Genetics and Human Life

In the spring of 1868, a man named Johann Miescher discovered a previously unknown substance in cell nucleus. Miescher called it nucleic acid. No one knew what its function might be. In fact, 75 years passed before DNA was recognized as the material that coded for inheritance in all organisms. How this came to be is interesting because it came from a series of seemingly unrelated clues and experiments.

For example, in 1928, Fred Griffith was trying to make a vaccine against a bacterium that causes a kind of pneumonia in humans. He isolated two strains of bacteria calling them S and R. Their appearance during culture gave them their names- S made colonies that looked smooth; R made colonies that looked rough.

He first injected mice with live R strain bacteria. The mice injected with the R strain did not develop pneumonia making the R strain appear to be harmless.

Next, mice were injected with live S strain bacteria. These mice died and when their blood was examined. It was found to contain a lot of live S cells. The S strain was pathogenic- it caused disease.

Griffith next killed some S strain bacteria by exposure to high temperature. Mice injected with the heat-killed bacteria did not die.

Finally, for some reason, Griffith mixed live R strain bacteria with heat-killed S strain bacteria and injected this into mice.

Strangely enough, the mice died and when their blood was examined, it was found to contain both live S and R strain bacteria.

What was going on? Well, it seems that the harmless bacteria had apparently picked up the instructions for infection and been transformed into pathogens. The transformation was permanent- that is, it was heritable.

A few years later, researchers working of extracts of S strain bacteria, found that the extract could transform harmless R strain bacteria. However, Avery and his coworkers found that they could block this transformation by adding a certain enzyme to the experiment.

The certain enzyme was DNAse, an enzyme that degrades DNA but has no effect on proteins which were the leading candidates in the search for the hereditary molecule at the time. Protein-degrading enzymes had no effect on transformation of the R strain bacteria.

In 1944, Avery reported that DNA was probably the stuff of inheritance.

Bacteriophages are a class of viruses that infect bacteria. By the early 1950s, experiments with these things were providing more evidence that DNA was the stuff of heredity.

Bacteriophages consist only of a coat protein and DNA. When bacteriophages attach to a host cell, they inject their DNA into it, causing the cell to make viral nucleic acids and proteins necessary to make new bacteriophages. The bacterial cell then bursts, releasing the new viruses (bacteriophages).

In 1952, Hershey and Chase devised a neat way to track what was going on. Bacteriophage coat proteins contain sulfur but no phosphorous; DNA contains phosophorous but no sulfur. They did two kinds of experiments.

In the first, they labeled bacteriophages with radioactive sulfur to tag their proteins.

In a second experiment, they labeled bacteriophages with radioactive phosphorous to tag their DNA.

Labeled bacteriophages were allowed to infect unlabeled bacterial cells suspended in a fluid. Hershey and Chase then whirled the fluid in a blender to knock the bacteriophages off of the bacterial cells.

Sulfur labeled protein remained in the suspending fluid; it was associated with the bacteriophage bodies. Phosphorous labeled DNA remained with the bacteria- it contained the hereditary instructions for producing new bacteriophages. Their experiments provided yet more evidence that DNA was the stuff of heredity.

You will recall that a DNA molecule is composed of four kinds of nucleotides and that the nucleotides are composed of a sugar (deoxyribose), a phosphate group and one of four nitrogenous bases. The nitrogenous bases are:

The nucleotides in DNA are structurally similar to each other but thymine and cytosine are smaller, single ringed structures; adenine and guanine are larger, double ringed structures.

The first convincing evidence for how the nucleotide bases were arranged came from x-ray diffraction images of DNA prepared by Rosalind Franklin. By her calculations, nucleotides had to be arranged into a long, thin molecule of uniform diameter.

Using Franklin's images and suggestions and adding several of their own ideas, Watson and Crick came up with a model for DNA consisting of two strands of nucleotides, twisted together into a double helix. Hydrogen bonds join the bases of one strand with those of the other.

For the entire length of the long DNA molecule, adenine always pairs with thymine and cytosine always pairs with guanine. There are only two kinds of base pairs in DNA:

A - T      G - C

However, the order of base pairs in a nucleotide strand can vary greatly.

The process by which DNA is duplicated prior to cell division is called replication. We looked at this recently on a fairly macro scale when I spoke of the cell cycle and the S phase where chromatids are formed. Replication makes the DNA that goes into these chromatids.

In order to replicate, the two nucleotide strands of the double helix have to unwind and expose their nitrogenous bases a bit. Cells have pools of free nucleotides and these pair with the exposed bases- where an A is exposed, a T pairs; where a C is exposed, a G pairs; and so forth.

Each parent strand of DNA remains intact. A companion strand is assembled and associates with the intact DNA strand. Because the parent strand is conserved, each "new" DNA molecule is really half-old, half-new.

DNA is not left to its own devices during replication. Enzymes and other proteins unwind the molecule, keep the two strands separated and assemble a new strand on each one.

DNA polymerases are major replication enzymes. They are rather special enzymes and not only govern nucleotide assembly- they also "proofread" the growing strands for mismatched base pairs which are replaced with correct ones.

The accuracy of replication is amazing but absolutely necessary. On average, for every 100 million nucleotides added to a growing strand, only one mistake squeezes by the DNA polymerases.

DNA is a lot like a book of instructions for each cell. The alphabet used is simple enough, A-T-G-C, but how do things actually get to be proteins? This is the process I want to address next.

You will recall my earlier mentioned definition of a gene- that it is a stretch of DNA- some nucleotides in a particular sequence or order.

The path from genes or DNA to protein involves two steps called transcription and translation.

In transcription, molecules of RNA are produced on templates of DNA in the nucleus. In translation, these RNA molecules are moved out of the nucleus into the cytoplasm where they associate with ribosomes and are used as templates themselves for the assembly of proteins.

A strand of RNA is almost, but not quite, like a single strand of DNA. Its nucleotides consist of a sugars (ribose), phosphate groups and the nitrogenous bases, adenine, cytosine, guanine and uracil. Uracil replaces thymine in RNA. Uracil base pairs with adenine just like thymine does in DNA.

During transcription, an RNA strand is assembled on a DNA template. Transcription differs from replication in two important respects. First, only one region of one DNA strand is used as the template and second, different enzymes (RNA polymerases) are involved rather than DNA polymerases.

Transcription starts at a promoter, a base sequence that signals the start of a gene. RNA polymerase binds with the promoter and moves along the DNA joining nucleotides into an RNA transcript. When the RNA polymerase reaches the end of the gene region, another enzyme releases the RNA transcript.

Three kinds of RNA molecules are transcribed from different regions of DNA. All three are necessary for translation, the next part of protein synthesis.

Ribosomal RNA or rRNA is a molecule that associates with particular proteins to form the ribosome- the anvil upon which proteins are assembled.

Messenger RNA or mRNA is the linear sequence of nucleotides that associates with a ribosome for translation into a protein.

Transfer RNA or tRNA is an carrier molecule of sorts, which picks up a specific amino acid and pairs with an mRNA code word for the amino acid picked up.

Only mRNA carries protein building instructions from the nucleus. And it has some alterations done before it leaves the shop. Newly formed mRNA has regions called introns (noncoding portions) and exons (portions that do get translated into protein). The introns get snipped out and the exons spliced together to make the functional mRNA. A diagram about how this might occur is provided for you.

The question to address now is just how are the protein building words coded in the linear sequence of mRNA now ready to go to work. There is a code--Crick, Brenner and others came up with how it works. It is a triplet code. That is, three bases are read at a time and each three base triplet codes for a particular amino acid.

A start signal built into DNA and transcribed to mRNA establishes the correct reading frame for blocking out every three nucleotides in the mRNA sequence.

Each base pair triplet in mRNA is called a codon. With few exceptions, the genetic code is universal for all forms of life. A codon that calls for a particular amino acid in a bacterium will call for the same amino acid in a human, in fungi and in plants.

Each kind of tRNA has its own anti-codon, a sequence of three nucleotide bases that can pair with a specific mRNA codon. Each tRNA also has a molecular combining site that can combine with a particular amino acid. A diagram that explains these associations is provided for you.

Translation goes through three stages called initiation, elongation and termination. Let's now look at these stages.

In the cytoplasm, pools of amino acids, tRNA and ribosomal subunits exist. An "initiator" tRNA binds to a small ribosomal subunit which in turn binds to the mRNA transcript. Next, a large ribosomal subunit joins with the small one to make an intact ribosome, an mRNA transcript and an initiator tRNA complex.

Then elongation begins. A start codon on the mRNA defines the reading frame starting point. A series of tRNAs deliver amino acids to the ribosome. A peptide bond forms between the growing polypeptide chain and each new amino acid added.

Chain termination occurs when a stop codon is reached and the ribosome and polypeptide chain are detached from the mRNA transcript. The polypeptide chain either joins the pool of free proteins in the cytoplasm or enters the cytomembranes for further processing.

In general, the base sequence in DNA must be preserved from one generation to the next or defects in growth and development may occur. Yet changes do occur in the DNA. Recall nature's mixing with crossing over; recall the effects of nondisjunction and deletions of chromosomes during meiosis.

Another kind of change that can occur is called gene mutation. Gene mutation is a deletion, addition, or substitution one to several bases in the nucleotide sequence of a gene.

Gene mutations are rare, chance events and most are bad news.

Some gene mutations are induced by mutagens, agents that can attack a DNA molecule and alter its structure. Ultraviolet radiation and certain chemicals are examples of mutagens.

Other gene mutations are spontaneous. For example, if adenine accidentally becomes paired with adenine during replication, enzymes with proofreading functions might well detect the mistake, but which adenine will they "fix?"

Single base modifications are not insignificant. Sickle-cell anemia is the result of a single mutation in the DNA strand coding for the beta chain of hemoglobin. Only one amino acid is affected--yet the substitution has profound consequences.

All the different cells of your body carry the same genes and they use most of them to synthesize proteins that are fundamental to the cell's structure and function.

At the same time, each cell type uses a small fraction of its genes in a highly specialized manner. Even though they all carry the genes for hemoglobin, only red blood cells have activated those genes. Even though all cell types have the genes for antibody production, only a certain subset of cells activate these genes.

All living cells control which genes are active and which gene products appear, at what times and in what amounts.

We know the most about gene control in the prokaryote, E. coli, of hamburger fame. Much, much less is know about gene controls in multicelled eukaryotes, mainly because patterns of gene expression vary within and between different body tissues.

Differentiation arises through selective gene expression in different cells. Depending on the cell type and its control agents, some genes might only be turned on at one particular stage in development and never again. Others might be left on all of the time; still others switched on and off at appropriate times.

Cell division is gene controlled. Not all cells divide at the same rate. Some, such as nerve cells, are arrested in interphase; others divide quite rapidly. No one knows exactly how, but genes govern cell growth and division.

And on rare occasions, a cell loses control over division. We refer to this loss of control as a cancerous transformation.

The next module will deal with recombinant DNA and genetic engineering. We humans are quite capable of manipulating genomes. A very large question is, should we?

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