Biology 2.6 IB Questions

What are linked by hydrogen bonds?

• Hydrogen and oxygen within a water molecule

• Phosphate and sugar within a DNA molecule

• Base and sugar between DNA nucleotides

• Hydrogen and oxygen in different water molecules

Answer explanation

Hydrogen and oxygen within a water molecule are held together by COVALENT bonds.

Phosphate and sugars are held together by COVALENT bonds.

DNA Nucleotides are linked by COVALENT, phosphodiester bonds.

Water molecules stick together using HYDROGEN bonds.

A molecule of DNA is found to contain 200 guanine bases, representing 25% of the total number of bases. How many phosphate groups does this molecule of DNA contain?

• 50

• 200

• 800

• 1000

Answer explanation

200 Guanine Bases is 25% of whole molecule. Guanine is one nitrogen base found in a nucleotide.

.25 x whole molecule = 200 nucleotides

whole molecule = 200 nucleotides / .25

whole molecule = 800 nucleotides

Every nucleotide has 1 phosphate, 1 base, and one pentose sugar.

What is the correct arrangement for the components of one strand in a DNA molecule? You can click on the image to zoom in.

• A

• B

• C

• D

Answer explanation

Phosphate and deoxyribose make the backbone of DNA, while the nitrogen bases are only attached to the sugar (deoxyribose) and make up the rungs of the DNA ladder.

The hydrolysis of a pure sample of an organic molecule produces a pentose sugar, thymine, guanine and cytosine. What other substances could be expected to be present in the hydrolysed sample?

• RNA

• Uracil

• Phosphate

• ATP

Answer explanation

Hydrolysis = break down (catabolism)

We are hydrolyzing (breaking down) a piece of DNA in this example.

DNA is the type of molecule being broken down because it contains thymine.

We know that nucleotides contain phosphate, deoxyribose, and a nitrogen base (a, t, g, c).

Nucleic acids are polymers of nucleotides. What parts of nucleotides are joined together in both DNA and RNA to make these polymers?

• Large nitrogenous bases with small nitrogenous bases

• Nitrogenous bases with hexose sugars

• Nitrogenous bases with phosphates

• Pentose sugars with phosphates

Answer explanation

The backbone of DNA is made up of pentose sugars being bonded to phosphates.

A short sequence of nucleotides reads GGCACAGCGCCAGACGA. In which type of molecule could this sequence be found?

• DNA molecule only

• RNA molecule only

• Both in a DNA and an RNA molecule

• In double-stranded DNA only

Answer explanation

RNA and DNA contain Adenine, Guanine and Cytosine.

RNA also contains Uracil, while DNA also contains Thymine.

There are no thymine or uracils in this strand, so it could be either DNA or RNA.

What is a similarity between DNA and RNA?

• Both are polymers of nucleotides.

• Both are composed of antiparallel strands.

• Both contain adenine, cytosine and thymine.

• Both contain ribose sugar.

Answer explanation

What is a similarity between DNA and RNA?

A. Both are polymers of nucleotides.

B. Both are composed of antiparallel strands.

-Incorrect because RNA is single stranded.

C. Both contain adenine, cytosine and thymine.

-RNA does NOT contain thymine.

D. Both contain ribose sugar.

-DNA has a deoxyribose sugar.