1.5 Nucleic acids

Question 1

What are the components of a DNA nucleotide

Answer 1

• Deoxyribose sugar (pentose sugar)
• Phosphate group
• Nitrogenous base (either adenine, thymine, cytosine or guanine)

Question 2

What are the components of an RNA nucleotide

Answer 2

• Ribose sugar (pentose sugar)
• Phosphate group
• Nitrogenous base (either adenine, uracil, cytosine or guanine)

Question 3

How are nucleotides joined together

Answer 3

• By phosphodiester bonds formed in condensation reactions.
• This forms dinucleotides which join to form polynucleotides.
• The bond forms between the deoxyribose sugar of one nucleotide and the phosphate group of another.

Question 4

What’s the difference between the structure of DNA and RNA

Answer 4

• A DNA molecule is a double helix composed of 2 polynucleotides joined together by hydrogen bonds between complementary bases.
• RNA is a relatively short polynucleotide chain.

Question 5

Why is DNA a stable molecule

Answer 5

• The phosphodiester backbone protects the more chemically reactive nitrogen containing organic bases inside the double helix.
• Hydrogen bonds form between the nitrogenous bases.
• There are 3 hydrogen bonds between cytosine and guanine, and 2 between adenine and thymine, so a higher proportion of C-G pairings make DNA more stable.

Question 6

What kind of replication is DNA replication

Answer 6

• Semi-conservative (in each new DNA molecule produced, one strand is from the original DNA molecule and one strand is new).
• Ensures genetic continuity between generations of cells.

Question 7

Describe the process of DNA replication

Answer 7

1. DNA helicase causes the two strands of DNA to separate by breaking the hydrogen bonds between complementary bases.
2. One of the strands is used as a template and complementary base pairing occurs between the template strand and free nucleotides (activated nucleotides).
3. Once activated nucleotides are bound the enzyme DNA polymerase joins them together by forming phosphodiester bonds. This forms 2 identical DNA molecules.

Question 8

What is an activated nucleotide

Answer 8

• They contain 3 phosphate groups rather than just one.
• Also known as nucleoside triphosphates.

Question 9

What is the role of the enzyme DNA helicase in DNA replication

Answer 9

• DNA helicase unwinds the DNA.
• It does this by breaking the hydrogen bonds between complementary base pairs on the two strands of DNA.
• This forms a replication fork.

Question 10

What is the role of the enzyme DNA polymerase in DNA replication

Answer 10

• DNA polymerase synthesises new strands of DNA from the two template strands.
• It does this by catalysing condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone.
• DNA polymerase breaks off the two extra phosphate groups from the activated nucleotides and uses the energy released to create the phosphodiester bonds.

Question 11

How is the new DNA strand formed on the leading strand

Answer 11

• DNA polymerase can only build the new DNA strand in the 5’ to 3’ direction.
• The DNA on the leading strand is unzipped in the 3’ to 5’ direction, so DNA polymerase will attatch to the 3’ end of the original strend and move towards the replication fork.
• This means the new strand will be formed in the 5’ to 3’ direction as DNA is anti-parallel.
• On the leading strand, the formation of the new DNA strand is continuous.

Question 12

How is the new DNA strand formed on the lagging strand

Answer 12

• DNA polymerase moves away from the replication fork (synthesis of the lagging strand is not continuous).
• DNA polymerase can only synthesis the lagging strand in short segments (called Okazaki fragments).
• The enzyme DNA ligase is needed to join these segments together, forming a continuous complementary DNA strand.