Molecular Biology: Enzymes, Proteins And Complexes

Front 1

Topoisomerase I

Back 1

- relieves tension in parent DNA molecule and unwinds strands

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DNA Polymerase I

Back 2

- Prokaryotic

- 1 subunit

- 5' → 3' endonuclease activity

- 3' → 5' endonuclease activity

- involved in replication and repair of DNA

- acts on lagging strands, joining Okazaki Fragments

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DNA Polymerase III

Back 3

- Prokaryotic

- at least 10 subunits

- 3' → 5' endonuclease activity

- main replicating enzyme

- synthesises DNA on leading strand

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DNA Polymerase α

Back 4

- Eukaryotic

- 4 subunits

- No endonuclease activity

- Acts as a primer during replication

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DNA Polymerase δ

Back 5

- Eukaryotic

- 2 or 3 subunits

- 3' → 5' endonuclease activity (proofreading)

- Main replicative enzyme

Front 6

Primase

Back 6

- Eukaryotic

- Prokaryotic

- produces RNA based primer

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DNA Helicase

Back 7

- Breaks H bonds between base pairs

- e.g. DnaB in bacterial cells

Front 8

Primosome

Back 8

- Formed in E. Coli when primase is attached to the prepriming complex to begin the formation of RNA primers

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Prepriming Complex

Back 9

- formed when DnaB helicase proteins bind to the site of origin

- DnaB breaks more base pairs and moves replication fork further from origin

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DNA-dependent DNA Polymerases

Back 10

- Family of enzymes that carry out DNA-dependent DNA synthesis

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Single Strand Binding Proteins

Back 11

- Proteins that protect single strands

- Strands may be attacked by nucleases (present to degrade single stranded virus DNA)

- Strands may reattach

- Eukaryotic example: Replication Protein A

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DNA Ligase

Back 12

- Catalyses the formation of a phosphodiester bond

- Joins DNA strands

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FEN1

Back 13

- Eukaryotic

- Cuts 5' end of Okazaki fragments of lagging strand

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Telomerase

Back 14

- Adds TTAGGG (in vertebrates) sequence to 3' end of DNA molecules in telomere regions

- Prokaryotic DNA is often circular so does not have ends for telomerase to work on

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DnaA

Back 15

- Prokaryotic

- Form 'barrels' of proteins near origin of replication

- DNA winds round barrels and breaks base pairs (origin is mostly A-T pairing which is relatively weak compared to C-G)

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DnaB

Back 16

- Prokaryotic

- Helicase enzyme

- Breaks more H bonds to move forks from origin

- Addition of these form prepriming complex

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Gamma Complex

Back 17

- Prokaryotic

- attaches and detaches pol III from the lagging strand during replication

- 9-10 subunits

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Proliferating Cell Nuclear Antigen

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- Eukaryotic

- PCNA complex holds Pol δ tightly to DNA

- Can be loosened so Pol δ can slide back along strand

- 'proliferating': during division, 'Antigen': studied in antibodes

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Tus Protein

Back 19

- Part of prokaryotic terminator sequences

- allows fork to pass in one direction only

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Nucleosomes

Back 20

- 'beads' on chromatin

- Octomer formed of histones: 2 x H2A, 2 x H2B, 2 x H3, 2 x H4

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DNA Glycosylase

Back 21

- Prokaryotic

- involved in base excision repair

- catalyses hydrolysis of n-glycosidic bond between base and sugar

- a family of enzymes, each with a specific excision role

- leaves an apyrimidinic/purinic (AP) site

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DNA Polymerase II

Back 22

- involved in prokaryotic nucleotide excision repair

- low rate of error

- can reinitiate DNA synthesis

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DNA Polymerase γ and DNA Polymerase β

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- Perform base excision repairs in eukaryotic cells

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Phosphodiesterase + AP endonuclease

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- Prokaryotic

- Removes pentose sugar left by removal of base by glycosylase, allowing new nucleotide addition by polymerase and ligase

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UvrABC Endonuclease

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- Involved in prokaryotic nucleotide excision

- moves up and down DNA molecule detecting general damage to helix

- uvrA checks for damage, uvrB and uvrC make cuts in molecule, urvB bridges gap in molecule for temporary protection before ligase/polymerase add new nucleotides

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MutH and MutS

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- Involved in prokaryotic mismatch repair that UvrABC Endonuclease can't detect

- MutH detects methylated parent strand

- MutS attaches to mismatched base in daughter strand

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Ku Proteins

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- Involved in eukaryotic homologous end joining

- Ku proteins cap ends of DNA molecule and attract one another, bringing the ends together

- Must act quickly or DNA drifts too far apart

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Histone H1

Back 28

- The linker histone

- attaches outside nucleosome

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Histones

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- Proteins in chromatin

- Form nucleosome octomers

- links the nucleosome and the DNA strand