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synthesis of a new strand of DNA.
The rcDNA genome is converted intocccDNA by cellular repair factors. Then, the cccDNA is transcribedto the pgRNA and subgenomic mRNAs (not shown). The mRNAs aretransported to the cytoplasm. The pgRNA is translated in thecytosol to form HBV core protein and the viral polymerase. Thesethree components assemble to form the core particle. The first(minus) DNA strand forms within the core particles via reversetranscription of the pgRNA to DNA; the pgRNA is degraded by viralRNase H as the plus strand is synthesized. (A) A3B inhibits thebinding of HnRnp K to the Enh II of HBV; (B) A3G may inhibit pgRNApackaging; (C) A3G renders HBV core protein-associated full-lengthpgRNA nuclease-sensitive; (D) A3G blocks DNA strand elongation andtargets a DNA-RNA hybrid. rcDNA, relaxed circular DNA; cccDNA,covalently closed circular DNA; pgRNA, pregenomic RNA; HBV,hepatitis B virus; APOBEC, apolipoprotein B mRNA editing enzyme,catalytic polypeptide-like; HnRnp K, heterogeneous nuclearribonucleoprotein K; Enh II, enhancer II.
There are three key differences between replication and transcription: (1) RNA molecules are much shorter than DNA molecules; only a portion of one DNA strand is copied or transcribed to make an RNA molecule. (2) RNA is built from ribonucleotides rather than deoxyribonucleotides. (3) The newly synthesized RNA strand does not remain associated with the DNA sequence it was transcribed from.
The new continuous complementary DNA strand synthesized ..
The DNA sequence that is transcribed to make RNA is called the , while the complementary sequence on the other DNA strand is called the or . To initiate RNA synthesis, the two DNA strands unwind at specific sites along the DNA molecule. Ribonucleotides are attracted to the uncoiling region of the DNA molecule, beginning at the 3′ end of the template strand, according to the rules of base pairing. Thymine in DNA calls for adenine in RNA, cytosine specifies guanine, guanine calls for cytosine, and adenine requires uracil. RNA polymerase—an enzyme—binds the complementary ribonucleotide and catalyzes the formation of the ester linkage between ribonucleotides, a reaction very similar to that catalyzed by DNA polymerase (). Synthesis of the RNA strand takes place in the 5′ to 3′ direction, antiparallel to the template strand. Only a short segment of the RNA molecule is hydrogen-bonded to the template strand at any time during transcription. When transcription is completed, the RNA is released, and the DNA helix reforms. The nucleotide sequence of the RNA strand formed during transcription is identical to that of the corresponding coding strand of the DNA, except that U replaces T.
The nucleus contains all the necessary enzymes, proteins, and nucleotides required for this synthesis. A short segment of DNA is “unzipped,” so that the two strands in the segment are separated to serve as templates for new DNA. DNA polymerase, an enzyme, recognizes each base in a template strand and matches it to the complementary base in a free nucleotide. The enzyme then catalyzes the formation of an ester bond between the 5′ phosphate group of the nucleotide and the 3′ OH end of the new, growing DNA chain. In this way, each strand of the original DNA molecule is used to produce a duplicate of its former partner (). Whatever information was encoded in the original DNA double helix is now contained in each replicate helix. When the cell divides, each daughter cell gets one of these replicates and thus all of the information that was originally possessed by the parent cell.
strand breaks in DNA, PARP-1 catalyzes the synthesis of ..
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