Re: Is it possible that our body make new genes

Hi Sreeram,

Thanks for your interesting question. I think the answers will become obvious to you if we take some time to think about the basic differences between prokaryotes and eukaryotes, particularly in terms of how they replicate their DNA and reproduce. This may seem like very basic biology but it's really key to answering your question, so bear with me.
Let's start with prokaryotes:

• Prokaryote means "before the nucleus". Prokaryotes are single-celled microorganisms (Bacteria or Archaea). DNA molecules in prokaryotes exist as chromosomes or plasmids, which may be linear or circular and are not compartmentalised inside a nuclear membrane.
• Prokaryotes reproduce asexually by binary fission. What this means is that a single cell makes a second copy of its DNA molecule(s), then divides into 2 cells, each of which take a copy with them.
• Prokaryotes can gain genetic material in different ways. Some of them can take up DNA from their environment. Some of them can exchange plasmids or chromosomes with other species. You mentioned transposons - these are mobile DNA elements that can move around in the chromosome. There are also integrons - gene cassettes that can pick up new genes from the environment. So prokaryotic genomes tend to be quite dynamic and mechanisms for acquiring or losing genes are numerous and widespread.
• Lastly, prokaryotes tend to have rapid generation times. Under ideal conditions, a bacterium such as E. coli can double every 20 minutes.

Now let's think about eukaryotes, by comparing them with the list of features that we looked at for prokaryotes.

• Eukaryotic DNA is packaged in chromosomes and contained in the cell nucleus. Eukaryotes can be single-celled (e.g. yeast, amoeba), but most of them are multicellular. In higher eukaryotes, cells differentiate into specialist types to make up different tissues.
• Higher eukaryotes reproduce sexually. They produce gametes (eggs and sperm), haploid cells which contain one copy of each chromosome. Gametes are generated by a process called meiosis, in which a cell copies its DNA, divides, then divides again. During this process, DNA is exchanged between pairs of chromosomes in a process called recombination, which means the genes in the gamete DNA may be slightly different to those in cells from other tissues in the adult individual. Non-gamete cells in the rest of the organism are normally diploid, containing 2 copies of each chromosome. They divide using a process named mitosis, which is not unlike binary fission, but leads to daughter cells of the same type, rather than gametes.
• Due to the higher complexity of eukaryotes and the packaging of their DNA, gene content tends to be more stable than in prokaryotes. There are very few natural mechanisms by which eukaryotes acquire or lose genetic material, although it is possible to introduce new genes artificially. Eukaryotes do contain transposons but they tend to be inactive, as their expression generally leads to unfavourable mutations.
• Complex multicellular eukaryotes have much longer generation times than prokaryotes. In contrast to a lab culture of E. coli doubling every 20 minutes, the human population is currently doubling about every 30 years.

OK, now we can get to your question! First, you state that in the example of antibiotic resistance, the resistance gene is not initially present. As we saw in the discussion of prokaryotes, there are two ways that a prokaryote might acquire antibiotic resistance: (1) get a resistance gene from another prokaryote or (2) generate a mutation in its chromosomal DNA which leads to increased resistance and is then passed on to its progeny when the cell divides. So really, it's wrong to think of a "resistance gene" which is either present or absent. What you have is a gene which through mutation, confers resistance and is then passed on to other cells. For example, the mutation may result in a protein product to which an antibiotic can no longer bind and so is rendered ineffective. Resistance genes can spread rapidly in a microbial population because under selection pressure (e.g. presence of an antibiotic), resistant cells can rapidly multiply and out-compete non-resistant cells.

Let's consider if the same process could occur in our bodies. It's quite possible that a gene could mutate and produce a gene product (usually a protein) with altered properties. But in our bodies, cells are differentiated into different kinds. So if for example, our mutation occurred in a liver cell, then only that cell and any liver cells derived from the division of that cell could contain the mutation. For a mutation to be passed onto our progeny, it would have to occur in the DNA of a gamete. Furthermore, because we reproduce slowly - our maximum rate is about one offspring per year - it will take a very long time for a favourable mutation to spread in a human population.

What's of more interest in organisms such as humans is a process named epigenetics. It's now generally accepted that the expression of certain genes can be modified in response to environmental stimuli and that these modifications are heritable - they can be passed onto offspring even though they do not involve mutation of nuclear DNA. There's an excellent Wikipedia entry on epigenetics.

In summary then, the processes that lead to the emergence of new characteristics in prokaryotes are less relevant in eukaryotes because (1) eukaryotic DNA is more stable and resistant to changes, (2) eukaryotes are multicellular, (3) reproduction and population growth are quite different in eukaryotes as compared to prokaryotes.