Module 2 | Genes and Health (TEMPORARY)

Question 1

2.5 Know the basic structure of mononucleotides.

Answer 1

A DNA mononucleotide contains one phosphate, a pentose sugar (called deoxyribose or ribose) and a base (either adenine, cytosine, thymine, guanine or uracil).

Question 2

2.5 Know the structure of DNA.

Answer 2

DNA is a polynucleotide composed of mononucleotides linked through condensation reactions. It is double stranded, and contains deoxyribose sugar and bases adenine (purine), thymine (pyrimidine), cytosine (pyrimidine) and guanine (purine). Purine bases have two nitrogen-containing rings, whereas pyrimidine bases have just one. Deoxyribose sugar and phosphates are linked by phosphodiester bonds, and complementary bases are linked by hydrogen bonds. DNA is arranged in a double helix structure with a sugar-phosphate backbone on each strand.

Question 3

2.5 Know the structure of RNA

Answer 3

RNA is a polynucleotide composed of mononucleotides linked through condensation reactions. It is single stranded, and contains ribose sugar, and bases adenine, uracil, cytosine and guanine.

Question 4

2.5 Know how complementary base pairing and the hydrogen bonding between two complementary strands are involved in the formation of the DNA double helix.

Answer 4

The formation of the double helix structure is where two complementary strands of nucleotides run in opposite directions and are held together by hydrogen bonds.

Question 5

2.6

Question 6

2.6

Question 7

2.6

Question 8

2.6

Question 9

2.7 Understand the nature of the genetic code.

Answer 9

The genetic code is a triplet code - each three bases codes for one amino acid. It also contains start and stop codons which either start or stop protein synthesis.
The genetic code is non-overlapping, meaning that each triplet is only read once and triplets don’t share any bases.
The genetic code is degenerate, meaning that more than one triplet codes for the same amino acid.

Question 10

2.8 Know that a gene is…

Answer 10

A sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain.

Question 11

2.9

Question 12

2.9

Question 13

2.9

Question 14

2.9

Question 15

2.9

Question 16

2.10

Question 17

2.10

Question 18

2.10

Question 19

2.10

Question 20

2.11 Understand the process of DNA replication.

Answer 20

Catalysed by a gyrase enzyme, the double helix begins to untwist a bit at a time and the DNA starts to unwind. This allows the DNA molecule to start to unzip, where the hydrogen bonds between the complementary nucleotide bases break, and this is catalysed by DNA helicase. This unzipping action results in a structure known as the replication fork where the “branching” prongs contain single-stranded DNA with exposed bases. Using the single-stranded DNA as a template, free phosphorylated nucleotides move in and line up opposite the exposed bases according to complementary base-pairing rules. The enzyme DNA polymerase catalyses the addition of the new nucleotide bases, in the 5’ to 3’ direction, to the single strands of DNA. Hydrolysis of the activated nucleotides releases the extra phosphate groups. This supplies the energy to form phosphodiester bonds between the deoxyribose sugar of one nucleotide and the phosphate group of the adjacent nucleotide. As the leading strand is synthesised in the same direction as the growing replication fork, it can be replicated continuously. As the direction of synthesis of the lagging strand is opposite to the direction of the growing replication fork, it is replicated discontinuously by forming short segments called Okazaki fragments. The Okazaki fragments are then joined during reactions catalysed by DNA ligase. Termination of the process occurs when the replication fork stops. The final product is two DNA molecules which are identical to each other as well as to the parent DNA molecule that initiated the process.

Question 21

2.11 Understand how Meselson and Stahl’s classic experiment provided new data that supported the accepted theory of replication of DNA and refuted competing theories.

Answer 21

Meselson and Stahl originally grew DNA in a culture containing N-15 for several generations, so all the bases contained this isotope. They then grew the DNA in a culture of N-14 for one generation. After this generation, the DNA contained one strand containing 15-N and one strand containing 14-N. After another generation, half of the DNA molecules were the same as in generation one, and the other half contained entirely 14-N, where the 14-N strand from generation one had been used as a template. This provided evidence for the semi-conservative model of DNA replication.