The Structure of DNA

Question 1

The nucleic acid DNA is a

Answer 1

polynucleotide – it is made up of many nucleotides bonded together in a long chain

Question 2

DNA molecules are made up of …… polynucleotide strands lying side by side, running in …….. directions – the strands are said to be?

Answer 2

2

opposite

antiparallel

Question 3

Each DNA polynucleotide strand is made up of

Answer 3

alternating deoxyribose sugars and phosphate groups bonded together to form the sugar-phosphate backbone.

Question 4

These bonds are covalent bonds known as phosphodiester bonds

The phosphodiester bonds link the

Answer 4

5-carbon of one deoxyribose sugar molecule to the phosphate group from the same nucleotide, which is itself linked by another phosphodiester bond to the 3-carbon of the deoxyribose sugar molecule of the next nucleotide in the strand

Question 5

Each DNA polynucleotide strand is said to have a …’ end and a ..’ end (these numbers relate to which carbon ……

Answer 5

3’ end 5’ end

on the pentose sugar could be bonded with another nucleotide)

Question 6

As the strands run in opposite directions (they are……..), one is known as the .’ to .’ strand and the other is known as the .’ to .’ strand

Answer 6

antiparallel

5’ 3’

3’ 5’

Question 7

The nitrogenous bases of each nucleotide project out ………….towards the interior of the ………….

Answer 7

from the backbone

double-stranded DNA molecule

Question 8

The two antiparallel DNA polynucleotide strands that make up the DNA molecule are held together by

Answer 8

hydrogen bonds between the nitrogenous bases

Question 9

These hydrogen bonds always occur between the same pairs of bases:
Which base pair has 2 hydrogen bonds

Answer 9

The purine adenine (A) always pairs with the pyrimidine thymine (T) – two hydrogen bonds are formed between these bases

Question 10

These hydrogen bonds always occur between the same pairs of bases:
Which base pair has 3 hydrogen bonds

Answer 10

These hydrogen bonds always occur between the same pairs of bases:
Which base pair has 2 hydrogen bonds

Question 11

This is known as complementary base pairing
These pairs are known as DNA base pairs

Question 12

DNA is described as a double helix
This refers to the

Answer 12

spiral shape, where two complementary strands of nucleotides run in opposite directions and are held together by hydrogen bonds.

Question 13

Make sure you can name the different components of a DNA molecule (sugar-phosphate backbone, nucleotide, complementary base pairs, phosphodiester bonds, hydrogen bonds) and make sure you are able to locate these on a diagram. Remember that phosphodiester bonds join the nucleotides in the sugar-phosphate backbone, and hydrogen bonds join the bases of the two complementary strands together. Remember that the bases are complementary, so the number of A = T and C = G. You could be asked to determine how many bases are present in a DNA molecule if given the number of just one of the bases. Note that knowledge of purines and pyrimidines is not required