2.6

Question 1

nucleotides

Answer 1

Nucleic acids are the genetic material of the cell and are composed of recurring monomeric units

Question 2

Each nucleotide is comprised of three principal components

Answer 2

• 5-carbon pentose sugar (pentagon)
• Phosphate group (circle)
• Nitrogenous base (rectangle)

Question 3

what are the phosphate group and nitrogenous base attached to?

Answer 3

Both the phosphate group and nitrogenous base are attached to the central pentose sugar

Question 4

where is the nitrogenous base attached?

Answer 4

The nitrogenous base is attached to the 1’– carbon atom (right point)

Question 5

where is the phosphate base attached?

Answer 5

The phosphate base is attached to the 5’– carbon atom (left point)

Question 6

There are two types of nucleic acids present in cells

Answer 6

DNA and RNA

Question 7

DNA

Answer 7

DNA (deoxyribonucleic acid) is a more stable double stranded form that stores the genetic blueprint for cells

Question 8

RNA

Answer 8

RNA (ribonucleic acid) is a more versatile single stranded form that transfers the genetic information for decoding

Question 9

how do DNA and RNA differ?

Answer 9

DNA and RNA are both polymers of nucleotides, however differ in a few key structural aspects:

• Number of strands present
• Composition of nitrogenous bases
• Type of pentose sugar

Question 10

DNA vs. RNA pentose sugar

Answer 10

DNA: deoxyribose
RNA: ribose

Question 11

DNA vs. RNA base composition

Answer 11

DNA: Adenine (A), Guanine (G), Cytosine (C), Thymine (T)
RNA: Adenine (A), Guanine (G), Cytosine (C), Uracil (U)

Question 12

DNA vs. RNA number of strands

Answer 12

DNA: double stranded (forms double helix)
RNA: single stranded

Question 13

Nucleic acids are composed of…

Answer 13

nucleotide monomers which are linked into a single strand via condensation reactions

Question 14

nucleic acid structure and bonds

Answer 14

• The phosphate group of one nucleotide attaches to the sugar of another nucleotide (at the 3’– hydroxyl (-OH) group)
• This results in a phosphodiester bond forming between the two nucleotides (and water is produced as a by-product)
• Successive condensation reactions result in the formation of long polynucleotide strands

Question 15

Two polynucleotide chains of DNA are held together via…

Answer 15

hydrogen bonding between complementary nitrogenous bases

Question 16

hydrogen bonding

Answer 16

• Adenine (A) pairs with Thymine (T) via two hydrogen bonds

- Guanine (G) pairs with Cytosine (C) via three hydrogen bonds

Question 17

The two strands of DNA are described as being…

Answer 17

antiparallel

Question 18

As the antiparallel chains lengthen, the atoms will organise themselves into the most stable energy configuration

Answer 18

This atomic arrangement results in the double-stranded DNA forming a double helix (~10 – 15 bases per twist)

Question 19

The structural organisation of the DNA molecule was correctly proposed in and by?

Answer 19

in 1953 by James Watson and Francis Crick

Question 20

Watson and Crick’s key experimental discoveries:

Answer 20

• DNA is composed of nucleotides made up of a sugar, phosphate and base – Phoebus Levene, 1919
• DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T) – Erwin Chargaff, 1950
• DNA is organised into a helical structure – Rosalind Franklin, 1953 (data shared without permission)

Question 21

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

Answer 21

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

Question 22

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

Answer 22

• 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

Question 23

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

Answer 23

• 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 recognised as a key contributor to the elucidation of DNA structure

Question 24

RNA functions

Answer 24

to transfer genetic instructions from the nucleus to the cytoplasm, where the information is decoded

Question 25

three types of RNA

Answer 25

• Messenger RNA (mRNA) – a transcript copy of a gene which encodes a specific polypeptide
• Transfer RNA (tRNA) – carries the polypeptide subunits (amino acids) to the organelle responsible for synthesis (ribosome)
• Ribosomal RNA (rRNA) – a primary component of the ribosome and is responsible for its catalytic activity