Biology - Class 5

American molecular biologists Matthew S. Meselson and Franklin W. Stahl carried out an experiment to determine how DNA replicates.

called "the most beautiful experiment in biology" because it so elegantly demonstrated the scientific method . Meselson and Stahl found that DNA in fact replicates semiconservatively. This finding also predicted the results when cells are allowed to undergo two rounds of replication in a medium containing only light 14N nitrogen. The heavy strand and light strand each serve as templates for a new light daughter strand.

Nucleic acid or protein sequences that are very similar from one organism to another are called:

conserved

leading strand

daughter strand that has its 3′ end pointed toward the replication fork, so as the parental double helix unwinds, this daughter strand can be synthesized as one long, continuous polymer.

leading and lagging strand

in DNA replication, because the template strand is made of two antiparallel strands, one daughter strand (the leading strand) is synthesized continuously and the other (the lagging strand) is synthesized in smaller pieces. Page 252

When DNA or genetic information is passed between bacteria (and bacteria take up this material from their surrounding environment), this process is referred to as:

transformation

single-strand binding protein

A protein that binds single-stranded nucleic acids.

helicase

A protein that unwinds the parental double helix at the replication fork.

Suppose Meselson and Stahl had done their experiment the other way around, starting with cells fully labeled with 14N light DNA and then transferring them to medium containing only 15N heavy DNA. What density of DNA molecule would you predict after one and two rounds of replication?

After one round of replication, you would predict only 14N/15N hybrid DNA, which has a density of 1.715 gm/cm3. After two rounds of replication, you would predict half the molecules to be 14N/15N hybrid DNA (density 1.715 gm/cm3) and half to be 15N/15N heavy DNA (density 1.722 gm/cm3).

RNA Primase

An RNA polymerase that synthesizes a short piece of RNA complementary to the DNA template and does not require a primer

topoisomerase II

An enzyme that breaks a DNA double helix, rotates the ends, and seals the break.

DNA polymerase

An enzyme that is a critical component of a large protein complex that carries out DNA replication. DNA polymerases exist in all organisms and are highly conserved, meaning that they vary little from one species to another because they carry out an essential function. A cell typically contains several different DNA polymerase enzymes, each specialized for a particular situation. But all DNA polymerases share the same basic function in that they synthesize a new DNA strand from an existing template.

How is DNA replicated? Experiment. Hypothesis

HYPOTHESIS DNA replicates in a semiconservative manner, meaning that each new DNA molecule consists of one parental strand and one newly synthesized strand. ALTERNATIVE HYPOTHESIS DNA replicates in a conservative manner, meaning that one DNA molecule consists of two parental strands, and the other consists of two newly synthesized strands.

daughter strand

In DNA replication, the parental strand whose sequence is used to synthesize a complementary daughter strand.

template strand

In DNA replication, the parental strand whose sequence is used to synthesize a complementary daughter strand.

semiconservative replication

The mechanism of DNA replication in which each strand of a parental DNA duplex serves as a template for the synthesis of a new daughter strand. That is, after replication, each new DNA duplex consists of one strand that was originally part of the parental duplex and one newly synthesized strand.

Each new DNA strand must begin with a short stretch of RNA that serves as a primer, or starter, for DNA synthesis. Why is a primer needed?

The primer is needed because the DNA polymerase complex cannot begin a new strand on its own; it can only elongate the end of an existing piece of DNA or RNA

replication fork

The site where the parental DNA strands separate as the DNA duplex unwinds.

link the replication fork to the synthesis of dan

The strand that terminates in the 5′ phosphate cannot grow in the direction of the replication fork because new DNA strands can grow only by the addition of successive nucleotides to the 3′end. That is, DNA always grows in the 5′-to-3′ direction. DNA polymerization occurs only in the 5′-to-3′ direction because of the chemistry of nucleic acid synthesis