DNA Structure

Question 1

What are the purines?

Answer 1

G and A; 2-ring structure

Question 2

What are the pyrimidines?

Answer 2

C and T; 1 ring structure

Question 3

How are nucleotides linked?

Answer 3

Covalently linked via phosphodiester bonds

Question 4

How is DNA read?

Answer 4

Top strand, 5” end begins with phosphate. 3” end with free -OH group.

Question 5

How do DNA and RNA differ?

Answer 5

RNA only has 1 strand, it can leave the nucleus. No thymine, uracil. 3 types (mRNA, tRNA, rRNA).

Question 6

How is DNA condensed?

Answer 6

• DNA is wrapped twice around a histone core, which is positively charged.
• Nucleosomes (8 histone molecules; H2A, H2B, H3, H4 form nucleosome core) coiled to form solenoid structures
• When not condensed, genes are expressed. This is euchromatin.
• Heterochromatin is highly condensed form.

Question 7

What are the key features of the DNA Helix?

Answer 7

2 independent polymers that are entirely complementary and anti-parallel.

There are 10 base pairs in a complete turn. 0.34nm between base pairs.

Purines and pyrimidines are planar and unsaturated.

Major grooves show exposed bases.

Question 8

Describe the Hydrogen Bonding in DNA

Answer 8

Electronegative oxygen atoms in carbon ring bond with dipole positive nitrogen atom.

Question 9

How many hydrogen bonds are there between G and C? How many between A and T?

Answer 9

3 and 2 respectively