DNA 1

Question 1

Structure of DNA

Answer 1

1. Double helix
2. DNA strands are antiparallel
3. Each strand consists of long chain of nucleotides, each comprising of a deoxyribose sugar, phosphate group and nitrogeneous base
4. Nitrogeneous base can be A/T/C/G
5. Each strand contains a sugar phosphate backbone - comprising of nucleotides arranged in sequence, held together by phosphodiester bonds between C3 of the sugar of 1 nucleotide and C5 of the sugar of another nucleotide

Question 2

Roles of DNA

Answer 2

1. Storage of genetic information
2. Directs protein synthesis

Question 3

Semi-conservative replication

Answer 3

1. 2 parental DNA strands separate due to breaking of hydrogen bonds between complementary bases.
2. Each strand acts as a template for the synthesis of a complementary daughter strand
3. Each new DNA molecule consists of 1 newly synthesised DNA strand and 1 original DNA strand

Question 4

Describe how replication of lagging strand occurs

Answer 4

1. The lagging strand is synthesised discontinuously, resulting in Okazaki fragments, as the 2 parental strands are antiparallel
2. Due to the shape of active site and enzyme specificity, DNA polymerase works only from the 5’ to 3’ direction
3. Hence, DNA polymerase can only add DNA nucleotides to 3’ -OH end of an existing strand
4. Removal of RNA primer of each Okazaki fragments results in gaps which are subsequently sealed by DNA ligase, which catalyses the formation of phosphodiester bonds between adjacent nucleotides

Question 5

Describe how the end replication problem arises

Answer 5

1. During DNA replication, DNA polymerase requires a free 3’ -OH end of an existing strand to add nucleotides
2. RNA primer is synthesised to provide the free 3’ -OH end for addition of nucleotides. RNA primer is then replaced by DNA nucleotides
3. However, RNA primer at the 5’ end of the newly synthesised strand cannot be replaced by DNA nucleotides as there is no free 3’ -OH end
4. Since DNA polymerase is unable to complete replication at the 5’ end of lagging strands, this creates a 3’ overhang at the end of the chromosome
5. Due to the end replication problem, the ends of chromosomes shorten with each round of DNA replication
6. Since telomeres are found at the ends of the chromosomes, shortening of chromosomal ends leads to shortening of the telomeres without any deleterious effect

Question 6

Describe the structure of eukaryotic genome

Answer 6

1. Negatively charged DNA wounds around a histone octamer, consisting of 2 molecules of H2A, H2B, H3 and H4 each, forming a nucleosome
2. Individual nucleosomes are connected by strands of linker DNA, forming a 10nm nucleohistone complex
3. With the aid of H1 histones, the string of nucleosomes coils to form a 30nm chromatin fibre
4. The 30nm chromatin fibre folds to form looped domains attached to a base of scaffolding proteins, forming the 300nm chromatin fibre, which is then further coiled and compacted to form a chromatid of a highly condensed chromosome structure

Question 7

Suggest 2 reasons why replication of a human chromosome would take so long

Answer 7

1. Length of DNA in humans is longer
2. DNA is wound around histones to form nucleosomes, hence decondensing has
   to take place first