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 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 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 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

Vocabulary: Chromatin Replication DNA polymerase 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

A SIMPLIFIED VERSION…                                  

A Little More Complicated…