Theta Replication

Theta (θ) replication is a common mode of DNA replication in prokaryotes, such as E. coli, and in
the circular DNA of mitochondria and plasmids. The process gets its name because the
intermediate structure formed during replication resembles the Greek letter "θ" (theta).

Key Features of Theta Replication

• Occurs in circular DNA molecules (e.g., bacterial chromosomes, plasmids, mitochondria).

• Bidirectional or unidirectional replication possible.

• Requires an origin of replication (OriC in E. coli).

• Produces a theta-shaped intermediate structure during replication.

• Semi-conservative replication, meaning each new DNA molecule consists of one parental
and one newly synthesized strand.

Steps of Theta Replication

1. Initiation

• Replication begins at a single origin of replication (OriC).

• The enzyme DnaA binds to the OriC, causing local unwinding of the DNA.

• DnaB (helicase) separates the two DNA strands, forming a replication bubble.

• Single-strand binding proteins (SSBs) stabilize the unwound DNA strands.

2. Elongation

• DNA Polymerase III synthesizes new DNA strands in the 5' → 3' direction.

• Leading strand: Synthesized continuously in the same direction as the replication fork.

• Lagging strand: Synthesized in short Okazaki fragments, which are later joined by DNA
ligase.

• DNA Gyrase (Topoisomerase II) prevents supercoiling ahead of the replication fork.

3. Termination

• Replication proceeds bidirectionally until the two forks meet at the termination sequence
(ter sites).

• Tus protein binds to ter sites, stopping the replication forks.

• DNA ligase seals the remaining gaps, completing the replication process.

• The two circular daughter DNA molecules may be interlinked (catenated) and are
separated by topoisomerase IV.
Bidirectional vs. Unidirectional Theta Replication

Type Description

Bidirectional Two replication forks move in opposite directions from the origin
Replication (common in bacteria like E. coli).

Unidirectional A single replication fork moves in one direction (less common, seen in
Replication some plasmids).

Comparison: Theta Replication vs. Rolling Circle Replication

Feature Theta Replication Rolling Circle Replication

Bacterial chromosomes,
Common in Some plasmids, bacteriophages (e.g., ΦX174)
plasmids, mitochondria

Template
Circular DNA Circular DNA
Shape

Replication
Semi-conservative Semi-conservative
Mode

Initiation Single origin (OriC) Nick in one strand

Strand Continuous synthesis of one strand,

Leading and lagging strand
Synthesis discontinuous synthesis of the other

End Product Two circular DNA molecules One continuous strand and one nicked strand

Conclusion

• Theta replication is a key process in bacterial replication, allowing rapid and efficient DNA
duplication.

• It is bidirectional in most cases, ensuring faster replication.

• Compared to rolling circle replication, theta replication is more common in bacterial

chromosomes and maintains DNA integrity without single-strand exposure.

Would you like details on the enzymes involved, regulation, or differences between prokaryotic
and eukaryotic replication?

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dna replication in eukaryotes

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DNA Replication in Eukaryotes

DNA replication in eukaryotes is a highly regulated, complex, and multi-step process that ensures
accurate duplication of the genome before cell division. It occurs in the S-phase of the cell cycle
and follows a semi-conservative mechanism, meaning each daughter DNA molecule consists of one
parental strand and one newly synthesized strand.

Key Features of Eukaryotic DNA Replication

• Multiple origins of replication (because eukaryotic genomes are large).

• Bidirectional replication from each origin.

• Occurs in the nucleus.

• Highly regulated by checkpoints in the cell cycle.

• Uses multiple DNA polymerases with specialized functions.

• Histone proteins are replicated and assembled into chromatin.

Steps of DNA Replication in Eukaryotes

1. Initiation

• Occurs at multiple origins of replication on each chromosome.

• The Origin Recognition Complex (ORC) binds to the origin.

• Helicase (MCM complex) unwinds the DNA, creating a replication bubble.

• Single-strand binding proteins (RPA) stabilize the unwound strands.

• Topoisomerase relieves supercoiling stress ahead of the replication fork.

2. Primer Synthesis

• DNA polymerases cannot start DNA synthesis on their own; they need an RNA primer.

• Primase (part of DNA polymerase α) synthesizes a short RNA primer (~10 nucleotides
long).

• DNA polymerase α extends the primer with a short stretch of DNA (~20 nucleotides).

3. Elongation

• DNA Polymerase δ synthesizes the lagging strand in short Okazaki fragments (~100-200
bp).

• DNA Polymerase ε synthesizes the leading strand continuously.

• Sliding clamp (PCNA - Proliferating Cell Nuclear Antigen) ensures processivity (prevents
DNA polymerase from detaching).

4. Removal of RNA Primers and Ligation

• RNase H and FEN1 remove RNA primers.

 • DNA Polymerase δ fills the gaps with DNA.

• DNA Ligase I seals the nicks between Okazaki fragments.

5. Termination

• When replication forks meet, replication stops.

• Topoisomerase II untangles the newly synthesized DNA.

• Telomerase extends the ends of linear chromosomes to prevent loss of genetic material.

Enzymes Involved in Eukaryotic DNA Replication

Enzyme Function

DNA Helicase (MCM Complex) Unwinds the DNA helix

Single-Strand Binding Proteins (RPA) Prevents reannealing of single strands

Topoisomerase Relieves supercoiling stress

Primase (part of Pol α) Synthesizes RNA primers

DNA Polymerase α Extends primers with short DNA stretches

DNA Polymerase δ Synthesizes lagging strand

DNA Polymerase ε Synthesizes leading strand

PCNA (Sliding Clamp) Enhances polymerase processivity

RNase H & FEN1 Removes RNA primers

DNA Ligase I Joins Okazaki fragments

Telomerase Extends telomeres to prevent chromosome shortening