4.1 DNA structure, replication & the genetic code

Question 1

describe the structure of DNA and explain how this structure relates to one of its functions.

Answer 1

• DNA is composed of two polynucleotide chains held together by a phosphodiester backbone. the phosphate and deoxyribose sugar wind around each other to form a double helix structure.
• the bases on the two DNA strands are held together by hydrogen bonds, which enables the molecule to coil and compact, so that large amounts of information can be stored within a small space.

Question 2

describe the structure of RNA and give an example of one of its functions.

Answer 2

• RNA is a single, relatively short polynucleotide chain, in which the pentose sugar is always ribose, and the organic bases are always adenine, guanine, cytosine, and uracil.
• one of the main functions of RNA is to transfer genetic information from DNA to the ribosomes, which then read the RNA to make polypeptides in a process known as translation.

Question 3

give two similarities and two differences in the structures of DNA and RNA.

Answer 3

similarities:

• both DNA and RNA are composed of many mononucleotides joined together.
• both DNA and RNA contain a phosphate group in the nucleotide structure.

differences:

• the pentose sugar found in DNA nucleotides is deoxyribose, whereas in RNA it is ribose.
• in DNA, the organic base adenine pairs with thymine, whereas in RNA, adenine pairs with uracil.

Question 4

give the three components found in an individual nucleotide.

Answer 4

• a pentose sugar, containing five carbon atoms.
• a phosphate group.
• a nitrogen-containing organic base.

Question 5

describe how dinucleotides are formed, and explain how this results in the formation of a polynucleotide.

Answer 5

• two mononucleotides can join as the result of a condensation reaction between the deoxyribose sugar of one mononucleotide and the phosphate group of another, forming a dinucleotide held together by a phosphodiester bond.
• the continued linking of mononucleotides in this way forms a long chain known as a polynucleotide.

Question 6

describe and explain the term ‘complementary base pairing’ in a DNA molecule.

Answer 6

• the bases on the two strands of DNA are held together by hydrogen bonds.
• the base pairing is specific, and is therefore described as ‘complementary’.

Question 7

give two reasons why DNA is described as a ‘stable’ molecule.

Answer 7

• the phosphodiester backbone protects the more chemically reactive organic bases inside the double helix.
• hydrogen bonds link the organic base pairs, forming bridges between the two phosphodiester strands, which contribute to the stability of the molecule.

Question 8

give three ways in which the DNA molecule is adapted to carry out its functions.

Answer 8

• the two separate strands of DNA are joined only by weak hydrogen bonds, allowing them to easily separate during DNA replication and protein synthesis.
• base pairing leads to DNA being able to replicate and transfer information as mRNA.
• by having the base pairs contained within the deoxyribose-phosphate backbone, genetic information is protected from being corrupted by outside chemicals, and physical forces.

Question 9

explain why the process of DNA replication known as ‘semi-conservative’.

Answer 9

because each strand contains half of the original DNA material, as well as new material formed from free nucleotides.

Question 10

give the requirements that must be met in order for semi-conservative replication to take place.

Answer 10

• the four types of nucleotide, each with their bases of adenine, guanine, cytosine, or thymine.
• both strands of the DNA molecule act as a template for the attachment of these nucleotides.
• the enzymes DNA helicase and DNA polymerase.
• energy provided by ATP.

Question 11

give the role of DNA helicase in semi-conservative replication.

Answer 11

DNA helicase causes the two strands of DNA helicase to separate by breaking the hydrogen bonds that join the complementary bases together.

Question 12

give the role of DNA polymerase in semi-conservative replication, and explain which type of bond is formed.

Answer 12

DNA polymerase joins together the free nucleotides, which have bonded to a specific base by complementary base pairing, forming phosphodiester bonds.

Question 13

what is a gene?

Answer 13

a section of DNA that contains the coded information for manufacturing polypeptides and functional RNA.

Question 14

explain why the genetic code is described as ‘degenerate’, ‘universal’ and ‘non-overlapping’.

Answer 14

degenerate - most amino acids are coded for by more than one triplet.

universal - each triplet codes for the same amino acid in all organisms.

non-overlapping - each base in the sequence is only read once.

Question 15

give three way in which eukaryotic DNA is different from prokaryotic DNA.

Answer 15

• prokaryotic DNA is smaller and shorter than eukaryotic DNA.
• prokaryotic DNA is circular in shape, eukaryotic DNA is linear in shape.
• prokaryotic DNA is not associated with proteins, eukaryotic DNA is associated with proteins called histones.

Question 16

what is a chromosome?

Answer 16

a threadlike structure of nucleic acids and proteins found in the nucleus of eukaryotic cells, which carry genetic information in the form of genes.

Question 17

what is a chromatid?

Answer 17

one of the two identical halves of a chromosome that has been replicated in preparation for cell division.

Question 18

give the difference between 3’ and 5’ carbon atoms.

Answer 18

the 3’ carbon atom has an attached hydroxyl group whilst the 5’ carbon atom has an attached phosphate group.

Question 19

explain why DNA strands are described as ‘antiparallel’.

Answer 19

• when nucleotides are arranged into the double strands of a DNA molecule, one strand runs in the 5’ to 3’ direction, whilst the other runs in the opposite direction, from 3’ to 5’.
• the two strands are therefore said to be antiparallel.

Question 20

explain why nucleic acids can only be synthesised in the 5’ to 3’ direction.

Answer 20

nucleic acids can only be synthesised in the 5’ to 3’ direction because DNA polymerase can only attach nucleotides to the hydroxyl group on the 3’ carbon molecule.