DNA structure Flashcards
Describe basic DNA Structure & Function:
βDNA is the store of genetic information
β it is made of a double helix (consists of major and minor grooves which are receptors for drugs)
βDNA bases encode genetic information
βDNA is present as chromatin in the nucleus It can be damaged by radiation and chemicals
what are the four bases and describe their composition
β adenine, thymine, guanine and cytosine
βAdenine and Guanine are Purines, they have 2 benzene rings.
βThymine and Cytosine are Pyrimidine, they have 1 benzene ring.
βAdenine and Thymine have 2 H bonds between them.
βGuanine and Cytosine have 3 H bonds between them.
how many forms does DNA have?
3 forms
A,B and Z
describe A DNA
A-FORM DNA βThe A form is wider than B βit is clockwise and has right-handed turns. βA-form is found in RNA and tRNA β it is 20-25% shorter than B DNA. β the grooves are identical in width β 11 base pairs per turn β more tightly coiled than B
describe B DNA
B-FORM DNA: β most common form of DNA found in cells βIt is clockwise β right handed helix β minor groove is 120 degrees β major groove is 240 degrees β 10 bases per turn
describe Z DNA
β Present in small amounts in the cell.
β Z DNA may be found in cells under certain conditions (eg: high salt conditions).
β 12 bps per turn.
β left handed helix.
β alternating purine and pyrimidine bases.
give a summary of the DNA helix
βtwo antiparallel polynucleotide strands form a right hand helix
β the bases are on the inside and the sugars and phosphates are on the outside
β the diameter is 2nM
β one strand is complementary in sequence to another
β the polynucleotide chains are held together by H bonds between pairs of bases
Describe bacterial DNA
β DNA is circular
β has 3 million base pairs
β supercoiled
β Bacterial cells also have plasmids that only carry certain genes. eg: those for antibiotic resistance.
β E.coli chromosome is circular and organized into 50 independently supercoiled domains.
what is bacterial DNA supercoiled by?
DNA gyrase
Describe eukaryotic DNA.
β humans have 3 billion base pairs of DNA
β DNA is complexed with histones and made into a fiber called chromatin
β the double helix is wound around 8 histone subunits
What are some reasons as to why mutations may occur in DNA?
SPONTANEOUS
β loss of bases
β hydrolysis of C to U
CHEMICALS
β change of base structure
β insert between bases (doxorubicin used as anticancer drugs)
RADIATION
β UV light produces thymine dimers
β ionizing radiation break DNA chromosomes to cause leukemia
what is the importance of DNA repair?
β maintains the genome stability.
β 50-100 enzymes/proteins to check that DNA is properly repaired.
what is an example of a disease due to failed repair mechanisms?
βXeroderma pigmentosum
β defect in excision repair that deals with UV damage to DNA
βprone to skin cancer
describe the Holliday junction and when it forms
βA Holliday Junction is a cross-shaped structure that forms during the process of genetic recombination.
β two double-stranded DNA molecules become separated into four strands in order to exchange segments of genetic information.
what is tetraplex DNA and where is it found?
DNA formed by the DNA folding back on itself found at the telomeres.