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Summary IB Biology HL Unit 3 Notes
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The notes hereby are created as a summary of IB Biology HL Unit 2 topic — cell biology. The notes are gathered mainly from IB books as well as other supplementary sourcers.
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,, ● Nonsense mutations occur when the DNA change creates a premature stop
codon which truncates the polypeptide.
● Frameshift mutations occur when the addition or removal of a base alters the
reading frame of the gene. This change will affect every codon beyond the point of
mutation and thus may dramatically change amino acid sequence.
● In multicellular organisms, mutations can be classed as either somatic or germ-line:
● Somatic mutations – occur in a single body cell and cannot be inherited; only tissues
derived from mutated cell will be affected
● Germline mutations – occur in gametes and can be passed onto offspring; every cell
in the entire organism will be affected
3.1.4 GENE MUTATION RATES
● Mutations can be spontaneous — caused by copying errors during DNA replication — or
induced by exposure to external elements.
● Examples of factors which can induce mutations include:
● Radiation – UV radiation from the sun, gamma radiation from radioisotopes, X-rays
from medical equipment
● Chemical – reactive oxygen species found in pollutants, alkylating agents found in
cigarettes
● Biological Agents – bacteria such as Helicobacter pylori, viruses such as human
papilloma virus
, ● Agents which increase the rate of genetic mutations are called mutagens, and can lead
to the formation of genetic diseases.
3.1.5 SICKLE CELL ANEMIA
● Sickle cell anaemia is an example of a disorder caused by a gene mutation.
● This is an autosomal recessive disease.
● The diseased allele arose from a base substitution mutation – where a single base was
changed in the gene sequence.
● Sickle cell anaemia results from a change to the 6th codon for the beta chain of
haemoglobin.
1. The DNA sequence changes from GAG to GTG on the non-transcribed strand, so the
template strand changes from CTC to CAC.
2. The mRNA sequence changes from GAG to GUG at the 6th codon position.
3. The sixth amino acid for the beta chain of haemoglobin is changed from glutamic acid
to valine.
● The symptoms of sickle cell disease are weakness, fatigue, and shortness of breath.
● The amino acid change alters the structure of haemoglobin, causing it to be clumped into
insoluble fibrous strands and hence, become insoluble. The insoluble haemoglobin cannot
carry oxygen as effectively, causing the individual to feel constantly tired.
● The formation of insoluble fibrous haemoglobin strands changes the shape of the red blood
cell to a sickle shape. The sickle cells may form clots within the capillaries as
haemoglobin now tends to crystallize, blocking blood supply to vital organs and
causing myriad health issues.
● The sickle cells are also destroyed more rapidly than normal cells, leading to a low red
blood cell count — anaemia.
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