DNA

Question 1

In all living organisms the ratio
GT:CA is constant but the ratio CG:TA varies from one
species to another. why? (3)

Answer 1

Number of A = T/C = G (1)
different organisms have different base sequences (1)
different amount of each base pairing (1)

Question 2

The DNA which codes for the human protein, -globin, has 850 base pairs but there are
only 141 amino acids in this protein.

Answer 2

Presence of non-coding DNA (1)

Question 3

Turtles have the same percentages of the four different bases as rats. why can they still be very different animals.

Answer 3

Sequence of bases is important (1)

Question 4

what is the structural difference in the DNA of an organism and that of a virus

Answer 4

DNA is single stranded in virus, whereas in organism it is ddouble stranded (1)

Question 5

In gamete production, what is the advantage of changing diploid cells into haploid cells?

Answer 5

Anaphase I;

Chromosomes are separating;

Question 6

why is DNA replication is described as semi-conservative (2)

Answer 6

When the DNA strand separates (1)

New molecules formed have one old strand and one new strand (1)

Question 7

What is the role of the spindle during the process of mitosis? (1)

Answer 7

Separates chromatid pairs (1)

Question 8

why is medical screening of people for the presence of these cancer-causing genes
is recommended. (2)

Answer 8

Identify those at risk from developing cancer (1)

So as to enable early diagnosis (1)

Question 9

Describe what is meant by a mutation, and explain how a mutation in a
suppressor gene might lead to the development of a malignant tumour. (6)

Answer 9

1. Mutation is a change in the DNA
2. Suppressor gene has different code;
3. Therefore different amino acid sequence;
4. And different protein structure
5. Cells divide by mitosis;
6. Tumour cells growth is rapid;
7. Tumour cells spread
8. Via blood system

Question 10

List the following phases of the cell cycle in the correct sequence.
anaphase interphase metaphase prophase telophase
(1)

Answer 10

1. Interphase
2. Prophase
3. Metaphase
4. Anaphase
5. Telophase;

Question 11

During which phase does the replication of DNA occur? 1)

Answer 11

Interphase

Question 12

Name the monomer from which nucleic acids are made(1)

Answer 12

nucleotide;

Question 13

Give two ways in which antibiotics can prevent bacterial growth. (2)

Answer 13

prevent cell wall synthesis (1)

stop DNA replication (1)

Question 14

Which part of the DNA molecule contains nitrogen? (1)

Answer 14

organic base

Question 15

Explain why the DNA from generation 1 is found in the position shown. (2)

Answer 15

it has been produced by semi-conservative replication (one old strand and one new )
one strand has 15N bases and the other 14N;
Accept light/ heavy N
(therefore) it is less dense / lighter;

Question 16

The structure of virus DNA is different from the DNA of the other two organisms.
Giving evidence from the table, suggest what this difference might be. (2)

Answer 16

viral DNA is single-stranded (1)

evidence from table e.g) not equal amount of A and T (1

Question 17

Explain how the structure of DNA is related to its functions.

Answer 17

1. Sugar-phosphate backbone so provides strength;
2. Large molecule so can store lots of information;
3. Helix so compact;
4. Base sequence allows information to be stored ;
5. Double stranded so replication can occur semi-conservatively;
6. Complementary base pairing so identical copies can be made;
7. Weak hydrogen bonds for strand separation;
8. MANY hydrogen bonds so STABLE

Question 18

Explain why DNA helicase is important in DNA replication.

Answer 18

1. Separates DNA strands by breaking Hydrogen bonds;

2. So strands can act as templates;

Question 19

Describe how DNA is replicated (6)

Answer 19

1. DNA helicase separates the DNA strands by breaking hydrogen bonds between bases exposing them
2. Both strands act as templates
3. Free nucleotides in the cytoplasm attach to the complementary bases: AT and GC
4. DNA polymerase joins the nucleotides together via phosphodiester bonds
5. Hydrogen bonds reform
6. New DNA molecule contains one new and one old strand (semi-conservative)