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DNA Replication
→ → → DNA replication → the process of copying a DNA molecule in order to produce 2 identical
DNA molecules
→ the DNA is replicated before the cell divides, so both daughter cells would have a copy of the DNA

→ semiconservative replication → the DNA daughter cells are made of one original DNA strands and
one newly made
→ → → Enzymes of DNA Replication
→ DNA Helicase → unwinds the DNA double helix & separates the DNA strands / breaks the hydrogen
bonds b/n the complementary base pairs [nucleotides]
→ Single Stranded Binding proteins[SSBs] → bind to separated DNA strands to stop them from
reattaching / prevent hydrogen bonds b/n base pairs from reforming
→ Topoisomerase → relieves tension in the unwinding DNA strands / moves ahead of the replication
fork & cuts one DNA strands to allow it to unwind & reattaches them
→ DNA Polymerase → builds new DNA strands (only in the 5’ to 3’ direction) / links the phosphate
of the next nucleotide to the 3’ end of the growing strand
→ Primase → builds RNA primer[short segment of RNA nucleotides] (DNA Polymerase cannot build
new DNA strands without a starting point[RNA primer])
→ DNA Polymerase III → builds complementary DNA strands / starts from the RNA primer & adds
complementary DNA nitrogenous bases to the template (one at a time) (only builds in the 5’ to 3’
direction)
→ DNA Polymerase I → replaces RNA primers with DNA bases[nitrogenous bases] / detaches the RNA
primer from the template by breaking the hydrogen bonds b/n the bases & adds complementary DNA
bases
→ DNA Ligase → joins DNA segments[deoxyribose sugar & phosphate & nitrogenous base] / forms the
covalent (phosphodiester) bonds b/n nucleotides, completing the new sugar-phosphate backbone
→ → → RNA → RiboNucleic Acid
→ single stranded
→ ribose sugar molecule
→ U instead of T [Uracil for Thymine]
→ origins of replication → where the
separation of strands started
→ replication fork → a single DNA
strand
→ replication bubble → two replication
forks
→ leading strand → the new DNA
strand that is continuously built into the
replication fork (5’ to 3’)
→ lagging strand → the new DNA
strand that is built discontinuously in
short segments (okazaki) out of the fork
with the help of primers (since it is built
in the opposite direction and has to wait
for the replication fork to be open)
→ okazaki segments → the short segments that are built on the lagging strand
→ → → Aging
→ at the end of the replication process, the primers are removed by Polymerase I→ the polymerase has
nothing to build off of and the DNA that could have been created in the place of the primers is lost
→ Telomere → extra nucleotides at the end of the DNA that protects it from deterioration (losing more
genetic information); overtime telomeres too are lost, which contributes to the Aging (loss of DNA)
→ → → Mistakes
→ DNA polymerase add the wrong base once per every 100 000 nucleotides

→ a mistake is fixed by: 1) being detected by the polymerase & 2) being replaced by other enzymes
→ if the polymerase misses the mistake: 1) Nuclease enzymes detect the mistake and cut the incorrect
base out & Polymerase fills in the gap & Ligase seals the gap left in the new DNA strand
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mRNA (messenger RNA) → carries an RNA-copy of a gene from DNA to ribosomes
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tRNA (transfer RNA) → transfers amino acids to the ribosome
→ → → Transcription (DNA gene is turned into mRNA; occurs in the nucleus)
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Elongation → RNA Polymerase makes pre-mRNA; RNA Polymerase adds complementary RNA
nucleotides onto the DNA template (builds pre-mRNA in the 5’ to 3’ direction)
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Mutations
→ → → Mutation → a permanent change in the DNA sequence (occur spontaneously & randomly)
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insertion → nucleotides get added to DNA
→ frameshift [change of “reading frame”]
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Physical → high energy radiation (X-ray, UV radiation)
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Biological → virus, bacteria
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