NOTES: 12.2 12.3 DNA Structure & Replication 12.2: DNA Structure it was known that DNA was made up of nucleotides joined into long strands by covalent bondsbut HOW were they connectedwhat 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) Chargaffs Rules Percentage of Bases in Four Organisms Source of DNA A
Streptococcus 29.8 31.6 20.5 18.0 Yeast 31.3 32.9 18.7 17.1 Herring 27.8 27.5 22.2 22.6 Human 30.9 29.4 19.9 19.8 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 Franklins 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! Whats holding the strands together?
HYDROGEN BONDS!!! Hydrogen Bonds form between A and T base pairs as well as between C and G base pairs Vocabulary: Chromatin Replication DNA polymerase
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 cells 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 longhow 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" forthe 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 (DNApolymerase) will "proof-read" the order of bases & make corrections A SIMPLIFIED VERSION A Little More Complicated