Nucleotides And Nucleic Acids

Question 1

Name the pentose sugars in DNA and RNA

Answer 1

DNA: deoxyribose
RNA: ribose

Question 2

Describe how polynucleotide strands are broken down and formed

Answer 2

Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone)
Hydrolysis reactions use a molecule of water to break these bonds
Enzymes catalyse these reactions

Question 3

Describe the structure of DNA

Answer 3

Molecule twists to form double hellix of 2 deoxyribose polynucleotide strands (so there are two sugar-phosphate backbones)
H-bonds form between complementary base pairs (AT and GC) on strands that run antiparallel

Question 4

Name the purine bases and describe their structure

Answer 4

Adenine - C5H5N5
Guanine - C5H5N5O
Two-ring molecules

Question 5

Name the pyrimidine bases and describe their structure

Answer 5

Thymine - C5H6N2O2
Cytosine - C4H5N3O
Uracil - C4H4N2O2
One-ring molecules

Question 6

Name the complementary base pairs in DNA and RNA

Answer 6

DNA: 2 H-Bonds between Adenine (A) and Thymine (T)
RNA: 2 H-Bonds between Adenine (A) and Uracil (U)
Both have 3 H-Bonds between Guanine (G) and Cytosine (C)

Question 7

Why is DNA replication described as semiconservative?

Answer 7

Strands from original DNA molecule acts as templates
New DNA molecule contains 1 old strand and 1 new strand (specific base pairing enables genetic material to be conserved accurately)

Question 8

Explain the role of DNA helicase in semiconservative replication

Answer 8

Breaks H-Bonds between base pairs to form 2 single strands each of which can act as a template

Question 9

How is a new strand formed during semiconservative replication

Answer 9

1. Free nucleotides from nuclear sap attach to exposed bases by complementary base pairing
2. DNA Polymerase joins adjacent nucleotides on new strand in a 5’ to 3’ direction via condensation reactions to form phosphodiester bonds
3. H-Bonds reform

Question 10

Identify features of the genetic code

Answer 10

Non-overlapping: each triplet is only read once
Degenerate: More than one triplet codes for the same amino acid (64 possible triplets for 20 amino acids)
Universal: Same bases and sequences used by all species

Question 11

How does a gene determine the sequence of amino acids in a protein

Answer 11

Consists of base triplets that code for a specific amino acid

Question 12

Describe how DNA can be purified by precipitation

Answer 12

Add ethanol and a salt to aqueous solution
Nucleic acids precipitate out of solution
Centrifuge to obtain pellet of nucleic acid
Wash pellet with ethanol and centrifuge again

Question 13

Where does transcription occur and what does it produce?

Answer 13

Occurs in nucleus
Produces mRNA

Question 14

Outline the process of transcription

Answer 14

1. RNA polymerase binds to promoter region on a gene
2. Section of DNA uncoils into 2 strands with exposed bases. Antisense strand acts as template
3. Free nucleotides are attracted to their complementary bases
4. RNA polymerase joins adjacent nucleotides to form phosphodiester bonds

Question 15

What happens after a strand of mRNA is transcripted

Answer 15

RNA polymerase detaches at terminator region
H-Bonds reform and DNA rewinds
Splicing removes introns from pre-mRNA in eukaryotic cells
mRNA moves out of nucleus via nuclear pore and attaches to ribosome

Question 15

What does translation produce and where does it occur?

Answer 15

Produces proteins
Occurs in cytoplasm on ribosomes (which are made of protein and rRNA)

Question 16

Outline the process of translation

Answer 16

1. Ribosome moves along mRNA until ‘start’ codon
2. tRNA anticodon attached to complementary mRNA
3. Condensation reaction between amino acids on tRNA form peptide bonds. Requires energy from ATP hydrolysis
4. Process continues to form polypeptide chain until ‘stop’ codon is reached

Question 17

Describe the structure of Adenosine Triphosphate (ATP) and Adenosine Diphosphate (ADP)

Answer 17

Nucleotide derivative of adenine
ATP has 3 inorganic phosphate groups
ADP has 2

Question 18

What is a mutation

Answer 18

An alteration to the DNA base sequence
Mutations often arise spontaneously during DNA replication