1.5 DNA

Question 1

What are the functions of DNA?

Answer 1

• Carries the genetic code for all organisms, and is passed on throughout generations
• Genetic code tells cells how to build proteins.
• Proteins then determine the functions of cells.
• Cells determine the characteristics of organisms.

= This is an effective method of storing information; unchanging, and lasts a very long time.

Question 2

Why do we use DNA rather than RNA?

Answer 2

• Deoxyribose adds stability to DNA

Question 3

How do we make DNA stable?

Answer 3

• Deoxyribose sugars.
• Phosphodiester backbone protects the more chemically reactive bases.
• Hydrogen bonds stabilise double helix.
• Force between, above, and below bases - stabilises structure further.

= The more C-G, the more stable it is.

Question 4

DNA vs RNA

Answer 4

• Both made from nucleotides, but slightly different nucleotides.
• DNA is used for storing information
• RNA used to transfer genetic info from DNA to ribosomes (+OH in structure)
  = RNA does not produce double helix; single molecule = less stable

Question 5

What are the RNA bases?

Answer 5

A, G, C, and Uracil (U) only found in RNA.

Question 6

What are the three types of RNA?

Answer 6

• Messenger RNA (mRNA)
• Ribosomal RNA (rRNA)
• Transfer RNA (tRNA)

Question 7

How are nucleotides joined?

Answer 7

Through a condensation reaction.

Question 8

What do we mean by 5’ to 3’?

Answer 8

5’ (5 prime) is the top of a DNA nucleotide (5th carbon is where the phosphate is joined to)
3’ is the bottom of DNA (3rd carbon is where other nucleotides bind to),

-> This name comes from the carbon ring.

Question 9

Give me the features of a double helix

Answer 9

= 2 strands are complimentary, but run in opposite directions.
(e.g. first strand is 5’-3’, second strand is 3’-5’)

• Strands wind around, forming double helix.
  (Bases help to hold structure together with hydrogen bonding).

Question 10

Give me the structure of DNA

Answer 10

• The phosphate group is around the DNA
• The bases are complimentary and always in pairs.
• Shape is a double helix.

Question 11

What are the monomers of DNA?

Answer 11

• Nucleotides; they are the building blocks of DNA.
• Deoxyribonucleic acid.

Question 12

What is the full name of RNA?

Answer 12

Ribonucleic acid.

Question 13

What are the four bases of DNA, and how many hydrogen bonds are there?

Answer 13

Adenine binds to Thymine (A-T; 2 hydrogen bonds)
Cysine binds to Guanine (C-G; 3 hydrogen bonds)

Question 14

How do we bind two strands of DNA?

Answer 14

Through hydrogen bonds between pairs of bases.

Question 15

What is conservative replication?

Answer 15

Both strands of DNA stay together and new double strand is formed.

Question 16

Semi-conservative replication

Answer 16

1 old strand, 1 new strand

Question 17

Dispersive replication

Answer 17

Parts of old & new join together

Question 18

What are the problems with DNA?

Answer 18

1. Too small to see
2. How can you tell which DNA is new or old?

Question 19

How do we overcome these problems?

Answer 19

• Scientists marked the DNA with heavy N15 instead of N14
  = DNA can be centrifuged (separation technique by mass)

Question 20

What happens when DNA is centrifuged?

Answer 20

Heavy substances end up at the bottom, and lighter substances at the top.

Question 21

What did the scientists do?

Answer 21

1. Grew bacteria on a medium that contained N15, this meant that any bacteria would only have N15 on its DNA.
2. Transferred bacteria to the medium that contained N14. This meant that the new bacteria would only have N14. They allowed it to replicate once.
3. They then repeated this and allowed bacteria to replicate twice.

Question 22

What happened when this sample was centrifuged?

Answer 22

N15 is higher than the N14 - when it was centrifuged, it created two bands.

Original DNA - N15 - bottom
New DNA - N14 - top.

Question 23

What happens when we conservatively replicate a sample twice?

Answer 23

First time - 1 old, 1 new. (Original and new strand)

Second time - 1 old, 3 new. (1/4 original, 3/4 new)
= This creates two distinct bands.

Question 24

What happens when we semi-conservatively replicate a sample twice?

Answer 24

First time - 1 strand new, 1 strand old (x2)

Second time - 1 strand new, 1 strand old (x2) | Completely new strands (x2)
= Creates two distinct bands.

Question 25

What happens when we dispersively replicate a sample twice?

Answer 25

First time - even proportion of old & new in the strands.

Second time - a higher proportion of new in the 2nd replication (less original).

Question 26

How does DNA replicate?

Answer 26

Semi-conservatively

Question 27

How does DNA replication happen?

Answer 27

1. DNA Helicase unzips DNA, breaking hydrogen bonds between bases. DNA unzips in the shape of a fork.
2. DNA primase makes a short strand of primer using RNA. This is the starting point of DNA replication.
3. DNA Polymerase binds to the primer. This makes a new continuous strand of DNA, 5’ to 3’. It does this only on the leading strand.
4. On the lagging strand, the DNA is replicated in Okazaki fragments. DNA primase makes a primer and it moves backward towards the 5’ end. DNA polymerase makes the DNA 5’ to 3’.
5. Exonuclease removes / breaks down the primer, RNA primer is then replaced with DNA using DNA polymerase.
6. DNA ligase joins up the fragments on both the leading and lagging strand. It would then coil up into the double helix shape.