IB Topics 3 and 7
#1. DNA Structure (an overview)
 DNA has three main components
 1. deoxyribose (a pentose sugar)
 2. base (there are four different ones)
 3. phosphate
#2. The Bases
 They are divided into two groups
 Pyrimidines and purines
 Pyrimidines (made of one 6 member ring)
 Thymine
 Cytosine
 Purines (made of a 6 member ring, fused to a 5
member ring)
 Adenine
 Guanine
 The rings are not only made of carbon (specific
formulas and structures are not required for IB)
#3. Nucleotide Structure
 Nucleotides are formed by the condensation of a
pentose sugar, phosphate and one of the 4 bases
 The following illustration represents one nucleotide
#3. Nucleotide Structure
 Nucleotides are linked together by covalent bonds
called phosphodiester linkage
#4. DNA Double Helix and Hydrogen Bonding
 Made of two strands of nucleotides that are joined
together by hydrogen bonding
 Hydrogen bonding occurs as a result of complimentary
base pairing
 Adenine and thymine pair up
 Cytosine and guanine pair up
 Each pair is connected through hydrogen bonding
 Hydrogen bonding always occurs between one
pyrimidine and one purine
#4. DNA Double Helix and Hydrogen Bonding
 Complimentary base pairing of pyrimidines and
purines
#4. DNA Double Helix and Hydrogen Bonding
#4. DNA Double Helix and Hydrogen Bonding
•Adenine always pairs
with thymine because
they form two H bonds
with each other
•Cytosine always pairs
with guanine because
they form three
hydrogen bonds with
each other
#5. DNA Double Helix
 The ‘backbones’ of DNA molecules are made of
alternating sugar and phosphates
 The ‘rungs on the ladder’ are made of bases that are
hydrogen bonded to each other
#6. Antiparallel strands
5’
3’
The strands
run opposite
of each
other.
The 5’ end
always has
the
phosphate
attached.
3’
5’
Assignment (in your notebook)
 1. Draw the structure of ribose and number the carbons
 2. Draw a schematic representation of a nucleotide. Label
the sugar, base and phosphate.
 3. What are the complimentary base pairs to a DNA strand
that has the following order A T A C C T G A A T?
 4. Draw a schematic representation of an unwound DNA
double helix using the base pairs from your answer in
question 3.
 Include the number of hydrogen bonds between each base
pair. Be sure to label all of the bases and the 5’ and 3’ ends
of the structure.
#6. When phosphodiester links
are formed . . .
 A. When the covalent bonds are formed between
nucleotides the attach in the direction of 5’→3’
 B. The 5’ end of one nucleotide attaches to the 3’ end
of the previous nucleotide
#7. Nucleosome structure
 Nucleosome are the basic unit of chromatin
organization
 In eukaryotes DNA is associated with proteins
 (in prokaryotes the DNA is naked)
 Nucleosomes = basic beadlike unit of DNA
packing
 Made of a segment of DNA wound around a
protein core that is composed of 2 copies of each
of 4 types of histones
#7. Nucleosome structure
 Nucleosomes have:
 8 histones in the core
 DNA wrapped twice
around the core
 One histone holding the
nucleosome together
 A DNA ‘linker’
continuing towards the
next nucleosome
#7. Nucleosome structure
 The DNA has a negatively charged backbone (because
of the phosphate groups)
 The proteins (the histones) are positively charged
 The DNA and proteins are electromagnetically
attracted to each other to form chromatin
#8. Genes
 Genes=units of genetic information (hereditary
information)
 Order of nucleotides make up the genetic code
 Genes can contain the information for one polypeptide
 Genes can also regulate how other genes are expressed
 All cells of an organism contain the same genetic
information but they do not all express the same genes
 THIS IS CELL DIFFERENTIATION
 Cells differentiate by genes that are activated
#8. Genes
 Repetitive sequences-part of the non-coding section of
DNA
 Function-unknown
 Can be used in DNA profiling (DNA fingerprinting)