![]() After ligase has connected all nicks, the new strand is one long continuous DNA strand, and the daughter DNA molecule is complete.ĭNA Replication: This is a clip from a PBS production called “DNA: The Secret of Life.” It details the latest research (as of 2005) concerning the process of DNA replication.Ribonucleic acid, or RNA. In the final stage of DNA replication, the enyzme ligase joins the sugar-phosphate backbones at each nick site. However, this creates new nicks (unconnected sugar-phosphate backbone). Once the primers are removed, a free-floating DNA polymerase lands at the 3′ end of the preceding DNA fragment and extends the DNA over the gap. The enzymes FEN1 and RNase H remove RNA primers at the start of each leading strand and at the start of each Okazaki fragment, leaving gaps of unreplicated template DNA. The group of cellular enzymes that remove RNA primers include the proteins FEN1 (flap endonulcease 1) and RNase H. RNA primers need to be replaced with DNA, and nicks in the sugar-phosphate backbone need to be connected. Once all the template nucleotides have been replicated, the replication process is not yet over. These unattached sections of the sugar-phosphate backbone in an otherwise full-replicated DNA strand are called nicks. However, DNA polymerase cannot catalyze the formation of a phosphodiester bond between the two segments of the new DNA strand, and it drops off. Eventually, the leading strand of one replication bubble reaches the lagging strand of another bubble, and the lagging strand will reach the 5′ end of the previous Okazaki fragment in the same bubble.ĭNA polymerase halts when it reaches a section of DNA template that has already been replicated. Each origin of replication forms a bubble of duplicated DNA on either side of the origin of replication. Once DNA replication is finished, the daughter molecules are made entirely of continuous DNA nucleotides, with no RNA portions.Įukaryotic chromosomes have multiple origins of replication, which initiate replication almost simultaneously. Once RNA primer has been synthesized at the template DNA, primase exits, and DNA polymerase extends the new strand with nucleotides complementary to the template DNA.Įventually, the RNA nucleotides in the primer are removed and replaced with DNA nucleotides. This short stretch of RNA nucleotides is called the primer. ![]() Primase initiates polynucleotide synthesis and by creating a short RNA polynucleotide strand complementary to template DNA strand. All newly synthesized polynucleotide strands must be initiated by a specialized RNA polymerase called primase. This process will continue until the DNA polymerase reaches the end of the template strand.ĭNA polymerase cannot initiate new strand synthesis it only adds new nucleotides at the 3′ end of an existing strand. For example, when DNA polymerase meets an adenosine nucleotide on the template strand, it adds a thymidine to the 3′ end of the newly synthesized strand, and then moves to the next nucleotide on the template strand. Only the nucleotide complementary to the template nucleotide at that position is added to the new strand.ĭNA polymerase contains a groove that allows it to bind to a single-stranded template DNA and travel one nucleotide at at time. The template strand specifies which of the four DNA nucleotides (A, T, C, or G) is added at each position along the new chain. The DNA fragments are joined by DNA ligase (not shown).ĭuring elongation, an enzyme called DNA polymerase adds DNA nucleotides to the 3′ end of the newly synthesized polynucleotide strand. On the leading strand, only a single RNA primer is needed, and DNA is synthesized continuously, whereas on the lagging strand, DNA is synthesized in short stretches, each of which must start with its own RNA primer. An RNA primer is synthesized by primase and is elongated by the DNA polymerase. \( \newcommand\): Replication Fork Formation: A replication fork is formed by the opening of the origin of replication helicase separates the DNA strands.
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