Replication is semi-conservative: each daughter molecule has one old strand and one new strand.
DNA supercoils are removed by DNA gyrase. The two halves of gyrase rotate and consume energy (ATP). Gyrase breaks DNA and rejoins strands after turning.
Two subunits of DNA gyrase:
Helicase = removes helical turns, but does not break it. Gyrase actually breaks the strands.
Single Stranded Binding Protein (ssb) = keeps the two strands apart
In replication, the original DNA strand is used as a template. Incoming bases hydrogen bond to their complementary bases. The incoming nucleotides are 5' tri-phosphates.
DNA synthesis goes 5' to 3' on the new strand of DNA. The new strand is antiparallel to the old strand.
The 3' -OH group of the growing end joins with the 5' phosphate group of the incoming nucleotide.
BASE:
Adenine
Guanine
Thymine (Uracil in RNA)
Cytosine
NUCLEOSIDE = BASE and SUGAR:
deoxyAdenosine
deoxyGuanosine
deoxyThymidine (Uridine)
deoxyCytidine
NUCLEOTIDE = BASE and SUGAR and PHOSPHATE
Adenosine Monophosphate (AMP)
GMP
TMP (UMP in RNA)
CMP
DNA Polymerase III or Pol III
-makes most new DNA
-senses if bases are mispaired and rejects mistakes
-contains 2 parts
a) synthetic subunit
During replication a "fork " is formed with a leading strand which is synthesized continuously and a lagging strand which is made in segments called Okasaki fragments. The Okasaki fragments are then joined by DNA Polymerase I.
DNA Polymerase I
-fills in gaps in lagging strand
-DNA repair
DNA ligase
-seals nicks
-used in genetic engineering
RNA priming - DNA polymerase cannot start a new chain. Primase makes a short piece of RNA. DNA Pol III then makes DNA and the RNA primer is degraded.
Division of Bacterial Chromosome
-occurs via binary fission
After the new DNA has replicated, the chromosomes attach to the cell wall and are pulled apart.
Eukaryotic Cell Division = Mitosis
a) nucleus of mother cell disassembles
b) chromosomes divide
c) partition of chromosomes
d) division of mother cell
e) assembly of nucleus in each daughter cell
Eukaryotic nucleus contains:
nucleolus - site where RNA (not mRNA) is made
chromatin - DNA and proteins
nuclear pore
nuclear envelope (2 layers)
Division of Eukaryotic Chromosome -
Begins from the origin of replication (rich in A and T pairs). The replication fork proceeds and forms a replication bubble. There can be several replication bubbles on a single chromosome.
Eukaryotic Cell Cycle
Manufacturing the DNA takes longer than the cell division process itself.
G1-phase - there is only one pair of chromosomes
S-phase - the DNA is duplicated, so there are 2 double helices, and the centromere is still not divided
G2-phase - the cell rests
Interphase - one chromatid is present
Mitosis: Prophase
a) nuclear membrane disintegrates
b) chromosomes condense
Mitosis: Metaphase
a) spindle forms
b) chromosomes line up at equator
two chromatids are now present
Mitosis: Anaphase
a) chromosomes divide completely
b) they move apart
Mitosis: Telophase
a) the nuclear membrane reassembles
...and finally, the cell splits apart
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Last updated: 18-Feb-99 / laa
