Lecture 4: DNA and chromosomes

Question 1

What are the 4 stages of the eukaryotic cell cycle?

Answer 1

1. M Phase
2. G1 Phase
3. Synthesis Phase
4. G2 Phase

Question 2

What are the 6 stages of the M phase?

PpMATc

Answer 2

1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase
6. Cytokinesis

Question 3

What is the purpose of M phase in the cell cycle?

Answer 3

Replication of cells and their genetic material via mitosis

Question 4

What happens during prophase?

Answer 4

Chromosomes condense and mitotic spindles form.

Question 5

What happens during prometaphase?

Answer 5

Nuclear membrane disintegrates and spindles attach to kinetochores.

Question 6

What happens during metaphase?

Answer 6

Chromosomes align at equator.

Question 7

What happens during anaphase?

Answer 7

Sister chromatids separate and are pulled towards spindle poles.

Question 8

What happens during telophase?

Answer 8

Chromosomes arrive at poles and nuclear envelope reforms.

Question 9

What happens during cytokinesis?

Answer 9

Cytoplasm divides resulting in two genetically near-identical cells.

Question 10

What happens during the G1 phase?

Answer 10

Cell synthesises mRNA and proteins in preparation for subsequent steps leading to mitosis.

Question 11

What is the purpose of S phase in the cell cycle?

Answer 11

The S phase is responsible for replication of genetic material via semi-conservative replication by complementary base-pairing of nucleotide bases.

Question 12

How is DNA replicated in the S phase?

Answer 12

1. DNA helicase unzips the double helix to expose the nucleotides.
2. DNA Primase adds a small RNA primer
3. DNA Polymerase then continuously adds complementary nucleotides to the 3’ end of leading and lagging strands.
4. Exonuclease then removes RNA primers and DNA Polymerase replaces with DNA nucleotides.
5. DNA Ligase seals the fragments of DNA in both strands.

Question 13

How does DNA replication differ in the leading and lagging strands?

Answer 13

DNA polymerase adds complementary nucleotides to the 3’ end of lagging strand in fragments known as Okazaki fragments. Process require multiple RNA primers as DNA replication is occurring in the opposite direction.

Question 14

What happens during the G2 phase?

Answer 14

Period of rapid cell growth and protein synthesis during which the cell prepares itself for mitosis.

Question 15

How does the cell prevent the accumulation of mutations?

Answer 15

1. Proof-reading by DNA polymerase during DNA replication

2. Excision repair systems act throughout cell life repairing DNA damage

Question 16

What are mutations?

Answer 16

Any large/small change to DNA sequence that causes a deviation away from ‘normal’ which can either be bad or good. Usually associated with disease.

Question 17

How do you characterise mutations?

Answer 17

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

Question 18

What are single nucleotide polymorphisms (SNP)?

Answer 18

Single base change in DNA sequence.

Question 19

What are the 2 types of SNPs?

Answer 19

1. Synonymous: base change does not result in a change in AA.
2. Non-Synonymous: base change alters AA sequence which can ultimately affect the final protein.

Question 20

What are monogenetic diseases?

Answer 20

Monogenic genetic diseases are single gene disorders.

Question 21

Werner Syndrome?

Answer 21

Caused by mutations in a DNA Helicase.

Leads to premature aging and increased risk of cataracts, osteoporosis and cancer.

Question 22

Xeroderma Pigmentosum?

Answer 22

Caused by an autosomal recessive mutation in UV repair whereby unable to remove thymine dimers.
Leads to acute sun sensitivity, hypo- and hyper-pigmentation, and intellectual disability.

Question 23

Sickle-Cell Anaemia?

Answer 23

Caused by single nucleotide substitution (A to T) in HBB gene.
Leads to misshapen blood cells.

Question 24

Cystic Fibrosis?

Answer 24

Caused by 3 base pair deletion in CFTR gene on chromosome 7, resulting in loss of phenylalanine AA.
Leads to impairment of chloride transport and abnormal lung mucus leading to infection.

Question 25

Huntington’s Disease?

Answer 25

Neurodegenerative disease caused by increased CAG trinucleotide repeats in the Huntingtin (HTT) gene.
Leads to polyglutamine plaque which is toxic to neurons and causes cell death.
Patients experience uncontrollable muscular movements, loss of memory and difficulties with speech and swallowing.
Severity f disease is dependent on number of CAG repeats.