IB SL BIOLOGY TOPIC 3 GENETICS COMPLETE STUDY GUIDE 2023.

IB BIOLOGY TOPIC 3 GENETICS COMPLETE STUDY GUIDE 2023. HKEXCEL LTD. 1 Topic 3.1 – Genes Genetics x It is the study of variation and inheritance. The basic unit of inheritance is a gene. Gene x A gene is defined as a heritable factor that controls a specific characteristic for an organism, or a section of DNA that codes for the formation of a polypeptide. x A gene occupies a specific position on a chromosome. x A typical animal or plant cell nucleus contains thousands of genes. The current estimate of the number of genes of the following organisms are as follows: ƒ E.coli (bacteria): 3200 ƒ Homo sapiens (Humans): 23000 ƒ Oryza sativa (Rice): 41000 Allele x It is a specific form of a gene, which occupies the same gene locus or position as other alleles of that gene, but differs from other alleles by small differences in its base sequence. Genome x The genome is the whole of the genetic information of an organism. lOMoARcPSD| HKEXCEL LTD. Gene mutation x Gene mutation is defined as a change in the sequence of bases in a gene. x The process of DNA replication is rather complex, and mistakes do sometimes occur, in which a nucleotide may be left out, or an extra one may be added, or even the wrong one inserted. x These errors are termed as gene mutations. x The insertion of an incorrect nucleotide in the DNA strand is called a base substitution mutation. x When the DNA containing an incorrect nucleotide is transcribed and translated, errors may occur in the polypeptide chain that is produced. Sickle-cell anemia x Sickle-cell anemia is a genetic disease which affects the red blood cells present in the body. x It occurs due to a mutation on the Hb gene, which codes for a polypeptide chain of 146 amino acids, which forms part of hemoglobin (hemoglobin is a vital protein required for transport of oxygen by blood to all bodily cells). x In the case of sickle cell anemia, base substitution mutation occurs, in which the codon GAG found in the normal Hb gene gets mutated to GTG. x This means, the DNA segment of Hb gene which should contain adenine (A), gets replaced by thymine (T). x When the mutated gene is transcribed, a codon on the messenger RNA (mRNA) will be different. x Instead of the normal codon GAG, the messenger RNA (mRNA) will contain the codon GUG. x This in turn is followed by an error during the process of translation. x In a healthy individual the codon GAG on the messenger RNA matches with the anticodon CUC on the transfer RNA carrying the amino acid glutamic acid. x However, if the mutated gene is present then GUG on the messenger RNA matches with the anticodon CAC on the transfer RNA which carries the amino acid valine. x So the base substitution mutation has caused glutamic acid to be replaced by valine on the sixth position on the polypeptide. x This results in hemoglobin S being present in red blood cells instead of normal hemoglobin A. x This leads to a change on the phenotype, as instead of normal donut (rounded) shaped red blood cells being produced, some of the red blood cells will be sickle shaped. x As a result these sickle shaped red blood cells cannot carry oxygen as efficiently as normal red blood cells would. Effects x Oxygen carrying capacity of