12.2: DNA Structure ● it was known that DNA was made up of nucleotides joined into long strands by covalent bonds…but HOW were they connected…what was the structure?
DNA Structure ● Made up of nucleotides (monomer) ● Each nucleotide composed of 3 parts: • 5-carbon sugar (DEOXYRIBOSE) • Phosphate group • Nitrogen-containing base ● Adenine (A) ● Thymine (T) ● Guanine (G) ● Cytosine (C)
Chargaff’s Rules What do these data suggest to you?
● A = T (A pairs with T) ● C = G (C pairs with G)
What DNA Looks Like: ● Rosalind Franklin: used X-ray diffraction patterns to discover that DNA strands twisted around each other like a HELIX
● Watson & Crick: while trying to build a 3-D model of DNA, Watson saw Franklin’s photograph and measurements and within weeks, he and Crick figured out the structure of DNA
DNA Structure = DOUBLE HELIX (“twisted ladder”) ● Backbone • deoxyribose & phosphate group of each nucleotide ●“Steps of Ladder” • bases (A, G, C, T) ●Any base sequence is possible!
What’s holding the strands together? ●HYDROGEN BONDS!!! ● Hydrogen Bonds form between A and T base pairs as well as between C and G base pairs
12.3 - DNA Replication Vocabulary: Chromatin Replication DNA polymerase • Key Concept: • What happens during DNA Replication?
The Review ● Prokaryotic cells do not have a nucleus • DNA molecules are located in the cytoplasm • Usually a circular DNA molecule and it is referred to as the cell’s chromosome ● Eukaryotic cells have a nucleus • Can have 1000x more DNA than prokaryotic cells • DNA is located in the form of a number of chromosomes • # of chromosomes varies widely from species to species
DNA molecules are long…how does DNA fit in the nucleus? ● It forms chromosomes! ● DNA coils around proteins called histones and then this chromatin supercoils around itself until chromosomes form
DNA Replication ● before a cell divides, it must copy its DNA so that all cells have a copy of the genetic instructions ● this process involves A LOT of ENZYMES! ● each half of the DNA double helix can serve as a "template" for the replication of another DNA double helix molecule • The strands are said to be complementary
DNA REPLICATION: THE PROCESS 1) An enzyme unwinds & "unzips" DNA (separates the 2 DNA strands) by breaking the hydrogen bonds between base pairs Hydrogen Bonds Breaking!
2) a complementary base is inserted to each side of the DNA strand with the help of DNA POLYMERASE Why does the “blue” always pair with the “green?”
3) The sugar-phosphate groups are covalently bonded to the growing DNA chain (new "backbone") 1 nucleotide COVALENT BOND
4) an enzyme (DNA polymerase) will "proof-read" the order of bases & make corrections