B2 2.6-2.7 Flashcards
Nucleic Acids
Very large molecules constructed by linking nucleotides to form a polymer.
Nucleotide Structure
- A pentose sugar.
- A phosphate group (negatively charged, acidic).
- A base- contains nitrogen.
How are nucleotides bonded?
Covalently
How are nucleotides made into polymer chains?
- Covalent bonds form between the pentose sugar and the phosphate groups of the molecules.
- This creates a strong backbone of alternating sugars and phosphates, with a base on each one.
How many types of nucleotides are there?
Four, one for each base.
How many sequences of bases are possible along a DNA molecule?
Literally any sequence
What does the base sequence do?
Stores information
What do the Phosphates and Sugars do?
Support the structure of the base sequence.
How are DNA and RNA different?
- DNA has 1 less Oxygen atom.
- DNA is double stranded, RNA single stranded.
DNA bases vs RNA bases
Adenine Cytosine Guanine DNA: Thymine RNA: Uracil
What does DNA consist of?
Two strands of antiparallel covalently bonded nucleotides.
Why are the strands of DNA antiparallel?
Because they run in opposite directions to one another, one from 5’ to 3’, the other from 3’ to 5’.
What is a word for DNA’s structure?
Double Helix structure.
Complementary Base Pairing
Each specific base can only pair with one other:
Adenine + Thymine (Uracil)
Guanine + Cytosine
How are the strands of DNA held together?
By hydrogen bonds between the bases.
What are molecular models?
Current theories of the time.
They are a representation of an idea, and help to fabricate them physically.
What was the first model for DNA structure?
- First model consisted of three-stranded DNA- a triple helix with bases on the outside and magnesium holding it together with ionic bonds to the phosphate groups.
- This was later rejected due to an inconsistency in magnesium levels.
- It also did not consider Chargaff’s Principle (also known as the base pair rule-bases should be in a 1:1 ratio).
What did Crick and Watson do when the first DNA structure was rejected?
- They then used cardboard cut outs to realise how the different elements of DNA would fit together, creating the Double Helix structure.
- They then prepared a second model, which ‘just looked right’.
- This began a molecular biology revolution.
When a cell divides…
… so do the strands of a double helix
How does complementary base pairing work?
- The base strand on the old strand determines that of the new strand- only nucleotides with a complementary base to the next base on the old strand can be added, e.g. Adenine to an existing Thymine base.
- The H-Bonds in between them stabilize the structure.
Who provided the first evidence for semi-conservative replication?
Meselson and Stahl in 1958- published evidence.
