Trombone Model of DNA Replication

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In this type of DNA replication, the DNA opens in to a 'leading strand' and a 'lagging strand'. The leading strand is complemented with usual base pairing. The lagging strand ,however, is at places complemented with RNA fragments'(primers)'. The rest is complemented with DNA deoxyribonucleotides in the 3'-5'direction (Okazaki fragments). Later the RNA primers are replaced by DNA.

Thus, DNA replication occurs within a fork structure in which the lagging strand loops around in a proposed “trombone                      model” to help orient the replication enzymes for repeated synthesis and joining.

It is considered that this mechanism helps maintain the integrity if DNA during the endless replication cycles it goes through.

The trombone model of DNA (deoxyribonucleic acid) proposed by Alberts et al. A Proper synthesis of both leading and lagging strands of DNA depend on a constant association of DNA polymerase with the replication fork. The trombone model explains how both leading and lagging strand synthesis takes place parallel to each other.

The replication involves three steps, namely initiation, elongation and termination. In the initiation helicases unwinds DNA, which results formation of super coils. These super coils can resolve by topoisomerases. The SSB bound to free strands and prevent rewinding.   

Then elongation is carried out in 5’ to 3’ direction, in which a circle reorients the DNA lagging-strand. Thus, its polymerase can replicate DNA in parallel with the leading-strand polymerase while bound to the replisome.

The adjoined picture shows the trombone model of DNA:   



The list of key components is as follows:

DNA helicase
Single strand binding proteins (SSB)
DNA topoisomerase
DNA polymerase
DNA ligase