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 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? And so DNA is a molecule that can be replicated to make almost perfect copies of itself. Which is all the more amazing considering that there are almost three billion base pairs of DNA to be copied. Replicating all of the DNA in a single human cell takes several hours of just pure copying time.
At the end of this process, once the DNA is all replicated, the cell actually has twice the amount of DNA that it needs, and the cell can then divide and parcel this DNA into the daughter cell, so that the daughter cell and the parental cell in many case are absolutely genetically identical.
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. A pairs with T, and C pairs with G. The color of the rectangle represents the chemical identity of the nitrogenous base. A grey horizontal cylinder is attached to one end of the rectangle in each nucleotide and represents a sugar molecule. The nucleotides are arranged in two rows and the nitrogenous bases point toward each other. A set of four nucleotides are in both the upper and lower rows.
From left to right, the nucleotides in the top row are adenine green , cytosine orange , thymine red , and guanine blue. From left to right, the complementary nucleotides in the bottom row are: thymine red , guanine blue , adenine green , and cytosine orange.
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