Natural selection and genetic modification Revision Notes

From Five Kingdoms to Three Domains

Early classification systems, like the five kingdoms (Monera, Protista, Fungi, Plantae, Animalia), relied on visible features and cell structure. Monera included all prokaryotes. Genetic analysis, especially of ribosomal RNA (rRNA), revealed fundamental differences within prokaryotes, leading to the three-domain system: Bacteria, Archaea, and Eukaryota. This change better reflects evolutionary relationships.

The Role of Genetic Analysis (rRNA)

Genetic analysis compares DNA and RNA sequences. Ribosomal RNA (rRNA) is particularly useful because it's present in all organisms and evolves slowly, making it a good 'molecular clock' for deep evolutionary relationships. Significant differences in rRNA sequences demonstrated that prokaryotes were not a single group and separated them into Bacteria and Archaea, with Archaea having closer genetic links to Eukaryota than to Bacteria.

Characteristics of the Three Domains

The three domains are Bacteria (true bacteria), Archaea (extremophiles, genetically distinct from bacteria but prokaryotic), and Eukaryota (all organisms with membrane-bound organelles, including Protista, Fungi, Plantae, Animalia). This modern system highlights the deep evolutionary splits identified through molecular evidence, specifically genetic analysis.

Selective Breeding: Process, Benefits, and Risks

Selective breeding (artificial selection) involves choosing organisms with desirable inherited characteristics and breeding them together. Over subsequent generations, offspring displaying the desired traits are selected and bred, leading to an accumulation of these characteristics. Benefits in agriculture include developing crops with higher yields, disease resistance, or improved flavour, and livestock with faster growth rates or increased meat/milk production. Risks include reduced genetic diversity within populations, making them more susceptible to new diseases or environmental changes, and the potential for undesirable traits to become prevalent due to close inbreeding.

Genetic Engineering: Process, Benefits, and Risks

Genetic engineering involves modifying an organism's genome using biotechnology. This can include inserting a gene from one species into another, deleting a gene, or altering existing genes. Benefits include creating crops resistant to pests, herbicides, or environmental stresses, and increasing nutritional content. In medicine, it allows for the production of hormones (e.g., insulin), vaccines, and gene therapies. Risks include potential long-term health effects of consuming genetically modified organisms (GMOs), unintended consequences for biodiversity if modified genes spread to wild populations, ethical concerns regarding the manipulation of life, and the high cost of development and regulation making it inaccessible to some regions.

Comparing Genetic Engineering and Selective Breeding

While both aim to improve organisms, genetic engineering is much faster and allows for the introduction of genes from different species (transgenic organisms), which is impossible with selective breeding. Selective breeding works with existing genetic variation and is slower. Genetic engineering offers greater precision in modifying specific traits. Ethical concerns are often heightened for genetic engineering due to direct gene manipulation compared to the more traditional and perceived 'natural' process of selective breeding.

Restriction Enzymes and Sticky Ends

Restriction enzymes are specialised proteins that act like molecular scissors. They identify and cut DNA at specific nucleotide sequences, creating fragments. The cuts often result in 'sticky ends,' which are short, single-stranded overhangs of DNA. These sticky ends are crucial because they can readily base-pair with complementary sticky ends on another piece of DNA, facilitating the insertion of a new gene.

Vectors and DNA Ligase

A vector is a DNA molecule used to carry foreign genetic material into another cell. Plasmids (small, circular DNA molecules found in bacteria) are commonly used as vectors. They are cut with the same restriction enzyme as the target gene, creating complementary sticky ends. Once the target gene's sticky ends have annealed with the vector's sticky ends, the enzyme DNA ligase is used. DNA ligase acts as a molecular glue, forming phosphodiester bonds to seal the gene into the vector, creating a recombinant DNA molecule. This recombinant DNA can then be introduced into host cells for replication and gene expression.

Stages of Genetic Engineering

The main stages involve: 1. Isolation of the desired gene. 2. Cutting the gene and vector (e.g., plasmid) with the same restriction enzyme to produce complementary sticky ends. 3. Ligation of the gene into the vector using DNA ligase to form recombinant DNA. 4. Introduction of the recombinant DNA into a host cell (e.g., bacterium). 5. Identification and multiplication of the host cells that have taken up the recombinant DNA. 6. Expression of the gene by the host cell to produce the desired product or trait.

What is Selective Breeding?

Selective breeding is the process where humans choose organisms with desirable characteristics and breed them together. The aim is to produce offspring with these enhanced characteristics. This is a practice that has been occurring for thousands of years, long before the scientific understanding of genetics.

Impact on Food Plants

For food plants, selective breeding has led to significant improvements. Key impacts include increased crop yields, enhanced resistance to common diseases and pests, better nutritional value (e.g., higher vitamin content), and improved growth characteristics such as faster maturation time or larger fruit size. For example, modern maize varieties produce far more corn per plant than their wild ancestors due to selective breeding.

Impact on Domesticated Animals

In domesticated animals, selective breeding has resulted in breeds tailored for specific human purposes. This includes animals that produce more meat, milk, or eggs, animals with better quality wool or fur, and those with desired temperaments (e.g., companion animals like dogs) or specific capabilities (e.g., working dogs or horses). For instance, dairy cows have been bred to produce exceptionally high volumes of milk.

Advantages and Disadvantages

A major advantage is the efficient production of food and other resources that meet human needs. However, a significant disadvantage is the reduction in genetic diversity within selectively bred populations. This can make entire populations highly susceptible to new diseases or environmental stresses, as they lack the varied genetic traits that might offer resistance or adaptability.