Click4Biology: 4.1 Chromosomes, genes, alleles and mutations

Chromosomes, genes, alleles and mutations

4.1.1 Eukaryotic Chromosomes

4.1.2 Definitions

4.1.3 Gene mutation.

4.1.4 Sickle cell anaemia





4.1.1 State that eukaryote chromosomes are made of DNA and proteins.(1)

State means to give a specific name, value or other brief answer without explanation or calculation.

a) DNA

b) Proteins called histones.





4.1.2 Define gene, allele and genome. (1)

Define means to give the precise meaning of a word, phrase or physical quantity.






4.1.3 Define gene mutation. (1)

Define means to give the precise meaning of a word, phrase or physical quantity.

Mutation was coined by Hugo de Vries a dutch researcher who at the time was testing the work of mendel. His research at the begriming of the 20th Century even suggested that sudden changes in gene might explain an evolutionary 'jumping' mechanisms rather than the gradual mechanisms suggested by Darwinism. Subsequently it has been shown that the plant that de Vries was working on has unusual genetic behaviour.

Mutations are not rare, even as you read these pages you are accumulating thousands of mutations and if you are male the mutation rate is even higher. Most mutations will not improve condition but just occasionally against all the odds this is the case. Mutation of course creates the raw material for the process of evolution. Cellular machinery acts against mutation with the presence of all sort of enzymes that correct the frequent errors. Nevertheless look around you and take in the diversity of life, at any level you care to consider the cause of this variation is mutation.

The production of mutation is in its self a series of random mistakes. Mutagens are the cause of mutations, radiation is a well know examples but there is also chemical-mutagenesis as discovered by Charlotte Albach. As mentioned the mutation is random and not directional, an animal exposed to a cold environment is just as likely to produce mutations which have advantages in warm climates as it is to a cold climate but it is even more likely to produce a mutation that has nothing to do with climate.

That said the rate of mutation is balance between the environmental mutagen attaching the genome and the corrective mechanisms such as the polymerase enzymes or corrective DNA enzymes. But why does mutation occur at all? Or for that matter why is the rate not very very high? The answer is not clear but perhaps the rate of mutation is a consequence of evolution itself. No mutation would result in an inflexible genome incapable of responding (as a gene pool) to the environmental changes of time. Too much mutation would render the organsims largely non functional and compromise its survival. The 'allowed' rate provides sufficient variation to respond to environmental change whilst not compromising the integrity of the organisms physiology. An interesting discover which in directly support this view is the discovery that some bacteria




4.1.4 Explain the consequence of a base substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anemia.(3)

Explain means to give a detailed account of causes, reasons or mechanisms.

Sickle cell anaemia is a genetic disease.

Frequency ia about 1 in 655 African Americans

The disease is inherited not contracted by infectious routes.

Sickle cell anaemia at the tissue level:


(a) Normal haemoglobin has two of four proteins changed in the mutation.



(b) The normal biconcave disc shape of the red blood cell is changed to a 'sickle' shape.


(c) In addition to not carrying oxygen correctly (anaemia) the cells also causes local clots (infarctions) such as is shown in the kidney tubules. This leads to necrosis (death) of the tubules, kidney damage, kidney failure and possible to death.






Genetics of Sickle Cell.