Complementary single-stranded ends

The staggered cuts made by this enzyme produce complementary single-stranded ends, which have specific affinity for each other and hence are known as cohesive or sticky ends. Any DNA fragment can be inserted into this plasmid if it has the same cohesive ends. Such a fragment can be prepared from a larger piece of DNA by using the same restriction enzyme as was used to open the plasmid DNA (Figure 5.10). 

The DNA of bacteriophage A, is a linear double-strand molecule that has complementary single-strand ends. These molecules can form closed circular molecules when two cohesive ends on the same molecule join, and they can form linear dimers, trimers, or longer molecules when sites on different molecules are joined. 

Involve the construction of complementary overlapping ends by partial fillup of different restriction sites (28), expected to reduce the influence of the tag concentration on the circularization efficiency of different size classes of DNA. Other alternatives mentioned before (14) could involve the use of site-specific recombination enzymes to circularize DNA (29), or the possible use of tags containing complementary single-stranded end sequences to allow circularization by hybridization. 

Cellular protein biosynthesis involves the following steps. One strand of double-stranded DNA serves as a template strand for the synthesis of a complementary single-stranded messenger ribonucleic acid (mRNA) in a process called transcription. This mRNA in turn serves as a template to direct the synthesis of the protein in a process called translation. The codons of the mRNA are read sequentially by transfer RNA (tRNA) molecules, which bind specifically to the mRNA via triplets of nucleotides that are complementary to the particular codon, called an anticodon. Protein synthesis occurs on a ribosome, a complex consisting of more than 50 different proteins and several stmctural RNA molecules, which moves along the mRNA and mediates the binding of the tRNA molecules and the formation of the nascent peptide chain. The tRNA molecule carries an activated form of the specific amino acid to the ribosome where it is added to the end of the growing peptide chain. There is at least one tRNA for each amino acid. 

Stichy-ended DNA Complementary single strands of DNA that protrude from opposite ends of a DNA duplex or from the ends of different duplex molecules (see also Blunt-ended DNA, above). 

Sticky-ends Complementary single-stranded tails projecting from otherwise double-helical nucleic acid molecules. 

These cohesive ends can be used to join together different restriction fragments. It is easy to see that the complementary single-stranded tails can form base pairs to regenerate the original hexanucleotide... 

The single-stranded ends of the fragment are then complementary to those of the cut plasmid. The DNA fragment and the cut plasmid can be annealed and then joined by DNA ligase, which catalyzes the formation of a phosphodiester bond... 

Figure 3-30. Action of restriction enzymes. EcoRI cleaves this palindrome. Two fragments are produced that contain complementary single-stranded regions (sticky ends).

Restriction enzymes (endonucleases) typically cut DNA at specific 4- to 8-bp palindromic sequences, producing defined fragments that often have self-complementary single-stranded tails (sticky ends). 

Fig. 6.2. The principle of PCR. (1) Denaturation the two DNA strands are separated at 95 °C (2) Anneahng of primers the primers are hybridised to their complementary sequences at 50-60 °C (3) Primer extension at 72 °C, the polymerase catalyses the synthesis of the complementary single stranded DNA by extending the 3 -end of the hybridised primer.

Wild-type (25-mer) 5 -TCnTAAnTCTAATATATTTAGAAttc-3 and its complementary sequence 5 -TTCTAAATATATTAGAAATTAAAGAgaa-3. The core unwinding element is underlined. The lowercase letters indicate overlapping single-stranded ends that anneal during concatemerlzation. 

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