What are individual nucleotides made up of?
- A pentose sugar
- A phosphate group
- Nitrogen containing organic base, ACTGU
How are mononucleotides formed?
-Condensation reactions join the pentose sugar, the phosphate group and the organic base molecule
- Removing molecules of water
What are DNA helix made up of?
Polynucleotides
How are polynucleotides formed?
- Two mononucleotides join by a condensation reaction
- This occurs between the deoxyribose sugar of one mononucleotide and the phosphate group of another mononucleotide
- Forms a phosphodiester bond creating a dinucleotide
- Continued linking of these nucleotides form a poly nucleotide
Compare the components of a DNA nucleotide and an RNA nucleotide
- The pentose sugar in DNA nucleotides is dexoyribose whereas in RNA it is ribose
- They both have a phosphate group attached
- RNA molecules have one of the organic bases AUGC
- DNA molecules have one of the organic molecules ATGC
- ## Each organic base contains nitrogen
Describe the basic structure of a DNA helix
- Double helix structure
- Two polynucleotide chains
- Held together with hydrogen bonds between complementary base pairs
Describe the basic structure of an RNA molecule
- A short polynucleotide chain
What is the purpose of semi conservative replication of DNA?
- Ensures genetic continuity between generations of cells
- This is due to it preserving one original DNA strand as a template during replication
- Each new DNA molecule contains one old DNA strand and a newely syntheised strand
Which scientists did work on the structure of DNA?
Watson and Crick followed up on pioneering work carried out by rosalin franklin
How may DNA be likened to a ladder?
- Alternating phosphate and deoxyribose molecules make up the uprights of the ladder
-Organic base molecules make up the rungs of the ladder - Each upright runs in an opposite direction meaning they are antiparallel
Why is DNA considered a stable molecule?
- The phosphodiester backbone protects the more chemically reactive organic bases in the double helix
- Hydrogen bonds link the organic bases forming rungs between the phosphodiester uprights, the higher the number of hydrogen bonds between each base pair the more stable the molecule is
How are DNA molecules suited to their functions?
- Stable structure, can pass from generation to generation without significant change, mutations can repair
- Two strands are joined by H bonds meaning they can be easily broken to separate in dna replication and protein synthesis
- Large, alot of genetic information carried
- Base pairs are contained within both of the strands meaning they are protected by the phosphodiester backbone from any other chemicals that will react with bases
Why do bases such as adedine and cytosine not join together
- Bases link by hydrogen bonds
- Hydrogen bonds may not form between bases that are not complementary
Describe the process of semi conservative replication
- Enzyme DNA helicase breaks hydrogen bonds linking the complementary base pairs of DNA between the two strands
- As a result the double helix separates into two different
polynucleotide strands and unwinds - Each polynucleotide strand then acts as a template of which complementary free nucleotides bind by specific base pairing to the bases on template strands by attraction, forming hydrogen bonds between specific bases
- Adjacent nucleotides are then joined together in a condensation reaction by the enzyme DNA polymerase formimg a phosphodiester backbone
- A polynucleotide chain is now formed
If an inhibitor of DNA polymerase was introduced into a cell explain what the effect would be on the cell
- The linking together of the new nucleotides would not take place
- Nucleotides would still match up to their complementary nucleotides on the original DNA strand, however adjacent nucleotides would not join to form a new polynucleotide strand
Describe the structure of DNA 5 marks
- Polynucleotide strands wrapped around eachother in a double helix structure
- Adjacent nucleotides form polynucleotides with phosphodiester bonds
- Each nucleotide is made up of a nitrogen containing organic base, a pentose sugar and a phosphate group
- Complementary nucleotides join the double helix strand by hydrogen bonds between AT and CG
Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule 2 marks
- Condensation reaction/ loss of water occurs
- Between the phosphate group of one nucleotide and the deoxyribose sugar from the other nucleotide
- Catalysed by DNA polymerase
Describe how the separation of strands occurs during DNA replication
-DNA helicase
- Breaks the hydrogen bonds between complementary base pairs, AT and CG
Give features of DNA and how these are important in the semi conservative base pairing of DNA
- Weak hydrogen bonds between bases allow the two strands to unzip
- 2 strands so each stand in the DNA can act as a template
- Complementary base pairing allows for accurate replication
Contrast the nucleotide structure of ATP and DNA
- ATP has 3 phosphate groups in a chain whereas DNA only has one phosphate group
- DNA has a deoxyribose sugar whereas ATP has a ribose sugar
- ATP the base is always adedine whereas DNA has bases which can vary
Use your knowledge of enzyme action and dna replication to describe why new nucleotides can only be added in the 5 to 3 end
- DNA polymerase is an enzyme which is specific , and is only complementary to the 5 end as the 3 and 5 end have different shapes
Describe other differences between rna and dna
DNA is found in chromosomes in the nucleus whereas RNA is found in the cytoplasm
DNA is an extremely long molecule whereas RNA is an extremely short molcule
DNA is two polynucleotide strands wrapped around eachother whereas RNA is only one polynucleotide strand
What makes DNA replication semi conservative?
- Each of the two copies contains one strand from the original DNA molecule plus one new strand
