1. REVISION:
GENETICS
Topic 4.2
IB Biology
Miss Werba

2. COMPARING PROKARYOTIC & EUKARYOTIC CHROMOSOMES
4.1.1
COMPARING PROKARYOTIC & EUKARYOTIC CHROMOSOMES
FEATURE
PROKARYOTIC
EUKARYOTIC
Shape
Circular
Linear
Contains
DNA
DNA & proteins
Location
Cytoplasm
Nucleus
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3. DEFINITIONS Gene: Segment of DNA that controls a characteristic
4.1.2
DEFINITIONS
Gene: Segment of DNA that controls a characteristic
Genome: Collective name given to all the genes of an organism
Gene pool: Collective name given to all the genes carried by the members of a population
Locus: Position of a gene on the chromosome
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4. DEFINITIONS Allele: Name given to the different versions of genes
4.1.2
DEFINITIONS
Allele: Name given to the different versions of genes
Alleles only differ by a few bases
Alleles of a gene always occupy the same locus
eg. alleles controlling blood type
controlled by 3 alleles
A (ΙA), B (ΙB) and O (i)
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5. GENE MUTATIONS Gene mutations can be: Base substitutions:
4.1.3
GENE MUTATIONS
Gene mutations can be:
Base substitutions:
one base is changed for another
reading frame remains the same
Frameshift mutations: reading frame changes
Insertions
insertion of one or more bases
Deletions
deletion of one or more bases
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6. GENE MUTATIONS Frameshift mutations are more serious.
4.1.3
GENE MUTATIONS
Frameshift mutations are more serious.
They result in changes to all of the codons (triplets of bases on the mRNA) following this mutation.
As the codons change, the amino acids that are coded for will be different.
The resulting protein will also be affected.
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7. 4.1.4
SICKLE CELL ANAEMIA
Gene mutation affects the beta (β) chain of the haemoglobin protein
Base substitution mutation in 6th amino acid in the beta chain
Changes the mRNA codon from GAG to GUG
Amino acid changes from glutamic acid to valine
The resulting polypeptide is different:
‘normal’ haemoglobin = HbA
‘mutant’ haemoglobin = HbS
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8. 4.1.4
SICKLE CELL ANAEMIA
HbS crystallises at low oxygen levels (eg. in the capillaries).
Causes the blood cells carrying HbS to change shape to a sickle shape.
The shape does not change back once the oxygen levels increase.
The sickle-cell blood cells can block the capillaries and are less efficient at transporting oxygen.
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9. 4.1.4
SICKLE CELL ANAEMIA
Can result in acute anaemia, heart damage, kidney damage, or even death in homozygotes
The sickle-cell trait is more common among people from West-Africa and in the African-American population
Carrier-testing is advised for people in predisopsed families and amniocentesis can be performed to check for disorder in utero.
Correlation between malaria and SC anaemia – disease causing plasmodium cannot reproduce in RBCs with HbS, meaning that SC carriers have a reduced chance of contracting malaria.
J WERBA – IB BIOLOGY 9