nucleic acids

Question 1

Describe the structure of a nucleotide

Answer 1

• A phosphate group
• Nitrogen-containing base
• Pentose sugar

Question 2

Name the pentose sugar in DNA and RNA

Answer 2

• DNA: deoxyribose
• RNA: ribose

Question 3

State the role of DNA in living cells

Answer 3

1. Base sequence of genes codes for functional RNA and amino acid sequence for polypeptides
2. Genetic information determines inherited characteristics and influences the structure and function of organisms

Question 4

State the role of mRNA, RNA and tRNA in living cells

Answer 4

1. mRNA = complementary sequence to 1 gene from DNA with introns spliced out, codons can be translated into a polypeptide by ribosomes
2. rNA = component of ribosomes
3. tRNA = supplies complementary amino acids to mRNA codons during translation

Question 5

How do polynucleotides form

Answer 5

Condensation reactions between nucleotides form strong phosphodiester bonds

Question 6

Describe the structure of DNA

Answer 6

• Double helix of 2 polynucleotide strands
• Contains deoxyribose
• Hydrogen bonds between complementary purine and pyrimidine base pairs on opposite strands

Question 7

Name the complementary base pairs in DNA

Answer 7

adenine + thymine
guanine + cytosine

Question 8

Which bases are purines and which are pyrimidines?

Answer 8

Adenine and guanine = 2 ring purine bases

Thymine and cytosine = 1 ring pyrimidine bases

(Purines are larger than pyrimidines because they have a two-ring structure while pyrimidines only have a single ring)

Question 9

Name the complementary base pairs in RNA and their bonds

Answer 9

2 hydrogen bonds between adenine and uracil

3 hydrogen bonds between guanine and cytosine

Question 10

Describe how the structure of DNA is adapted for its functions

Answer 10

1. Sugar phosphate backbone and many hydrogen bonds provide stability
2. Long molecule so stores a lot of information
3. Helix is compact for storage in the nucleus
4. Base sequence of triplets codes for amino acids
5. Double-stranded for semi-conservative replication
6. Complementary base pairings for accurate replication
7. Weak hydrogen bonds so strands separate for replication

Question 11

Describe the structure of messenger RNA (mRNA)

Answer 11

• Long ribose polynucleotide
• Contains uracil instead of thymine
• Single-stranded and linear so has no complementary base pairs
• Codon sequence is complementary to exons of 1 gene from 1 DNA strand

Question 12

Describe how the structure of mRNA makes it adaptable to its function

Answer 12

1. Breaks down quickly so no excess polypeptide forms
2. Ribosomes can move along strands and tRNA can bind exposed bases
3. Can be translated into a specific polypeptide by ribosomes

Question 13

Describe the structure of transfer RNA (tRNA)

Answer 13

• Single strand of 80 nucleotides
• Folded into a clover shape
• Anticodon on one end, amino acid binding site on the other

Question 14

What is the function of an anticodon on tRNA

Answer 14

Anticodon binds to complementary mRNA codon

Question 15

Why is DNA replication described as semi-conservative

Answer 15

• Strands from the original DNA molecule act as a template
• New DNA molecule contains 1 old strand and 1 new strand

Question 16

Outline the process of semi-conservative DNA replication

Answer 16

1. DNA helices break hydrogen bonds between bases which unwinds the double helix producing 2 strands that act as templates
2. Free-floating DNA nucleotides are attracted to the exposed bases via specific complementary base pairing, and hydrogen bonds form between these
3. DNA polymerase joins adjacent nucleotides on a new strand by condensation which creates phosphodiester bonds

Question 17

Describe the Meselson-Stahl experiment

Answer 17

1. Bacteria were grown in a medium containing heavy isotope 15N for many generations
2. Some bacteria were moved to a medium containing light isotope 14N, and samples were extracted after 1 and 2 cycles of DNA replication
3. Centrifugation formed a pellet, and heavier DNA bases settles closer to the bottom of the tube