REVISION GENETICS Topic 4 2 IB Biology Miss

REVISION: GENETICS Topic 4. 2 IB Biology Miss Werba

4. 1. 1 COMPARING PROKARYOTIC & EUKARYOTIC CHROMOSOMES FEATURE PROKARYOTIC EUKARYOTIC Shape Circular Contains DNA Location Cytoplasm Linear DNA & proteins Nucleus J WERBA – IB BIOLOGY 2

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 J WERBA – IB BIOLOGY 3

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) J WERBA – IB BIOLOGY 4

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 J WERBA – IB BIOLOGY 5

4. 1. 3 GENE MUTATIONS Frameshift mutations are more serious. They result in changes to all of the codons (triplets of bases on the m. RNA) following this mutation. As the codons change, the amino acids that are coded for will be different. The resulting protein will also be affected. J WERBA – IB BIOLOGY 6

4. 1. 4 SICKLE CELL ANAEMIA Gene mutation affects the beta (β) chain of the haemoglobin protein Base substitution mutation in 6 th amino acid in the beta chain Changes the m. RNA codon from GAG to GUG Amino acid changes from glutamic acid to valine The resulting polypeptide is different: ‘normal’ haemoglobin = Hb. A ‘mutant’ haemoglobin = Hb. S J WERBA – IB BIOLOGY 7

4. 1. 4 SICKLE CELL ANAEMIA Hb. S crystallises at low oxygen levels (eg. in the capillaries). Causes the blood cells carrying Hb. S 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. J WERBA – IB BIOLOGY 8

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 Hb. S, meaning that SC carriers have a reduced chance of contracting malaria. J WERBA – IB BIOLOGY 9