B5.3 How do genes control growth and development within the cell?
DNA is a nucleic acid in the shape of a double helix. Long strands of DNA make up chromosomes – these are found in the nucleus of a cell. DNA is a chemical code – our bodies need proteins for growth and development, and the DNA controls which proteins are made. The code consists of four different chemicals, or bases, that always pair up in the same way.
T always pairs with A
G always pairs with C
The order of these pairs of bases along the DNA molecule codes for all different proteins. A section of DNA that codes for one particular protein is called a gene.
To enable genes to code for proteins, the bases A, T, G and C get together not in pairs but in triplets. This is how it works:
- Each protein is made up of large numbers of amino acid molecules
- Each triplet of bases codes for one particular amino acids
- So amino acids are made in the number and order dictated by the number and order of base triplets
- Finally, the amino acid molecules join together in a long chain to make a protein molecule. The number and sequence of amino acids determines which protein results
The DNA has sequences of genes, which code for proteins – however the proteins themselves are manufactured in the cytoplasm of the cell. Therefore there is a mechanism for transferring the information stored in the genes into the cytoplasm.
The DNA molecule is too large to leave the cell so the relevant section of DNA is unzipped and the instructions are copied onto smaller molecules which can pass through the nuclear membrane of the nucleus into the cytoplasm.
The smaller molecules are called messenger RNA (mRNA) – these leave the nucleus and carry the instructions to the ribosomes, which follow the instructions to make the specific protein.
All body cells, including stem cells contain exactly the same genes. However although all the body cells in an organism contain the same genes, many genes in a particular cell are not active (switched off) because the cell only produces the specific proteins it needs.
In specialised cells only the genes needed for the cell can be switched on but in embryonic stem cells any gene can be switched on during development to produce any type of specialised cell
Adult stem cells and embryonic stem cells have the potential to produce cells needed to replace damaged tissues. For example, stem cells can be used to replace brain tissue in a patient with Parkinson’s disease, or to grow new skin tissue following a burn.
To produce the large number of stem cells needed for, it is necessary to clone cells from five-day-old embryos. The stem cells are collected when the embryo is made up of approximately 150 cells – the rest of the embryo is destroyed. At the moment, unused embryos from IVF treatments are used for stem cell research
There is an ethical issue to whether it is right to use embryos to extract stem cells in this way – should embryos be classed as people?
One view is that if an embryo is left over from IVF (and would therefore never grow into a human being) it would be acceptable for stem cell research to be carried out on it as long as the parents gave their consent. However, another view is that destroying an embryo amounts to destroying a life. The Government regulates and makes laws on such matters.
Mammalian cloning – in carefully controlled conditions of mammalian cloning it is possible for scientists to reactivate (switch on) inactive genes in the nucleus to form cells of all tissue types. This gives the potential to grow new tissue that is genetically the same as the patient