2.1.3 Nucleotides and Nucleic Acids

Question 1

What is DNA?

Answer 1

Deoxyribonucleic acid

Question 2

What is the structure of DNA?

Answer 2

• A polymer made up of repeating units called nucleotides
• Each nucleotide consists of a common sugar, a phosphate group and one of four bases (either A, T, C or G)
• There is complementary base pairing where A and T pair up and C and G pair up
• Antiparallal

Question 3

What does antiparallel mean?

Answer 3

• Chains are running in opposite directions
• One runs in the 5’3’ direction and the other in the 3’5’ direction

Question 4

What is the structure of a DNA nucleotide?

Answer 4

• Contains deoxyribose sugar
• H on carbon 2
• Nitrogenous base can be either A, T, C or G

Question 5

What is the structure of an RNA nucleotide?

Answer 5

• Contains ribose sugar
• OH on carbon 2
• Nitrogenous base can be either A, U, C or G

Question 6

What are purines?

Answer 6

• Contain 2 carbon-nitrogen rings
• A and C

Question 7

What are pyrimidines?

Answer 7

• Contain 1 carbon-nitrogen ring
• T and C
• A purine base will always pair with a pyrimidine base

Question 8

What are the differences between RNA and DNA?

Answer 8

• RNA contains ribose and DNA contains deoxyribose
• RNA contains uracil and DNA contains thymine
• RNA is single stranded and DNA is double stranded

Question 9

How are polynucleotides formed?

Answer 9

• Phosphodiester bond
• Between the phosphate group of one nucleotide and the deoxyribose sugar of the next
• Catalysed by DNA polymerase

Question 10

How do two single polynucleotide strands join together to make a double helix?

Answer 10

• Hydrogen bonding between base pairs
• Complementary base pairing is A-T and C-G
• There are 2 hydrogen bonds between A and T and three hydrogen bonds between C and G
• Two antiparallel polynucleotide strands twist to form a double helix

Question 11

What is ATP?

Answer 11

Adenosine triphosphate

Question 12

What is the structure of ATP?

Answer 12

• Contains adenine, ribose and 3 phosphates
• The phosphates are joined by a phosphoanhydride bond
• Adding a phosphate group is called phosphorylation

Question 13

How is ATP formed?

Answer 13

• Made when adenosine diphosphate (ADP) is bonded to a third inorganic phosphate (Pi) using the energy released from glucose
• With ATP synthase enzyme

Question 14

What is the role of ATP?

Answer 14

• Stores most of the energy in the third bond of the molecule in small manageable amounts
• Energy is released when that bond is broken

Question 15

How is energy released from ATP?

Answer 15

• Phosphoanhydride bond between the second and third phosphate is hydrolysed
• By ATPase enzyme
• Forms ADP and inorganic phosphate

Question 16

What does semi-conservative replication mean?

Answer 16

• New DNA molecule comprises of one original and one new strand
• Each strand of DNA acts as a template strand

Question 17

What is the process of semi-conservative replication?

Answer 17

• DNA helicase causes two strands to separate by breaking the H bonds between the polynucleotide strands
• The helix separates to form 2 single strands
• Each original strand acts as a template and free nucleotides are attracted to exposed complementary bases
• Nucleotides and bases are joined together by DNA polymerase to form sugar phosphate backbone as hydrogen bonds form between bases
• 2 identical strands are formed as all nucleotides are joined to form a complete polynucleotide
• Each strand retains half the original DNA (semi-conservative)

Question 18

What were the three possible theories for DNA replication?

Answer 18

Conservative = one molecule of both original strands and one molecule of two new strand
Semi-conservative = two DNA molecules with one original and one new strand
Dispersive = two DNA molecules that are a mixture of parental and daughter DNA

Question 19

What is the evidence for semi-conservative replication?

Answer 19

In 1958, Matthew Meselsohn and Franklin Stahl demonstrated that DNA replication was semi-conservative through an experiment using E.coli bacteria

Question 20

What were the details of the experiment to prove semi-conservative replication?

Answer 20

• E.coli bacteria were grown in a medium containing a heavy isotope of nitrogen (15N)
• The bacteria used 15N to make the purine and pyrimidine bases
• The bacteria were then moved to a medium containing 14N

Question 21

What are the results from the semi-conservative replication experiment?

Answer 21

• Used density gradient centrifugation to separate molecules into bands
• Parental generation was the heaviest as both strands made with 15N
• 1st generation was lighter (hybrid band) as DNA made of one strand of 15N and one of 14N
• 2nd generation had one lighter and one hybrid band as one DNA molecule was made with only 14N
• 3rd generation had a thicker lightest band and thinner hybrid/intermediate band

Question 22

What is the explanation of the results from the semi-conservative replication experiment?

Answer 22

• Parental generation had both strands made with 15N
• 1st generation had 2 DNA molecules with one strand of 15N and one strand of 14N
• 2nd generation had 4 DNA molecules where some DNA was made of 2 strands of 14N and some made of 14N and 15N

Question 23

What are genes?

Answer 23

• Each gene contains a different sequence of bases
• Codes for a particular polypeptide
• Each gene is on a fixed position (locus) on a strand of DNA

Question 24

What is the triplet code?

Answer 24

• The sequence of 3 bases is the code for one particular amino acid
• The order of bases controls the order that the amino acids are assembled to produce a particular protein

Question 25

What is a triplet?

Answer 25

Each amino acid on a protein is coded for by a sequence of 3 nucleotide bases on mRNA called a codon

Question 26

What does degenerate mean?

Answer 26

There is usually more than on codon that codes for each amino acid

Question 27

What does non-overlapping mean?

Answer 27

Each base sequence is only read once

Question 28

What does universal code mean?

Answer 28

The same codon codes for the same amino acid in all organisms

Question 29

What is the structure of tRNA?

Answer 29

- Clover shaped - One strand folded back on itself - Anti-codon is complimentary to the codon on the mRNA - Amino acid is attached at the amino acid activation site, is specific and is determined by the anti-codon

Question 30

What is the process of transcription?

Answer 30

1. DNA helicase separates the two strands of DNA by breaking the hydrogen bonds 2. RNA polymerase moves along one of the strands of DNA (template strand) causing RNA nucleotides to align with complimentary bases and join together with a phosphodiester bond 3. As RNA polymerase moves along, the DNA strands rejoin behind it

Question 31

What starts and stops translation?

Answer 31

Start and stop codons

Question 32

What is the process of translation?

Answer 32

1. One end of the mRNA attaches to the ribosome 2. A tRNA molecule that has a complementary anticodon also attaches to the ribosome 3. A second tRNA molecule binds to the next codon 4. The ribosome moved along the mRNA and joins the two amino acids together by a peptide bond (catalysed by rRNA) 5. This continues until a stop codon is reached