Core Concepts: Necleic Acids and Protein Synthesis

Question 1

What is a nucleotide

Answer 1

A pentose sugar bonded to phosphate and a nitrogenous base

Question 2

What is DNA (structure)

Answer 2

DNA is an abbreviation for deoxyribonucleic acid. In DNA the sugar is deoxyribose, and the base is one of these four: adenine, thymine, guanine or cytosine

A DNA molecule is composed of two complementary polynucleotide strands. A polynucleotide is a polymer of nucleotides.

The sugar-phosphate molecules are joined by condensation reactions, making a phosphodiester linkage.

The sugar-phosphate molecules form the two sugar-phosphate ‘backbones’ of the molecule. The bases project into the centre of the molecule.

The two strands are joined by hydrogen bonds between the complementary base pairs (A-T and C-G).

Question 3

What is DNA (structure)

Answer 3

DNA is an abbreviation for deoxyribonucleic acid. In DNA the sugar is deoxyribose, and the base is one of these four:
* adenine
* thymine
* guanine
* cytosine

Question 4

What is RNA

Answer 4

RNA is an abbreviation for ribonucleic acid. In RNA, the sugar is ribose, and the base is one of these four:
* adenine
* uracil
* guanine
* cytosine

Question 5

What is ATP Structure)

Answer 5

ATP is an abbreviation for adenosine triphosphate. In ATP, the sugar is ribose, the base is adenine and there are three phosphate groups

Question 6

What nucleotides have ribose as a component

Answer 6

ATP
RNA

Question 7

What is ATP

Answer 7

ATP is the ‘universal energy currency’. This means it provides energy to all reactions in all cells in all species.
ATP is important to organisms as this energy is released in a single hydrolysis reaction that is catalysed by ATPase. This energy is released in small useable quantities; 30.6kJ mol-1 is released as the terminal phosphate bond is broken forming ADP and Pi

Question 8

How does ATP release energy

Answer 8

ATP can be hydrolysed to ADP and Pi, this reaction releases energy. The enzyme that catalyses this reaction is ATPase. 30.6 kJmol-1 energy is released when ATP is converted to ADP and Pi

Question 9

How is ATP formed

Answer 9

• ATP is formed in an endergonic reaction
• ADP and phosphate (Pi) are combined to form ATP
• The energy to form the bond comes from exergonic (energy releasing reactions) in cellular respiration
• The reaction is a condensation reaction; water is eliminated when the bond is formed
• An enzyme that catalyses this reaction is ATP synthetase

Question 10

What are the two main categories of base in nucleic acids

Answer 10

• Purines
• Pyridimines

Question 11

Is a purine base a single ring or double ring

Answer 11

double ring

Question 12

Is a pyridimine base a single ring or double ring

Answer 12

single ring

Question 13

What are the purine bases

Answer 13

• Guanine
• Adenine

Question 14

what are the pyridimine bases

Answer 14

• Thymine
• Cytosine
• Uracil

Question 15

Out of A C G T bases, which complementary base pair

Answer 15

A T
C G

Question 16

What are the two functions of DNA in cells

Answer 16

• As the base sequence codes for amino acid sequences in protein synthesis
• Replicating prior to cell division so that each daughter cell gets equal DNA

Question 17

What is the structure of DNA and how do the two DNA strands run relative to eachother

Answer 17

DNA is a double helix
The two strands run antiparalell

Question 18

What are the types of RNA

Answer 18

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

Question 19

What were Meselson and Stahl’s 3 possible mechanisms for DNA replication

Answer 19

• Conservative replication - the DNA molecule would be copied from the original, leaving the original DNA molecule as it was and having a new copy.
• Dispersive replication - sections of the DNA molecule would be copied and spliced together, making each new DNA molecule a mix of original and new DNA.
• Semi-conservative replication - the two polynucleotide chains would part, and new nucleotides attach to each of the chains, leading to each new molecule having one original chain and one new one.

Question 20

How did Meselson and Stahl’s experiment work

Answer 20

Meselson and Stahl used E.coli bacteria as the source of DNA. E.coli are easily grown in a flask of culture medium and replicate their cells (and DNA) every 20 minutes under optimal conditions.

‘New’ and ‘original’ nucleotides were distinguished by the isotope of nitrogen (N) in the nitrogenous bases. 15N is heavier in mass than 14N, so DNA molecules containing bases with 15N and DNA molecules containing 14N can be separated by mass in centrifugation.

First, the scientists grew E.coli in a culture medium with only 15N isotopes until, after many generations, all of the bases contained 15N. When a sample was centrifuged, the band of DNA molecules was near the bottom of the tube, as shown in the diagram below. This is because 15N is heavier, so it settles at the bottom of the tube

A sample of the E.coli was then transferred to a new culture medium containing 14N (light) nucleotides and allowed to replicate once

Finally, a sample was withdrawn from the flask and centrifuged. The DNA was found at an intermediate position between 15N molecules and 14N molecules, as shown below

This first generation showed that replication was not conservative but that each new DNA molecule produced by replication consisted of half new (14N) nucleotides and half original (15N) nucleotides

Question 21

What mechanism does DNA replicate from

Answer 21

Semi-Conservative