Deoxyribonucleic acid, or DNA as we all know it, is a remarkable molecule that holds our genetic information. This genetic information carries the blueprint for an organism's development, its characteristics, and is the basis of inheritance - ever wondered why children share similar features to their parents? It is because of DNA!
Storage of genetic information
You can find DNA primarily in the nucleus as chromosomes, as DNA is an extremely long molecule. DNA is also found in the mitochondria and chloroplast of plants in the form of circular DNA. To make it easier to analyse DNA, we refer to short sections of DNA as genes and different versions of the same genes as alleles. It is the base sequence of these genes that we are interested in as it determines the amino acid sequence of proteins, as illustrated below in the diagram.
Base sequence of DNA and amino acid sequence of proteins
Transport of genetic information
The genetic information stored in the base sequence of genes is transferred through protein synthesis. This process requires transcription and translation and requires other molecules like RNA to be present.
This process of transferring genetic information is a little different in prokaryotic cells. Prokaryotic cells, such as bacteria, carry their genes in extrachromosomal DNA called plasmids. These are circular pieces of DNA that are transferred to other bacteria through processes called conjugation and transformation.
In the previous section, we discussed that DNA enables offspring to share similar features to their parents. So why then do children also have different features from their parents? This is where genetic variation comes in, and this describes the differences in DNA between individuals.
What is Genetic Variation?
Genetic variation explains the differences between individuals in a species caused by changes in the DNA. The main causes of this phenomenon are mutations, meiosis and random fertilisation.
Mutations describe the changes to the base sequence of genes. Below, you will see an example of a section of mutated DNA. This has the ability to change an organism's characteristics; for example, a mutation in the gene coding for a moth's wing colour can turn the original brown to a darker black colour. Mutations can have a disadvantageous effect, an advantageous effect or no effect on an organism. When the allele is advantageous, the organism has an increased chance of survival and a phenomenon called natural selection occurs. We'll take a look at this in the next section.
Original and mutated DNA sequence
Meiosis and sexual reproduction also introduce genetic variation into a population. Meiosis, a type of cell division, involves processes called crossing over and independent assortment. These describe the exchange of DNA between chromosomes and their alignment in the cell, both of which result in changes to the DNA.
All male sperm and female eggs (gametes) contain slightly different DNA (due to meiosis), and the fertilisation between these gametes is a completely random process. This is great for genetic variation because it means the resulting offspring will also have its own unique set of DNA.
Natural selection
Natural selection describes the process whereby organisms that possess characteristics that improve their ability to survive will live on to reproduce. These characteristics are granted primarily through genetic mutations, which give these organisms advantageous alleles. For example, moths with a genetic mutation that grants them a darker wing colour can camouflage from their prey and survive better than moths with a lighter wing colour.
Predation is termed selection pressure as it is a factor that affects the chances of an organism's survival. Availability of food and shelter are also selection pressures that drive natural selection.
Other forms of variation
Differences between individuals are not only caused by DNA changes. Environmental factors can also modify the characteristics of an organism. Different environments will have different living conditions.
For example, plants that are exposed to more sunlight have a better chance of growing larger than plants that are in the shade for most of the day.
Genetic variation and environmental variation intersect, and they both play a role in the resulting characteristics of an organism, also called the phenotype. Therefore, it is the combination of these factors which explains the variation in individuals.
How do we measure diversity in genetic variation?
The genetic variation found within a species can be quantified by:
To do this, we have to extract DNA or proteins, which are then analysed.
Biodiversity describes the variation of species we see within a community. This can be quantified using species richness. Species richness measures the number of different species in a community, for example, the number of different species found in a waterfall. Another form of measurement is the index of diversity, a formula that describes the relationship between the number of different species in a community (species richness) to the number of organisms in each species (species evenness).
Genetic Information - Key takeaways
- All genetic information is carried in DNA.
- Genetic variation is caused by mutations, crossing over, independent assortment and random fertilisation.
- Natural selection is the process by which organisms that possess advantageous characteristics survive and reproduce. These advantageous traits are caused by genetic variation and environmental variation.
- Diversity can be measured using species richness and the index of diversity.
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