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Chromosomes, Genes, Alleles and Mutations

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Chromosomes, Genes, Alleles and Mutations

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    1. Chromosomes, Genes, Alleles and Mutations Topic 4.1

    2. Eukaryotic Chromosomes The chromosome is composed of two main molecules. a) DNA b) Proteins called histones. This image was taken shortly after DNA a replication but before the prophase. It is composed of two daughter chromatids joined at the centromere. The chromosome is super coiled by a factor around x16,000. The DNA molecule is about 1.8m long but is located in the nucleus which is only 10um in diameter!

    3. Genes, Alleles and Genome Gene: a heritable factor that controls a specific characteristic. Allele: one specific form of a gene, differing form other alleles by one or a few bases only and occupying the same gene locus as other alleles of that gene. Genome: the whole genetic information of the organism.

    4. Gene Mutation

    5. Gene mutation is a change in the base sequence of an allele. The changed base sequence may produce a different amino acid sequence in the protein translated. The changed base sequence may not change the protein because of the degenerate nature of the genetic code. The expression of the mutated gene may or may not be beneficial to the organism. Substances that cause mutation are called mutagens and include chemicals and radiation.

    6. The consequence of a base substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anemia (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.  

    7. The gene loci for the normal beta chain of haemoglobin is on chromosome 11. The normal allele carried the triplet GAG at the sixth amino acid position for the beta chain (146 amino acids). This transcribed and translates into the negatively charged Glutamic acid. The mutation changes a single base ( T replaces A) and this transcribes and translates into the amino acid Valine. Valine has a neutral charge and the result is a change in the shape of the beta chain with long needle like structures forming. This gene is noted for many mutations and it is estimated that some 5% of humans carry one or other variants.

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