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Home > Biology > Options > Genetics: the code broken? > Genetics: The Code Broken: 8. The timing of gene expression

9.7 Option– Genetics: The Code Broken? : 8. The timing of gene expression

Syllabus reference (October 2002 version)
8. The timing of gene expression is important in the developmental process	Students learn to: identify the role of genes in embryonic development   summarise the role of gene cascades determining limb formation in birds and mammals   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	Students: 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

Extract from Biology Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.

[Edit:26Sep05]

Prior learning : Stages 4 and 5 Science, 5.8.2 c) and d) HSC 9.3.3 and 4

identify the role of genes in embryonic development

• On fertilisation, two haploid sets of chromosomes join to produce a zygote containing all the genes necessary to grow into a new individual.
   
• As mitosis occurs, the resulting embryo grows into a mass of undifferentiated cells. As the embryo develops, cells differentiate and become specialised in their size, shape and function according to which genes are used to regulate these developments.
   
• The activities of genes are regulated by other genes to control all cell activities, production of proteins and enzymes, at different times of development. A variety of proteins, activators and repressors, link up with different parts of DNA to cause interaction between various other proteins and genes. The result is that some genes are affected and expressed and others are repressed or inhibited. Genes are turned on or off according to their position in the embryo, function and age of the developing embryo/foetus/individual.

summarise the role of gene cascades determining limb formation in birds and mammals

• During limb development in birds and mammals, an appropriate sequence of genes is turned on to form such things as bones, muscles, ligaments, tendons, nerves and blood vessels. As each gene is turned on, certain substances are produced that turn on the next gene in the sequence. This process is repeated and continues as the limb develops. This process of a sequence of genes being turned on which then causes other genes to be turned on is called a gene cascade.
   
• The CSIRO is currently studying the development of limbs in chickens. During embryonic development, cells that form limbs are specifically programmed to form either a leg or a wing. At this stage, scientists know about gene sequences after the cells are programmed to form a part of the body such as a leg, but they do not know very much about the gene cascade in the preprogrammed stage of development. They do not know the particular chemicals that will determine which cells will form a limb in the first place.

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 data sources such as the Internet, biology text books and science journals.
• Gather information from the identified sources.
• Process the information by assessing the reliability of information from various sources. If two have different information cross check them from one or two other sources.
• Analyse information you have gathered and processed to make generalisations about evolutionary relationships identified by gene analysis.
• Use the available evidence to assess the evolutionary relationships that gene analysis has provided.

describe the evidence which indicates the presence of ancestral vertebrate homologues in lower animal classes

• Homologous structures such as pentadactyl limbs in vertebrates provide evidence for evolution from common ancestors. In a similar way, there are DNA sequences that are similar in many organisms. These DNA sequences are called homologue genes, homeobox or Hox genes. These genes regulate the development of an organism by producing proteins that switch other genes on and off.
   
• An example of a homologue is the gene cascade for skeletal and neurological development in limbs. It is similar in organisms such as humans, chickens, rodents, insects, nematodes and molluscs. Experiments have confirmed that the homologue gene from an amphibian can regulate the corresponding gene in mammals. A mammal homologue gene can regulate the corresponding gene in insects such as fruit flies.
   
• These homologue genes exist in many eukaryotic organisms, both vertebrates and invertebrates as well as some fungi and plants. This suggests common ancestry between all eukaryotic organisms.

discuss the evidence available from current research about the evolution of genes and their actions

• Homologues, homeotic or Hox genes are found in most or all groups of multicellular animals and show similar DNA sequences suggesting that these genes evolved in a common ancestor. These genes, being similar in both structure and function, are expressed in similar sequences on chromosomes.
   
• The study of DNA sequences provides evidence for the evolution of genes. Sequences of bases in genes that do not change or change very slowly over time are used to measure relationships between groups of organisms. For example, in mice, the gene for the development of eyes is similar to the gene in insects.
   
• Previously, amino acid sequences in proteins such as haemoglobin, have been used to determine evolutionary relationships. The protein myoglobin in insects provides oxygen to cells and is very similar to a protein with a similar function in primitive fish. Studies of all of these proteins responsible for carrying oxygen suggest an evolutionary pattern for their development. The analysis of the DNA that makes up the genes that code for proteins has provided more evidence of evolutionary changes in certain groups of organisms.
   
• The study of mutations of homeotic genes shows that a small mutation can suddenly affect an organism as a gene cascade is altered. A mutation in a homeotic gene can cause one part of the body to develop into another. For example, in Drosophila fruit flies, one mutation in a homologue gene can result in legs growing on the head rather than antennae. In humans, a mutation in a homeotic gene for the development of the bones of the skull results in rigidity in the joints between these bones. This can result in abnormal brain growth and possible mental retardation. The base sequence for this gene is very similar in many animals.
   
• These gene changes can then result in often dramatic alterations in organisms which may then lead to the rapid evolution of new body structures. This is evidence for the expansion of diversity of living things and the theory of punctuated evolution.