NAME______________________________Period_____DATE___________________
Chapter 12: DNA and RNA Cornell Notes
pp. 287-308
12-1 DNA (p.291-294)
1.: The Components and Structure of DNA
Scientists discovered that genes were made of DNA.
They wondered how DNA could do the 3 critical (important) things that genes were known to do:
1. Genes had to carry info from one generation to the next
2. Genes put information to work by determining the inheritable characteristics of an
organism.
3. Genes have to be easily copied because genes are replicated every time a cell divides
For DNA to do all of that, it would have to be a very special molecule
DNA is a long molecule made up of units called nucleotides.
The 3 basic components of a nucleotide are:
o 5-carbon sugar (deoxyribose)
o Phosphate group
o Nitrogenous base
There are four kinds of nitrogenous bases:
o Adenine (A)
o Guanine (G)
o Cytosine (C)
o Thymine (T)
The backbone of a DNA chain is formed by sugar and phosphate groups of each nucleotide.
2.: Chargaff’s Rules
 Erwin Chargaff discovered that the percentages % of G’s and C’s are almost equal in any
sample of DNA. The same is true for the other two nucleotides, A’s and T’s.
 A=T and C=G became known as Chargaff’s rule
 No one had any idea why this was!
3.: X-ray Evidence
 In the early 1950’s Rosalind Franklin began to study DNA.
 She used a technique called X-ray diffraction to get information about the structure of the
DNA molecule.
 She discovered that DNA was a double helix.
4.: The Double Helix
 Two scientists, Watson and Crick were trying to understand the structure of DNA by
building 3-D model of the molecule.
 Once given Franklins results, Watson and Crick discovered the actual structure of DNA
and made a model of it.
 Watson and Crick’s model of DNA was a double helix, in which two strands were wound
around each other.
 A double helix looks like a spiral staircase or a twisted ladder
 Watson and Crick discovered that hydrogen bonds could form between certain
nitrogenous bases and provide just enough force to strands the two strands together
 Hydrogen bonds can form ONLY between certain base pairs: Adenine and Thymine
Cytosine and Guanine
 Watson and Crick called this principle, base pairing
**Now there was a reason that A=T and G=C! (Chargaff’s Rule)
Structure of DNA
Nucleotide
Hydrogen
bonds
Sugar-phosphate
backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
12-2 Chromosomes and DNA Replication (pp. 295-300)
5.: DNA and Chromosomes
 Prokaryotes – no nucleus no organelles
 DNA located in the cytoplasm
 usually have one circular DNA molecule that contains nearly all of the cell’s
genetic info.
 Ex. bacteria
 Eukaryotes – Nucleus & Organelles
 1000 times the amount of DNA as prokaryotes.
 Eukaryotic DNA is located nucleus
 DNA is in the form of a number of chromosomes
6.: How big is DNA?
 DNA molecules are very long
 To fit inside of cells, the DNA must be folded to fit into a space only
____________________ of its length!
 The __________________________ of a human cell contains more than
__________________________ of DNA!!
 To fit into a cell nucleus, the chromosomes, made of __________________________ and
__________________________ are tightly packed together to form
__________________________.
When can you see chromosomes?
 During most of the __________________________ __________________________, the
chromatin fibers are spread out in the nucleus and __________________________
_________________________.
 During __________________________, however, the fibers are drawn together forming
the tightly packed chromosomes, so you can see them!
7.: DNA Replication
 The double helix structure of DNA explains how DNA can be copied, or replicated.
 Each strand of the DNA double helix has all the information needed to reconstruct the
other half by the mechanism of base-pairing.
 If you can separate the two strands, the rules of base-pairing would allow you to
reconstruct the base sequence of the other strand

The two strands of DNA are
complementary to each other.
DNA Strand
DNA Strand
Base-Pairing Rules:
A–T
C–G
8.: Copying DNA
 Before a cell divides it copies its DNA in a copying process called replication
 This process ensures that each resulting cell will have a complete set of DNA molecules.
 During DNA replication:
The DNA molecule separates into two strands then produces two new complementary
strands following the rules of base pairing.
Each strand of double helix of DNA serves as a model for the new strand.
For example:
 The two strands of the double helix separate.
 If the base on the old strand is adenine, thymine is added to the newly forming strand. If
the base is cytosine, then guanine is added.
TCGCTGCTGATCAATC
will produce the complementary strand…
AGCGTCGACTAGTTAG
The result will be two DNA molecules identical to each other and to the original molecule
**Each DNA molecule resulting from replication has one original strand and one new strand.
 The principle enzyme involved in DNA replication is called DNA polymerase because it
joins individual nucleotides to produce a DNA molecule, which is a polymer.
 DNA polymerase is a protein
 DNA polymerase also “edits” each new DNA strand, helping to make sure that each
molecule is a perfect copy of the original DNA!!!
*"Rungs" are the Nitrogenous bases joined together by base pairing rules