2. #1. DNA Structure (an overview)
DNA has three main components
1. deoxyribose (a pentose sugar)
2. base (there are four different ones)
3. phosphate
3. #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)
4. #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
5. Nucleotides are linked together by covalent bonds
called phosphodiester linkage
#3. Nucleotide Structure
6. #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
7. Complimentary base pairing of pyrimidines and
purines
#4. DNA Double Helix and Hydrogen Bonding
9. •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
#4. DNA Double Helix and Hydrogen Bonding
10. 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
#5. DNA Double Helix
11. #6. Antiparallel strands
The strands
run opposite
of each
other.
The 5’ end
always has
the
phosphate
attached.
5’ 3’
3’ 5’
12. 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.
13.
14. #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
15. #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
16. 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
17. 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
#7. Nucleosome structure
18. #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
19. Repetitive sequences-part of the non-coding section of
DNA
Function-unknown
Can be used in DNA profiling (DNA fingerprinting)
#8. Genes
