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Text Box: MOLECULAR GENETICS OF EUKARYOTES   Organization of Chromosomes -         Chromatin is wrapped around proteins called histones. -         There are 5 types of histones. -         Histones prevent DNA from tangling.  -         We do not yet understand how DNA wraps itself around histones   Gene Expression in Eukaryotes In prokaryotes (bacteria) gene expression depends on the environment.  The cell turns genes on or off depending on levels of nutrients, toxins, etc, in the environment.  Eukaryotes are usually much more complex so gene expression depends on the environment and varying demands as the organism grows and develops.  Different genes are turned on or off depending on the stage of development. For example, a human fetus produces fetal hemoglobin while in the uterus.  This is required to obtain oxygen from the mother’s blood stream, since it has a higher affinity for oxygen that adult hemoglobin.  After birth the child begins producing alpha and beta chains of adult hemoglobin.  Humans must be able to turn these genes on or off at the right times.  Regulating Gene Expression   1) Condensation of the Chromosome and Gene Expression             Heterochromatin is  dark-staining and tightly packed.  It remains condensed during interphase and thus is not transcribed and so does not code for making protein. It probably helps move chromosomes around during mitosis and meiosis.             Euchromatin is light-staining loosely and packed (during interphase). This allows for mRNA to transcribe particular genes.             As cells differentiate from zygote to adult, the amount of euchromatin decreases as the cell becomes more specialized and needs less genes.  In cells that do not make a particular protein the gene for that protein would appear as heterochromatin.  For example, the gene to make the hormone thyroxin only appears as euchromatin in the thyroid gland cells that are in interphase.    2) Methylation and Gene Expression             Adding a methyl group (-CH3) to any cytosine nucleotide inhibits gene expression.  Methyl groups are added by special enzymes.    3) Regulation By Specific Binding Proteins             In eukaryotes transcription (DNA à  mRNA) is controlled by many different regulatory proteins that turn genes on or off.  These proteins are difficult to discover since they often bind to places on the chromosome that are quite far away from the actual gene they control.    Four Features of the Eukaryotic Genome   1) The amount of DNA varies enormously between species.  Humans have 25 x as much DNA as fruit flies, yet salamanders have 20 x as much DNA as humans.   2) Eukaryotes contain much more DNA than is used.  Prokaryotes and viruses express virtually all of their DNA.  Humans only express 1% of their DNA.    3) About 1/2 of eukaryotic DNA consists of nucleotide sequences that are repeated hundreds or even millions of times.  This defies the laws of Mendellian genetics that suggest that an organism should only have 2 copies of each gene.   4) Within a gene that codes for a protein there may be sections of DNA that do not code for anything.  The noncoding sequences or intervening sequences are called introns.  The part of the gene that is expressed (codes for protein) are called exons.      Often the noncoding introns are still transcribed by mRNA but are cut out before translation.  Introns may promote crossing over within a gene since the section of DNA is much longer.  Many DNA sequences are repeated over and over.  For example, half of the DNA in one type of crab is made up of ATATAT… repeated over and over.  Between 20 and 30% of human DNA is made of repeats.