dna structure Flashcards
Describe basic DNA Structure & Function:
→DNA is the store of genetic information
→ it is made of a double helix (consists of major and minor grooves which are receptors for drugs)
→DNA bases encode genetic information
- cellular dna is supercoiled and present as chromatin
what are the four bases and describe their composition
→ adenine, thymine, guanine and cytosine
→Adenine and Guanine are Purines, they have 2 benzene rings.
→Thymine and Cytosine are Pyrimidine, they have 1 benzene ring.
→Adenine and Thymine have 2 H bonds between them.
→Guanine and Cytosine have 3 H bonds between them.
how many forms does DNA have
3 forms
A,B and Z
describe A DNA
Right-handed helical structure 2 antiparallel polynucleotides form a RH helix)
- More densely compacted than B form DNA.
- similar to B form DNA, sugar phosphate portions are found on the outside and bases on the inside.
- 11.6 base pairs per turn
- B DNA takes this form under dehydrating conditions.
- Narrow and deep major grooves
- Wide and shallow minor grooves
- tRNA folds in an A-form helix
Describe B DNA
Right-handed helical structure ( 2 antiparallel polynucleotides form a RH helix)
- Most common/predominant structure in cells and nature
- DNA prefers this form under natural physiological conditions- optimum pH and salt concentrations.
- 10 base pairs per turn,
- major groove and minor groove allows access for proteins that want to read the dna sequence.
- Major groove is wide and deep (easy to study DNA here)
- Minor groove is narrow and shallow
- the sugar phosphate portions of the chains are found on the outside of the helix and the bases are on the inside of the helix
Describe Z DNA
Left-handed helical structure
- present in small amounts in the cell
- Formed by 5’…GCGCGCGCGCGC OR 5’…GTGTGTGTGTGTGT
- Nucleotide pairs are dimers
- Double helix winds in a zigzag pattern due to alternate base stacking.
- Flat major groove
- Narrow and deep minor groove
- 12bp per turn
- alternating purine and pyrimidine bases.
- can be detectd in cells favoured by high salt. Some metal ions can flip the sequence into Z-DNA . DNA supercoiling can also flip the DNA
Give a summary of the dna helix structure.
2 antiparallel strands that runs from 5’-3’ and is linked by 3’5 phosphodiester bonds.
Forms a right handed helix
Bases on inside/sugar-phosphate backbone on the outside. Oxygen is linked to the 3’ carbon of deoxyribose and phosphate is linked to the 5’ carbon of the next deoxyribose. Because of the nature of the 3’5’ link, the dna strand has polarity. Dna is also an acid as phosphate groups are protonated and can give up either protons and become negatively charged. Diameter is 2nm.
One strand is complementary to the other.
The polynucleotide chains are held together by H bonds between base pairs.
DNA helix is not rigid but conformationally mobile such as a b and z
describe bacterial DNA and how it fits inside the bacterium .
- dna is circular and comprises of 3 mil base pairs
- Is supercoiled – the dna ribbon is itself twisted in space
- Have plasmids that only carry certain genes, e.g those for antiobiotic resistance.
- E.coli chromosome is circular and organised into approx. 50 independetly supercoiled domains.
- a combination of loop organisation and super coiling allows the dna to be compact and fit in the bacterium .
what is bacterial DNA supercoiled by?
DNA gyrase
Describe eukaryotic DNA.
→ humans have 3 billion base pairs of DNA
→ DNA is complexed with histones and made into a fiber called chromatin
→ the double helix is wound around 8 histone subunits
What are some reasons as to why mutations may occur in DNA?
SPONTANEOUS
→ loss of bases
→ hydrolysis of C to U
CHEMICALS
→ change of base structure
→ insert between bases (doxorubicin used as anticancer drugs)
RADIATION
→ UV light produces thymine dimers
→ ionizing radiation break DNA chromosomes to cause leukemia
what is the importance of DNA repair
→ maintains the genome stability.
→ 50-100 enzymes/proteins to check that DNA is properly repaired.
what is an example of a disease due to failed repair mechanisms?
→Xeroderma pigmentosum
→ defect in excision repair that deals with UV damage to DNA
→prone to skin cancer
describe the Holliday junction and when it forms
- Arises naturally in living cells through DNA strand exchange between two homologous chromosomes during the process of genetic recombination
- 4 way junction when two chromosomes are brought together.
- Allows for the exchange of genetic information
- Requires an enzyme to separate
describe how the nucleotides are arranged
→linked by 3’ 5’ phosphodiester bonds
→ DNA runs from 5’ to 3’
→ the two strands of DNA that are bonded together by H bonds run antiparallel to each other
