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| Question | Answer |
| Development from a single cell to many different cells and organs | - All life starts off as an egg (zygote) - Once fertilised, the egg cell makes many cells through multiple rounds of cell division. 1. Fertilisation: 1 sperm enters the egg and releases nuclear contents (genetic material) within. This sets off a chemical reaction, a wave of calcium across the egg, making it impermeable to other sperm. Divides, forming a zygote 2. Zygote then moves from the oviduct, where cell divisions begin, and by the time it reaches the uterus, where it implants, it is a blastocyst. |
| Proliferation and differentiation | - During embryogenesis the organism gets bigger (proliferation), and more complex (differentiation). - all cells in the organism contain the same genetic information, although they make up a number of different cell types with different shapes and functions. - in order to differentiate, cells must be equipped with different proteins. These are encoded by genes, so, in order to differentiate, cells must express different genes. |
| Proliferation | -Fertilisation -Cleavage (1st rounds of division) -Blastula (hollowed embryo) -Gastrulation (formation of germ layers) |
| How cell layers become tissue and organs: Gastrulation | - gastrulation is the reorganisation of the blastula into a 3 layered embryo Germ layers: *Ectoderm: goes around the outside, forms skin and neural tube *Mesoderm: Layer under the ectoderm, forms muscle, bones and some organs *Endoderm: inside layer. Forms digestive trancts and respiritory system - each step of differentiation is built upon the previous step, becoming more specialised as you go along |
| Pattern formation: How shapes and form emerge during development | - Patterns are generated early on a small scale, ranging from just 10s to a few 100s of cells. - Adult pattern is due to the growth of the basic pattern - Key events in human development occure during the earliest stages of embrogenesis: germ layers differentiating, pattern laid down, further growth. - However, development can go wrong in a number of ways as the process is so long and complex. |
| How genes control development | - to understand how genes control development, model organisms can be used, as many genes are conserved between organisms. EG: - fly - zebra fish - frog - chick - mouse |
| Experiments investigating development in the frog (xenopus) | - synchronised developent in frogsspawn; many frog eggs divide at the same time. - eggs are outside the body so stages of development can be easily studied |
| Investigating how genes regulate development in a frog | - Pax-6: transcription factor expressed at the regions of the embryo where the eye is going to form - Finding out where the genes are expressed gives an idea as to what they do in development. - hypothesese can be tested by overexpressing genes of interest into specific areas. Adding RNA for the gene in an area that is not usually expressed. - this is done using a whole-mount in situ, where a lineage tracer shows where RNA has been injected. However, the way we can prove this, inhibiting PAX-6 must result in no eye devlopment, which it did. |
| Results of mutations of the PAX-6 gene in humans | - no iris forms so light cannot be controlled - photophobia - issues with optic nerves |
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