Question

Question 10 A human chromosome has 3 origins of replication. Assuming that all 3 origins of replication are used at the same time, what is the maximum number of helicases present on DNA during DNA replication (not including DNA repair)? 12 O 3 O2 Question 11 Which of the following correctly compares primase and telomerase? O Both primase and telomerase synthesize DNA using a DNA template. O Primase synthesizes RNA using an RNA template, but telomerase synthesizes DNA using a DNA template. Primase synthesizes RNA using a DNA template, but telomerase synthesizes DNA using an RNA template. Both primase and telomerase synthesize RNA using a DNA template. Question 12 How is the primer for the leading strand replaced during DNA replication? O 3 to 5 exonuclease repair catalyzes the formation of DNA where the primer used to be. After the primer is removed, a DNA polymerase adds onto the 3' end of an Okazaki fragment. Aspecial DNA polymerase adds onto the 5' end of the leading strand. Telomerase acts to prevent the loss of this sequence information. Question 13 Why might inhibitors of telomerase activity be beneficial if targeted to cancer cells? They would change the RNA sequence added onto the 3'end, They would change the DNA sequence added onto the 3'end. They would allow cancer cells to replicate. They would stop cancer cells from replicating.

Answer 1

Answer 10) The correct option is d (6)

Helicase is an enzyme responsible for unwinding of double helix of DNA during replication and thereby provide free single stranded DNA for daughter strand synthesis. In eukaryotes DNA replication is bidirectional, starting from a origin of replication. Hence, for one origin of replication two Helicases are required for unwinding of DNA in two opposite direction. Therefore, if there are 3 origin of replication present, a total of 6 Helicases will be required.

Option a, b, and c are incorrect: These options are incorrect because single origin of replication, which is bidirectional requires two Helicases which moves in opposite direction. Hence 6 is correct and 12, 3 or 2 number of Helicases are incorrect.

Answer 11) The correct option is c (Primase synthesizes RNA using a DNA template but telomerase synthesizes DNA using RNA template).

Primase is an enzyme used during DNA replication. It's function is to synthesize short RNA primers because DNA polymerase can not synthesize first nucleotide of the daughter strand and needs a free 3'OH end for polymerization. Primase synthesizes RNA primers using single-stranded DNA as template.

Telomerase is responsible for synthesis of telomere which is present at the end of chromosome and are known to protect the chromosomes from degradation. Telomerases synthesize DNA using RNA as template which it carries.

Option a is incorrect: because primase synthesizes RNA primers.

Option b is incorrect because primase uses single stranded DNA (parent strand) as template for synthesis of RNA primers.

Option d is incorrect: because telomerase synthesize DNA and not RNA.

Answer 12) The correct option is c (a special DNA polymerase add on to the 5' end of the leading strand)

DNA synthesis on leading strand is continuous and therefore has single RNA primer to be removed. This primer is removed by exonuclease activity od DNA polymerase by adding to on to the 5' end of DNA as DNA synthesis takes place in 3' to 5' direction.

Option a is incorrect: exonuclease activity is responsible for removing the nucleotide and not for synthesis. Hence, this statement is incorrect.

Option b is incorrect: Okazaki fragments are not present on th leading strand rather they are present on the lagging strand. Hence, this option is incorrect.

Option d is incorrect. Telomerase has no role in primer removal or filling of gaps. Hence, this option is incorrect.

Answer 13) The correct option is d (They would stop cancer cells from replicating).

Telomerase is an enzyme responsible for telomere synthesis which ultimately save chromosomes from degradation. If telomerase activity is lost, telomere length start decreasing with every round of DNA replication. At a particular length of telomere, cells stop dividing and induces apoptosis thereby checking cancer cell proliferation. Hence, If the telomerase activity is inhibited they would stop cancer cells from replicating.

Option a is incorrect: Telomere is part of DNA and not RNA. hence, this option is not a correct explanation.

Option b is incorrect: Telomere is a non-coding repetitive DNA sequences with primary role in protection of chromosome ends. Telomerase activity inhibition will not affect sequence rather it will affect telomere synthesis resulting in chromosome shortening.

Option c is incorrect: Telomerase inhibition of cancer cells is one of the emerging drug targets for cancer treatment because it inhibits cancer cell proliferation. Hence, this option is incorrect.