DNA and Chromosomes

Question 1

What are the stages of Cell Cycle? (4)

Answer 1

1. Mitosis and Cytokinesis (nuclear and cytoplasmic division)
2. G1 phase - Growing proteins and organelles
3. S phase - DNA replication of 23 pairs of chromosomes to 46 pairs
4. G2 phase - preparation for

Question 2

What are the 6 stages of Mitosis? (6)

Answer 2

(PMAT)

1. Prophase
2. Pre-metaphase
3. Metaphase
4. Anaphase
5. Telophase
6. Cytokinesis

Question 3

Describe Prophase (3)

Answer 3

Prophase:

• Chromosomes condense chromatins to 2 sister chromatids joined at a centromere.
• Centrosomes migrate to end of the cell with spindles forming.

Question 4

Describe Premetaphase and metaphase (3)

Answer 4

Premetaphase: Nuclear envelope starts to disappear due to phosphorylation.

Metaphase:

• Chromosomes line up in the middle of the cell.
• Spindle fibres attach at kinetochore in preparation for anaphase.

Question 5

Draw and label a metaphase chromosome (2 sister chromatids joined together) (5)

Answer 5

Allocate:

1. Telomere
2. Centromere
3. Short arm and long arm
4. Kinetochore
5. Spindle fibres

Question 6

Describe Anaphase (2)

Answer 6

• The 2 sister chromatids are pulled apart by the microtubules towards centrosomes.
• Forming 2 chromosomes at each end.

Question 7

Describe Telophase and cytokinesis (3)

Answer 7

Telophase :

1. Chromosomes arrive at centrosome.
2. Nuclear envelope starts to reform around chromosomes

Cytokinesis: Cytoplasm and cell membrane divides to form 2 new identical cells.

Question 8

What proteins control are cell cycle checkpoints? (2)

Answer 8

1. cyclins
2. Protein kinases

Controlled by phosphorylation of kinase or cyclin complex.

Question 9

Why are cell cycle checkpoints important? (2)

Answer 9

• Errors are checked for at G1, S, G2 or mitosis e,g damaged DNA, improper replication or poor attachment to spindles.
• Failure to check (stop, wait or go) leads to cancer.

Question 10

How many base pairs are replicated in DNA, and at what accuracy and speed? (2)

Answer 10

1. 6 billion base pairs

2. 100 nt/s accuracy and speed at multiple points and time about 1 hour.

Question 11

Describe the steps of DNA Replication in Leading Strand (4)

Answer 11

1. DNA chain in unwound by enzyme (topoisomerase)
2. Helicase breaks hydrogen bonds between base pairs to form replication fork.
3. DNA primase adds RNA primer.
4. DNA polymerase binds to primer to add nucleotides from 5’ to 3’ end. This happens continuously in leading strand.

Question 12

Describe replication in lagging strand (4)

Answer 12

1. As lagging strand runs in opposite direction (3’ to 5’), nucleotides can only be added in fragments. (Okazaki fragments).
2. DNA primase adds RNA primer.
3. DNA polymerase attaches to primer, adding nucleotides to fill in gap to form okazaki fragment.
4. DNA primase attaches further up line and repeats steps.

Question 13

Describe final steps of DNA replication (3)

Answer 13

1. Enzyme Exonuclease removes all RNA primers from both strands of DNA.
2. Another DNA polymerase attaches to fill in gaps left with DNA.
3. DNA ligase seals up fragments of DNA to form continuous strand.

Question 14

Why is DNA replication called semi-conservative? (1)

Answer 14

It is made up of one conserved (old) strand of DNA and one new strand.

Question 15

What is an example of a condition resulting from DNA helicase in DNA replication? (3)

Answer 15

Werner Syndrome (mutation in Helicase)

• Causes premature ageing
• Increase risk of cataracts, cancer, atherosclerosis etc.

Question 16

How is accumulation of mutation prevented during Replication and in cell cycle? (2)

Answer 16

• DNA polymerase can proof-read during replication

- Excision repair systems in place throughout cell cycle (G1, S, G2 etc).

Question 17

How does DNA repair occur? (4)

Answer 17

• Damage in strand in recognised by DNA polymerase
• DNA Nuclease removes damaged region
• DNA polymerase makes new strand using bottom as template.
• DNA ligase seals strand.

Question 18

Examples of External sources that can cause DNA Damage?

Answer 18

UV, mutagenic chemicals.

Question 19

Example of inherited disease characterised by problem with DNA repair? (4)

Answer 19

Xeroderma Pigmentosum (acute sun sensitivity, poor pigmentation, cancers at young age, degeneration, intellectual disability)

1. It is Autosomal recessive disorder.
2. Caused by two thymines bounded together.

Question 20

How are mutations Characterised? (3)

Answer 20

1. Effect on heritability (somatic or germline)
2. Scale of mutation (chromosome or SNP)
3. Effect on normal function (loss or gain of function)

Question 21

How can single gene mutations occur and example of corresponding disease that could result? (3)

Answer 21

1. Substitution (sickle cell anaemia) Loss of function
2. Deletion (cystic fibrosis) loss of function
3. Insertion (huntingtons disease) gain of function

Question 22

Describe Sickle Cell (3)

Answer 22

1. Substitution Mutation in Beta globin gene; loss of function mutation.
2. Leads to change of tetiary structure of red blood cells, which do not survive as long and clog up capillaries
3. Around 12,500 cases in UK

Question 23

Describe Cystic Fibrosis (3)

Answer 23

1. Deletion Mutation, 70% of patients share 3bp deletion in CFTR (cystic fibrosis transmembrane conductance regulator) or chr 7.
2. Impairs chloride transport, cause build up of mucus in lungs that lead to infection.
3. Loss of Function

Question 24

Describe Huntingtons disease (3)

Answer 24

1. Insertion mutation, CAG tri-nucleotide repeats >40. (carrier 36-39). Gain of function.
2. Damages nerve cells in areas of brain
3. Leading to uncontrollable muscular movements, loss of memory and depression, difficulties with speech and swallowing.