Steps

1. Initiation
   • A replication initiator protein recognizes a specific sequence at the double-stranded origin.
   • It makes a nick in one DNA strand.
   • This creates a free 3′-OH end and leaves the initiator protein attached to the 5′ end.
2. DNA synthesis and strand displacement
   • DNA polymerase extends the 3′-OH end, using the intact circular strand as template.
   • As polymerase moves around the circle, it displaces the original nicked strand as single-stranded DNA.
3. Completion
   • After one genome length has been copied, the initiator protein cleaves and rejoins the newly synthesized strand to form a circular double-stranded DNA molecule.
   • If synthesis continues for more than one round, it produces a long linear DNA containing repeated genome units, called a concatemer.
4. Second-strand synthesis
   • The displaced single-stranded DNA is converted into double-stranded DNA by synthesis of its complementary strand, often starting at a single-strand origin.

Simple schematic

Circular dsDNA
      ↓ nick one strand at origin
Free 3′-OH end
      ↓ DNA polymerase extends it
Template strand goes around the circle
      ↓
Old strand is displaced as ssDNA
      ↓
Cut and ligate after one unit length
      ↓
New circular dsDNA + ssDNA template for another copy

Key features

• Requires a circular nucleic-acid template.
• Initiation involves a nick, rather than an RNA primer as the main first event.
• Produces a displaced single-stranded strand.
• Can rapidly generate multiple copies and concatemers.
• In herpesviruses, concatemeric DNA is later cleaved into genome-length molecules for packaging into capsids, as described in Sherris & Ryan's Medical Microbiology, Replication section.

Rubella

• Virus: Rubella virus, an enveloped, positive-sense single-stranded RNA virus in the Matonaviridae family.
• Transmission: Respiratory droplets.
• Incubation: Usually 14 to 21 days, average about 17 days.
• Infectious period: Approximately 7 days before to 7 days after rash onset. CDC clinical overview

Clinical features

• Low-grade fever, malaise, sore throat
• Tender posterior auricular, posterior cervical, and suboccipital lymph nodes
• Fine pink maculopapular rash beginning on the face and rapidly spreading downward to the trunk and limbs
• Rash typically lasts about 3 days
• Arthralgia or arthritis, particularly in adolescents and adults, especially women
• Forchheimer spots: small petechiae on the soft palate, sometimes present

Important complication: congenital rubella syndrome

1. Sensorineural deafness
2. Eye defects: cataract, retinopathy, glaucoma
3. Congenital heart disease: especially patent ductus arteriosus and peripheral pulmonary artery stenosis

Diagnosis

• Rubella-specific IgM antibody
• Rising rubella-specific IgG titres in paired serum samples
• RT-PCR from throat, nasal, urine, or other appropriate specimens, especially during suspected outbreaks

Treatment and prevention

• Treatment: No specific antiviral therapy. Provide rest, fluids, and antipyretics as appropriate.
• Isolation: Isolate affected patients for 7 days after rash appears.
• Prevention: Live attenuated rubella vaccine, usually given as MMR. It is contraindicated during pregnancy; susceptible people should be vaccinated before pregnancy or postpartum according to local guidance. Prevention of CRS is the main goal of rubella immunization programs. CDC clinical guidance

Rubella: microbiology point of view

1. Causative agent

• Rubella virus
• Genus: Rubivirus
• Family: Matonaviridae
  • Older textbooks may list it under Togaviridae.
• It has one serotype, so vaccine-induced immunity is broadly protective.

2. Structure

3. Replication

1. Virus enters through respiratory mucosa.
2. Viral envelope fuses with host-cell membrane and releases positive-sense RNA into the cytoplasm.
3. Viral RNA acts as mRNA and is translated to form viral replicase proteins.
4. A complementary negative-sense RNA intermediate is synthesized.
5. This negative-sense RNA acts as template for new positive-sense genomic RNA and subgenomic RNAs.
6. Structural proteins are synthesized, virions assemble, acquire envelope from intracellular membranes, and exit from the cell.

4. Pathogenesis

Respiratory droplets
       ↓
Replication in nasopharynx and regional lymph nodes
       ↓
Viremia
       ↓
Spread to skin, lymph nodes, joints, and placenta in pregnancy

• Viremia causes systemic spread.
• The rash is mainly immune-mediated, occurring as antibodies develop and viral clearance begins.
• In pregnancy, virus may cross the placenta during maternal viremia.
• Fetal damage results from persistent infection, cellular injury, and interference with cell division and organ development. The fetus may shed virus for months after birth.

5. Transmission and communicability

• Transmitted via respiratory droplets.
• Infectious from about 7 days before to 7 days after rash onset.
• Congenitally infected infants can shed virus in respiratory secretions and urine for a prolonged period, potentially up to a year.

6. Laboratory diagnosis

A. Acute acquired rubella

• Rubella-specific IgM in serum indicates recent infection.
  • Best collected about 5 days after onset of rash. An earlier negative result can require repeat testing.
• Rubella-specific IgG
  • A significant rise in paired acute and convalescent samples supports recent infection.
  • IgG is also used to assess immunity, especially before pregnancy.
• RT-PCR
  • Detects viral RNA.
  • Suitable specimens include throat/nasopharyngeal swab, nasal swab, and urine.
  • Useful for confirmation, genotyping, and outbreak surveillance.

B. Congenital rubella syndrome

• Rubella-specific IgM in neonatal serum confirms congenital infection because maternal IgM does not cross the placenta.
• Persistence of rubella-specific IgG beyond the age when maternal antibody should decline also supports congenital infection.
• RT-PCR can be performed on throat/nasopharyngeal samples or urine.

7. Culture

• Virus can be isolated in cell culture, but this is rarely used for routine diagnosis.
• Rubella often produces little or no obvious cytopathic effect.
• Identification may require interference tests, immunostaining, or molecular methods.
• In practice, serology and RT-PCR are preferred.

8. Immunity and vaccine

• Natural infection generally produces long-lasting immunity.
• The vaccine is a live attenuated rubella vaccine, usually administered as MMR or MMRV in relevant childhood age groups.
• It induces both humoral and cellular immunity.
• As a live vaccine, it is contraindicated in pregnancy. Nonimmune women should receive vaccination before conception or postpartum according to local recommendations.

High-yield microbiology summary

Rubella virus:
Enveloped, +ssRNA, Rubivirus, Matonaviridae
↓
Respiratory entry → lymph nodes → viremia
↓
Mild exanthem and lymphadenopathy
↓
Placental transmission in pregnancy
↓
Congenital rubella syndrome
↓
Diagnosis: IgM, rising IgG titre, RT-PCR
↓
Prevention: live attenuated MMR vaccine