Mistakes in Meiosis

What are Chromosomes?

What are chromosomes? Human cell = >6 billion nucleotide base pairs (~2 meters) Wrapped around protein = chromatin DNA/protein = chromosome E. Coli bacteria have approx 5 million base pairs. Human cells have approximately 1000X as much DNA. http://www.ornl.gov/sci/techresources/Human_Genome/faq/compgen.shtml

How many chromosomes do humans have? Humans are diploid (2n) Two of each chromosome, one from each parent. n = 23 unique chromosomes (haploid #) 2(n) = 46 total chromosomes Curly hair allele Strait hair allele The two copies of each chromosome in human cells are homologous Different versions - same genes in same locations but different DNA sequence. Different versions (alleles) of a gene may promote different traits (e.g. hair type).

How do cells get the right number of chromosomes? DNA How do cells get the right number of chromosomes? DNA Cell Division Duplicate cell components Organelles Cytoplasm Chromosomes DNA How does a cell do this? Won’t worry about organelles, focus on DNA DNA 2. Separate the material into two daughter cells

Correct Cycle Diploid Cells Haploid Cells Mitosis Cells divide 1x Meiosis Cells divide 2x 2n 2n 2n 2n n n Diploid Cells n n n n Haploid Cells

Nondisjunction Nondisjunction The failure of homologous chromosomes to separate properly during meiosis. What should happen Nondisjunction

The results If the abnormal gametes is fertilized the results

Polyploidy the condition where cells have multiple sets of chromosomes usually 3-4 sets Most often found in plants, rare in animals Polyploidy is the condition where cells have more than one set of chromosomes, as supposed to haploidy. In most organisms which reproduce by sexual reproduction the usual state of cells is diploidy. Some plants (especially flowering plants) exibit triploidy and tetraploidy, or even octoploidy e.g. in strawberry plants and sugar cane. Plants are more likely than animals to exhibit polyploidy because their gametes are produced from somatic cells rather than germ-line cells, and therefore go through more cell cycles, but polyploidy can also occur in some animals. Polyploidy has been observed in insects, fish, amphibians, reptiles, and once in mammals (a rat), though it was once thought polyploidy would be fatal to mammals. Polyploidy is a route by which plants could increase their chromosome numbers, and the size of their genome. Over time changes in the duplicated chromosomes are liable to build up until they can be distinguished as different chromosomes, and the organism can once again be described as diploid ('diploidisation', during which the haploid chromosome number of the organism becomes difficult to define). Polyploidy will duplicate the genes on each chromosome, and during the subsequent changes many copies will be 'turned off', increading pseudogenes and the proportion of 'Junk DNA' in the genome. Polyploid plants are often larger and have other accentuated charactoristics, making them useful in agriculture and horticulture. Polyploidy can be artificially induced in plants by botanists and horticulturalists, e.g. with the chemical colchicine, which provents chromosomes from separating during mitosis.

Examples of Polyploidy Diploid & tetraploid many plants Octoploidy strawberries Triploid seedless watermelon Seedless watermelon is the result of a cross between 4n and 2n – 3n offspring is sterile

Trisomy The zygote has an extra chromosome Organisms with an extra chromosome sometimes survive

Examples of trisomy in humans Trisomy 21: Downs Syndrome Trisomy 18: Edwards syndrome Trisomy 13: Patau syndrome XXY: Klinefelter's syndrome Trisomy 1: rare Trisomy 2: associated with loss of pregnancy Trisomy 3: lethal Trisomy 4: lethal Trisomy 5: lethal Trisomy 6: rare, no heath concerns in know cases (six total cases) Trisomy 7: never reported in live birth childern Trisomy 8: Trisomy 9: Trisomy 10: lethal Trisomy 11: Trisomy 12:majority have normal outcomes Trisomy 13: Patau syndrome 0.03 Trisomy 14: Trisomy 15: lethal Trisomy 16: lethal Trisomy 17: not reported Trisomy 18: Edwards syndrome .2% Trisomy 19: Trisomy 20: not viable Trisomy 21: Downs Syndrome Trisomy 22: Trisomy 23: Trisomy x: XXY: normal development, slightly taller, decreased sperm quality Trisomy X: super female syndrome

Trisomy 1: rare Trisomy 2: associated with loss of pregnancy Trisomy 3: lethal Trisomy 4: lethal Trisomy 5: lethal Trisomy 6: rare, no heath concerns in know cases (six total cases) Trisomy 7: never reported in live birth childern Trisomy 8: Trisomy 9: Trisomy 10: lethal Trisomy 11: Trisomy 12:majority have normal outcomes Trisomy 13: Patau syndrome 0.03 Trisomy 14: Trisomy 15: lethal Trisomy 16: lethal Trisomy 17: not reported Trisomy 18: Edwards syndrome .2% Trisomy 19: Trisomy 20: not viable Trisomy 21: Downs Syndrome Trisomy 22: Trisomy 23: Trisomy x: XXY: normal development, slightly taller, decreased sperm quality Trisomy X: super female syndrome

Monosomy The zygote is lacking a chromosome Organisms lacking one or more chromosomes rarely survive

Examples of monosomy in humans Monosomy X: Turners Syndrome

Human Karyotype