DNA Structure PowerPoint

1. IB Topics 3 and 7

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

8. #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.

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