1.5 Nucleic Acids

Question 1

Function of DNA.

Answer 1

To hold or store genetic information. (contains the instructions for the growth and development of all organisms)

Question 2

Function of RNA.

Answer 2

To transfer the genetic code found in DNA out of the nucleus and carry it to the ribosomes in the cytoplasm. (ribosomes = site of translation)

Question 3

Structure of DNA nucleotides.

Answer 3

• phosphate group
• deoxyribose (pentose) sugar
• one of four nitrogenous bases (Adenine,Thymine,Cytosine,Guanine)

Question 4

Structure of RNA nucleotides.

Answer 4

• phosphate group
• ribose (pentose) sugar
• one of four nitrogenous bases (Adenine,Uracil,Cytosine,Guanine)

Question 5

What is a purine?

Answer 5

Bases with a double-ring structure (A and G).

Question 6

What is a pyrimidine?

Answer 6

Bases with a single-ring structure (T, U and C).

Question 7

Nucleotides are joined via …

Answer 7

condensation reactions.

Question 8

What is the bond between two nucleotides called?

Answer 8

Phosphodiester bond.

Question 9

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule.

Answer 9

Condensation reaction between deoxyribose and phosphate, catalysed by DNA polymerase.

Question 10

The chain of alternating phosphate groups and pentose sugars produced as a result of many phosphodiester bonds is known as the …

Answer 10

sugar-phosphate backbone of the DNA or RNA molecule.

Question 11

Structure of DNA.

Answer 11

• polynucleotide
• made up of two polynucleotide strands (double-stranded) lying side by side, running in opposite directions – the strands are said to be antiparallel (one strand is 3’ to 5’ and the other is 5’ to 3’)
• sugar-phosphate backbone
• hydrogen bonding holds two strands together (between the nitrogenous bases)
• complementary base pairing
  (A - T , C - G)
• double-helix structure

Question 12

Structure of RNA.

Answer 12

• polynucleotide
• relatively short compared to DNA
• sugar-phosphate backbone
• single-stranded

Question 13

Which scientists came up with the theory of semi-conservative DNA replication?

Answer 13

Watson and Crick

Question 14

What is semi-conservative replication?

Answer 14

The process by which DNA is replicated. Following replication, each new DNA molecule produced has one strand of DNA conserved from the original DNA. (i.e. one strand from original DNA, one strand from new DNA).

Question 15

What is the importance of semi-conservative replication?

Answer 15

The process of semi-conservative replication ensures genetic continuity between generations of cells.

Question 16

Describe the process of semi-conservative replication of DNA.

Answer 16

1. DNA helicase unzips the DNA double helix by breaking the hydrogen-bonds between the base pairs.
2. Each of these single polynucleotide DNA strands acts as a template for the formation of a new strand made from free nucleotides that are attracted to the exposed DNA bases by complementary base pairing.
3. Then, DNA polymerase joins these new nucleotides together, forming phosphodiester bonds.
4. Original strand and new strand are joined together by hydrogen bonding between the base pairs.

Question 17

Give 3 features of DNA and explain how each one is important in the semi-conservative replication of DNA.

Answer 17

1. Weak hydrogen bonds → broken easily so allows the two strands to separate
2. Two polynucleotide strands → each can act as a template
3. Complementary base pairing → allows accurate replication

Question 18

What was Meselson and Stahl’s experiment?

Answer 18

Stage 1: Culture of bacteria containing 15N DNA.
Stage 2: Sample of DNA spun in a centrifuge and heavy N settled at the bottom of the test tube forming a band.
Stage 3: Sample containing only 15N DNA allowed to replicate in a broth containing only 14N DNA.
Stage 4: Sample of DNA extracted and spun in a centrifuge.
Results: PROOF of semi-conservative replication as after a round of replication a band formed in the middle of the test tube (hybrid of light and heavy N).