A primer is a strand of nucleic acid that serves as a starting point for DNA synthesis.
It is required for DNA replication because the enzymes that catalyze this process, DNA
polymerases, can only add new nucleotides to an existing strand of DNA.
The polymerase starts replication at the 3'-end of the primer, and copies the opposite strand.
In most cases of natural DNA replication, the primer for DNA synthesis and replication
is a short strand of RNA.
Many of the laboratory techniques of biochemistry and molecular biology that involve DNA polymerase,
such as DNA sequencing and the polymerase chain reaction, require DNA primers.
These primers are usually short, chemically synthesized oligonucleotides, with a length
of about twenty bases.
They are hybridized to a target DNA, which is then copied by the polymerase.
Mechanism in vivo
The leading strand of DNA is that strand of the DNA double helix that is orientated in
a 5' to 3' manner.
Therefore, its complement must be synthesized in a 3'→5' manner.
Because DNA polymerase III cannot synthesize in the 3'→5' direction, the lagging strand
is synthesized in short segments known as Okazaki fragments.
Along the lagging strand's template, primase builds RNA primers in short bursts.
DNA polymerases are then able to use the free 3'-OH groups on the RNA primers to synthesize
DNA in the 5'→3' direction.
The RNA fragments are then removed by DNA polymerase I for prokaryotes or DNA polymerase
δ for eukaryotes and new deoxyribonucleotides are added to fill the gaps where the RNA was
present.
DNA ligase then joins the deoxyribonucleotides together, completing the synthesis of the
lagging strand.
Primer removal In eukaryotic primer removal, DNA polymerase
δ extends the Okazaki fragment in 5' to 3' direction, and when it encounters the RNA
primer from the previous Okazaki fragment, it displaces the 5′ end of the primer into
a single-stranded RNA flap, which is removed by nuclease cleavage.
Cleavage of the RNA flaps involves either endonuclease 1 cleavage of short flaps, or
coating of long flaps by the single-stranded DNA binding protein replication protein A
and sequential cleavage by Dna2 nuclease and FEN1.
This mechanism is a potential explanation of how the HIV virus can transform its genome
into double-stranded DNA from the RNA-DNA formed after reverse transcription of its
RNA.
However, the HIV-encoded reverse transcriptase has its own ribonuclease activity that degrades
the viral RNA during the synthesis of cDNA, as well as DNA-dependent DNA polymerase activity
that copies the sense cDNA strand into antisense DNA to form a double-stranded DNA intermediate.
Uses of synthetic primers
DNA sequencing is used to determine the nucleotides in a DNA strand.
The Sanger chain termination method of sequencing uses a primer to start the chain reaction.
In PCR, primers are used to determine the DNA fragment to be amplified by the PCR process.
The length of primers is usually not more than 30 nucleotides, and they need to match
the beginning and the end of the DNA fragment to be amplified.
They direct replication towards each other – the extension of one primer by polymerase
then becomes the template for the other, leading to an exponential increase in the target segment.
It is worth noting that primers are not always for DNA synthesis, but can in fact be used
by viral polymerases, e.g. influenza, for RNA synthesis.
PCR primer design Pairs of primers should have similar melting
temperatures since annealing in a PCR occurs for both simultaneously.
A primer with a Tm significantly higher than the reaction's annealing temperature may mishybridize
and extend at an incorrect location along the DNA sequence, while Tm significantly lower
than the annealing temperature may fail to anneal and extend at all.
Primer sequences need to be chosen to uniquely select for a region of DNA, avoiding the possibility
of mishybridization to a similar sequence nearby.
A commonly used method is BLAST search whereby all the possible regions to which a primer
may bind can be seen.
Both the nucleotide sequence as well as the primer itself can be BLAST searched.
The free NCBI tool Primer-BLAST integrates primer design tool and BLAST search into one
application, so does commercial software product such as ePrime, Beacon Designer.
Computer simulations of theoretical PCR results may be performed to assist in primer design.
Many online tools are freely available for primer design, some of which focus on specific
applications of PCR.
The popular tools Primer3Plus and PrimerQuest can be used to find primers matching a wide
variety of specifications.
Highly degenerate primers for targeting a wide variety of DNA templates can be interactively
designed using GeneFISHER.
Primers with high specificity for a subset of DNA templates in the presence of many similar
variants can be designed using DECIPHER.
Mononucleotide and dinucleotide repeats should be avoided, as loop formation can occur and
contribute to mishybridization.
Primers should not easily anneal with other primers in the mixture; this phenomenon can
lead to the production of 'primer dimer' products contaminating the mixture.
Primers should also not anneal strongly to themselves, as internal hairpins and loops
could hinder the annealing with the template DNA.
When designing a primer for use in TA cloning, efficiency can be increased by adding AG tails
to the 5' and the 3' end.
The reverse primer has to be the reverse complement of the given cDNA sequence.
The reverse complement can be easily determined, e.g. with online calculators.
Degenerate primers
Sometimes degenerate primers are used.
These are actually mixtures of similar, but not identical primers.
They may be convenient if the same gene is to be amplified from different organisms,
as the genes themselves are probably similar but not identical.
The other use for degenerate primers is when primer design is based on protein sequence.
As several different codons can code for one amino acid, it is often difficult to deduce
which codon is used in a particular case.
Therefore primer sequence corresponding to the amino acid isoleucine might be "ATH",
where A stands for adenine, T for thymine, and H for adenine, thymine, or cytosine, according
to the genetic code for each codon, using the IUPAC symbols for degenerate bases.
Use of degenerate primers can greatly reduce the specificity of the PCR amplification.
The problem can be partly solved by using touchdown PCR.
Degenerate primers are widely used and extremely useful in the field of microbial ecology.
They allow for the amplification of genes from thus far uncultivated microorganisms
or allow the recovery of genes from organisms where genomic information is not available.
Usually, degenerate primers are designed by aligning gene sequencing found in GenBank.
Differences among sequences are accounted for by using IUPAC degeneracies for individual
bases.
PCR primers are then synthesized as a mixture of primers corresponding to all permutations.
There are a number of programs available to perform these primer predictions;
CODEHOP can be run on a server though the server is no longer supported.
It uses the block format.
HYDEN is an executable that runs on windows through command prompt.
iCODEHOP also uses the block format but has been taken down.
FAS-DPD has to be run through a Java operating system that can be set up in a virtual machine(VM).
See also Oligonucleotide synthesis — the methods
by which primers are manufactured References
External links Tm calculator
Primer3 Primer-BLAST