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DNA Replication
What is replication?Unique property of double stranded
molecules of DNASemi-conservative synthesis of DNA based
on a templateDuplication of genetic material The main mechanism that assures
transmission of information from cell to cell and from one generation – to another.
Why replication is required?
Assures exact transmission of information;
Determines self-reproduction; Represents the support of heredity; Assures material linkages between
generation.
When replication takes place?
What are the principles of replication?
Based on template;Complementary;Antiparallel;Two directions;Semi-conservative;Fidel;Very complex.
Bidirectional synthesis
Principles of replication:The monomers (nucleotides) are added to 3’ of
one strand by enzyme DNA polymerase;Newly syntheses strand is complementary to
template strand;Synthesis is semi-conservative: each new
double-stranded molecule contains an old strand (template strand) a new strand (daughter strand).
Semi-conservative mechanism
Components required for replicationsDNA molecule - templateOrigin of replication – point ORIEnzymesNucleotides (dNTP and NTP) SSB proteins
Main enzymes required for replicationDNA-polymerase (I, II,
III – in prokaryotes, a, g, e – in eukaryotes)
Primase DNA-helicases Topoisomerase DNA-ligaseTelomerase
DNA Polymerase
Topoisomerase
What are the main steps of replication?
InitiationElongationTermination
Initiation of replication:Identification of point ori;Formation of replication eye containing two
replication forks:Unfolding of DNADenaturation of DNA
Synthesis of RNA primersInitiation of polymerization of DNA
Point of initiation of replication - ORI
• consists of ~300 bp
• contains sites for interaction with specific proteins required for initiation of replication
ElongationEnlarging of replication eye;Synthesis of DNA in two directions:
Continuous synthesis of leader strand;Interrupted synthesis of lagging strand; synthesis of
Okazaki fragments on lagging strand.
TerminationMeeting of opposite replication forks;RNA primers are removed;Feeling in the gaps;Ligation of all fragments;Renaturation of DNA;
1 mol. DNA 2 mol. DNA.
The replicon – replication unit
• Contains 100-300 kb.
• Contains ORI point
• Contains two replication forks
• In eukaryotic there are numerous replicons
Characteristics of eukaryotic DNA-polymerases DNA polymerase α δ ε β γ
Location Nucleus Nucleus Nucleus Nucleus Mitochondria
Synthetic activityPrimer
extensionSynthesis of
DNADNA repair DNA repair Replication
Additional functions
- Exonuclease Exonuclease - Exonuclease
Peculiarities of replication in eukaryotesIn nucleus replication is carried down by DNA-
polymerases a, dIn mitochondria replication is done by DNA-
polymerase g. Chromosomal DNA contains many points ORI; Replication speed – 20-100 bases/second;Human nuclear genome contains 60000 ORI
points at every 100-150 kb.
Replication of nuclear human DNA is carried out in 6-9 hours
Example:If 1st chromosome will contain 1 ori point,
replication will take place during 15 hours:Contains 260 000 000 bp;Each replication fork will contain 130 000 000
bp Replication speed - 100 bp/sec;
Peculiarities of replication of telomeres
Telomeres:Sequences of nucleotides at the end of chromosomes;(GGGTTAA)n;Form loops;
Telomerase:DNA-polymerase RNA-dependent;Contains snRNA – template for synthesis of telomeric
DNA.
Replication of telomeres
Werner Patient
26 years old
30 years old
Doctor and patient – 30 years old 45 years old