A Brief History of Molecular Biology by Barbara

A Brief History of Molecular Biology by Barbara Wimpee

The Big Picture: Proteins are the machinery of the cell, carrying out all of the cell’s essential functions. DNA (deoxyribonucleic acid) programs the synthesis of proteins. So proteins are the “hardware, ” and DNA is the “software. ” How do we know this?

It was Gregor Mendel, in the 1860’s, who showed that the characteristics of organisms are specified by discreet factors, which we now call genes.

Around the same time (1860’s), Friedrich Miescher discovered DNA.

Early 1900’s: the laboratory of Thomas Hunt Morgan showed definitively that genes are on chromosomes, arranged in linear order. Drosophila melanogaster, the fruit fly.

But what exactly is a gene? How do genes specify the characteristics of organisms? This requires an understanding of the molecules involved.

1940’s and 1950’s: The Dawn of Molecular Biology

One Gene, One Protein

George Beadle Edward Tatum In the 1940’s Beadle and Tatum showed that each gene determines the synthesis of a specific protein.

The Chemical Nature of Genes

Around the same time, Oswald Avery showed that DNA is the genetic material.

In biology, structure and function are intimately related. So to understand how genes specify proteins, the next big question was: “What is the structure of DNA? ”

Some necessary pieces of the puzzle:

Erwin Chargaff determined an essential feature of DNA biochemistry. “Chargaff’s Rules” The amount of adenine (A) = the amount of thymine (T) The amount of guanine (G) = the amount of cytosine (C)

Rosalind Franklin’s X-ray crystallography showed that DNA must be a helix if some sort, with specific dimensions. Franklin’s X-ray diffraction photo of DNA

Francis Crick (left) and James Watson (right) used Chargaff’s Rules and Franklin’s X-ray photo to propose a structure for DNA.

This paper (Watson and Crick, 1953) changed everything.

DNA is a double helix. The key to its structure is complementary base pairing; A with T G with C

Where we are so far: Beadle and Tatum showed that each gene specifies a protein. Avery showed that DNA is the genetic material. Watson and Crick showed that DNA is a double helix. The next big question: “How does DNA specify protein sequence? ”

In the 1950’s, it became clear that the “other” nucleic acid, RNA (ribonucleic acid), plays a central role in protein synthesis. In 1960, Jacob and Monod coined the term “messenger RNA” (abbreviated m. RNA). Jacques Monod Francois Jacob

We now know that m. RNA is a working copy of a DNA gene, specifying the amino sequence of a protein.

For m. RNA to specify amino acids, there must be a code that converts “nucleic acid language” into “protein language. ” It was Nirenberg and Khorana (working separately) who cracked the code. By the mid 1960’s, it was complete. Nirenberg Khorana

The next major breakthrough occurred in the 1970’s: molecular cloning, developed by Berg and (separately) by Cohen and Boyer. Paul Berg Herb Boyer and Stanley Cohen

Using recombinant DNA technology, single genes from any organism could be inserted into plasmids (essentially “mini-chromosomes”) and grown in unlimited amounts in bacteria. http: //aliqariali. blogspot. com/2015/10/cloning-and-its-importane. html

For the first time, molecular biologists could study any gene in isolation from all the other genes.

In the 1980’s, Kerry Mullis developed a shortcut that allows the amplification of a specific gene in the test tube, rather than in bacteria. This process is called the Polymerase Chain Reaction (PCR). PCR can be used instead of molecular cloning for many (not all) purposes.

The Polymerase Chain Reaction http: //www. wcpl. com/pcr. asp

DNA Sequencing How do we know the “spelling” of specific genes?

Walter Gilbert Fred Sanger In 1977, Gilbert and Sanger (working separately) developed methods for determining the nucleotide sequence of DNA.

The earliest DNA sequencing required running radioactively labeled DNA on a long gel, overlaying with Xray film to make an autoradiogram, and reading the sequence visually.

1990’s: Leroy Hood pushed for the development of automated DNA sequencing. This would greatly increase the speed and throughput of DNA sequencing.

An automated Sanger DNA Sequencer (ABI 3730) Automated Sanger sequencing

Output from an automated DNA sequencer

Next Generation DNA Sequencing This technology skips the cloning step, and instead sequences an enormous number of different DNA molecules directly and simultaneously.

Illumina Hi. Seq Orders of magnitude higher throughput than Sanger automated sequencing.

Oxford Nanopore DNA Sequencer

In brief summary: DNA sequence specifies RNA sequence specifies the amino acid sequence of proteins. We now have the capability to sequence entire genomes.

As we will now see, molecular techniques can be used to look at gene activity in cells and tissues, merging molecular biology and histology.