Section 16Variation in Chromosome Nunberand Structure• The karyotype can change due to rare events.• Changes are inherited.Consequently:• The karyotype can vary somewhat within a species.• The karyotype evolves.• Different species, and less often different individuals within a species,have different numbers of chromosomes and different arrangements ofgenes on the chromosomes.Indian Muntjac 2N = 6/7Chinese Muntjac 2N = 46VARIATIONS IN CHROMOSOME NUMBERVariations in chromosome number include euploidy (varying numbers ofcomplete chromosome sets) and aneuploidy (partial chromosome sets).Euploids have varying numbers of complete chromosome sets; varieties includediploid, haploid, auto- and allopolyploid, polytene. Aneuploids have partialchromosome sets.Ploidy of organism = number of chromosome setsEuploid: multiples of complete setshaploid N A B Cdiploid 2N A A B B C Cpolyploid > 2Ntriploid 3N A AA B B B C C Ctetraploid 4N A A A A B B B B C C C CAneuploids: diploid ± partial setmonosomic 2N - 1 A A B B C monosomic Ctrisomic 2N + 1A A A B B C C trisomic AAneuoploidy important because:• genetic tool• results in genetic defectsPolyploidy important because:• plant evolutionpolyploidization -> new species, especially in grasses• plant breeding polyploidization -> new useful varietyBread wheat (Triticum aestivum) is allotetraploid with diploid chromosome setsfrom 3 different parent species, each of which had 2N = 14, so total in wheat is42.Variations in chromosome number are usually caused by errors in mitosis,meiosis, or fertilization.Errors in mitosis, meiosis, or fertilization cause polyploidy. E.g.:(a) 2 sperm + 1 egg --> 3N(b) Failure of anaphase separation in mitosis in germ line --> 4N gametocyte -->2N gametes. Can be induced with agents that block spindle formation ordissolve spindle.Aneuploids result from nondisjunction.3. Changes in chromosome number often cause abnormalities in geneexpression (and hence the phenotype) and in meiosis.Gene dosage effects: aneuploidy results in unbalanced genomes and abnormaldevelopment; hence aneuploidy is usually lethal or detrimental.Embryo with one extra or one too few chromosome has unbalanced genome -->abnormal development. Genome evolved to work with two functional copies ofeach gene. If have too few or too many of some genes, those make too much ortoo little gene product, and may upset metabolic pathways; e.g. if some enzymesare too abundant, may make too much product.Aneuploidy in humans is common medical problem.Occurs in ca. 3.5% of all embryos.Aneuploidy for large chromosome in humans usually causes unbalance for oneor many important genes, usually lethal --> stillbirth. Accounts for ca. 20% of allstillbirths.Aneuploidy for small chromosomes maybe viable. E.g. Down's syndrome =trisomy 21. (Or part of 21, attached to another chromosome.)Causes of trisomy 21:≈ 75% female MI or MII nondisjunction≈ 25% male MI or MII "Female eggs arrested in prophase of MI at birth. Eggs age --> increasingfrequency of nondisjunction with increasing maternal age, especially > 45 years.Meiosis in aneuploids and autopolyploids may be abnormal because of problemswith synapsis, resulting in sterility. Don’t have time to cover in course, can omitcorresponding sections of reading.VARIATIONS IN CHROMOSOME STRUCTUREAlso called chromosomal mutations (OK when they first occur), chromosomalabnormalities or aberrations (not strictly applicable because sometimes ≥ 2different structures are present in population in high frequency; can't say thateither one is aberrant or abnormal).Changes in chromosome structure occur in both prokaryotes and eukaryotes. Wewill focus on eukaryotes.1. Transposable elements = TEs AKA “jumping genes”Tranposition:• TE moves to new location• rare event; frequency is on order of mutation rate or somewhat higher, butnot so frequent that they interfere with mapping• duplicative transposition puts a copy of the TE in the new position, leavingthe old one behind; nonduplicative transposition moves the TE.1 2 3 4 5 6 7 8 9 10 --duplicative transposition--> 1 2 3 4 5 6 7 8 9 2 10 --non-duplicative transposition--> 1 3 4 5 6 7 8 9 2 10• Duplicative transposable elements can spread in genome even if they are ofno selective advantage to the organism, and even if they are mildlydetrimental. Sometimes called selfish DNA.Transposable elements were first discovered by Barbara McClintock (firstwoman elected to National Academy of Science).in maize; Nobel Prize.Mechanisms:• Some have transposase gene -> transposase protein which cuts TE out andinserts in target sequence.• Retrotransposons transcribed to make RNA copy, then reverse transcriptasemakes DNA copy of the RNA, and DNA is integrated into target sequence.The act of transposition itself may cause rearrangements of adjacent sequences.TE inserted in gene or in controlling sequences can make gene inactive, just like amutation.If transposon excises cleanly, gene activity can be restored.The copies of a transposable element are sites of homology at which crossing-over can occur within a chromosome and between homologous ornonhomologous chromosomes --> changes in chromosome structure. Crossing-over probably in interphase.2. Repeats = repeated sequencesRepeats are segments of DNA that are present two or more times in the genomeof an organism.(1) Many short repeats arise in the course of evolution due to mutations, just bychance:(2) Short (simple sequence) tandem repeats arise by replication slippage:ATTTCG –replication--> ATTTTCGATGATGATG  ATGATGATGSome human hereditary defects are due to increases in number of tandemrepeats; e.g.Myotonic muscular dystrophySome regions of chromosomes are rich in short tandem repeats, notablycentromeres.These are the bane of DNA sequencers; this is why a large part of the humangenome still hasn’t been sequenced.(3) Longer repeated sequences arise by duplicative transposition.Polytene ChromosomesSome insects, including Drosophila:During differentiation of some tissues (salivary glands, Malpighian tubules, etc.)cells go through repeated S phases (e.g. 10) without mitosis --> polyploid orpolytene nuclei.Polytene chromosomes: all copies held tightly together and in alignment.Stain, see bands where DNA is more concentrated.PHYSICAL LANDMARKS!3. Duplication = process of duplicating a segment (usually a whole gene or genes); also usedto denote the repeated copies1 2 3 4 5 6