Nucleic Acid

Question 1

Nucleotides are linked by ……. bond, between the …… of one sugar and the …… of an adjacent sugar

Answer 1

phosphodiester bonds

3-OH , 5-phosphate ester

Question 2

DNA exists as ……., with …… interactions between the …. & …. in the opposite strands

Answer 2

double helix
non covalent
purine, pyrimidine

Question 3

The primary structure of DNA or RNA refers to ……..

Higher orders are linked by …… between purine and pyrimidine bases

Answer 3

sequence of nucleotides in DNA and RNA

non covalent bonds

Question 4

Nucleotides consist of 3 parts: … , …. & ….

Answer 4

nitrogen base, pentose & phosphate group

Question 5

The linkage of the nitrogen base to the pentose forms ……, the addition of phosphate forms ….

Answer 5

nucleoside, nucleotide

Question 6

What are the types of nitrogenous bases?

Answer 6

1. Purines: two rings with nitrogen atoms. two types, Adenine A & guanine G
2. Pyrimidine: single ring with nitrogen. three types: cytosine C, thymine T (only in DNA) & uracile U (only in RNA)

Question 7

RNA contains …. sugar, while DNA contains …. . The difference is …….

Answer 7

ribose, deoxyribose

deoxyribose has a hydrogen atom instead of hydroxyl attached to carbon 2

Question 8

Phosphate groups are linked to pentose by …..

Answer 8

ester linkage to carbon 5

Question 9

What are the types of phosphodiester links in nucleic acid?

Answer 9

3’-5’ phosphodiester linkage: most common in DNA and RNA
2’-5’ phosphodiester linkage: involved in RNA splicing
5’-5’ phosphodiester linkage: found in cap structure at the 5’ end of eukaryotic mRNA

Question 10

Nucleotides have polarity. T/F

Answer 10

True

at one end there is a free 3’ hydroxyl group, at the other end there is a 5’ phosphate group

Question 11

What are the differences between DNA and RNA?

Answer 11

1. pentose: DNA has 2’-deoxyribose, while RNA has a ribose
2. DNA is double stranded, while RNA is single
3. DNA has G,A,C,T . . While RNA has G,A,C,U
4. DNA is a repository for genetic information. While RNA is a working copy of DNA (transfers genetic information to protein sequences)

Question 12

In purines, the ….. of the ring system is linked to carbon 1 on sugar, while in pyrimidines the …. of the ring is linked to carbon 1

Answer 12

nitrogen 9

nitrogen 1

Question 13

The secondary structure for DNA is ……, while for RNA is …..

Answer 13

double stranded structure

the single strand can bend back on itself

Question 14

The sugar is connected to the base by ….. bond, while one sugar to another is connected by ….

Answer 14

glycosidic

phosphodiester

Question 15

Double structure can occur between ….., …. & ….

Answer 15

1. DNA double helix
2. single stranded RNA (bent on itself)
3. RNA and DNA complementary to each other

Question 16

The double DNA strands always run in ……. fashion

Answer 16

antiparallel

Question 17

G-C bonds provide more stability to DNA than A-T & A-U because…..

Answer 17

G-C is connected by 3 hydrogen bonds, while the others by two

Question 18

Hydrophobic interactions occur between adjacent ….. & …… add stability to the double helix

Answer 18

pyrimidines and purines

* This is called base stacking

Question 19

Base stacking is interrupted by …….

Answer 19

intercalating agents, like Actinomycin D, which prevents replication (DNA-DNA) and transcription (DNA-mRNA)

Question 20

Nucleases enzymes cut …….. . Exonucleases remove ……., while endonucleases cut …..

Answer 20

the phosphodiester bonds
terminal nucleotides
between internal nucleotides

Question 21

Most DNA forms ………

Answer 21

right handed, antiparallel helix

Question 22

Define B DNA

Answer 22

is the typical right handed, anti-parallel double stranded, with the bases stacked and perpendicular to the axis of the helix. The helix has two grooves that provide site for DNA-protein interactions
* Also called Watson-Crick model

Question 23

Define Z DNA

Answer 23

Left handed, anti parallel helix. Usually rich with G-C sequences.
* It is a rare form with the function unknown

Question 24

Define A DNA

Answer 24

Resembles B DNA. but the bases are not perpendicular but tilted to axis.

• This is the anhydrous type on B DNA
• Found in DNA-RNA hybrids

Question 25

Circular DNA has no ..... so it resists ..... actions. It may be in relaxed form or .....

Answer 25

ends, exonuclease | supercoiles form

Question 26

Supercoiling is important in ......

Answer 26

DNA packaging

Question 27

What is difference between positive and negative supercoiling?

Answer 27

1. Negative: twist the helix in counter clock wise turns. DNA is usually negatively supercoiled. Occurs in both circular and stranded chromosomes 2. Positive: DNA is twisted in the same direction as the double helix

Question 28

Define the topoisomerase enzymes

Answer 28

Relieve the tension caused by supercoiling during replication, by cutting the phosphodiester bonds in the double strands and then resealing

Question 29

Nucleotides are synthesized by ....... or .....

Answer 29

1. salvage pathway: The preferred method of synthesis in adults 2. de novo synthesis: occurs in rapidly dividing cells

Question 30

In de novo synthesis of nucleotides, the precursors are .........

Answer 30

amino acids, ammonia, bicarbonate, ribose-5-phosphate & THF | * THF acts as a methylating agent in nucleotides synthesis

Question 31

Deoxyribonucleotides are synthesized by reduction of ......, catalyzed by ...., .... & ....

Answer 31

ribonucleoside diphosphates | * This reaction is catalyzed by NADPH, reductase & thioredoxin

Question 32

Purine is synthesized ...... ribose-5-phosphate throughout the assembly, while pyrimidines are .......

Answer 32

attached to | completely assembled before attaching to ribose-5-phosphate

Question 33

The parent purine nucleotide is ......, while pyrimidines parent is .....

Answer 33

IMP: give rise to both AMP & GMP (they also exert inhibition on purine synthesis) OMP: forms both UTP & CTP, while Thymidine nucleotides (for DNA) is synthesized by methylation of dUMP by THF. CTP & UTP inhibit pyrimidine synthesis

Question 34

Nucleotides are degraded by removal of ...... & ....., while the amino group is removed as ..... by the action of ..... & .....

Answer 34

phosphate & pentose ammonia guanine deaminase & adenosine deaminase

Question 35

How is the purine degraded?

Answer 35

The ring can not be degraded. It is excreted as uric acid

Question 36

How is the pyrimidine degraded?

Answer 36

Thymidine: becomes butyrate | Uracil & cytosin: become beta-alanine

Question 37

Folate is needed for the formation of the nucelotides ....., ..... & .....

Answer 37

adenosine, guanine & thymidine | * folate deficiency affects many tissues with high mitotic activity

Question 38

The catabolism of nucleotides gives ...... ATP

Answer 38

zero

Question 39

The intermediate in the synthesis of purines and pyrimidines is .......

Answer 39

5-phosphoribosyl-1-pyrophosphate * PRPP either converted to purine or * converted to orotic acid --->orotate monophosphate ---->pyrimidine

Question 40

Uric acid is formed from .... degradation

Answer 40

Purine | * purine ----> Xanthine -----> uric acid , catalyzed by xanthine oxidase

Question 41

DNA and RNA are differentiated in the lab by ...........

Answer 41

Feulgen reaction, which is specific for deoxyribose

Question 42

A nucleosome consists of .......

Answer 42

DNA wrapped around a histone & held by ionic bonds * Histone is composed of Arginine & Lysine (+ve charged due to the free amino group) that bonds to the -ve phosphate of DNA