Sickle-cell anemia is one of the most common genetic diseases in the world. This occurs because of a mutation of the gene that codes for hemoglobin (Hb). Most humans have the allele HbA. At some time in the past, a base substitution mutation coverts the sixth codon of the gene from GAG to GTG. This happened in the ovary or testic cell and my have occured in more than one person. The allele with the mutation is called HbS and when this alle is transcribed, the mRNA produced has GUG instead of GAG as its sixth codon which now produced the amino acid valine instead of glutamic acid. This change causes hemoglobin to stick together in conditions with low oxygen conditions. This distorts the red blood cells into sickle shapes and these cells damage tissues by getting trapped in blood capillaries blocking them and reducing their blood flow. When sickle cells return to high oxygen conditions in the lung the hemoglobin chains break up and cells return to their normal shape. These changes occur time after time. Both the hemoglobin and plasma membrane are damage so the life of a red blood cell is shortened and the body cannot replace the red blood fast enough so anemia develops.
Chromosomes, genes and genomes
Many scientists noticed a group of genes located on the X chromosome of Drosophila. By crossing experiments carefully, they were able to show that the genes were arranged in a linear sequence along the X chromosome. Groups of genes were then assigned to other chromosomes in Drosophila and were again arranged in this way. This pattern has been found in many species and is called the locus of a gene. Locus is the particular position on homologous chromosomes of a gene. If two chromosomes have the same sequence of the genes, they are homologous. xHomologous chromosomes are chromosomes that have the same genes as each other and arranged in the same sequence but do not necessarily have the same alleles of those genes. They are not usually identical to each other because for some of the genes on them, the alleles are different. Since Morgan, it has been discovered tahat all genes on a chromosome are part of one DNA sequence. Now,the sequence of bases along a chromosome can be discovered and this is called a genome. The genome is the whole of the genetic information of an organism.
During mitosis and meiosis, chromosomes of eukaryotes can be seen using a microscope because they are quite large. This is because they are condensed so they have become shorter and fatter by coiling. Coiling involves histone proteins that are associated with DNA in eukaryotes and these proteins also help the process of controlling gene transcription.
Data-based question: differences in chromosome number
1. No chromosomes have 13 because all chromosomes come in pairs homologous pairs. Therefore species end up with an even number of chromosomes and no odd numbers. If they end up with an odd number, they would probably die.
2. This hypothesis can be both supported and rejected in some ways. For example, one can see that the threadworm is not very complex and therefore has 4 chromosomes, whereas a dog, which is more complex has 78 chromosomes. However, this does not take into consideration plants, so the hypothesis is not always true.
3. Some chromosomes may be logner than others or can be fused together therefore the genome can’t be determined from the amount of chromosomes.
4. Chimpanzees have one homologous pair more than humans. Two pair of chromosomes may have fused together and this caused to have 46 chromosomes instead of 48 chromosomes.