1.4 Nucleic Acids

Question 1

What does DNA stand for?

Answer 1

Deoxyribonucleic acid

Question 2

What is a genome?

Answer 2

Organism’s entire genetic content

Question 3

Eukaryotic vs prokaryotic DNA

Answer 3

In eukaryotes, DNA is found mainly in the nucleus but there is also some in chloroplasts and mitochondria.
In prokaryotes, the DNA is not enclosed in a membranous envelope.

Question 4

What does RNA stand for?

Answer 4

Ribonucleic acid

Question 5

What are ribosomes made of?

Answer 5

RNA and proteins

Question 6

What monomers make DNA and RNA?

Answer 6

Nucleotides

Question 7

What is the structure of nucleotides?

Answer 7

Each nucleotide is formed from a pentose sugar, a nitrogen-containing organic base and a phosphate group.

Question 8

What is the bond between nucleotides?

Answer 8

Phosphodiester bonds

Question 9

What are the differences between DNA and RNA structure?

Answer 9

DNA:
- A deoxyribose sugar
- Bases = adenine, cytosine, guanine, thymine
RNA:
- A ribose sugar
- Bases = adenine, cytosine, guanine, uracil

Question 10

Describe the structure of a DNA molecule

Answer 10

A DNA molecule is a double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs. The sugar and phosphate lie on the outside of the helix and form the DNA’s backbone.

Question 11

Describe the structure of an RNA molecule

Answer 11

An RNA molecule is a relatively short polynucleotide chain.
RNA only has one polynucleotide chain.
RNA is comprised of ribonucleotides that are linked by phosphodiester bonds.

Question 12

What are the base pairs?

Answer 12

Adenine (A) with Thyme (T) (Uracil (U) in RNA)
Guanine (G) with Cytosine (C)

Question 13

What does it mean that DNA strands are antiparallel?

Answer 13

The two polynucleotide strands are antiparallel because they run in opposite directions. The 5′ carbon end of one strand will face the 3′ carbon end of its matching strand.

Question 14

Describe the process of DNA replication

Answer 14

DNA helicase binds to DNA and breaks the hydrogen bonds between the two strands.
The DNA helix unwinds and the two strands separate.
Free-floating nucleotides form hydrogen bonds with the complementary bases.
Each strand is used as a template to produce complementary strands.
DNA polymerase forms phosphodiester bonds between the nucleotides.
Two new DNA strands are synthesised.

Question 15

What are the two enzymes involved in DNA replication?

Answer 15

• Helicase
• DNA polymerase

Question 16

Why is DNA replication described as semi-conservative?

Answer 16

After replication, the new DNA is made up of one original polynucleotide strand and a new, complementary strand.
This explains why DNA replication is described as semi-conservative because one original strand is conserved.

Question 17

Describe how a phosphodiester linkage forms

Answer 17

The phosphate residue attaches to the hydroxyl group of the 5′ carbon of one sugar and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide.
The bond is called a 5′–3′ phosphodiester linkage.

Question 18

Which two scientists discovered complementary base pairing?

Answer 18

Watson and Crick

Question 19

Which two scientists discovered semi-conservative replication?

Answer 19

Meselson and Stahl

Question 20

Describe the E.Coli growing experiment which shows that DNA replicates via semi-conservative replication

Answer 20

E. coli is grown for several generations in one medium containing a “heavy” isotope of nitrogen (15N) and another medium containing a ‘light’ isotope of nitrogen (14N). After each of the first few generations centrifuge cells.
The DNA will form separate bands according to its density. DNA grown in “heavy” 15N formed a band lower down the centrifuge tube than that grown in “light” 14N.
E. coli is moved from the 15N batch to a broth containing 14N only.
After one generation of growth in 14N, the single band observed will be in the space between DNA of cells grown exclusively in 15N and 14N.
The DNA harvested from cells grown for two generations in 14N will form two bands:
- One DNA band was between 15N and 14N.
- The other DNA band corresponded to the band of 14N DNA.
This shows semi-conservative replication.