You need the PowerPoint Animation Player.
Click here if you do not have it.
- Bacteria reproduce by binary fission.
- This process one cell divides to give two identical daughter cells
- Most of the genetic information in bacteria is found on a single circular chromosome which consists of DNA and proteins.
- This information must distributed equally to each of the daughter cells.
- This is only possible because DNA is a self-replicating molecule and is able to make an exact copy of itself before cell division occurs.
- One copy will go to each daughter cell.
- The structure of DNA plays an important role in the replication process.
- Two important features to keep in mind are:
- The two strands of DNA are complementary to one another.
- The base adenine will only pair with the bast thymine and the base guanine will only pair with the base cytosine.
- The two strands of DNA are held together by hydrogen bonds that exist between complementary bases.
- The two strands of DNA are also antiparallel to one another.
- One strand runs in the 5' to 3' direction, the other in the 3' to 5' direction.
- Replication begins on the bacterial chromosome at a specific sequence of nucleotides called the origin.
- Enzymes called helicases recognize this sequence and bind to his site.
- Helicases unwind that two strands by breaking the hydrogen bonds that hold the strand together.
- This area where DNA separates and the bases are exposed is called the replication fork.
- Single-stranded binding proteins attach in chains along the separated strands for stablization and to prevent rewinding.
- Free nucleoside triphosphates in the cytoplasm are paired up with their complementary base on the exposed parental strand.
- A nucleoside triphosphate is just like a nucleotide except it has three phosphates instead of one.
- This makes it very reactive because of the cluster of negative charge.
- Once it is aligned properly with its complementary base , the nucleoside triphosphate is joined to the growing strand by an enzyme called DNA polymerase.
- This enzyme catalyses the hydrolysis of the phosphates as the nucleotide is added to the strand, forming a phosphodiester bond.
- DNA polymerase has two special properties that must be taken into account:
- 1. It can not initiate the synthesis of a chain all by itself.
- DNA polymerase can only add a nucleotide to the 3' end of an already growing chain.
- Therefore, it would not be able to join the very first two replicated nucleoside triphosphates.
- To start synthesis it adds the first nucleoside triphosphate to a primer.
- This is a short stretch of RNA made by the enzyme primase.
- The RNA primer is a several nucleotides long and is complementary to the parental strand.
- DNA polymerase can then join a nucleoside triphosphate to the 3' end of the primer.
- The primer will be replaced later by a DNA strand and joined to the replicated strand by DNA ligase.
- The other special property that must be taken into account is that DNA polymerase can only synthesize in the 5' to 3' direction.
- This means it can only add bases to an exposed 3' end.
- This has important implications for antiparallel strands running in opposite directions:
- Remember that a replicated strand must be antiparallel to its parent strand.
- This is no problem for the parent strand that runs 3' to 5' because its replicate will be 5' to 3'.
- Therefore, DNA polymerase can continuously join nucleoside triphophates to the replicating strand in the way described previously.
- This 5' to 3' replicated strand is called the leading strand.
- This is a problem for the parent strand running 5' to 3' for its replicated strand must be 3' to 5'.
- DNA polymerase cannot add nucleoside triphophates in this direction.
- Instead it must work in the opposite direction away from the replication fork.
- This newly replicated strand is called the lagging strand because it is discontinuously synthesized away from the replication fork.
- We will now look at how synthesis of the lagging strand occurs:
- To begin with, primase synthesizes short RNA primers that are complementary to the 5' to 3' parental strand.
- DNA polymerase then extends these primers by joining free nucleoside triphosphates to the growing strand in the 5' to 3' direction.
- The complex, the RNA primer with the added DNA nucleotides, is called an Okazaki fragment.
- The primer will be digested by DNA polymerase and will be replaced by DNA.
- All the Okazaki fragments are joined by DNA ligase.
- The result is a new complementary strand running 3' to 5'.
- Each replicated strand winds with its template parental strand to form a double helix.
- Because each new DNA molecule contains one conserved parental strand and one new replicated strand, the process of replication is referred to as semiconsevative replication.
- The bacterial chromosome is circular.
- The replication process starts at a site called the origin.
- But instead of replicating in one direction around the chromosome, it replicates in two directions.
- This is called bidirectional replication as two replication forks move in opposite directions away from the origin.
- The forks will meet at the bottom of the chromosome and replication is terminated as the two chromosomes separate.
- The bacterial chromosome is also capable of initiating multiple replication forks.
- The processes of bidirectional replication and the concept of multiple
replication forks are discussed in more detail in the replication laboratory.
Enter The Lab To Learn More About Microbial DNA Replication