La complexification des génomes - Terminale Spécialité SVT

The entire course is available at www.moncoursdesvt.com Chapter: The Increasing Complexity of Genomes I – Demonstration of Genetic Transfer Observing the transmission of the pathogenic phenotype between different strains of bacteria has demonstrated the transfer of genetic information between individuals. II – Mechanisms of Genetic Material Transfer in Bacteria A – Transformation Free DNA in the environment can be incorporated into the bacterial genome. B – Transduction (or Viral Transfer) When a bacterium is infected by a bacteriophage virus, fragments of DNA from the host bacterium can be carried by the virus and integrated into the genome of the next infected bacterium. C – Conjugation Bacteria form bridges between themselves through which they exchange molecules, including DNA. D – Horizontal vs. Vertical Transfer The transmission of DNA between individuals of the same generation, not necessarily related, constitutes horizontal transfer, to be distinguished from vertical transfer, which occurs during reproduction through the passing of genetic material from one generation to the next. III – The Universality of DNA and its Evolutionary Consequences A – A Universal Carrier of Genetic Information Genetic transfers are observed even between groups of living beings that are very distant from a phylogenetic point of view. This is explained by the universality of the DNA molecule as the carrier of genetic information within the living world. B – Contributions of Genome Sequencing Sequencing and comparing genomes reveal that all species contain genes originating from other species, including species that are very distant phylogenetically. For example, genes originating from viruses, bacteria, fungi, and plants have been identified in humans. C – Applications in Biotechnology Biotechnology leverages these properties to transfer and express genes of interest in an organism other than the original one. For example, it is possible to transform bacteria or yeast to produce large quantities of molecules useful to humans (medicines, food molecules, etc.). IV – Role of Horizontal Gene Transfer A – Diversification of Life Horizontal gene transfer is very frequent and contributes to the diversification of the living world by enabling the receiving lineages to acquire new traits. Comparison of sequences shows that the syncytin of hominoids is very similar to a viral protein (greater than 80% identity). This is explained by horizontal gene transfer: a viral gene was integrated into the genome of the common ancestor of hominoids. This gene, which became syncytin, is essential for the formation of the placenta through the fusion of cell membranes. B – Public Health Issues The high frequency of genetic exchange poses a public health problem, as the intensive use of antibiotics in livestock farming and human health promotes the emergence of resistance genes that are then widely dispersed in the environment and transferred between bacteria. V – The Endosymbiosis Theory A – Origin of Mitochondria and Chloroplasts The observation of significant similarities in structure and function between cellular organelles and free-living bacteria led to the formulation of the endosymbiosis theory. This theory states that mitochondria and chloroplasts originated from bacteria that were integrated into the cytoplasm of other cells more than 2 billion years ago. B – Transmission of Organelles Capable of multiplying, these organelles are transmitted from one generation to the next with the cytoplasm of gametes (cytoplasmic inheritance) or by cell division. C – Reduction and Transfer of Organelle Genomes Over time, the organelle genome regresses; either some of its genes are eliminated, or some of its genes are transferred to the nucleus of the host cell. D – New Metabolic Capabilities The integration of other cells confers new metabolic capabilities to the host cell, such as the ability to perform respiration with mitochondria or to perform photosynthesis with chloroplasts. Serial endosymbioses have frequently occurred throughout the history of eukaryotes.