Chromosome Number PDF

Chromosome Number Chromosome Number Polyploidy - Addition of one or more complete sets of chromosomes. - Whole genomes are duplicated in polyploid organisms. Aneuploidy - Alteration in copy number of individual chromosomes. - Individual chromosomes are duplicated or lost in aneuploid organisms. B...

Chromosome Number Chromosome Number Polyploidy - Addition of one or more complete sets of chromosomes. - Whole genomes are duplicated in polyploid organisms. Aneuploidy - Alteration in copy number of individual chromosomes. - Individual chromosomes are duplicated or lost in aneuploid organisms. Basic Set Basic set (x): minimum number of non-homologous chromosomes carrying a complete set of genetic information. - Human basic set: x = 23. - One basic set contains every gene required to make a human. Humans are diploid; our cells have two basic sets. - Chromosomes occur in homologous pairs in somatic cells (2n = 2x). - Meiosis reduces this to one basic set in gametes (n = x). Ploidy Levels Polyploid organisms have more than Ploidy level dictates homologous two basic sets. chromosome copies. – 3x = triploid - Triploidy = three of each homolog. – 4x = tetraploid - Tetraploidy = four of each. – 5x = pentaploid - Pentaploidy = five of each. – 6x = hexaploid - etc. – et cetera Total chromosome number (2n) = (# basic sets)*(# chromosomes in basic set) - Meiosis reduces ploidy level by half in gametes. - Meiosis reduces total number by half to the haploid number (n). Ploidy Levels Wild strawberries are usually diploid. (x = 7, 2n = 14, n = 7) - 13 diploid species. - 5 tetraploids. - 1 hexaploid. Wild strawberry - 4 octoploids. - 1 decaploid. “Garden” strawberries are usually octoploid. (x = 7, 2n = 56, n = 28) Garden strawberry Auto- vs. Allo- polyploidy Autopolyploid - Multiple copies of the same chromosome set. - All sets come from the same species, whether different individuals or the same individual. Allopolyploid - Chromosome sets from two or more species. - Two species must create hybrid offspring for an allopolyploid to be created. Auto-polyploidy Common in plants, especially autotetraploidy (4x basic sets), due to mitosis errors in somatic tissue. If sister chromatids in a somatic cell fail to separate during anaphase, then a lineage of tetraploid daughter cells will arise. Nondisjunction Total nondisjunction means all chromosomes fail to segregate. Three common causes: - Cohesin binding sister chromatids together fails to adequately degrade. - Condensin fails to condense chromosomes during prophase. - Topoisomerase fails to adequately unwind supercoiled DNA during replication. Can occur in mitosis or meiosis to some or all chromosomes. Autotetraploidy Tetraploid somatic tissue Nondisjunction in seed embryos may cause autotetraploid individuals. Diploid ovules Nondisjunction later in development can cause localized autotetraploid Diploid pollen segments. Plants are often self-fertilizing hermaphrodites. Consequences 1 Increased ploidy level leads to larger, fleshier fruit. True for kiwi species and other common fruits. Fuzzy hexaploid kiwi fruit (x = 29, 2n = 174). Hexaploid kiwi & diploid kiwi fruits. Versus wild diploid kiwi fruits (x = 29, 2n = 58). Consequences 2 Adding basic sets increases nuclear space to contain DNA. Requires larger cytoplasmic cell volume to compensate. Polyploid plant tissues are larger than diploid counterparts. Without increasing cell numbers. (A – D) diploid kiwi, (E – G) tetraploid kiwi, (H – M) hexaploid kiwi. Polyploidy Consequences Polyploidy has little effect on individual viability. - Larger cells. - Larger tissues. - Few downsides. Polyploidy may have negative effects on fertility. - Even-numbered polyploids (2n = 4x, 6x, 8x, etc.) experience normal reproduction. - Odd-numbered polyploids (2n = 3x, 5x, 7x, etc.) are invariably sterile. Autotriploid Food Cavendish is the most common variety of banana. - Gametes are non-viable because of parent autotriploidy. - When fertilized, seeds are spontaneously aborted. Cavendish banana Wild banana (x = 11, 2n = 33) (x= 11, 2n = 22) Autotriploid Food Autotriploid seedless Wild-type seeded watermelon watermelon (x = 11, 2n = 22) (x = 11, 2n = 33) Autotriploidy When nondisjunction occurs during meiosis, only a tiny number of gametes are diploid. Fertilization by normal monoploid gametes is likely. Offspring are autotriploid (2x + x = 3x). Autotriploidy Occurs when diploid gametes (2x) are fertilized by monoploid (x) gametes. - Tetraploid parent + diploid parent = triploid offspring (3x). - Likely outcome when non-disjunction events happen in anaphase I of meiosis. Common in plants. - Autotriploid plants are viable, but sterile. - Offspring of autotriploid plants are non-existent. - Autotriploid plants can be “rescued” from sterility. Meiosis I Revisited Homologous chromosomes are separated by reductional division in meiosis I. Homologs chromosomes must occur in pairs for this to work. Extra homologs (singletons) segregate randomly. Polyploid Problems With odd-numbered polyploids, homologous chromosomes do not occur in pairs, so problems will always occur: - Meiosis gets arrested at the spindle (SAC) checkpoint because meiotic spindles fail. - Or gametes are unbalanced and inconsistent (most likely). Every gamete has a random set of parental chromosomes with incomplete basic sets. Autotriploid Meiosis Here, 3 homologs divide into A1/A2 & A3… Homologs aligned in metaphase I or A2/A3 & A1 daughter cells. There are a number of other possible outcomes. Polyploid Problems Unpaired homologous chromosomes do not segregate into balanced gametes during anaphase I of meiosis. - Unpaired chromosomes segregate randomly. - Any two randomly-selected gametes will almost certainly differ in homolog copy numbers. - The odds of finding a matching reciprocally unbalanced gamete are very small. Offspring off odd-numbered polyploids are non-viable because of gene dosage problems. Gene Dosage Normal embryonic development involves 1000s of genes working in concert. Gene “products” are balanced in meticulously evolved cellular proportions. For every gene, amount of product is proportional to its copy number. Gene Dosage Increasing copies of a gene with respect to the rest of the genome alters the balance. Altering gene dosage throws off the stoichiometry of gene products in a cell. Development is altered in some way, sometimes fatally. Allopolyploidy Chromosome sets from two (or more) different species. - Starts with hybridization. - Normal monoploid gametes from two species fuse and make diploid offspring. - Offspring are viable but, again, sterile. - Later, a mitotic error can “rescue” the infertile hybrid with polyploidy. Rescue Hybrid offspring sterile because chromosomes do not have paired homologs. Mitotic error (total nondisjunction) during development can double every chromosome. Balanced gametes can arise from tetraploid tissue. Allopolyploids Chromosomes from two different species (A & B) can’t pair and undergo normal meiosis. Gametes are produced at random unless rescued by chromosome doubling. Bread Wheat Modern bread wheat is an allohexaploid created via the duplication of three separate genomes. + = + 6x = 42 Aneuploidy Differences in the copy number of individual chromosomes. - Euploid cells contain all chromosomes in their basic set, and a number of sets determined by ploidy level. - Aneuploid cells are not euploid. Total chromosomes are is not a multiple of normal haploid number (n). Assuming a cell is normally diploid. – 2n – 1 = monosomic aneuploid. – 2n + 1 = trisomic aneuploid. – 2n – 2 = nullisomic aneuploid. Animal Aneuploidy Animals are highly sensitive to aneuploidy and the effects of gene dosage. In fruit flies, only monosomics and trisomics of chromosome 4 are viable. Chromosome 4 is tiny. Chr 4 gene dosage imbalances are D. melanogaster have four tolerable. chromosomes. Causes of Aneuploidy If nondisjunction occurs during meiosis I reductional division, gametes will be ½ n+1 and ½ n-1. Causes of Aneuploidy If nondisjunction occurs during meiosis II mitotic division, Gametes will be ½ normal (n), ¼ n+1, and ¼ n-1. Human Aneuploidy Aneuploidy happens often. Most human aneuploid gamete types do not produce viable offspring. - ~ 25% of all conceptions are spontaneously aborted. ending in miscarriage, often without the mother knowing. - > 50% are caused by aneuploid chromosomal aberrations. Surviving aneuploid offspring are restricted to cases involving the smallest autosomes and sex chromosomes. Human Aneuploidy Autosomal monosomy is 100% fatal in humans. Most autosomal trisomics are likewise lethal and not seen. Surviving trisomics involve the three smallest chromosomes, correlating with number of genes involved and relative severity of altered gene dosage. Aneuploid # Protein Coding Condition Life Expectancy Chromosome Genes Down Syndrome Trisomy 21 ~ 230 60+ years Patau Syndrome Trisomy 13 ~ 321 130 days Edwards Syndrome Trisomy 18 ~ 269 several weeks Maternal Age Aneuploidy strongly correlates with maternal age. Humans born with oocytes suspended in prophase I. Meiosis concludes just before eggs are released. More likely to fail after decades of suspension, leading to nondisjunction. Human Aneuploidy Sex chromosome aneuploidy is more common. - Y-chromosome is tiny (~63 coding sequences), so extra copies don’t severely affect gene dosage. - X-chromosomes have a lot of genes (~846), but gene dosage is balanced naturally. X-inactivation In animals with XY sex determination, X- inactivation maintains gene dosage. One X chromosome is silenced at random in every cell of females. When X is trisomic, all but one are mostly inactivated. Example: tortoiseshell cat coat coloration. Human Aneuploidy Sex chromosome aneuploids more common. May cause altered phenotypes and reduced fertility. 1. X0 = Turner syndrome (monosomic) 1 in 2500 births. Infertility, short physical stature, and high risk of congenital defects. 2. XXY = Klinefelter syndrome (trisomic) 1 in 1000 births. Physically male, feminized features, and reduced fertility. 3. XXX = Triple X syndrome (trisomic) 1 in 1000 births. Normal fertility. Majorly undiagnosed. 4. XYY = XYY syndrome (trisomic). 1 in 1000 births. Normal fertility. Occasional learning disabilities. Majorly undiagnosed. Chromosome Number 1. Polyploidy versus aneuploidy (versus euploidy). 2. Allo- versus auto- polyploidy. 3. Polyploid problems: unbalanced gametes and gene dosage. 4. Polyploid food. 5. Allopolyploid rescue & bread wheat. 6. Causes of aneuploidy & maternal age. 7. Viable human aneuploids.