DNA Structure Flashcards
Describe the structural features of DNA
- Double helix structure
- Complementary polynucleotide chains
- Major and minor grooves act as receptors for drugs
- DNA has polarity
- Runs in 5’-3’ direction
Why is DNA more stable than RNA?
Each phosphodiester has a negative charge
- repels OH- group hydrolytically attacking the phosphate
backbone
- DNA doesn’t have OH on 2nd C so not hydrolysed easily
How is DNA stored in the nucleus?
As chromatin
Describe the 4 levels of DNA structure and how they can be viewed in a lab
Primary structure
- Sequence of bases
(found by DNA sequencing)
Secondary structure
- helical structure in forms A.B, and Z
(found by Xray and chemistry)
Tertiary structure
- DNA supercoiling
(viewed by electron microscopy)
Quartenary structure
- interlocked chromosomes
Describe the A form structure of DNA
Tightly coiled
Right hand helix
e.g. tRNA
Describe the structure of B form DNA
Most common form
Major and minor grooves present
Right hand helix
How are right hand helices formed?
Two anti parallel polynucleotide chains are held by hydrogen bonds. The strands are complementary in sequence
Describe the Z form DNA structure
Left hand helix
formed by alternating purine and pyrimidine bases
Is DNA structure stationary?
No, DNA can interconvert between forms
What are Pyrimidine bases?
Single ring structures
e.g. Cytosine, Thymine and Uracil
What is meant by Purine bases?
Double ring structures
e.g. Guanine and Adenine
How are complementary base pairs formed?
2 Hydrogen bonds between A and T
3 Hydrogen bonds between C and G
How is the DNA backbone linked?
Repeating sugar phosphate units are linked by 3’ - 5’
phosphodiester bonds
How are the DNA base pairs stored in chromosomes?
Humans have 3 billion base pairs organised into 23 pairs
of linear chromosomes
How is DNA so compact?
DNA is complexed with histones and made into chromatin
What is a nucleosome?
A length of DNA coiled around a core of histones - the
building blocks of chromatin
Describe the structure of DNA histone complexes formed
DNA is wrapped around 2 molecules of each 2A, 2B, 3 and 4 histone subunits
Histone 1 binds to the outside and to linker DNA
How is a holiday junction formed?
4 stranded junction formed when DNA from 2 different
chromosomes are brought together
What is tetraplex DNA?
Formed at telomeres by folding DNA back in itself
What is the benefit of tetraplex DNA?
Protects ends of chromosomes joining to another
Describe the structure of a tRNA molecule
tRNA folds into an A form helix made of ribonucleotides
List the features of bacterial DNA
- circular
- supercoiled DNA
- organised into 50 independently supercoiled domains
What is the benefit of supercoiled DNA?
Supercoiling stores energy
What enzyme is used to cause supercoiling?
Gyrase using ATP
