BB1720 DNA: structure and composition

Question 1

How was DNA discovered?

Answer 1

• analysis which showed that chromosomes in the dividing cells contain hereditary information
• the double helix was discovered using xray diffraction

Question 2

What is the structure of DNA?

Answer 2

• contains a major and minor groove
• 10 bases per turn
• one full turn rises 3.4nm
• 2nm wide
• can have right of left handed helicase shape or original

Question 3

function of major groove

Answer 3

• more exposed bases for proteins to bind and recognise the correct sequence
• transcription regulators bind

Question 4

function of minor groove

Answer 4

helps binding to histones and wrapping DNA

Question 5

chemical composition of DNA

Answer 5

• 2 polynucleotide chains, form double helix
• held together by hydrogen bonds between complementary base pairs
• nucleotides composed of deoxyribose sugar, phosphate group, base

Question 6

what are purines

Answer 6

double ring structures
adenine
guanine

Question 7

what are pyrimidines

Answer 7

single ring structure
cytosine
thymine

Question 8

what is a phosphate group

Answer 8

forms phosphodiester bonds between nucleotides

Question 9

What did the xray diffraction tell scientists about DNA?

Answer 9

• the cross represents the double helix
• the fundamental repeats of the helix are 0.34nm
• the spacing between the spots indicate the pitch is 3.4nm
• the missing layer line indicates there’s more than one strand and they are not symmetrically placed

Question 10

DNA nomenclature

Answer 10

• strands are antiparallel
• 5’ phosphate 3’ OH
• two strands are complementary
• 5’ ACGT 3’ -> direction of synthesis of DNA strand
• 3’ TGCA 5’ -> DNA template

Question 11

Which base is not found in DNA?

Answer 11

uracil

Question 12

How many base pairs are there in one full turn of the DNA double helix in its B form?

Answer 12

10

Question 13

The three major forms of the DNA double helix are A, B and Z form. Which is considered the normal form in the cell?

Answer 13

B

Question 14

Which technique was used to determine the double-helical structure of DNA?

Answer 14

X-ray crystallography

Question 15

The double-helical structure of DNA is formed by which forces?

Answer 15

a combination of hydrogen bonds and stacking interactions

Question 16

DNA from any cell of any organism should have a 1:1 ratio of _______ and _______ bases (Chargaff’s rule)

Answer 16

pyrimidine, purine

Question 17

If 30% of the bases in a given DNA molecule are adenine. What is the correct percentage of guanine bases in the DNA molecule?

Answer 17

20%

Question 18

The distance between a single base pair in B DNA is?

Answer 18

34 nm