The Watson and Crick model of DNA Flashcards
Of what does DNA consist?
A linear chain of Adenine(A), Cytosine (C), Guanine (G) and Thymine (T) subunits = bases.
How many types of bases exist?
2.
Which are the 2 types of bases?
Purines (A, G).
Pyrimidines (C, T and U).
What is the characteristic of Purine bases?
Double-ring structure.
What is the characteristic of Pyrimidine bases?
Single-ring structure.
Of what are bases a part?
Larger subunits = nucleosides and nucleotides.
What is the nucleotide monophosphate?
Building-block of DNA polymer.
By what is DNA made?
DNA Polymerase.
What does DNA Polymerase use?
Nucleotide triphosphates.
What does DNA Polymerase release?
A diphosphate.
What are the key chemical structures in DNA?
Bases.
Pentose sugar.
Phosphate.
Where is pentose sugar connected?
Bases.
What does pentose sugar form?
Nucleoside.
Where is phosphate connected?
To nucleoside.
What do the ribose sugar group of nucleic acids have?
H group at position 2’ = ‘2 prime’.
OH group at position 3’.
Phosphate group at position 5’.
What does the structure of: H, OH and phosphate give the DNA chain?
Specific orientation = 3’ to 5’.
What is the DNA chain?
A series of nucleotide monophosphates linked together in a specific orientation with 5’-P end and 3’-OH end.
How are bases always occurred?
In particular molar ratios.
How are molar ratios known as?
‘Chargaff’s Rules’.
Who established ‘Chargaff’s Rules’ and when?
Erwin Chargaff.
In 1947, 1951.
What did Chargaff do?
- Chemically reduced purified DNA samples to bases.
2. Assessed relative quantities with layer chromatography.
What did Phoebus Levene suggested in 1909?
DNA was a series of tetranucleotides.
DNA was a monotonous polymer.
What did Chargaff show?
DNA could not be composed of a series of ‘tetranucleotides’, because ratios of 4 bases were different from 1:1:1:1 predicted ratio.
What did Chargaff show about the rations of purines to pyrimidines?
=1.
No matter the source of DNA.
How is the ratio of purines to pyrimidines = 1, no matter the DNA source, known?
‘Base Pair Rule’.
How should the amount of guanine to cytosine be?
Equal.
G = C.
How should the amount of adenine to thymine be?
Equal.
A = T.
How could DNA be visualised at single molecule level?
By electron microscopy.
What did electron microscopy suggest?
DNA was a long, thin and flexible polymer.
By what was the structure of DNA finally resolved?
By X-ray crystallography.
Who used the X-ray crystallography for DNA structure and when?
James Watson.
Francis Crick.
Rosland Franklin.
Early 1950s.
What can refract X-rays?
Solids.
How are the X-rays refracted when the solid is composed of a regular array of molecules?
In ordered, predictable manner.
How is the pattern of X-rays diffracted by solids used?
Reconstruct electron densities.
Destruct atomic model for crystallized molecule.
By what and who was the DNA structure revealed?
X-ray studies.
Rosalind Franklin 1950.
James Watson and Francis Crick 1953.
What did Watson and Crick do?
Used Rosalind’s data to build physical models.
What did Watson and Crick suggest based on Chargaff’s ratios of deoxyribose nucleotides and Franklin’s X-ray data?
A structural model of DNA where 2 chains of DNA were bound together.
What is DNA?
A duplex of 2 strands.
Of what is DNA composed?
2 oppositely orientated antiparallel strands.
Of what are the stairs of ladder made?
Paired bases.
A-T : 2H bonds.
C = G : 3H bonds.
= Base pairs.
Of what are the sides of ladder made?
Alternating ribose sugar units and phosphate groups.
What does each end of the duplex have?
One 5.
One 3’ end.
Into what is duplex twisted?
A helix.
Where is the ladder twisted?
Central core.
Why is the ladder twisted at central core?
To produce right-handed helix.
What is the helix?
23.7 A wide.
To what does the helix extend?
34 A per 10bp of sequence.
What does helix make when extends?
One complete turn every 10.4-10.5 base pairs.
What has been observed in vitro and in cells?
DNA different structures.
What happens to the pitch and helix direction in different DNA structures?
They change.
In how many forms does DNA exist?
3.
Which are the 3 different forms of DNA?
- A.
- B.
- Z-DNA.
What is the B form of DNA?
More relaxed.
More common.
Where is Z-DNA form of DNA found?
In some cancerous cells.
What are the types of the ladder sides?
Major groves.
Minor groves.
What are the outer edge of the 2 helices from one another?
Not evenly spaced.
What can the DNA-binding proteins ‘read’?
Major grove’s base-pairs.
What do phosphodiester bonds link?
Phosphate group of one nucleotide to sugar of adjacent nucleotide along double helixes’ side.
By which molecule are bases held together?
Hydrogen bonds.
Which factors stabilize base stacking?
Weak hydrophobic forces.
Into what is DNA packed?
Chromosomes.
What are the characteristics of DNA?
Too long.
Fragile.
Where is DNA ‘stuffed’ ?
In cells.
In nuclei.
Around what is DNA wrapped?
Specialised proteins.
Why are DNA-protein loops lopped together?
To form chromosomes.
When can DNA duplexes join?
During chromosomal repair.
What do DNA duplexes produce?
Halliday Junctions = ‘Chiasma’ in Prophase 1.
Into what do ‘Chiasma’ need to be resolved back?
Into 2 sister chromosomes before cell division.
What can proteins recognise in general?
DNA.
How else can proteins recognise DNA?
Through specific DNA sequences via major groove.
What does major groove stabilise?
DNA-protein complexes.
What do major groove allow when stabilising DNA-protein complexes?
Interactions. DNA replication. Repair. Gene expression. Regulation.
