1. Topic 3: The chemistry of life
3.3 DNA structure
IB Biology SFP - Mark Polko

2. IB Biology SFP - Mark Polko
ASSESSMENT STATEMENTS
3.3.1 Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate.
3.3.2 State the names of the four bases in DNA.
3.3.3 Outline how DNA nucleotides are linked together by covalent bonds into a single strand.
3.3.4 Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds.
3.3.5 Draw and label a simple diagram of the molecular structure of DNA.
IB Biology SFP - Mark Polko

3. IB Biology SFP - Mark Polko
3.3.1 Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate.
Genetic information is stored by nucleic acids. There are two kinds of nucleic acids:
deoxyribonucleic acid (DNA)
ribonucleic acid (RNA)
For the majority of species genetic information is stored in DNA in the nucleus while RNA is found in the cytoplasm. Some viruses and prokaryotes store genetic information in RNA.
Nucleic acids are long-chain molecules (like proteins, but longer) and their building blocks are called nucleotides (like amino acids but, again, more complicated). Nucleotides themselves are complex molecules consisting of three molecules linked together:
a pentose sugar
a phosphate
an organic base
IB Biology SFP - Mark Polko

4. IB Biology SFP - Mark Polko
3.3.1 Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate.
There are three components.
The sugar
Two possibilities: ribose gives RNA and deoxyribose gives DNA.
The phosphate
H3PO4 (P forms 5 bonds with other atoms.)
The organic base
Also known as the nitrogenous base.
The nucleotide is usually schematically represented
IB Biology SFP - Mark Polko

5. IB Biology SFP - Mark Polko
3.3.2 State the names of the four bases in DNA.
There are five different bases:
Adenine
Cytosine
Thymine
Guanine
Uracil
A, C, T and G are found in DNA; A, C, U and G are found in RNA. The structural formulae of these nitrogenous bases are shown. You do not need to memorise these structures but you need to remember the names.
IB Biology SFP - Mark Polko

6. IB Biology SFP - Mark Polko
3.3.2 State the names of the four bases in DNA.
Complementary base pairing
Adenine, Thymine and Uracil are capable of forming two Hydrogen bonds while Cytosine and Guanine can form three. So Adenine (A) can form a base pair with Thymine (T) (or uracil (U) in RNA) and Cytosine (C) can form a base pair with Guanine (G).
All organic bases have a complicated molecular structure involving ring compounds. Adenine and Guanine are purines, ‘big’ 2-ring structures; Cytosine, Thymine and Uracil are pyrimidines, ‘small’ 1-ring structures.
IB Biology SFP - Mark Polko

7. IB Biology SFP - Mark Polko

8. IB Biology SFP - Mark Polko
3.3.3 Outline how DNA nucleotides are linked together by covalent bonds into a single strand.
LINK
Since the reactions involved are (again) condensation reactions, the equation becomes:
phosphate + sugar + organic base → nucleotide + 2 water
The nucleotides can be linked together to form a single chain by a condensation reaction between the phosphate of one nucleotide and the sugar of another. The sugar and phosphate form the ‘backbone’ of the nucleic acid with the organic bases sticking out.
Covalent bonds are formed between the phosphate and sugar and a single strand is made with a ‘backbone’ of phosphate and (deoxy)ribose and an organic base attached to every ribose.
IB Biology SFP - Mark Polko

9. IB Biology SFP - Mark Polko
3.3.4 Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds.
Knowing that the structure of DNA is a ‘double helix’ (a twisted ladder) we need to fit two DNA molecules together. Since some of the organic bases can form bonds with certain others, we can combine the DNA strands this way. The sugar and phosphate ‘backbones’ run antiparallel forming the sides of the ladder with the organic base pairs as rungs in between.
IB Biology SFP - Mark Polko

10. IB Biology SFP - Mark Polko
3.3.4 Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds.
In a DNA molecule, the backbone is made of alternating phosphate - sugar groups. The ‘rungs’ of the twisted ladder which DNA resembles are the organic base pairs..
So the bonds between the components of a nucleotide are covalent bonds, formed by condensation reactions. The bonds between nucleotides are also covalent bonds, formed by condensation. However the bonds keeping the two strands of the DNA together are hydrogen bonds. Hydrogen bonds are much weaker than covalent bonds but there are so many hydrogen bonds between the two strands of DNA that they are kept together securely.
Here she goes again!
IB Biology SFP - Mark Polko

12. IB Biology SFP - Mark Polko
3.3.5 Draw and label a simple diagram of the molecular structure of DNA.
Draw with me!
Or with her again…
IB Biology SFP - Mark Polko

13. Topic 3: The chemistry of life
3.3 DNA structure
IB Biology SFP - Mark Polko