Rvc

Strand continues

Thus the two ends of a polynucleotide chain differ. At one end of a strand is a free (not connected to another nucleotide) 5' phosphate group (—OPO3); this end is called the 5' end. At the other end is a free 3' hydroxyl group (—OH); this end is called the 3' end. In a DNA double helix, the 5' end of one strand is paired with the 3' end of the other strand, and vice versa. In other words, were you to draw an arrow for each strand running from 5' to 3', the arrows would point in different directions; thus it is said that the strands are antiparallel.

The double helical structure of DNA is essential to its function

The genetic material performs four important functions, and the DNA structure proposed by Watson and Crick was elegantly suited to three of them.

► The genetic material stores an organism's genetic information. With its millions of nucleotides, the base sequence of a DNA molecule could encode and store an enormous amount of information and could account for species and individual differences. DNA fits this role nicely.

► The genetic material is susceptible to mutation, or permanent changes in the information it encodes. For DNA, mutations might be simple changes in the linear sequence of base pairs.

► The genetic material is precisely replicated in the cell division cycle. Replication could be accomplished by complementary base pairing, A with T and G with C. In the original publication of their findings in the journal Nature in 1953, Watson and Crick coyly pointed out, "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

► The genetic material is expressed as the phenotype. This function is not obvious in the structure of DNA. However, as we will see in the next chapter, the nucleotide sequence of DNA is copied into RNA, which is in turn converted into a linear sequence of amino acids—a protein. The folded forms of proteins provide much of the phenotype of an organism.

Three modes of DNA replication appeared possible

The prediction that the DNA molecule contains the information needed for its own replication was demonstrated by the work of Arthur Kornberg, then at Washington University in St. Louis. He showed that DNA can be synthesized in a test tube containing just three substances:

► The substrates, deoxyribonucleoside triphosphates dATP, dCTP, dGTP, and dTTP

► The enzyme DNA polymerase

► DNA, which serves as a template to guide the incoming nucleotides

There were three possible patterns that could result in complementary base pairing during DNA replication:

► Semiconservative replication, in which each parent strand serves as a template for a new strand, and the two new DNAs each have one old and one new strand (Figure 11.8a)

► Conservative replication, in which the original double helix serves as a template for, but does not contribute to, a new double helix (Figure 11.8b)

► Dispersive replication, in which fragments of the original DNA molecule serve as templates for assembling two

Original DNA After one round of replication

Original DNA After one round of replication

Semiconservative replication would produce molecules with both old and new DNA, but each molecule would contain one complete old strand and one new one.

Semiconservative replication would produce molecules with both old and new DNA, but each molecule would contain one complete old strand and one new one.

Conservative replication would preserve the original molecule and generate an entirely new molecule.

Conservative replication would preserve the original molecule and generate an entirely new molecule.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment