Atlas of Plant and Animal Histology
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The cell. 8. Cell cycle.

S PHASE

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DNA replication occurs during S phase.

Replication starts simultaneously at many points of the DNA strand. These points are the replication origins.

DNA replication is semiconservative.

One of the single strands at the replication fork is replicated slower than the other.

S phase starts when the restriction checkpoint of the G1 phase is passed. Then, two important things happen: replication of DNA and duplication of centrioles (in animal cells).

DNA replication

DNA is made up of two single strands of deoxyribonucleotides or bases. The two single DNA strands are joined together by hydrogen bonds established by complementary bases (adenine-thymine, cytosine-guanine), that gives a helical double DNA strand. The two single DNA strands are oriented in an anti-parallel manner. That is, the 3' end of one of the strands is close to the 5' end of the other strand, so that there are 3' and 5' ends of single strands in every end of the double strand. For DNA replication, the two single strands become separated after breaking the hydrogen bonds so that both single strands may be replicated.

Replication of DNA does not start from just one point, this would take too long. Instead, there are many replication origins, which are sites where replication starts at about the same time. Cells have molecular mechanisms to avoid that replication starts twice in the same replication origin. Otherwise, the new cells may have more than two copies of some DNA segments, which might be dangerous for the cell or for the organism. The beginning of replication in every replication origin is a two steps mechanism: first, the molecular machinery is assembled and ready for starting the replication; second, a signal must arrive to trigger the beginning of the DNA replication process.

For DNA replication, the two single strands get separated by an enzyme known as helicase. After that, two primase enzymes join to the exposed single strands and synthesize a short complementary fragment of RNA of about ten nucleotides, one in each single strand. These short fragments are known as primers, and without them DNA polymerases are not able to copy the DNA. Primers recruit δ y ε DNA polymerases, which add complementary nucleotides to the 3' end of each primers toward the 5' end of DNA single strand. In this way, a new complementary DNA strand is synthesized in each of the open single DNA strands. That is why the replication is semi-conservative, the two new double strands will have a new and an old DNA single strands. Subsequently, the primers will be removed by RNAases and these short segments of unpaired DNA are copied by DNA polymerases, which are coming along the single DNA strand from other replication origin.

Two replication forks are formed in one replication origin after the separation of the two single DNA strands, and the two strands are copied at the same time, but in different directions. DNA polymerases add nucleotides only in the 5' to 3' direction of the new synthesized strand (3' to 5' of the copied strand). This means that, whereas the replication process in one of the two strands is continuous and rather straightforward, in the other one it is a bit more complicated because after adding a primer and copying a segment, a new primer must be synthesized while the replication fork moves away. In this second strand, after the primer is substituted by DNA, there are many segments of new DNA which are later connected by the ligase enzymes. The segments are known as Okazaky fragments.

It must be kept in mind that not all the DNA is replicated at same time. During the S phase, it has been estimated that only 10 % to 15 % of the total DNA is being copy at same time. The replication process is stopped when the quality control machinery detects breakages of the DNA. There are many other processes running at the same time as the DNA replication, such as histone synthesis, which must also be duplicated, and the synthesis of a new centrosome in animal cells for the organization of the mitotic spindle.


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Updated: 2016-10-29. 11:37