Which description of dna replication is correct

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 human body is composed of trillions of cells, all with their own specialised function. If you have any other comments or suggestions, please let us know at comment yourgenome. Can you spare minutes to tell us what you think of this website? Open survey. In: Facts In the Cell. The strand with the Okazaki fragments is known as the lagging strand.

As synthesis proceeds, an enzyme removes the RNA primer, which is then replaced with DNA nucleotides, and the gaps between fragments are sealed by an enzyme called DNA ligase. You isolate a cell strain in which the joining together of Okazaki fragments is impaired and suspect that a mutation has occurred in an enzyme found at the replication fork.

Which enzyme is most likely to be mutated? Because eukaryotic chromosomes are linear, DNA replication comes to the end of a line in eukaryotic chromosomes. As you have learned, the DNA polymerase enzyme can add nucleotides in only one direction. In the leading strand, synthesis continues until the end of the chromosome is reached; however, on the lagging strand there is no place for a primer to be made for the DNA fragment to be copied at the end of the chromosome.

This presents a problem for the cell because the ends remain unpaired, and over time these ends get progressively shorter as cells continue to divide. The ends of the linear chromosomes are known as telomeres, which have repetitive sequences that do not code for a particular gene. As a consequence, it is telomeres that are shortened with each round of DNA replication instead of genes.

The discovery of the enzyme telomerase Figure 9. The telomerase attaches to the end of the chromosome, and complementary bases to the RNA template are added on the end of the DNA strand. Once the lagging strand template is sufficiently elongated, DNA polymerase can now add nucleotides that are complementary to the ends of the chromosomes. Thus, the ends of the chromosomes are replicated.

Telomerase is typically found to be active in germ cells, adult stem cells, and some cancer cells. For her discovery of telomerase and its action, Elizabeth Blackburn Figure 9. Telomerase is not active in adult somatic cells. Adult somatic cells that undergo cell division continue to have their telomeres shortened. This essentially means that telomere shortening is associated with aging.

In , scientists found that telomerase can reverse some age-related conditions in mice, and this may have potential in regenerative medicine. Telomerase reactivation in these mice caused extension of telomeres, reduced DNA damage, reversed neurodegeneration, and improved functioning of the testes, spleen, and intestines. Thus, telomere reactivation may have potential for treating age-related diseases in humans.

Recall that the prokaryotic chromosome is a circular molecule with a less extensive coiling structure than eukaryotic chromosomes. The eukaryotic chromosome is linear and highly coiled around proteins. While there are many similarities in the DNA replication process, these structural differences necessitate some differences in the DNA replication process in these two life forms. DNA replication has been extremely well-studied in prokaryotes, primarily because of the small size of the genome and large number of variants available.

Escherichia coli has 4. This means that approximately nucleotides are added per second. DNA replication is one of the most basic processes that occurs within a cell. Each time a cell divides, the two resulting daughter cells must contain exactly the same genetic information, or DNA, as the parent cell. To accomplish this, each strand of existing DNA acts as a template for replication.

Replication occurs in three major steps: the opening of the double helix and separation of the DNA strands, the priming of the template strand, and the assembly of the new DNA segment. During separation, the two strands of the DNA double helix uncoil at a specific location called the origin. Several enzymes and proteins then work together to prepare, or prime , the strands for duplication.

The following description of this three-stage process applies generally to all cells, but specific variations within the process may occur depending on organism and cell type. Figure 2: While helicase and the initiator protein not shown separate the two polynucleotide chains, primase red assembles a primer. This primer permits the next step in the replication process. The sugar-phosphate backbones of each strand are depicted as a segmented grey cylinder. Nitrogenous bases on each strand are represented by blue, orange, red, or green horizontal rectangles attached to each segment of the sugar-phosphate backbone.

The bases form rungs of red-green or blue-orange between the grey cylinders. Helicase is bound to the ends of several nitrogenous bases on the lower strand. Beside it, four nitrogenous bases, each attached to a sugar molecule, have been annealed to complementary nitrogenous bases on the bottom strand.

About three dozen individual nucleotides float in the background. Meanwhile, as the helicase separates the strands, another enzyme called primase briefly attaches to each strand and assembles a foundation at which replication can begin.

This foundation is a short stretch of nucleotides called a primer Figure 2. As DNA polymerase makes its way down the unwound DNA strand, it relies upon the pool of free-floating nucleotides surrounding the existing strand to build the new strand. The nucleotides that make up the new strand are paired with partner nucleotides in the template strand; because of their molecular structures, A and T nucleotides always pair with one another, and C and G nucleotides always pair with one another.

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.