DNA ReplicationDNA Replication

What is DNA replication?What is DNA replication?When does it happen?When does it happen?

DNA replication is the process by DNA replication is the process by which the DNA molecule duplicates which the DNA molecule duplicates itself to create itself to create identical identical copies during copies during the S phase prior to cell division – the S phase prior to cell division – either mitosis or meiosis. either mitosis or meiosis.

Semi-Conservative Semi-Conservative ReplicationReplication

DNA replication is also called DNA replication is also called semi-semi-conservativeconservative replication replication One DNA molecule is used as a One DNA molecule is used as a templatetemplate to produce to produce

two newtwo new, identical molecules. , identical molecules. Thus, each new DNA molecule consists of one Thus, each new DNA molecule consists of one

“parent” strand and one newly synthesized strand.“parent” strand and one newly synthesized strand.

Steps in DNA SynthesisSteps in DNA Synthesis

AnnealAnneal-the pairing of complementary -the pairing of complementary strands of DNA through hydrogen bonding.strands of DNA through hydrogen bonding.

DNA replication requires a number of DNA replication requires a number of different different stepssteps, each associated with a , each associated with a different different enzymeenzyme. .

The rules of The rules of complementary base pairingcomplementary base pairing (A-T, G-C) helps explain how DNA is (A-T, G-C) helps explain how DNA is replicated prior to cell division. replicated prior to cell division.

Ensures that DNA REPLICATION occurs rapidly

1. Gyrase- Relieves pressure in coil caused by unwinding- Initiates unwinding of DNA

2. Helicase- Unwinds DNA at replication fork by breaking the H bonds

3. SSB- Destabilizes helix- Facilitates unwinding by blocking H-bonding- Stability of replication fork

4. RNA Primer, Primase

- Initiates complementary chain growing

5&7 DNA Polymerase

- Constructs growing complementary chain (5’ 3’)

6. DNA Ligase

- joins the gaps in the Okazaki fragments

Okazaki Fragment- Short fragment of DNA that is the result of the synthesis of the lagging strand

Lagging

Strand- Uses 5’3’ template as a guide

Leading

Strand- Uses 3’ 5’ template as a guide

DNA polymerase III

Enzyme/Protein Effect

DNA gyrase Relieves any tension from the unwinding of the double helix

Single-stranded binding proteins (SSB) Keep separated strand of DNA apart by blocking hydrogen bonding.

helicase Breaks the hydrogen bonds between the base pairs

primase Synthesizes RNA primers that will be used by DNA polymerase as a starting point to build the new complementary strands

Cannot initiate a new complementary strand by itselfRequires an initial starting 3’ end to start elongation. Synthesize DNA in the 5’ to 3’ direction. Add free deoxyribonucletides triphosphates to a 3’ end of the elongation strand.Uses the energy from breaking the bonds b/w first and second phosphate to drive the condensation reaction that adds the complementary nucleotide.Act as quality control checker by proofreading the new strands of DNA

DNA polymerase I Removes RNA primers from the leading strand and replaces them with the appropriate DNA nucleotides to one end of the growing complementary strand of nucleotides and builds the DNA in a 5’ to 3’ direction

exonuclease Remove sections of DNA that are incorrectly paired to the complementary strand. This must be done quickly to avoid mistakes in the replication of DNA.

ligase Joins the gaps in the Okazaki fragments by binding the backbones (phosphate to sugar) of the fragments

A leading strand: A leading strand: replication replication proceeds proceeds continuously along continuously along it toward the it toward the replication fork. replication fork.

A lagging strand: A lagging strand: Composed of short segments Composed of short segments

of DNA, known as of DNA, known as Okazaki Okazaki fragmentsfragments, is built , is built discontinuouslydiscontinuously away from away from the replication fork. the replication fork.

The process occurs in The process occurs in separate short segments separate short segments because DNA polymerase because DNA polymerase II can only add nucleotides to can only add nucleotides to the 3’ end of a growing DNA the 3’ end of a growing DNA strand. strand.

The overall direction of The overall direction of growth of the lagging strand growth of the lagging strand must be from its 3’ end must be from its 3’ end toward its 5’ end, which is toward its 5’ end, which is opposite to the direction of opposite to the direction of nucleotide addition by DNA nucleotide addition by DNA polymerases. polymerases.

DNA REP-overview.MOV

DNA Replication.MOV