Twin helical strands form the DNA backbone

The separated strands serve as templates for the synthesis of new DNA.

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As the strands continue to unwind and separate in both directions around the entire DNA molecule, new complementary strands are produced by the hydrogen bonding of free DNA nucleotides with those on each parent strand. As the new nucleotides line up opposite each parent strand by hydrogen bonding, enzymes called DNA polymerases join the nucleotides by way of phosphodiester bonds. Actually, the nucleotides lining up by complementary base pairing are deoxynucleotide triphosphates, composed of a nitrogenous base, deoxyribose, and three phosphates. As the phosphodiester bond forms between the 5' phosphate group of the new nucleotide and the 3' OH of the last nucleotide in the DNA strand, two of the phosphates are removed providing energy for bonding (see Fig. 6). In the end, each parent strand serves as a template to synthesize a complementary copy of itself, resulting in the formation of two identical DNA molecules (see Fig. 7). In bacteria, Par proteins function to separate bacterial chromosomes to opposite poles of the cell during cell division. They bind to the origin of replication of the DNA and physically pull or push the chromosomes apart, similar to the mitotic apparatus of eukaryotic cells. Fts proteins, such as FtsK in the divisome, also help in separating the replicated bacterial chromosome.

An embryonic cell divides again and again

Where there was one cell there are two, then four, then eight,..

DNA polymerase III replaces the primase and is able to add DNA nucleotides to the RNA primer. As the free deoxyribonucleoside triphosphates line up by complementary base pairing with the nucleotides on each parent strand of the unwound DNA in the replication fork, the phosphate on the 5 carbon of the newest building block lining up then forms a phosphodiester bond with the 3 carbon of the last nucleotide in the growing strand. During the process, two phosphates are lost . Because the parent strands are antiparallel and DNA can only be replicated in a 5 to 3 direction, the the two new strands must be synthesized in opposite directions.

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In addition, DNA polymerase enzymes cannot begin a new DNA chain from scratch. They can only attach new nucleotides onto 3' OH group of a nucleotide in a preexisting strand. Therefore, to start the synthesis of the leading strand and each DNA fragment of the lagging strand, an RNA polymerase complex called a primase is required. The , which is capable of joining RNA nucleotides without requiring a preexisting strand of nucleic acid, first adds several comlementary RNA nucleotides opposite the DNA nucleotides on the parent strand. This forms what is called an RNA primer (see Fig. 10).


Nucleic Acids - Biology Junction

DNA synthesis requires a primer usually made of RNA. A primase synthesizes the ribonucleotide primer ranging from 4 to 12 nucleotides in length. DNA polymerase then incorporates a dNMP onto the 3' end of the primer initiating leading strand synthesis. Only one primer is required for the initiation and propagation of leading strand synthesis.

nucleic acids & protein synthesis notes b1 - Biology …

In general, DNA is replicated by uncoiling of the helix, strand separation by breaking of the hydrogen bonds between the complementary strands, and synthesis of two new strands by . Replication begins at a specific site in the DNA called the origin of replication (oriC).

topic #13 DNA to RNA to Protein Flashcards | Easy Notecards

2. DNA polymerase then incorporates a dNMP onto the 3" end of the primer and initiates lagging strand synthesis. The polymerase extends the primer for about 1,000 nucleotides until it comes in contact with the 5' end of the preceding primer. These short segments of RNA/DNA are known as Okazaki fragments.

Study topic #13 DNA to RNA to Protein flashcards

Leading and lagging strand synthesis is thought to be coordinated at a replication fork. The two polymerases are held together by another set of proteins, , which are near the fork that is being unwound and simultaneously primed by helicase-primase. Both polymerases are bound by a processivity factor, . Upon completing an Okazaki fragment the lagging strand polymerase release the factor and dissociates from the DNA. The complex then loads the new factor/primer complex onto the lagging strand polymerase which initiates a new round.....