DNA and RNA

Question 1

How are nucleotides joined together?

Answer 1

Two mononucleotides are joined between the pentose sugar of one mononucleotide and the phosphate group of the other by a condensation reaction, a phosphodiester bond is formed, water molecule is released and a dinucleotide is formed. If more than two are joined a polynucleotide (nucleic acid) is formed.

Question 2

Describe the differences between DNA and RNA?

Answer 2

DNA is double stranded and RNA is single stranded.
DNA has the bases adenine, thymine, cytosine and guanine and RNA has the bases adenine, uracil, cytosine and guanine.
DNA contains a deoxyribose sugar and RNA contains a ribose sugar.
DNA is a long molecule compared to RNA which is a short molecule.

Question 3

Describe the structure of DNA?

Answer 3

DNA is a polymer of nucleotides. Each nucleotide formed from deoxyribose, a phosphate group and an organic nitrogen containing base. Phosphodiester bonds join nucleotides together, the bond forms between the phosphate of one nucleotide and the deoxyribose sugar of another. DNA is a double helix made from two strands held by hydrogen bonds between adenine, thymine and cytosine, guanine (complementary base pairs).

Question 4

What are the steps in DNA replication (Step 1/4) ?

Answer 4

DNA helicase breaks the hydrogen bonds between the complementary base pairs. This causes the DNA to unwind and the double helix separates into two strands.

Question 5

What are the steps in DNA replication (Step 2/4) ?

Answer 5

Both strands act as a template and free DNA nucleotides are attracted to exposed bases to which they bind using complementary base pairing. Adenine with thymine and guanine with cytosine.

Question 6

What are the steps in DNA replication (Step 3/4) ?

Answer 6

DNA polymerase then joins adjacent nucleotides on the new strand together by catalysing a condensation reaction that causes the formation of phosphodiester bonds between the deoxyribose sugar of one nucleotide and the phosphate group of another.

Question 7

What are the steps in DNA replication (Step 4/4) ?

Answer 7

Each of the new molecules contains one parental strand and one new strand. This is known as semi-conservative DNA replication.

Question 8

How is DNA suited to its function-Strong covalent bonds?

Answer 8

The stronger covalent bonds create the sugar phosphate backbone are on the outside of the double helix structure and the weaker hydrogen bonds are protected on the inside, so the DNA is stable and strong.

Question 9

How is DNA suited to its function-It is a large molecule?

Answer 9

This means that DNA can store lots of genetic information.

Question 10

How is DNA suited to its function- DNA has a coiled helix structure?

Answer 10

The shape means that it is compact and can therefore store lots of DNA in a small space.

Question 11

How is DNA suited to its function-Complementary base pairing?

Answer 11

The base pairing, Adenine with thymine and guanine with cytosine, means that replication is accurate forming identical copies.

Question 12

How is DNA suited to its function-What the base sequence codes for?

Answer 12

The base sequence codes for the order of amino acids in proteins, which allows the information to be stored.

Question 13

How is DNA suited to its function-It is double stranded?

Answer 13

It allows both strands to act as templates, this means that replication can act semi-conservatively.

Question 14

How is DNA suited to its function-Has weak hydrogen bonds?

Answer 14

The weak hydrogen bonds allow the helix to be easily unzipped, this allows for DNA to be replicated.

Question 15

How is DNA suited to its function-The large number of hydrogen bonds?

Answer 15

The large number of hydrogen bonds between bases join the two DNA strands together, this means that the DNA structure is stable.

Question 16

Explain the directional structure of a DNA strand?

Answer 16

Due to the ends of each strand being either a sugar that’s attached to the 5th carbon (5 prime) or a hydroxyl group (3 prime).

Question 17

Explain how DNA polymerase works?

Answer 17

DNA polymerase has a specifically shaped active site, that is only complementary to the 3’ end of the template strand, so it adds nucleotides in the 3’ to 5’ direction. This means that the new strand is built in the 5’ to 3’ direction, as strands run antiparallel. This means one strand will be continuously built and the other strand is built in the other direction. On the second strand the DNA polymerase has to detach and re-attach so often moves more slowly.

Question 18

Explain the three proposed theories of DNA replication:
-Semi-conservative
-Conservative
-Dispersive

Answer 18

Semi-conservative replication produces two helices that each contain one old and one new DNA strand.
Conservative replication produces one helix made entirely of old DNA and one helix entirely of new DNA.
Dispersive replication would produce two helices in which the individual strands are patchworks of old and new strands.

Question 19

Describe Meselson-Stahl’s experiment?

Answer 19

E.coli is first grown in a medium containing 15N (heavy isotope). After many generations growing in 15N the nitrogen containing bases all contain 15N. The bacteria were then switched to a medium of 14N (light isotope). DNA made after the switch will be made up of 14N. Small samples of each generation are extracted to measure their density using a density gradient centrifugation.

Question 20

What evidence was there that semi-conservative replication is how DNA is replicated?

Answer 20

The density of generation one is between the density of 15N and 14N so each strand must contain one original and one new strand.

Question 21

How did they rule out conservative replication after generation one?

Answer 21

If it was conservative there would be only two separate bands containing 15N at the bottom and 14N at the top and nothing with a density between these two isotopes.

Question 22

How was dispersive replication ruled out after generation 2?

Answer 22

If it was dispersive replication there would be one large band with a density between 15N and 14N and the DNA strands would all be a mixture of the old and new strands but generation two contains two separate bands of different densities.