Replication initiation and template preparation
Duplication of the bacterial chromosome cannot start at a random location. The landing site for the replicative complex is a strictly defined region — the origin (origin of replication). A specific nucleotide sequence in this zone promotes the initial unwinding of the double helix. One or two replication forks can start moving from this region, and the process continues until the entire molecule is completely duplicated.
A whole team of proteins works to successfully separate the parental DNA strands:
- Helicase — an enzyme that physically unwinds the double helix at the branch point. This process requires significant energy expenditure.
- Topoisomerase (gyrase) — relieves structural strain and prevents the formation of secondary coils (supercoiling) ahead of the advancing fork.
- SSB proteins (Single-Strand Binding proteins) — immediately bind to the exposed single-stranded regions. They prevent the DNA from snapping back into a helix and reliably protect vulnerable strands from degradation by nucleases.
Biochemical mechanism of synthesis and the role of the primer
Direct synthesis of new strands is carried out by the enzyme DNA polymerase. However, this enzyme has an important biochemical limitation: it is capable of adding new nucleotides exclusively to the 3'-hydroxyl group (3' end) of an existing chain. DNA polymerase cannot initiate synthesis "from scratch".
To start, it requires a special primer. A short RNA chain that is fully complementary to the DNA template takes on this role. This primer is synthesized by the enzyme DNA primase (or RNA primase). The uniqueness of primase is that it can initiate RNA synthesis on single-stranded DNA without any prior support. As a result, ribonucleotides are always initially present at the 5' end of each new strand, and only after their placement can DNA polymerase proceed to fully elongate the strand.
Asymmetry of the replication fork
Because the two strands in a DNA molecule are antiparallel to each other, and synthesis can proceed exclusively in the 5' to 3' direction, the processes on the two strands occur fundamentally differently. Two strands are distinguished within the replication fork:
- Leading strand (Leading strand). Here, the direction of synthesis completely coincides with the vector of helix unwinding. Therefore, DNA polymerase moves behind the helicase continuously, assembling a long, continuous strand.
- Lagging strand (Lagging strand). On this strand, the direction of synthesis is opposite to the direction of movement of the replication fork. Synthesis is forced to proceed discontinuously, in separate short segments named Okazaki fragments.
Maturation of the lagging strand and cell division
For the lagging strand to turn into a fully continuous molecule, it must undergo maturation. First, all RNA primers used to start each Okazaki fragment are removed. The resulting gaps are accurately filled in by DNA polymerase. At the final stage, the enzyme DNA ligase seals the separate fragments into a single covalent strand.
The completion of the chromosome replication cycle is a signal for the start of bacterial cell division. In bacteria, segregation (separation) of the nucleoid occurs without the involvement of the spindle apparatus characteristic of eukaryotes. The newly synthesized DNA strands attach to the cytoplasmic membrane. There is a strict spatial relationship between these attachment sites and the zone of active cell wall growth. As the membrane grows, it physically "pulls apart" the two anchored copies of the genome in opposite directions toward the poles of the future daughter cells. This binary division ultimately ensures the exponential growth of the bacterial population.
Briefly: answers to topical questions
What types of DNA polymerases are found in bacteria and what are their main functions?
In bacteria (using E. coli as an example), 5 types of DNA polymerases (I, II, III, IV, V) are distinguished. In various combinations, these enzymes are responsible for DNA replication and repair.
- DNA polymerase I — removes RNA primers via exonuclease activity and fills the resulting gaps with deoxyribonucleotides.
- DNA polymerase III — elongates the DNA chain from the primer, forming Okazaki fragments on the lagging strand.
The functions of the remaining types (II, IV, V) are not detailed, but overall they ensure the processes of synthesis and restoration of genetic material.
Which enzyme is responsible for removing RNA primers in the lagging strand in bacteria?
The removal of RNA primers in the lagging strand is carried out by enzymes with nuclease activity.
- DNA polymerase I — removes RNA primers via exonuclease activity and fills the resulting gaps with deoxyribonucleotides.
- 5'-3' endonuclease — participates in the removal of the RNA primer.
After the removal of RNA primers, the resulting gaps are filled by DNA polymerase, and the separate Okazaki fragments are joined into a single strand by DNA ligase.
Helicase — Hacker (hacks and unwinds the helix). Topoisomerase — Brake (brakes supercoiling, relieves tension). Primase — First (lays down the first primer). Polymerase — Foreman (builds the main strand). Ligase — Tape (glues Okazaki fragments together).
Class and exam questions
Why cannot DNA polymerase initiate synthesis independently?
The biochemical mechanism of DNA polymerase requires a free 3' end (3'-hydroxyl group) to attach a new nucleotide. Therefore, it always needs a starting platform in the form of an RNA primer synthesized by primase.
What are Okazaki fragments?
These are short segments of new DNA synthesized on the lagging strand. Their discontinuous formation is due to the fact that synthesis proceeds in the 5'→3' direction, which is opposite to the direction of replication fork unwinding.
How do chromosome copies segregate into cells if bacteria lack a spindle apparatus?
The synthesized DNA copies attach to the cytoplasmic membrane. Due to the active growth of the membrane and cell wall between the chromosome attachment sites, they are physically separated to opposite sides of the forming daughter cells.
What else to study on this topic
- Structural features and nucleotide sequence of the origin region
- Detailed composition of the complex of more than 20 enzymes that ensure duplication
- Mechanism of RNA primer removal during lagging strand maturation
- Dynamics of the movement of two replication forks toward each other on a circular chromosome
- Mathematical aspects of geometric progression during binary population division
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