An illustration to show replication of the leading and lagging strands of DNA. Image credit: Genome Research Limited. Each genome contains all of the information needed to build that organism and allow it to grow and develop.
DNA or deoxyribonucleic acid is a long molecule that contains our unique genetic code. Like a recipe book it holds the instructions for making all the proteins in our bodies. Cells are the basic building blocks of living things. The ends of the parent strands consist of repeated DNA sequences called telomeres. Telomeres act as protective caps at the end of chromosomes to prevent nearby chromosomes from fusing. A special type of DNA polymerase enzyme called telomerase catalyzes the synthesis of telomere sequences at the ends of the DNA.
Once completed, the parent strand and its complementary DNA strand coils into the familiar double helix shape. In the end, replication produces two DNA molecules , each with one strand from the parent molecule and one new strand. DNA replication would not occur without enzymes that catalyze various steps in the process.
Enzymes that participate in the eukaryotic DNA replication process include:. Each molecule consists of a strand from the original molecule and a newly formed strand. Prior to replication, the DNA uncoils and strands separate. A replication fork is formed which serves as a template for replication. This addition is continuous in the leading strand and fragmented in the lagging strand. Once elongation of the DNA strands is complete, the strands are checked for errors, repairs are made, and telomere sequences are added to the ends of the DNA.
Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. The enzymes recognize the incorrectly added nucleotide and excise it; this is then replaced by the correct base.
If this remains uncorrected, it may lead to more permanent damage. How do mismatch repair enzymes recognize which of the two bases is the incorrect one? Thus, DNA polymerase is able to remove the wrongly incorporated bases from the newly synthesized, non-methylated strand. In eukaryotes, the mechanism is not very well understood, but it is believed to involve recognition of unsealed nicks in the new strand, as well as a short-term continuing association of some of the replication proteins with the new daughter strand after replication has completed.
Figure 8. In mismatch repair, the incorrectly added base is detected after replication. The mismatch repair proteins detect this base and remove it from the newly synthesized strand by nuclease action. The gap is now filled with the correctly paired base. Figure 9. Nucleotide excision repairs thymine dimers.
When exposed to UV, thymines lying adjacent to each other can form thymine dimers. In normal cells, they are excised and replaced. Once the bases are filled in, the remaining gap is sealed with a phosphodiester linkage catalyzed by DNA ligase. This repair mechanism is often employed when UV exposure causes the formation of pyrimidine dimers. Answer the question s below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.
Use this quiz to check your understanding and decide whether to 1 study the previous section further or 2 move on to the next section. Privacy Policy. Skip to main content. Search for:. Click through this tutorial on DNA replication. In Summary: Basics of DNA Replication The model for DNA replication suggests that the two strands of the double helix separate during replication, and each strand serves as a template from which the new complementary strand is copied.
Figure 4. Primer and Template. Show Answer True. Most biological reactions rely on the enzyme to speed up the reaction. Show Answer Answer a. DNA is a double helix made up of two long chains of deoxyribonucleotides. This phenomenon is known as complementary base pairing Figure 4 , and it results in the production of two complementary strands of DNA. Base pairing ensures that the sequence of nucleotides in the existing template strand is exactly matched to a complementary sequence in the new strand, also known as the anti-sequence of the template strand.
Later, when the new strand is itself copied, its complementary strand will contain the same sequence as the original template strand. Thus, as a result of complementary base pairing, the replication process proceeds as a series of sequence and anti-sequence copying that preserves the coding of the original DNA. In the prokaryotic bacterium E. In comparison, eukaryotic human DNA replicates at a rate of 50 nucleotides per second. In both cases, replication occurs so quickly because multiple polymerases can synthesize two new strands at the same time by using each unwound strand from the original DNA double helix as a template.
One of these original strands is called the leading strand, whereas the other is called the lagging strand. The leading strand is synthesized continuously, as shown in Figure 5. In contrast, the lagging strand is synthesized in small, separate fragments that are eventually joined together to form a complete, newly copied strand.
This page appears in the following eBook. Aa Aa Aa. How is DNA replicated? What triggers replication? Figure 1: Helicase yellow unwinds the double helix.
The initiation of DNA replication occurs in two steps. First, a so-called initiator protein unwinds a short stretch of the DNA double helix. Then, a protein known as helicase attaches to and breaks apart the hydrogen bonds between the bases on the DNA strands, thereby pulling apart the two strands. As the helicase moves along the DNA molecule, it continues breaking these hydrogen bonds and separating the two polynucleotide chains Figure 1. How are DNA strands replicated?
Figure 3: Beginning at the primer sequence, DNA polymerase shown in blue attaches to the original DNA strand and begins assembling a new, complementary strand. Figure 4: Each nucleotide has an affinity for its partner.
0コメント