8.2 Structure of DNA
KEY CONCEPT
DNA structure is the same in all organisms.
8.2 Structure of DNA
DNA is composed of four types of nucleotides.
• DNA is made up of a long chain of nucleotides.
• Nucleotide: One “step” of the DNA ladder. Consisting of
phosphate group, sugar, and base.
• Over 3 billion nucleotides in human DNA
• Each nucleotide has three parts.
– a phosphate group
phosphate group
nitrogen-containing
base
– a deoxyribose sugar
– a nitrogen-containing base
deoxyribose (sugar)
8.2 Structure of DNA
Watson and Crick determined the three-dimensional
structure of DNA by building models.
• They realized that DNA is
a double helix that is
made up of a sugarphosphate backbone on
the outside with bases on
the inside.
8.2 Structure of DNA
• Watson and Crick’s discovery built on the work of Rosalind
Franklin and Erwin Chargaff.
– Franklin’s x-ray images suggested that DNA was a
double helix of even width.
– Chargaff’s rules stated that A=T and C=G.
8.2 Structure of DNA
Nucleotides always pair in the same way.
• The base-pairing rules show
how nucleotides always pair
up in DNA.
– A pairs with T
– C pairs with G
• Because a pyrimidine
(single ring) pairs with a
purine (double ring), the
helix has a uniform width.
G
C
A T
8.2 Structure of DNA
• The nitrogen containing bases are the only difference in
the four nucleotides.
A
T
G
C
• Sugar (Deoxyribose) connects
with the phosphate group
• 4 Different Bases:
– Adenine (A)
– Thymine (T)
– Guanine (G)
– Cytosine (C)
• Rule: A always joins with T
• Rule: C always joins with G
8.2 Structure of DNA
• The backbone is connected by covalent bonds.
• The bases are connected by hydrogen bonds.
• There are an average of 25 hydrogen bonds within each complete
turn of the double helix providing a stability of binding about as
strong as what a covalent bond would provide.
hydrogen bond
covalent bond
8.2 Structure of DNA
• Every human cell has
46 chromosomes (23
from mother and 23
from father).
• DNA are wound up
like a Slinky to form
these chromosomes.
8.2 Structure of DNA
How DNA affects our lives:
• DNA doesn’t actually DO anything. Just as a
book has no use unless someone reads it,
the info in DNA has no purpose unless it is
read.
• The information in DNA is used to build
proteins.
8.2 Structure of DNA
• are the building blocks of your cells
• can speed up reactions when they act as enzymes
• perform important functions (e.g. hemoglobin –
transports oxygen in your blood)
• consist of amino acids
– All proteins in your body are made by the same 20 amino acids.
– What separates one protein from another are shape, size, and
which amino acids are in it.
– These amino acids come from the proteins that we eat, then are
broken down to be used by the body.
8.2 Structure of DNA
The Information of DNA
• DNA is encoded in what is called base 4.
This is because there are four possible
chemicals that can be in one particular spot.
– We count in base 10 because we have 10 different
possibilities for a number 0-9.
– We speak in base 26 because of our 26 letters.
– You might think that with only 4 “letters” DNA can’t say
much but each strand of DNA has approximately
3000000000 letters.
– That means that there are 43000000000 (4 x 4, 3 billion times)
possible combinations for humans. Which means that no
human has ever looked like any other in all of human
history (except of course for twins because they share the
same DNA).
• Defined: Process of copying DNA
• During interphase
• Step 1: DNA Helicase (enzyme)
“unzips” the two DNA strands
(breaks the H bonds)
• Step 2: Free floating nitrogen bases
(A, T, C, G) match up with
complimentary base pairs.
• Step 3: DNA Polymerase (enzyme)
reconnects the two strands.
• Enzymes also “proofread” each new
DNA strand to make sure it is a
perfect copy.
• Mutation: change in
DNA sequence
• Mutations can be
corrected
– Enzyme “proofreads”
DNA and
check/correct errors
• Pre-repair: 1 in 10,000
nucleotides has an error
• Post-repair: 1 in 1
billion nucleotides has
an error
Quick Review
• DNA nucleotides
form long chains
• DNA nucleotides
have 4 different
bases (A, T, C, G)
• DNA makes exact
copies of itself