DNA structure

Question 1

Describe basic DNA Structure & Function:

Answer 1

→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

→DNA is present as chromatin in the nucleus It can be damaged by radiation and chemicals

Question 2

what are the four bases and describe their composition

Answer 2

→ 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.

Question 3

how many forms does DNA have?

Answer 3

3 forms

A,B and Z

Question 4

describe A DNA

Answer 4

A-FORM DNA 
→The A form is wider than B 
→it is clockwise and has right-handed turns. 
→A-form is found in RNA and tRNA
→ it is 20-25% shorter than B DNA.
→ the grooves are identical in width 
→ 11 base pairs per turn 
→ more tightly coiled than B

Question 5

describe B DNA

Answer 5

B-FORM DNA: 
→ most common form of DNA found in cells 
 →It is clockwise 
→ right handed helix 
→ minor groove is 120 degrees
→ major groove is 240 degrees 
→ 10 bases per turn

Question 6

describe Z DNA

Answer 6

→ Present in small amounts in the cell.
→ Z DNA may be found in cells under certain conditions (eg: high salt conditions).
→ 12 bps per turn.
→ left handed helix.
→ alternating purine and pyrimidine bases.

Question 7

give a summary of the DNA helix

Answer 7

→two antiparallel polynucleotide strands form a right hand helix
→ the bases are on the inside and the sugars and phosphates are on the outside
→ the diameter is 2nM
→ one strand is complementary in sequence to another
→ the polynucleotide chains are held together by H bonds between pairs of bases

Question 8

Describe bacterial DNA

Answer 8

→ DNA is circular
→ has 3 million base pairs
→ supercoiled
→ Bacterial cells also have plasmids that only carry certain genes. eg: those for antibiotic resistance.
→ E.coli chromosome is circular and organized into 50 independently supercoiled domains.

Question 9

what is bacterial DNA supercoiled by?

Answer 9

DNA gyrase

Question 10

Describe eukaryotic DNA.

Answer 10

→ 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

Question 11

What are some reasons as to why mutations may occur in DNA?

Answer 11

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

Question 12

what is the importance of DNA repair?

Answer 12

→ maintains the genome stability.

→ 50-100 enzymes/proteins to check that DNA is properly repaired.

Question 13

what is an example of a disease due to failed repair mechanisms?

Answer 13

→Xeroderma pigmentosum
→ defect in excision repair that deals with UV damage to DNA
→prone to skin cancer

Question 14

describe the Holliday junction and when it forms

Answer 14

→A Holliday Junction is a cross-shaped structure that forms during the process of genetic recombination.

→ two double-stranded DNA molecules become separated into four strands in order to exchange segments of genetic information.

Question 15

what is tetraplex DNA and where is it found?

Answer 15

DNA formed by the DNA folding back on itself found at the telomeres.

Question 16

describe how the nucleotides are arranged

Answer 16

→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

Question 17

What are 4 main points of DNA structure?

Answer 17

→ Primary: The sequence of DNA bases (ATCG) and their combinations.

→ Secondary: conformational form, A,B,C, or Z which is detected using X-Ray Crystallography.

→ Tertiary​: DNA supercoiling, detected using electron microscopes.

→ Quaternary​: interlocked chromosomes.

Question 18

When is A - form DNA found in the cell?

Answer 18

A - form occurs under dehydrating conditions​.

Question 19

What are the potential consequences of DNA repair defects?

Answer 19

→ DNA repair defects = cancer likely.
→ Hereditary DNA repair defects, due to mutations, so unable to repair DNA properly.
→ Mutations in DNA lead to formation of tumours.