Topic 8

A gene mutation is the alteration of a base in the sequence of bases for one gene. This is likely to occur during DNA replication, which is during interphase in the cell cycle.

Gene mutations occur spontaneously, but the frequency of these occurring can be increased by certain factors which are called mutagenic agents.

Because mutations alter the gene, they can result in a different amino acid sequence in the encoded polypeptide.

If the amino acid sequence changes then when protein is modified into the tertiary structure it will form hydrogen and ionic bonds in different places and fold differently. This will result in a different 3D shape, and therefore a non-functioning protein. Alteration to gene can result in a mutation that causes cancer.

These mutagenic agents include radiation such as a and ฿ particles and x-ray and gamma rays. Ultraviolet light is not ionising, but it is still high enough so can cause damage and disrupt the structure of DNA.

This is the term given to chemicals that can alter the structure of DNA and interfere with transcription. These include chemicals in tobacco smoke, mustard gas and peroxides.

• Addition • Deletion • Substitution • Inversion • Duplication • Translocation

The impact of adding one base is that all subsequent codons are altered. This is known as a frame shift. This type of mutation can be very harmful because all the altered codons could potentially code for different amino acids and result in a very different sequence of amino acids resulting in a non-functioning protein.

The deletion of a base in a sequence. This causes a frame shift to the left. This could result in a different polypeptide chain and a non-functioning protein.

One bases has been changed for a different base, but the number of bases remains the same and there is no frame shift. This results in only one codon changing, and due to the genetic code being degenerate it may still code for the same amino acid and therefore have no impact.

A section of bases detach from the DNA sequence, but when they re-join they are inverted, so this section of code is back to front. This results in different amino acids being coded for in this region.

A section of bases on one chromosome detaches and attaches onto a different chromosome. This is a substantial alteration and can causes significant impacts on gene expression and therefore the resulting phenotype.

Stem cells are undifferentiated cells that can continually divide and become specialise. Differentiation is the process by which stem cells become specialised.

These stem cells that can divide and produce any type of body cell. During development, totipotent cells translate only part of their DNA, resulting in cell specialisation. Totipotent cells occur only for a limited time in early mammalian embryos.

These stem cells are found in embryos and can become almost any type of cell.

There are issues with this as sometimes the treatment doesn’t work, or the stem cells continually divide to create tumours. Additionally, ethically there is debate on whether it is right to make a therapeutic clone of yourself to make an embryo to get the stem cells to cure a disease, and then destroy the embryo.

They are used in research with the prospect of using them to treat human disorders. These stem cells could be used to regrow damaged cells in humans, such as replace burnt skill cells, or beta cells for type I diabetics or neurones in Parkinson’s disease sufferers.

Multipotent and unipotent these stem cells are found in mature mammals and can divide to form a limited number of different cell types. Multipotent cells, such as in bone marrow, can differentiate into a limited number of cells, where as unipotent cells can only different into one type of cell.

Embryos up to 16 days after fertilisation, contain stem cells that are pluripotent and can differentiate into any type of cells. Umbilical cord blood contains stem cells which are multipotent, like adult stem cells. The placenta has stem cells that are multipotent and can develop into a limited number of specialised cells. Adult stem cells, such as in the bone marrow, can produce different cells to repair those within a particular tissue or organ.

Induced pluripotent stem cells (iPS cells) can be produced from adult somatic cells using appropriate protein transcription factors to overcome some of the ethical issues with using embryonic stem cells.

IPS cells are created from adult unipotent cells. These cells, which can be from almost any body cell, are altered in the lab to return them to a state of pluripotency. To do this, the genes that were switched off to make the cell specialised must be switched back on. This is done using transcriptional factors. They are very similar to embryonic pluripotent stem cells, but do not cause the destruction of an embryo and the adult can give permission.

In eukaryotic organisms, one way gene expression (protein synthesis) is controlled is by epigenetics.

Epigenetics is the heritable change in gene function, without changing the DNA base sequence. These changes are caused by changes in the environment and can inhibit transcription

Diet stress and toxins. This can control gene expression in eukaryotes