AQA The Genetic Code | Maths and Physics Tutor

One piece of experimental evidence for the semi-conservative mechanism for replication of DNA involves the use of bacteria labelled with the heavy isotope of nitrogen, 15N. (i) Which component of DNA would be labelled with 15N? (ii) Explain how DNA labelled with 15N could be separated and distinguished from DNA containing 14N.

(i) base/named base (ii) 1. DNA centrifuged 2. in solution of caesium chloride 3. separate depending on its mass/density/weight 4. compared with position of band formed by 14N/normal DNA 5. DNA with 15N further down the tube

Describe and explain how the structure of DNA results in accurate replication.

1 two strands therefore semi-conservative replication (possible) 2 base pairing/hydrogen bonds holds strands together 3 hydrogen bonds weak/easily broken, allow strands to separate 4 bases (sequence) (exposed so) act as template /can be copied 5 A with T, C with G / complementary copy 6 DNA one parent and one new strand

The cells in the heart and liver of a person are genetically identical but different in appearance. Use your knowledge of genes to suggest why these cells are different in appearance.

1. Different genes are expressed in each 2. Producing different enzymes / proteins

Give three ways in which the structure of the DNA molecule enables it to carry out its functions.

1. Sugar - phosphate backbone gives strength 2. Coiling gives compact shape 3. Sequence of bases allows information to be stored 4. Long molecule / coiling stores large amount of information 5. Complementary base pairing enables information to be replicated / transcribed 6. Double helix protects weak hydrogen bonds / double helix makes molecule stable 7. Many hydrogen bonds together give molecule stability; Prevents code being corrupted 8. Hydrogen bonding allows chains to split for replication / transcription OR molecule unzips easily for replication / transcription.

A cancerous tumour is formed by uncontrolled mitotic division. This results in a mass of cells with an inadequate blood supply. Drugs are being developed which only kill cells in a low oxygen environment. Suggest how these drugs could be useful in the treatment of cancer.

1. cancer cells killed, normal body cells survive 2. cancer cells low oxygen (as blood supply cannot satisfy demand)

Describe the behaviour of chromosomes during mitosis and explain how this results in the production of two genetically identical cells.

1. chromosomes shorten/thicken/supercoiling 2 chromosomes (each) two identical chromatids/strands/copies (due to replication) 3 chromosomes/chromatids move to equator/middle of the spindle/cell 4 attach to individual spindle fibres 5 spindle fibres contract / centromeres divide / repel 6 (sister) chromatids/chromosomes (separate) move to opposite poles/ends of the spindle 7 each pole/end receives all genetic information/ identical copies of each chromosome 8 nuclear envelope forms around each group of chromosomes/ chromatids/at each pole

Explain why specific base pairing is important in DNA replication.

1. identical/exact copies made 2. same base sequence as original DNA 3. both strands act as template/complementary base pairing occurs on both strands

Describe two differences in the effect of the drug on the cancer cells, compared with healthy cells in the following weeks.

1. longer time to recover 2. reduced rate of mitosis / divide more slowly/increased doubling time

Suggest one reason for the lower number of cancer cells compared to healthy cells at the end of the first week.

1. more (cancer cells) killed 2. cancer cells divide more (often) 3. so are more likely to be killed, more susceptible

Describe the structural features shown in the drawing which account for the ability of the molecule to replicate itself.

1. two chains/strands 2. base pairing present/A-T/C-G 3. hydrogen bonds easily broken

Describe two features of DNA which make it a stable molecule.

1. two strands with specific base pairing; 2. large number of hydrogen bonds (between strands) 3. helix/coiling reduces chance of molecular damage / protects H bonds 4. strong sugar-phosphate backbone

Explain how the differences between the DNA sequences from the quagga and the mountain zebra may have arisen as a result of mutation.

Mutagenic agents: x-rays high energy radiation / particles chemical substances Examples of mutations deletion: substitution addition Idea of: incorrect pairing during replication

The sequence of bases on one strand of DNA is important for protein synthesis. What is its role?

determines (sequence of) amino acids / specific protein produced / mRNA formation

What is meant by specific base pairing?

each base only pairs with one other/one specific base / complementary base pairing

Describe the molecular structure of DNA and explain how a sequence of DNA is replicated in the bacteria.

nucleotides composition of a nucleotide: - 4 bases named - sugar-phosphate 'backbone' - two (polynucleotide) strands - specific base-pairing example e.g. A-T / C-G; hydrogen bonding 'uncoiling' / 'unzipping semi-conservative replication DNA polymerase new complementary strands form / identical DNA molecule produced; DNA inserted into plasmids which are self-replicating

Explain why DNA replication is described as semi-conservative.

one strand of original molecule in each new molecule/DNA

Give one advantage of DNA molecules having two strands

stability / protects bases / replication