Exam 3: 29 Nucleic Acids (2/3 complete)

Question 1

10. For a circular double-stranded DNA, which of the following quantity is not going to change unless a strand is broken?

A) Its linking number
B) Its twisting number
C) Its writhe number
D) All the above

Answer 1

A) Its linking number
Lk is always an integer and, importantly, it remains constant for a closed circular DNA molecule unless a strand is broken!!

Linking number (Lk): This represents the number of times one DNA strand winds around the other in a right-handed direction. Imagine holding one strand and tracing its path; Lk is how many times you completely encircle the other strand. Lk is always an integer and, importantly, it remains constant for a closed circular DNA molecule unless a strand is broken.

Question 2

Linking number in DNA supercoiling means…

A) the number of times the long axis of the double helix crosses over itself

B) the number of times that a strand of DNA winds in the right-handed direction around the helix axis when the axis lies in a plane

C) the number of times one strand completely wraps around the other strand

Answer 2

B) the number of times that a strand of DNA winds in the right-handed direction around the helix axis when the axis lies in a plane

Question 3

Writhing number in DNA supercoiling means…

A) the number of times the long axis of the double helix crosses over itself

B) the number of times that a strand of DNA winds in the right-handed direction around the helix axis when the axis lies in a plane

C) the number of times one strand completely wraps around the other strand

Answer 3

A) the number of times the long axis of the double helix crosses over itself

Question 4

Twisting number in DNA supercoiling means…

A) the number of times the long axis of the double helix crosses over itself

B) the number of times that a strand of DNA winds in the right-handed direction around the helix axis when the axis lies in a plane

C) the number of times one strand completely wraps around the other strand

Answer 4

C) the number of times one strand completely wraps around the other strand

Question 5

If DNA appears as a simple, planar circle, so there is no supercoiling; what will the Writhe number be?

A) will equal zero
B) will equal the number of times the DNA crosses over itself

Answer 5

A) will equal zero

Wr = 0: If the DNA appears as a simple, planar circle, there’s no supercoiling, and Wr is 0.

Question 6

If DNA displays supercoiling; what will the Writhe number be?

A) will equal zero
B) will equal the number of times the DNA crosses over itself

Answer 6

B) will equal the number of times the DNA crosses over itself

Wr ≠ 0: If the DNA crosses over itself, count the number of times the long axis crosses. Right-handed supercoils are assigned negative Wr values, while left-handed supercoils have positive values.

Question 7

Are right-handed supercoils assigned positive or negative Wr values?

Answer 7

Right-handed supercoils are assigned negative Wr values,

Question 8

Are left-handed supercoiling assigned positive or negative Wr values?

Answer 8

Left-handed supercoils have positive values.

Question 9

Which is more stable?

A) A=T
B) C (tripple bond) G

Answer 9

C (3x=) G most stable hydrogen bonds

Question 10

Sugar+Base =

A) Nucleoside
B) Nucleotide

Answer 10

A) Nucleoside

Question 11

Sugar+Base+Phosphate=

A) Nucleoside
B) Nucleotide

Answer 11

B) Nucleotide

Question 12

What kind of Nitrous base are: Adenine and Guanine

A) Pyrmindes
B) Purines

Answer 12

B) Purines

Question 13

What kind of Nitrous base are: Cytosine, Uracil, Thymine?

A) Pyrmindes
B) Purines

Answer 13

A) Pyrmindes

Question 14

How do you distinguish Ribose sugar from Deoxyribose sugar?

Answer 14

Ribose sugar: OH at 2’
Deoxyribose: H at 2’

Question 15

What is an ester- linkage?

Answer 15

Monophosphate group (AMP)

Question 16

What is an anhydride- linkage?

Answer 16

Diphosphate group (ADP)
or
Triphosphate group (ATP)

Question 17

How many rings do Pyrmindes (Cytosine, Uracil, Thymine) have?

Answer 17

Single rings

Question 18

How many rings do Purnines (Adenine, Guanine) have?

Answer 18

Double rings

Question 19

True or False:
Nucleosides contain a phosphate group.

Answer 19

False,
Sugar + Base = Nucleoside
Sugar + Base + Phosphate = Nucleotide

Question 20

Clicker:
In proteins, amino acids are linked by peptide bonds; in nucleic acids, nucleotides are linked by

A. N-glycosidic bonds
B. Phosphodiester bonds
C. Hydrophobic interactions
D. None of the above

Answer 20

B. Phosphodiester bonds

Nucleotides are joined together in nucleic acids through phosphodiester bonds. In this linkage, the OH group of the sugar acts as a nucleophile and attacks the phosphate group of another nucleotide.

Question 21

Clicker:
For a circular double-stranded DNA, which
of the following quantities is not going to
change unless a strand is broken?

A. Its linking number
B. Its twisting number
C. Its writhe number
D. All of the above

Answer 21

A. Its linking number

Question 22

Endonuclease…

A) cut at the ends of nucleic acid strands, removing nucleotides one by one

B) cut within a strand of DNA as specific sequences of nucleotides

Answer 22

B) cut within a strand of DNA as specific sequences of nucleotides

Endonucleases, on the other hand, cut within a strand of DNA. They can recognize specific sequences of nucleotides and cleave the DNA at those sites.

Question 23

Exonuclease….

A) cut at the ends of nucleic acid strands, removing nucleotides one by one

B) cut within a strand of DNA as specific sequences of nucleotides

Answer 23

A) cut at the ends of nucleic acid strands, removing nucleotides one by one

Exonucleases cut at the ends of a nucleic acid strand. They can cut at either the 5’ end or the 3’ end of the strand, removing nucleotides one by one.

Question 24

(Question from discussion)

Which of the following is a reason why DNA serves as a more suitable form of genetic material (compared to RNA)?

A)It lacks a 2OH

B)It is double stranded

C)DNA polymerase’s proofreading ability

D)All of the above

Answer 24

D)All of the above

Question 25

(Question from discussion)
Which form is found in most cellular DNA?

A)B-DNA
B)A-DNA
C)Z-DNA

Answer 25

A)B-DNA

• Most cellular DNA is in the B-DNA form.
• A-DNA is found in double-stranded RNA or DNA/RNA hybrids.
• Z-DNA is based on the crystal structure of CGCGCG.