2.6 Structure of DNA and RNA

Question 1

The basic structures of DNA and RNA are very ___

Answer 1

Similar as they are both made of monomers called nucleotides.

Question 2

Structure of DNA and RNA

Answer 2

DNA and RNA are composed of three parts, namely a pentose sugar (meaning that it has five carbon atoms), a phosphate group, and a nitrogenous base.

Question 3

Differences between the structure of DNA and RNA

Answer 3

• A DNA nucleotide makes up the basic unit of DNA and RNA; although in RNA, the pentose sugar is ribose instead of deoxyribose.
• DNA and RNA also differ in their bases. DNA has four nitrogenous bases: guanine (G), adenine (A), thymine (T), and cytosine (C).
• RNA has the same bases except for thymine, which is replaced by uracil (U).

Question 4

How are bases classified?

Answer 4

• Bases can be classified based on the number of rings present in their structure.
• Purines are bases that have two rings in their structure, while pyrimidines contain only one ring.
• Thymine, cytosine, and uracil are examples of pyrimidines, while adenine and guanine are classified as purines.

Question 5

Structure of a DNA molecule

Answer 5

• Contains the sugar deoxyribose
• Bases involved are adenine, thymine, guanine, and cytosine.
• Is a double-stranded molecule.

Question 6

Structure of an RNA molecule

Answer 6

• Contains the sugar ribose.
• Bases involved are adenine, uracil, guanine, and cytosine (thymine has been replaced by uracil).
• Is a single-stranded molecule

Question 7

Diagram of the simplified structure of a DNA nucleotide.

Answer 7

Question 8

Drawing diagrams of single nucleotides of DNA and RNA

Answer 8

You should be able to draw simple diagrams of the structure of single nucleotides of DNA and RNA, using circles, pentagons, and rectangles to represent phosphates, pentoses, and bases, respectively.

Question 9

The nucleotide units link together through a ___

Answer 9

• Phosphodiester bond (a covalent bond) to form a single strand, a polynucleotide.
• The phosphodiester bond is always formed between the phosphate group attached to the 5’-C of one sugar and the hydroxyl (OH) group attached to the 3’-C of another sugar.

Question 10

What do 3’ and 5’ refer to?

Answer 10

• 3’ and 5’ refer to the carbon position on the sugar in DNA.
• Carbon 1 is attached to the base and from there on carbon atoms are counted in a clockwise direction.

Question 11

An important fact about DNA is that it is made up of two ___

Answer 11

Strands held together by hydrogen bonds to form a double helix, while RNA is only a single-stranded molecule.

Question 12

Diagram of DNA (double-stranded)

Answer 12

Question 13

How is the DNA structure usually described?

Answer 13

Double helix

Question 14

What is the name of the pentose present in DNA molecules?

Answer 14

Deoxyribose sugar

Question 15

RNA consists of uracil instead of ___

Answer 15

Thymine

Question 16

DNA is the ___

Answer 16

Genetic material of living organisms.

Question 17

Guanine always pairs with

Answer 17

Cytosine

Question 18

Adenine always pairs with ___

Answer 18

Thymine

Question 19

The complementary base pairing rule

Answer 19

• Guanine always pairs with cytosine.

- Adenine always pairs with thymine.

Question 20

How are two nitrogenous bases held together?

Answer 20

• The two strands are held together by hydrogen bonds between G and C (3 hydrogen bonds) and between A and T (2 hydrogen bonds).
• In order for bases to be facing each other, the two strands must run in opposite directions (i.e. they are antiparallel ) to each other.
• Thus, one strand of DNA runs from 5’ to 3’, and the opposite strand runs from 3’ to 5’. The 3’ and 5’ refer to the exposed carbon (on the sugar) at the ends of the DNA chains

Question 21

Diagram of DNA double helix

Answer 21

Question 22

Crick and Watson (Using a model to elucidate the DNA structure)

Answer 22

• Crick and Watson built on what was known about the chemistry of DNA to determine its actual structure.
• They used the information from the DNA X-ray diffraction patterns produced by Rosalind Franklin and Maurice Wilkins to deduce that the DNA molecule must have a regular double helix structure.
• Additionally, they integrated Chargaff’s base ratio, a finding that showed A is always equal to T and C always equal to G in the DNA molecule to work out the complementary base pairing of A to T and C to G.
• Once they had all the information they built a 6-foot (about 1.8 m) model out of metal scrap and, as expected, everything clicked into place.
• One very important aspect of using a model is the ability to visualise the molecule and to quickly see how well it fits the available evidence.

Question 23

If a DNA sample were composed of 10% thymine, what would be the percentage of guanine?

Answer 23

40

If a DNA sample contains 10% thymine that means there would have to be 10% adenine as well, as thymine pairs with adenine. That leaves 80% for cytosine and guanine combined; so the correct answer is half of 80 (which is 40)

Question 24

Which method was used in the discovery of the DNA double helix?

Answer 24

X-ray

Question 25

In a DNA deoxyribose sugar of a single nucleotide, which carbon is connected to the base?

Answer 25

The first carbon

The 5th carbon makes up the 5’ end and the 3rd carbon makes up the 3’ end of a nucleotide. The carbons are counted clockwise starting from the carbon attached to the base and finishing on the carbon attached to the phosphate.

Question 26

What are the three main components of a DNA nucleotide?

Answer 26

Phosphate, Deoxyribose sugar, Nitrogenous Base

Question 27

Which molecule is found in both DNA and RNA?

Answer 27

Phosphate

Question 28

Define “nucleic acid backbone”

Answer 28

Nucleic acids are composed of a backbone of sugar molecules and phosphate groups. The nitrogen bases are attached to the sugars.

Question 29

Which base replaces thymine in RNA?

Answer 29

Uracil

Question 30

Watson and Crick- making DNA models

Answer 30

Using trial and error, Watson and Crick were able to assemble a DNA model that demonstrated the following:

• DNA strands are antiparallel and form a double helix
• DNA strands pair via complementary base pairing (A=T; C=G)
• Outer edges of bases remain exposed (allows access to replicative and transcriptional proteins)

As Watson and Crick’s model building was based on trial and error, a number of early models possessed faults:

• The first model generated was a triple helix
• Early models had bases on the outside and sugar-phosphate residues in the centre
• Nitrogenous bases were not initially configured correctly and hence did not demonstrate complementarity

The final construction of a correct DNA molecule owed heavily to the X-ray crystallography data generated by Franklin

• This data confirmed the arrangement of the DNA strands into a helical structure
• The data was shared without Franklin’s knowledge or permission and contributed profoundly to the final design
• Hence, Franklin is now recognized as a key contributor to the elucidation of DNA structure