L4 DNA and Chromosomes

Question 1

What does the M Phase consist of, in abstract terms?

Answer 1

Mitosis (nuclear division)

Cytokinesis (cytoplasmic division)

Question 2

M Phase - what are all 6 stages called? [6]

Answer 2

Prophase 
Prometaphase 
Metaphase 
Anaphase 
Telophase 
Cytokinesis

Question 3

What is a telomere?

Answer 3

Structure at the end

Get shorter as you age

Question 4

What is a centromere?

Answer 4

Where two chromatids meet

Question 5

What is a kinetochore?

Answer 5

Give rise to spindle microtubules

Pull chromatids apart

Question 6

Prophase - what happens?

Answer 6

Replicated chromosomes condense

Mitotic spindle (outside nucleus) assembles between 2 centrosomes as they move apart

Question 7

Prometaphase - what happens?

Answer 7

Starts breakdown of nuclear envelope abruptly

Chromosomes attach to spindle microtubules via kinetochore and undergo active movement

Question 8

Metaphase - what happens?

Answer 8

Chromosomes aligned at equator of spindle - midway between spindle poles.
Paired kinetochore microtubles attach to opposite poles of spindle

Question 9

Anaphase - what happens?

Answer 9

Sister chromatids synchronously separate and each is pulled slowly towards the spindle pole attached
Kinetochore microtubles get shorter and spindle poles move apart –> segregation

Question 10

Telophase - what happens?

Answer 10

Sets of chromosomes arrive at spindle poles. Nuclear envelope forms - 2 nuclei
Division of cytoplasm begins with assembly of contractile ring

Question 11

Cytokinesis - what happens?

Answer 11

Cytoplasm divided by contractile ring of actin and myosin filaments, pinches in cell to create 2 cells

Question 12

Cell cycle checkpoints - what are they?

Answer 12

Controlled by cyclins and protein kinases (Cdks) – phosphorylation of cdk/cyclin complexes

Question 13

Chemotherapy drugs target which stages of cell replication?

Answer 13

S and M phases – kill rapidly replicating cells

Question 14

p53 - what is its significance?

Answer 14

~50% cancers have mutations in p53

Question 15

DNA replication - why is it important?

Answer 15

Must replicate 6 billion bp
Accuracy and speed required (~100nt/s)
Complementary bp
Polar strands (opp direction)

Question 16

Nucleotide added to which end?

Answer 16

3’ end

Question 17

DNA synthesised direction/energy?

Answer 17

5’ to 3’

Breakdown of triphosphate bond = energy

Question 18

Leading/lagging strand - what do they mean?

Answer 18

Leading strand – continuous synthesis
Lagging strand – discontinuous synthesis
(Okazaki fragments)

Question 19

Origins of replication

Answer 19

~10,000 origins of replication: speeds up process to ~1h

Question 20

DNA replication enzymes [6]

Answer 20

1. DNA Helicase: Unwinds double helix
   
   2. DNA Polymerase: adds nucleotides to 3’ end of leading strand
   3. Exposed lagging strand protected by single-strand DNA binding proteins
   4. DNA Primase: adds small RNA primer to lagging strand
   5. DNA Polymerase: adds nucleotides to 3’ end of lagging strand
      DNA Ligase: joins together small gaps

Question 21

Werner syndrome - what is it?

Answer 21

1/200,000 in USA
DNA Helicase mutation - premature aging disorder
Errors in DNA replication and repair

Increased risk of cataracts, atheroscleoris, osteoporosis and cancer

Question 22

DNA polymerase is responsible for…

Answer 22

Monitor bp between new/old strands & catalyse nucleotide addition reaction
Proofreading
Clips off mismatch

Question 23

Primers are responsible for…

Answer 23

Created by primase
10 nucleotide long
RNA primer needed for leading and new primer for lagging at intervals
DNA polymerase elongates fragment until next primer (deoxyribonucelotide)

Question 24

What does nuclease do?

Answer 24

Degrade RNA primer & repair polymerase replaces it with DNA

Question 25

What does DNA helicase do?

Answer 25

uses energy from ATP hydrolysis to properl itself forward Pries double helix apart DNA topoisomerase relieve tension and produce nicks in DNA backbone

Question 26

DNA topoisomerase - function?

Answer 26

relieve tension and produce nicks in DNA backbone

Question 27

Sliding clamp - function?

Answer 27

keep DNA polymerase firmly attached to template - form a grip around new double helix & allows polymerase enzyme to move

Question 28

Preventing mutations - how?

Answer 28

Proof-reading capacity of DNA polymerase during DNA replication Excision repair systems act throughout cell life repairing DNA damage Polymerase can detect distortion in shape due to wrong base pair (rungs no longer same length)

Question 29

DNA damage - how?

Answer 29

Large number of genes devoted to DNA repair Internal sources: Products of normal cell function External sources: Mutagenic chemicals (benzene, cigarette smoke) UV Ionising radiation

Question 30

Characterising mutation - how?

Answer 30

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

Question 31

Monogenic genetic disease - overview

Answer 31

Substitution: Sickle-cell anaemia Deletion: Cystic fibrosis Insertion: Huntington’s disease

Question 32

CF

Answer 32

Deletion • Abnormal lung mucus leading to infection • 9,000 cases in the UK • Impaired chloride transport (loss of function) • 70% of patients share 3bp deletion in CFTR gene on chr 7 (cystic fibrosis transmembrane conductance regulator)

Question 33

Sickle cell disease

Answer 33

Substitution Single nucleotide substitution in HBB gene (beta chain of haemoglobin) • Misshapen blood cells do not survive as long (can cause anaemia) and clog up capillaries ~12,500 SCD in the UK

Question 34

Huntington's disease

Answer 34

• Neurodegenerative disease (starts to appear age 30-50) uncontrollable muscular movements loss of memory and depression difficulties with speech and swallowing • Damage of the nerve cells in areas of the brain • ~7,000 cases in the UK Caused by increase in number of CAG trinucleotide repeats (encoding glutamine) in the Huntingtin (HTT) gene

Question 35

Polyglutamine

Answer 35

residues stick together creating a toxic product (gain of function) which causes neuron cell death through multiple mechanisms

Question 36

Xeroderma pigmentosum

Answer 36

``` • Most mutations in DNA repair lethal • XP: mutation in UV repair Unable to remove thymine dimers Autosomal recessive disorder • Acute sun sensitivity • Hypo- and hyper-pigmentation • Multiple cancers at young age • Mental retardation Progressive degeneration ```