L4 DNA and Chromosomes Flashcards
What does the M Phase consist of, in abstract terms?
Mitosis (nuclear division)
Cytokinesis (cytoplasmic division)
M Phase - what are all 6 stages called? [6]
Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis
What is a telomere?
Structure at the end
Get shorter as you age
What is a centromere?
Where two chromatids meet
What is a kinetochore?
Give rise to spindle microtubules
Pull chromatids apart
Prophase - what happens?
Replicated chromosomes condense
Mitotic spindle (outside nucleus) assembles between 2 centrosomes as they move apart
Prometaphase - what happens?
Starts breakdown of nuclear envelope abruptly
Chromosomes attach to spindle microtubules via kinetochore and undergo active movement
Metaphase - what happens?
Chromosomes aligned at equator of spindle - midway between spindle poles.
Paired kinetochore microtubles attach to opposite poles of spindle
Anaphase - what happens?
Sister chromatids synchronously separate and each is pulled slowly towards the spindle pole attached
Kinetochore microtubles get shorter and spindle poles move apart –> segregation
Telophase - what happens?
Sets of chromosomes arrive at spindle poles. Nuclear envelope forms - 2 nuclei
Division of cytoplasm begins with assembly of contractile ring
Cytokinesis - what happens?
Cytoplasm divided by contractile ring of actin and myosin filaments, pinches in cell to create 2 cells
Cell cycle checkpoints - what are they?
Controlled by cyclins and protein kinases (Cdks) – phosphorylation of cdk/cyclin complexes
Chemotherapy drugs target which stages of cell replication?
S and M phases – kill rapidly replicating cells
p53 - what is its significance?
~50% cancers have mutations in p53
DNA replication - why is it important?
Must replicate 6 billion bp
Accuracy and speed required (~100nt/s)
Complementary bp
Polar strands (opp direction)
Nucleotide added to which end?
3’ end
DNA synthesised direction/energy?
5’ to 3’
Breakdown of triphosphate bond = energy
Leading/lagging strand - what do they mean?
Leading strand – continuous synthesis
Lagging strand – discontinuous synthesis
(Okazaki fragments)
Origins of replication
~10,000 origins of replication: speeds up process to ~1h
DNA replication enzymes [6]
- DNA Helicase: Unwinds double helix
- DNA Polymerase: adds nucleotides to 3’ end of leading strand
- Exposed lagging strand protected by single-strand DNA binding proteins
- DNA Primase: adds small RNA primer to lagging strand
- DNA Polymerase: adds nucleotides to 3’ end of lagging strand
DNA Ligase: joins together small gaps
Werner syndrome - what is it?
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
DNA polymerase is responsible for…
Monitor bp between new/old strands & catalyse nucleotide addition reaction
Proofreading
Clips off mismatch
Primers are responsible for…
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)
What does nuclease do?
Degrade RNA primer & repair polymerase replaces it with DNA
What does DNA helicase do?
uses energy from ATP hydrolysis to properl itself forward
Pries double helix apart
DNA topoisomerase relieve tension and produce nicks in DNA backbone
DNA topoisomerase - function?
relieve tension and produce nicks in DNA backbone
Sliding clamp - function?
keep DNA polymerase firmly attached to template - form a grip around new double helix & allows polymerase enzyme to move
Preventing mutations - how?
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)
DNA damage - how?
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
Characterising mutation - how?
Effect on heritability (somatic or germ line)
Scale of mutation (chromosome or SNP)
Effect on normal function (loss or gain of function)
Monogenic genetic disease - overview
Substitution: Sickle-cell anaemia
Deletion: Cystic fibrosis
Insertion: Huntington’s disease
CF
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)
Sickle cell disease
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
Huntington’s disease
• 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
Polyglutamine
residues stick together creating a toxic product (gain of function) which causes neuron cell death through multiple mechanisms
Xeroderma pigmentosum
• 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
