Topic 4.1: Chromosomes, genes, alleles, and mutations

1. Topic 4.1: Chromosomes, genes, alleles, and mutations

2. Assessment Statements: • 4.1.1: State that eukaryotic chromosomes are made of DNA and proteins • 4.1.2: Define gene, allele and genome • 4.1.3: Define gene mutations • 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 anaemia

3. How is DNA organized • Chromosomes are bundles of long strands of DNA • If you could unwind a chromosome, it would be like unraveling a ball of string • In eukaryotes that reproduce sexually, chromosomes always come in pairs • Humans have 46 chromosomes in 23 pairs • The DNA un eukaryotes is associated with proteins which helps to keep the DNA organized

4. Gene • A gene is a heritable factor that controls a specific characteristic • Heritable means passed on from parent to offspring and characteristic refers to genetic traits such as your hair color or your blood type • The estimated 30,000 genes which you possess are organized into chromosomes

5. Gene • The genes which determine eye color have more than one form • Variations (like in eye color) of a gene are called alleles • An allele is one specific form of a gene, differing from other alleles by one or a few bases • Alleles of the same gene occupy a corresponding place (locus) on each chromosome of a pair

6. Gene • In order to find out which gene does what, a list must be made showing the order of all the letters in the DNA code (sequence of bases) • A complete set of an organism’s base sequence is called its genome • The complete genome of a few organisms have been fully written out.

7. Mutations • A mutation is a random, rare change in genetic materials • One type involves a change of the sequence of bases of DNA • If DNA replication works correctly, this should not happen

8. Mutations • Base substitution mutation • The consequence of changing one base could means that a different amino acid is placed in the growing polypeptide chain • This may have little or no effect on the organisms or it may have a major influence on the organisms physical characteristics • http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter15/mutation_by_base_substitution.html

9. Mutations • In fruit flies • One base substitution mutation can cause: • Extra pair of wings • Change in eye color • The number of legs • Shape of the wings

10. Mutations • In humans: • one base substitution mutation can cause: • Mutations is sometimes found in the gene which creates hemoglobin for red blood cells • This mutation gives a different shape to the hemoglobin molecule • The mutated red blood cell with the characteristic curved shape made its discoverers think of a sickle (a curved knife used to cut tall plants) • Called sickle cell anaemia

11. Mutations • Sickle cell anemia • One base is substituted for another so that the codon GAG becomes GTG • During translation instead of adding glutamic acid it added valine instead • This results in a different shape of the polypeptide—hemoglobin molecule has a different shape which causes the red blood cell have a different shape

12. Mutations • Symptoms of sickle cell anemia • Weakness, fatigue, and shortness of breath • Oxygen cannot be carried as efficiently by the irregularly-shaped red blood cells • Hemoglobin tends to crystallize within the red blood cells, causing them to be less flexible • The affected red blood cells can get stuck in capillaries so blood flow can be slowed or blocked

13. Mutations • People affected by sickle cell anima have a risk of passing the mutated gene to their offspring • Mutated gene is mostly found in populations originating from West Africa or from the Mediterranean