DNA

Question 1

What are the two nucleic acids?

Answer 1

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

Question 2

What are the monomers of DNA and RNA?

Answer 2

Nucleotides.

Question 3

What is the formation of a nucleotide?

Answer 3

• Deoxyribose sugar
• Phosphate group
• Nitrogenous base

Question 4

What are the 4 nitrogenous bases and what is their pairing rule?
How many bonds do they form and what type of bond?

Answer 4

Adenine - Thymine (2 hydrogen bonds)
Guanine - Cytosine (3 hydrogen bonds)

Question 5

Describe the structure of DNA.

Answer 5

DNA consists of two polynucleotide strands joined together by hydrogen bonding to form a double helix. The nucleotides (made up of a deoxyribose sugar, phosphate group and nitrogenous base) are joined together in a chain by phosphodiester bonds formed in condensation reactions. The sugar and phosphate form the backbone of the polynucleotide strands, whereas the bases of one strand are paired by hydrogen bonds to their complementary base on the other strand.

Question 6

Describe/explain the appearance of a chromosome.

Answer 6

Two identical sister chromatids produced by DNA replication, joined at a centromere.

Question 7

What is a homologous pair?

Answer 7

2 chromosomes (maternal and paternal), with the same genes at the same loci. They may have different alleles.

Question 8

Give an advantage of the structure of DNA and explain why it is useful.

Answer 8

The twisted helix structure makes it a compact store of genetic information.

Question 9

Functions of DNA?

Answer 9

• Genes, due to specific base sequences, code for specific proteins which provide phenotypes (characteristics).
• Self-replication essential for cell division.
• Mutations in DNA provide variation and lead to evolution.

Question 10

How is DNA’s structure adapted to its function?

Answer 10

• Sugar-phosphate backbone: gives strength
• Helix: compact shape, protects sequence of bases
• Double-stranded: each strand serves as template in replication, protects sequence of bases, gives molecule stability
• Large molecule: lots of info can be stored
• Many hydrogen bonds: stability, individual bonds are weak allowing helix to unzip easily for replication
• Sequence of bases: codes for specific amino acid sequence in proteins
• Complimentary base pairing: enables accurate replication of info.

Question 11

Structure of RNA?

Answer 11

Shorter chain than DNA. Contains ribose rather than deoxyribose. Instead of the base thymine there is URACIL. Single stranded.

Question 12

Name and describe the process of DNA replication.

Answer 12

Semi-conservative replication:
1. DNA HELICASE breaks hydrogen bonds between the bases of the two polynucleotide strands. These strands now act as template strands.
2. The bases are now exposed. Free nucleotide bases in the nucleus line up to their complementary bases on the template strand.
3. Adenine bonds with thymine, guanine bonds with cytosine.
4. DNA POLYMERASE forms phosphodiester bonds between the nucleotides of the new strand -> new sugar-phosphate backbone. Condensation reaction.
5. There is now 1 parent strand and 1 newly synthesised strand.

Question 13

Evidence for semi-conservative replication?

Answer 13

1. Bacteria grown in N-15 (heavy, parent) then N-14 (light, new). Centrifuged.
2. 1st generation: intermediate DNA band (one parent, one new strand).
3. 2nd generation: intermediate DNA band and light DNA band (still some parent strands, new molecules contain only new strands).

Question 14

What are the methods of replication that were rejected and why?

Answer 14

Conservative: later gens would show heavy and light bands (didn’t happen).
Dispersive: all future gens remain intermediate (not observed).