Need help with the genetics option... if anyone wants to take the time to try and explain them
:
identify the role of genes in embryonic development
describe the evidence which indicates the presence of ancestral vertebrate homologues in lower animal classes
discuss the evidence available from current research about the evolution of genes and their actions
identify data sources, gather, process and analyse information from secondary sources and use available evidence to assess the evidence that analysis of genes provides for evolutionary relationships
• Identify the role of genes in embryonic development:
– During the development of the embryo, different cell types are produced in a precise pattern in space and time
– The differences in cells of the embryo results from the expression of different genes – certain genes are switched on or off during development
– The genes that are turned on during embryonic development are influenced by the POSITION of the cells in the embryo
– Chemicals which diffuse into the cells alter gene expression.
– A variety of proteins called activators and repressors combine with different parts of the DNA causing interactions between various proteins and genes. As a result, some are expressed and others are inhibited.
– HOX genes (described later) also play an important part in controlling the development of body shape, structure and organisation of an organism.
• Describe the evidence which indicates the presence of ancestral vertebrate gene homologues in lower animal classes:
– Gene homologues are DNA sequences that are similar in many different species
- In the Drosophila fruit fly, 8 genes were identified that controlled structural development of the body of the fly.
- The physical order of the HOX genes on the chromosome matched the physical order of the body segments of the fly (i.e. the gene controlling the position of the head comes before the gene for the thorax, which is before the gene for the abdomen, etc.)
- Also, when the genes were intentionally mutated, severed structural mutations occurred, such as legs growing out of the fly’s head.
– Using DNA probes that were complementary to the homeobox genes, it was found that the same gene was found in a large range of animals, such as worms, insects, cats, cows and also humans.
– In vertebrates, the number of HOX genes is greater than in invertebrates.
– Invertebrates have HOX genes on only one chromosome, while vertebrates have them on 4 chromosomes; HOX A, B, C and D.
– When the HOX genes from both the insects and the mammals were compared, the following observations were made:
- In both, the physical order of the genes along the chromosome corresponded to the spatial (physical) order of the structure they coded for along the head to tail axis of the embryo
- The base sequence of HOX genes are similar in insects and in mammals.
- If the gene was transferred from insect to mammal, or vice versa, it would do the same job.
– Therefore, the gene was probably inherited from a common ancestor of both vertebrates and invertebrates
– The vertebrates have extra HOX genes possibly due to chromosome duplication or polyploidy along the course of evolution
– Since they have survived a long time without change, they must be very significant.
• Discuss evidence available from current research about the evolution of genes and their actions:
– Recent studies have shown the presence of homeobox genes in most groups of multicellular animals.
– This implies that the genes evolved in an ancestor common to all animals
– When comparing the HOX genes of flies and mammals, not only are they similar in structure, but they also have similar functions.
– DNA sequencing has also revealed an evolutionary progression in the change of DNA of some genes.
– E.G. The genes that code for haemoglobin proteins. Studies of DNA sequences that code for the globin proteins has produced the variety of haemoglobin present in different mammals today. These suggest an evolutionary sequence for the origin of the different globin genes.
• Use available evidence to assess the evidence that analysis of genes provides for evolutionary relationships:
– DNA sequencing has identifies similar genes in different organisms
– E.G. The genes that code for the enzymes of respiration are very similar for every living organism, from bacteria to humans. This suggests that all organisms evolved from a single ancestor, and the gene for respiration first evolved in this common ancestor.
– Also, the study of HOX genes provides evidence for evolution as the genes have remained similar for many organisms, and the function is the same regardless of what organism the gene is placed in.
– The similarity of genes shows relatedness of organisms in relation to evolution, e.g. humans and chimpanzees share 98% of their DNA.
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