1. 2.6 Structure of DNA and RNA
Essential idea: The structure of DNA allows efficient storage of genetic information.
There is 2m of DNA in each human cell, however the most cells in the human body have a diameter of 10 μm. This DNA is divided in chromosomes and coiled around proteins called histones so that it can be efficiently stored in each cell's nucleus. The human genome project which has decoded the case sequence for the whole 2m of the human genome requires a data warehouse (pictured) to store the information electronically. This should give a good idea of just how efficient DNA is at storing information and why it needs to be so.

2. Understandings, Applications and Skills
Statement
Guidance
2.6.U1
The nucleic acids DNA and RNA are polymers of nucleotides.
2.6.U2
DNA differs from RNA in the number of strands present, the base composition and the type of pentose.
2.6.U3
DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.
2.6.A1
Crick and Watson’s elucidation of the structure of DNA using model making.
2.6.S1
Drawing simple diagrams of the structure of single nucleotides of DNA and RNA, using circles, pentagons and rectangles to represent phosphates, pentoses and bases.
In diagrams of DNA structure, the helical shape does not need to be shown, but the two strands should be shown antiparallel. Adenine should be shown paired with thymine and guanine with cytosine, but the relative lengths of the purine and pyrimidine bases do not need to be recalled, nor the numbers of hydrogen bonds between the base pairs.

7. DNA - Chromosomes A chromosome is essentially a single DNA molecule.
DNA stands for "deoxyribonucleic acid”, a term which describes the type of sugar (deoxyribose) and the location in the cell (nucleus).

8. Nucelotides
The building blocks of DNA are "nucleotides" which are attached together like a twisted ladder to form a “double helix”.
DNA is a polymer. The Monomer Units of DNA are nucleotides.

9. Each nucleotide is composed of three parts:
phosphate group
deoxyribose sugar
nitrogenous base

10. There are four different kinds of bases so there are four different kinds of nucleotides:
Purines: A = Adenine and G = Guanine
Pyrimidines: T = Thymine and C = Cytosine
Base Pairing, is Specific, and always occurs between a Purine and a Pyrimidine.
Specifically A-T and C-G, This is because of the hydrogen bonding that occurs between these pairs of nucleotides.

12. The sugars and phosphate groups are on the outside of the molecule forming the "sugar-phosphate backbone". Each sugar is attached to the phosphate below by a “covalent” bond. Also known as a phosphodiester bond
These are STRONG BONDS which is important for preserving the integrity of the DNA sequence

13. DNA compared to RNA

14. So What’s the Point?
All living things have the DNA with the same 4 nucleotides, and the same structure as well as the same “code” for the amino acids that are the monomers for the polymer proteins they make.

15. In Human this time
DNA is the brains of every cell because it contains all of the genes that are the codes for making proteins. Proteins are the structural components hormones, and catalysts… for essentially everything your cells do, and therefor what you do.
DNA has two major purposes:
DNA REPLICATION & PROTEIN SYNTHESIS

16. Application: Crick and Watson’s elucidation (explanation) of the structure of DNA using model making.
The structural organisation of the DNA molecule was correctly proposed in 1953 by James Watson and Francis Crick
These British scientists constructed models to quickly visualise and assess the viability of potential structures
Their efforts were guided by an understanding of molecular distances and bond angles developed by Linus Pauling, and were based upon some key experimental discoveries:
DNA is composed of nucleotides made up of a sugar, phosphate and base – Phoebus Levene, 1919
DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T) – Erwin Chargaff, 1950
DNA is organised into a helical structure – Rosalind Franklin, 1953  (data shared without permission)
Making DNA Models
Using trial and error, Watson and Crick were able to assemble a DNA model that demonstrated the following:
DNA strands are antiparallel and form a double helix
DNA strands pair via complementary base pairing (A = T ; C Ξ G)
Outer edges of bases remain exposed (allows access to replicative and transcriptional proteins)

17. Con’t
As Watson and Crick’s model building was based on trial and error, a number of early models possessed faults:
The first model generated was a triple helix
Early models had bases on the outside and sugar-phosphate residues in the centre
Nitrogenous bases were not initially configured correctly and hence did not demonstrate complementarity
The Rosalind Franklin Controversy
The final construction of a correct DNA molecule owed heavily to the X-ray crystallography data generated by Franklin
This data confirmed the arrangement of the DNA strands into a helical structure
The data was shared without Franklin’s knowledge or permission and contributed profoundly to the final design
Hence, Franklin is now recognised as a key contributor to the elucidation of DNA structure

18. 2.6.U1 The nucleic acids DNA and RNA are polymers of nucleotides.

19. 2.6.U1 The nucleic acids DNA and RNA are polymers of nucleotides.
A nucelotide: a single unit of a nucleic acid
Nucleic acids are very large molecules that are constructed by linking together nucleotides to form a polymer.
There are two types of nucleic acid: DNA and RNA.

20. 2.6.U1 The nucleic acids DNA and RNA are polymers of nucleotides.
A nucelotide: a single unit of a nucleic acid
acidic
negatively charged
contains nitrogen
has one or two rings in it’s structure
covalent bond
five carbon atoms = a pentose sugar
If the sugar is Deoxyribose the polymer is Deoxyribose Nucleic Acid (DNA)
If the sugar Ribose the polymer is Ribose Nucleic Acid (RNA)
covalent bond

21. 2.6.U1 The nucleic acids DNA and RNA are polymers of nucleotides.
What is the relevance of this coffee cup to this topic?

22. 2.6.U1 The nucleic acids DNA and RNA are polymers of nucleotides.
There are four nitrogenous bases in DNA:
Adenine (A) Guanine (G) Thymine (T) Cytosine (C)
RNA Shares the same bases except that Uracil (U) replaces Thymine
n.b. when talking about bases always use the full name on the first instance

23. 2.6.U1 The nucleic acids DNA and RNA are polymers of nucleotides.
Nucleotides a linked into a single strand via a condensation reaction
bonds are formed between the phosphate of one nucleotide and the pentose sugar of the next
The phosphate group (attached to the 5'-C of the sugar) joins with the hydroxyl (OH) group attached to the 3'-C of the sugar
This results in a phosphodiester bond between the two nucleotides and the formation of a water molecule
Successive condensation reactions between nucleotides results in the formation of a long single strand

24. RNA DNA Bases Adenine (A) Guanine (G) Uracil (U) Cytosine (C)
2.6.U2 DNA differs from RNA in the number of strands present, the base composition and the type of pentose.
RNA
DNA
Bases
Adenine (A)
Guanine (G)
Uracil (U)
Cytosine (C)
Thymine (T)
Sugar
Ribose
Deoxyribose
Number of strands
Single stranded, and often, but not always, linear in shape
Two anti-parallel, complementary strands form a double helix

25. 2.6.U3 DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.

26. 2.6.U3 DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.

27. 2.6.U3 DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.

28. 2.6.U3 DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.
In Summary:
Each polynucleotide chain (strand) consists of a chain of nucleotides bonded covalently.
Two polynucleotide chains of DNA are held together by hydrogen bonds between complementary base pairs:
Adenine pairs with thymine (A=T) via two hydrogen bonds
Guanine pairs with cytosine (G=C) via three hydrogen bonds
In order for bases to be facing each other and thus able to pair, the two strands must run in opposite directions (i.e. they are anti-parallel)
As the polynucleotide chain lengthens, the atoms that make up the molecule will arrange themselves in an optimal energy configuration. This position of least resistance results in the double-stranded DNA twisting to form a double helix with approximately bases per twist.

29. 2.6.S1 Drawing simple diagrams of the structure of single nucleotides of DNA and RNA, using circles, pentagons and rectangles to represent phosphates, pentoses and bases.
Use this simple, but very effective You Tube video to learn how to draw the nucleotides making up a short section of a DNA molecule. To make sure you have learn this skill you need to practice it repeatedly.
n.b. ideally label lines should be drawn with and rule and should not have arrow heads.

30. It required too much magnesium (identified by Franklin)
2.6.A1 Crick and Watson’s elucidation of the structure of DNA using model making.
Whilst others worked using an experimental basis Watson and Crick used stick-and-ball models to test their ideas on the possible structure of DNA. Building models allowed them to visualize the molecule and to quickly see how well it fitted the available evidence.
It was not all easy going however. Their first model, a triple helix, was rejected for several reasons:
The ratio of Adenine to Thymine was not 1:1 (as discovered by Chargaff)
It required too much magnesium (identified by Franklin)
From their setbacks they realized:
DNA must be a double helix.
The relationship between the bases and base pairing
The strands must be anti-parallel to allow base pairing to happen
Because of the visual nature of their work the second and the correct model quickly suggested:
Possible mechanisms for replication
Information was encoded in triplets of bases

31. Find out more about the discovery of DNA:
2.6.A1 Crick and Watson’s elucidation of the structure of DNA using model making.
Watson and Crick gained Nobel prizes for their discovery. It should be remembered that their success was based on the evidence they gained from the work of others. In particular the work of Rosalind Franklin and Maurice Wilkins, who were using X-ray diffraction was critical to their success.
Find out more about the discovery of DNA:

32. Bibliography / Acknowledgments
Jason de Nys