNA 8 histone molecules 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 DNA polymerase DNA polymerase