Relative Sizes

In reality, however, many tiny individual units called cells make up these objects and almost all other components of plants and animals. The average human body contains over 75 trillion cells, but many life forms exist as single cells that perform all the functions necessary for independent existence. Most cells are far too small to be seen with the naked eye and require the use of high-power optical and electron microscopes for careful examination.

Repetitive DNA

Repetitive DNA studies of many organisms has revealed that a large proportion of eukaryotic genomes consists of repetitive DNA.

For example in the kangaroo rat

• the sequence (AAG) is repeated 2.4 billion times,


• the sequence (TTAGGG) is repeated 2.2 billion times and


• the sequence (ACACAGCGGG) is repeated 1.2 billion times.

What it does is unclear. These are called junk DNA sequences.

Note that junk is stuff you don't throw away because it might be useful some day;
garbage is stuff you don't want so you throw it away.

These sequences might have some function we don't know about so they have been called junk DNA. The fact that such sequences seem to accumulate in genomes has lead to the notion that repetitive DNA is selfish DNA, since the sequence makes additional copies of itself within the genome decoupled from the reproduction rate of the host.

The Chloroplast Genome

The genome of the chloroplasts found in Marchantia polymorpha contains 121,024 base pairs in a closed circle.

A chloroplast is the organelle that carries out photosynthesis and starch grain formation. It is a chlorophyll-containing organelle in plants that is the site of photosynthesis.

These make up some 128 genes which include:

• duplicate genes encoding each of the four subunits (23S, 16S, 4.5S, and 5S) of the ribosomal RNA (rRNA) used by the chloroplast
• 37 genes encoding all the transfer RNA (tRNA) molecules used for translation within the chloroplast.

Some of these are represented in the figure by black bars.
4 genes encoding some of the subunits of the RNA polymerase used for transcription within the chloroplast (the blue ones)
a gene encoding the large subunit of the enzyme ribulose bisphosphate carboxylase oxygenase (RUBISCO)

9 genes for components of photosystems I and II
6 genes encoding parts of the chloroplast ATP synthase
genes for 19 of the ~60 proteins used to construct the chloroplast ribosome

The Mitochondrial Genome

What does the genome of human mitochondria look like?

It contains 16,569 base pairs of DNA organized in a closed circle.

These encode:

• 2 ribosomal RNA (rRNA) molecules
• 22 transfer RNA (tRNA) molecules (shown in the figure as yellow bars; two of them labeled)
• 13 polypeptides.

The 13 polypeptides participate in building several protein complexes embedded in the inner mitochondrial membrane.

• 7 subunits that make up the mitochondrial NADH dehydrogenase
• 3 subunits of cytochrome c oxidase
• 2 subunits of ATP synthase cytochrome b

All these gene products are used within the mitochondrion, but the mitochondrion also needs proteins encoded by nuclear genes. These proteins are synthesized in the cytosol and then imported into the mitochondrion.

Why are there variations in Genome Size?

The genome size varies considerably from one species to another. The enormity of the c. 40 000-fold range in genome size have intrigued scientists for over half a century. Scientists have asked how and why genomes vary so extensively and whether it matters. But recent findings have extended a paleogenomics dimension to these questions. By using the size of fossil dinosaur bone cells as proxies for genome size, they have attempted to trace the evolution of genome size in reptiles over 200 million years. By analyzing currently available sequence data from a range of reptiles and birds, they have aimed to shed light on the genomic makeup of dinosaur genomes.

Size doesn't matter
Back in the later 1940's estimates of genome size were made, but as data increased, it soon became clear that there was a huge disparity between organismal complexity and genome size. In other words, a complex animal does not guarantee a large genome. In fact, the lowly liverwort has 18 times as much DNA as we have, and the slimy, dull salamander known as Amphiuma has 26 times our complement of DNA'.

Where does Genome size diversity come from?
Since then, there has been much progress in understanding the molecular basis behind how genomes vary so extensively in size. It is now widely accepted that genome size diversity arises from differences in the amount of non-coding repetitive DNA (e.g. pseudogenes, retrotransposons, transposons satellite repeats, and so on.).

Where does the size of the Genome come from?
The actual genome size of an organism is determined by the differential activity of mechanisms generating increases such as retrotransposon amplification, polyploidy, segmental duplications or generating decreases like illegitimate and unequal recombination, or differences in double-strand break repair in the DNA amount.

The Out of Nowhere Gene

New genes have been for decades believed to created from existing genes. New research that some new genes pop up Out of Nowhere. Researchers studying the Drosophilia fruit fly have discovered a gene without peers.

A new gene, called hydra, exists in only a small number of species of Drosophila fruit flies. Research indicates that the gene was created roughly 13 million years ago, when it is believed that the melanogaster subgroup species diverged from a common ancestor.

In addition to the appearance of new gene, some early evidence also shows that this new gene is functional i.e. not "junk" DNA. and may express itself as a protein involved in late stages of sperm cell development (spermatogenesis). Other scientists working with functional genes in any species also are expressed in male testes and appear related to spermatogenesis.

One problem that the researchers are working on is how the hydra gene was created. They believe that it came out of nowhere. Some speculate that the gene may have developed from a piece of DNA junk called a transposable element which some refer to as a "jumping gene". It may have been inserted into the genome by a virus. These transposons are known to copy and insert themselves into DNA sequences. One theory is that when a transposon sits next to a gene it carries part of the gene sequence it was next to and then if it jumps to a new location it inserts that gene sequence in the new location. Current thinking is that transposable gene elements appear to have no function or may be harmful and are eliminated by natural selection, however, other researchers believe that some transposons may be a source for creating new functional genes as well.

Genetic variation and Genetic stability

Scientists from the Max-Plank institute and from the Salk Institute and the university of Chicago have identified which genes are prone to variation and which are stable and do not have modification. They have developed a method to sift out whole genomes for all the environmental fixes and addendums accumulated over time. What the scientists are after is the regions that are currently targeted by natural selection or have been so during the evolutionary past.

This is an exciting prospect because it lends itself to a further accumulation of evidence about how evolution works. The entire genome isn't affected, only parts of it. Scientists studying the plant the mustard weed Arabidopsis thaliana have been able to identify genetic variations in 23 strains.

Why study arabidopsis?
About 10 years ago Arabidopsis was adopted by plant scientists as an easily manipulated model for other plants because it is simple to grow in the laboratory, has a short life cycle and a small genome. Arabidopsis only has about 120 million base pairs of DNA. Compared to corn, which might have as many as 2.5 billion base pairs of DNA and the human genome with roughly 3 billion pairs, one can study it more easily.

Effects on genes
Plants are under constant threat from heat, cold, high acidity or salinity, or pathogens such as viruses and leaf-munching insects. So how do plants survive? Plants mobilize physiological and biochemical defenses for their survival. Scientists expected certain classes of genes to be highly variable due to natural selection in different environments. And now two different studies revealed precisely which gene family members indeed were shaped by evolution. In general, genes that don't change over time are under strong negative selection because they perform important housekeeping functions, while genes that vary widely such as disease resistance genes are under strong positive selection.