T3L3 - DNA replication Flashcards
M Phase [2]
Mitosis (nuclear division) and cytokinesis (cytoplasmic division)
Interphase Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis
Cell cycle
M Phase
G1 Phase
S Phase
G2 Phase
Kinetochore
Spindle fibres form and pull chromatids apart
Telomeres get shorter as you age
Prophase
Replicated chromosomes consist of 2 chromatids which condense
Mitotic spindle assembles between 2 centrosomes
Prometaphase
Starts abruptly with breakdown of nuclear envelope
Chromosome attach to spindle microtubules via kinetochore and undergo active movement
Metaphase
Chromosomes align at equator of spindle - midway between spindle poles
Paired kinetochore microtubules on each chromosome attach to opposite poles
Anaphase
Sister chromatids synchronously separate and each is pulled slowly towards spindle pole attached to it
Kinetochore microtubules get shorter and spindles poles move apart
Telophase
2 sets of chromosomes arrive at poles of spindles
Nuclear envelope reassembles around each set and complete formation of 2 nuclei
Assembly of contractile ring
Cytokinesis
Cytoplasm divides in 2 by a contractile ring of actin and myosin filaments
S phase [8]
Every time cell divides must replicate 6 billion bp
Accuracy and speed required
(~100 nt/s)
Complementary base-pairing
DNA strands run in opposite directions (polar)
Nucleotides added to 3’ end
DNA synthesised in 5’ to 3’ direction
Energy provided from breakage of triphosphate bond
Phosphodiester bond formed
(-O-P-O-)
DNA 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
Characterising mutations [3]
Effect on heritability (somatic or germ line)
Scale of mutation (chromosome or SNP)
Effect on normal function (loss or gain of function)
Mutations can be prevented through [3]
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)
Primers in DNA synthesis
Primase creates primer (10 nucleotide long RNA)
Primase = RNA polymerase
RNA primer needed for leading strand
DNA polymerase adds deocyribonucleotide to 3’ side
Nuclease degrades RNA primer & repair polymerase replaces it with DNA
• DNA helicase uses energy from hydrolysis of ATP to propel itself forward -> pries double helix apart • Opening of DNA makes It tightly wound on otherside -> DNA topoisomerase needed to relieve tensions -> produce transient nicks in DNA backbone -> temporary release tension Sliding clamps keep DNA polymerase firmly attached to template
DNA damagae causes [3]
Undergoes thermal collusions with other molecules
Trillion purine bases lost (depurination) -> does not break phosphodiester backbone & remove purine bases
UV radiation promotes covalent linkage between adjacent pyrimidine bases -> thymine dimer
DNA repair [4]
Damaged DNA recognised & removed by mechanisms involving nucleases -> cleave covalent bonds that join damaged nucleotides to DNA strand
Repair DNA polymerase binds to 3’ hydroxoyl end of cut DNA strand -> elongates chains in 5’ to 3’ direction
After repair DNA polymerase has filled the gap, a break remains in the sugar phosphate backbone of repaired strand -> nick in helix sealed by DNA ligase (same as that which joins Okazaki fragments)
Mismatch pair: recognise mistake & remove newly made DNA
DNA replication
Initiator proteins bind to specific DNA sequences
Pry DNA strand apart and break hydrogen bonds
DNA being replicated contains Y shaped junction
DNA replication in bacterial and eukaryotic chromosomes is bidirectional
Polymerisation energy provided by
Hydrolysis of deoxyribonucleotside triphosphate - release pyrophosphate
Depurination
does not break phosphodiester backbone & removes purine base
Ultraviolet radiation promotes
covalent linkage between adjacent pyrimidine bases to form a thymine dimer
DNA topoisomerase
needed to relieve tensions -> produce transient nicks in DNA backbone -> temporary release tension
Sickle cell disease [3]
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 [4]
• 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
Single nucleotide polymorphism (SNP)
• Single base change in DNA sequence
• Normal genetic variation in population
• Synonymous: no change in amino acid sequence
Non-synonymous: change to amino acid sequence
Werner Syndrome
Mutations to mitosis or DNA replication genes usually lethal
Range of syndromes result from loss of minor components
Werner Syndrome (1/200,000 in USA)
Premature aging disorder
Mutation in a DNA Helicase (WRN)
Errors in DNA replication and DNA repair
Increase in risk of cataracts, atherosclerosis, osteoporosis and cancer
Model for the aging process
Life expectancy is anywhere from 45 to 50 years
