4.1 CHROMOSOMES, GENES, ALLELES AND MUTATIONS

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.

The chromosome is composed of two main molecules. a) DNA =b) Proteins called histones (8 histones in a cluster. 147 base pairs

double wrapped constituting a nucleosome (bead) facilitates supercoiling and regulates gene expression.)

This image was taken shortly after DNA a replication but before prophase. It is composed of two daughter chromatids joined at the centromere.

The chromosome is super coiled by a factor of around x16,000. The DNA molecule is about 1.8m long but is located in the nucleus which is only 10um in diameter!

4.1.2 Define gene, allele and genome. (1)Define means to give the precise meaning of a word, phrase or physical quantity.

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-Loci/locus: the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map. Gene mapping is the process of determining the locus for a particular biological trait.

-A gene is a unit of heredity in a living organism. It normally resides on a stretch of DNA that codes for a type of protein or for an RNA chain that has a function in the organism.-The genome is the entirety of an organism's hereditary information

 

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.

4.1.3 Define gene mutation. (1)Define means to give the precise meaning of a word, phrase or physical quantity.

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.

The entire reading frame is altered downstream of a deletion/insertion, which is disastrous for the cell.

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.

Mutation 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.

Look around you and observe the diversity of life. At any level, you can appreciate that the cause of this variation is mutation.

A mutation story:

http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html

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.

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

Sickle cell anemia is a genetic disease.

Frequency ia about 1 in 655 African Americans

The disease is inherited,not contracted, by infectious routes.

•Homozygous individuals are seriously afflicted with disorder•Heterozygous individuals are mildly affected BUT are more resistant to malaria• Epidemiology studies reveal that in malarial free zones, individuals with the allele are less likely to contribute to gene pool•In malarial zones individuals with the allele are more likely to survive and contribute to the gene pool• Environment “naturally selects” the best adapted individuals; favours particular alleles

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 (anemia) the sickle cells also cause 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 death.

The Genetics of Sickle Cell

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 transcribes 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 (level of folding) 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.

FYI…