Unit 1 - Nucleic Acids

Question 1

What is a nucleotide and what is it made from?

Answer 1

A nucleotide is the monomer of a nucleic acid. It is made up of a phosphate group, a pentose sugar and a nitrogen-containing organic base.

Question 2

Structure of DNA?

Answer 2

It has a phosphate group, deoxyribose (pentose sugar) and base (adenine, thymine, guanine and cytosine).

Question 3

Structure of DNA diagram?

Question 4

Structure of RNA?

Answer 4

Phosphate group, ribose (pentose sugar) and base (adenine, uracil, cytosine and guanine).

Question 5

Structure of RNA diagram?

Question 6

How are polynucleotides formed?

Answer 6

Formed from the condensation reaction of many nucleotides to form a chain.

Question 7

What bond does the condensation of nucleotides make?

Answer 7

A phosphodiester bond between the phosphate group of one nucleotide to the pentose sugar in the next nucleotide.

Question 8

Property of a phosphodiester bond?

Answer 8

It is a covalent bond which makes the sugar-phosphate backbone of the nucleic acid stable and strong.

Question 9

Where are the nitrogenous bases found?

Answer 9

They’re not in the backbone and they stick out from the backbone.

Question 10

Diagram of the formation of a polynucleotide?

Question 11

Structure of a DNA molecule?

Answer 11

It is a double helix structure. This consists of 2 polynucleotide chains held together by hydrogen bonding between specific complementary base pairings.

Question 12

Diagram of a DNA molecule?

Question 13

Why are complementary base pairs formed?

Answer 13

Due to their specific structures and bonding of the organic bases.

Question 14

What are the complementary base pairs in DNA?

Answer 14

Adenine bonds with Thymine and Guanine bonds with Cytosine.

Question 15

What are the complementary base pairs of RNA?

Answer 15

Adenine bonds with Uracil and Cytosine bonds with Guanine.

Question 16

What is the function of DNA?

Answer 16

It codes for amino acids (3bases=1amino acid), so long strands of DNA code for polynucleotide chains and proteins.

Question 17

Structure and function of RNA?

Answer 17

A RNA molecule is a relatively short polynucleotide chain, which is normally a single stranded helix. RNA is used to transfer genetic information from DNA to ribosomes.

Question 18

DNA consists of 2 antiparellel nucleotide strands. How do these strands remain parallel?

Answer 18

A 2 ring base must always bind with a single ring base which is why thymine (single ring) always bonds with Adenine (double ring) and cytosine (single ring) always bonds with guanine (double ring).

Question 19

What does 2 antiparallel nucleotide strands mean?

Answer 19

The 2 strands of DNA run in opposite directions, and one chain is basically upside down. Each end of the molecule is labelled with a 5’-end and a 3’-end (pronounced 5 prime end and 3 prime end).

Question 20

What importance do the 5’-end and 3’-end have?

Answer 20

Their important when making proteins as they ensure only 1 strand is read to make the protein and their important during DNA replication, as DNA polymerase is only complementary to the 3’-end of the molecule.

Question 21

How does the structure of DNA allow it to carry out its function?

Answer 21

1) sugar-phosphate backbone and double helix structure = provides strength and stability/protects bases and hydrogen bonding between bases.
2) long molecule = can store a lot of info.
3) helix structure = compact.
4) base sequence = codes for amino acids and therefore proteins.
5) double stranded = allows semi conservative replication, as each strand can act as a template.
6) complementary base pairing = allows accurate replication.
7) hydrogen bonds between bases are weak = hydrogen bonds can be broken so allows strands to separate for replication.
8) many hydrogen bonds in the whole molecule = strong and stable molecule.

Question 22

What does semi-conservative replication of DNA ensure?

Answer 22

It ensures genetic continuity between generations of cells.

Question 23

How does semi-conservative replication occur?

Answer 23

1) The enzyme DNA helicase attaches and moves along the DNA molecule, breaking hydrogen bonds between complementary bases.
2) The 2 strands separate and both strands then act as a template.
3) New individual DNA nucleotides are attracted to exposed complementary bases on template strands and bind by complementary base pairing.
4) The enzyme DNA polymerase now connects the new nucleotides together through a a condensation reaction forming phosphodiester bonds adjacent nucleotides.
5) Semi- conservative replication ensures that each new DNA molecule contains an original and a new strand and is identical to the original DNA.

Question 24

What was Hershey and Chase’s experiment?

Answer 24

It was designed to show that DNA is replicated by the semi-conservative method, and how we know that DNA is the hereditary material.
1) Phages replicate by attaching to the bacteria cell wall and injecting the bacteria with their DNA. This then instructs the bacteria to create a new generation of Phages.
2) In the 1st experiment radioactive 35S was used to label proteins to allow it to be monitored throughout the experiment (S is found in proteins, but not DNA).
3) No radioactive 35S was found in the next generation of Phages, therefore none of the protein was passed onto the next generation.
4) In the 2nd experiment, radioactive 32P was used to label DNA (P is found in DNA, but not in proteins).
5) Radioacitve 32P was found in the next gen, showing that DNA must be the hereditary material.

Question 25

What was Griffith’s experiment?

Answer 25

Griffith used mice and the bacteria that caused pneumonia, to show that DNA is the hereditary material. 1) The 1st mouse was injected with a safe strain of the virus. 2) The 2nd mouse was injected with a harmful strain of the virus. 3) The 3rd mouse was injected with the harmful strain that had been heat killed. 4) The 4th mouse was injected with both the heat killed harmful strain and the safe strain. The mouse injected with the heat killed harmful bacteria did not die. But the mouse injected with the heat killed harmful bacteria and the live safe strain did die. Therefore the info for how to be “harmful” must have been passed to the live bacteria from the heat killed bacteria.

Question 26

What was the Meselson Stahl experiment?

Answer 26

Bacteria were grown in a medium containing N15, a heavy isotope form of Nitrogen. The bacteria used the nitrogen atoms to make bases to make their own DNA. This DNA was extracted and centrofused in a test tube. This causes substances to separate because of their different masses. Dense substances settle at the bottom of the tube, less dense substances rise to the top. The bacteria were then transferred to a medium contained N14, a lighter form of nitrogen. The bacteria now used these nitrogen atoms when replicating DNA. After each cell division, the DNA was light and heavy. This was repeated for 3 cell divisions.

Question 27

What was in generation 0?

Answer 27

N15 heavy DNA only.

Question 28

After the 1st division what was in generation 1?

Answer 28

15N heavy DNA joined with 14N DNA.

Question 29

After the 2nd division what was in Generation 2?

Answer 29

15N heavy DNA joined with 14N DNA and 14N DNA joined with 14N DNA.

Question 30

After the 3rd division what was in generation 3?

Answer 30

N15 DNA bonded with N14 DNA and double the amount of N14 DNA joined with N14 DNA.

Question 31

How does this experiment prove that DNA replication is semi-conservative?

Answer 31

The DNA from generation 1 forms a band midway between the position of ‘heavy’ and ‘light’ DNA. This shows that the molecules have 1 heavy strand from the original DNA molecule and 1 new light strand.