Chromosome Mutations

Transcription

1 Chromosome Mutations

2 Variation in Chromosome Number Euploidy: having full sets of chromosomes Haploid Diploid Triploid Aneuploidy: having anything other than full sets of chromosomes Monosomy Trisomy

3 Variation in Chromosome Number Polyploidy: having more than two full sets of chromosomes Triploid (3n) Tetraploid (4n)

4 Gross Chromosome Anomalies

5 CHROMOSOME ABERRATIONS Normal human 2n = distinct pairs of homologous chromosomes Sex chromosomes are heteromorphic What happens when things aren t normal? Missing or extra chromosomes Missing or extra parts of chromosomes Rearrangement of segments of chromosomes

6 Variations in Chromosome Aneuploidy Number One or more individual chromosomes added or missing Polyploidy (Autopolyploidy) Multiple complete sets of chromosomes from same species Allopolyploidy Multiples of different genomes Note Table 6.1

7 Nondisjunction Reason for most aneuploidy Failure of chromosomes to separate during meiosis Primary nondisjunction Meiosis I Homologs fail to separate Secondary nondisjunction Meiosis II Chromatids fail to separate

8 Nondisjunction Fig. 6-1

9 Aneuploidy Only human conditions that typically survive are... 2n +/- 1 Examples: Klinefelter syndrome (47, XXY) Turner syndrome (45, X) Down syndrome (47, 21+) Patau syndrome (47, 13+) Edwards syndrome (47, 18+)

10 Trisomy 21 (47, 21+) Down Syndrome Fig. 6-3

11 Down Syndrome Trisomy 21 (47,21+) Mental retardation Similar physical characteristics Very affectionate Nondisjunction of maternal gametes Incidence increases with age

12 Down Syndrome Fig. 6-4

13 Spontaneous Abortion Aneuploidy is associated with reduced viability: 30% of spontaneously aborted fetuses have some kind of chromosome abnormality 90% of fetuses with chromosome abnormalities are spontaneously aborted 15-20% of all pregnancies are spontaneously aborted

14 Polyploidy (Autopolyploidy) Multiple complete sets of chromosomes Causes Failure of all chromosomes to segregate during meiosis (diploid gamete) Double fertilization Plants Most common Often results in desirable qualities I.e., larger size, larger fruit, more vigorous, seedless Maintained in plants that can be propagated asexually

15 Polyploidy (Allopolyploidy) Hybridization of two closely related species May be sustainable in nature if... Chromosome sets are non-homologous Form balanced gametes Ex., American cotton Fig. 6-8

16 Changes in Chromosome Structure

17 Chromosome Breakage Chromosomes may break and reattach Mistakes are often made during reattachment Spontaneous or induced Chemicals Radiation

18 Aberrations in Chromosome Structure Deletions Part of the chromosome is lost Fig. 6-11

19 Deletions Aberrations in Chromosome Duplications Structure A segment of a chromosome is duplicated within the genome Fig. 6-11

20 Aberrations in Chromosome Structure Deletions Duplications Inversions Part of a chromosome gets turned around Fig. 6-11

21 Aberrations in Chromosome Structure Deletions Duplications Inversions Translocations A segment of one chromosome gets moved to another chromosome Fig. 6-11

22 Deletions Terminal deletions The end of a chromosome is lost Ex., Cri-du-chat syndrome Partial monosomy Part of chromosome 5 lost (46,5p-) Symptoms Mental retardation Internal anatomic malformations Malformed glottis & larynx

23 Cri-du-chat Syndrome 46, 5p- Fig. 6-11

24 Deletions Intercalary deletions More central portion of a chromosome is lost Requires formation of compensation loop during synapsis Results in homozygous loss of chromosome segment Fig. 6-10

25 Duplications Some are a normal part of the genome Gene redundancy Ex., rrna E. coli: 0.7% of genome = rdna

26 Duplications Some are due to unequal crossover events Results in duplication and a deletion Fig. 6-12

27 Duplications Ex., Bar-eye Drosophila Duplication of region of X chromosome causes reduction in compound eye facets Fig. 7-13

28 Duplications May be a mechanism for evolution of new genes Duplication of genes allows original to maintain its function, while copy can mutate to form a new gene

29 Inversions Rearrangement of genetic information Fig. 6-14

30 Inversions Potential problems during meiosis Homologs cannot synapse normally One has to form an inversion loop

31 Inversions Potential problems during meiosis Homologs cannot synapse normally One has to form an inversion loop Crossover within the loop can result in abnormal chromosomes

32 Inversions Paracentric inversion Centromere not part of inversion loop Results in Normal chromosome Dicentric chromosome with duplication & deletion Inversion Acentric chromosome with duplication & deletion See Fig. 6-15

33 Pericentric inversion Centromere is part of inversion loop Results in Normal chromosome 2 chromosomes with duplication & deletion Chromosome with inversion Inversions See Fig. 6-15

34 Translocations Nonreciprocal translocation Part of one chromosome breaks off and attaches to another chromosome Reciprocal translocation Exchange of genetic material between two nonhomologous chromosomes

35 Reciprocal Translocation Causes unusual homolog pairing during synapsis Cross-like pattern Fig. 6-16

36 Reciprocal Translocation Orientation of homologs can result in unbalanced gametes Fig. 6-16

37 Robertsonian Translocation Break on p arm of two non-homologous acrocentric chromosomes Small segments are lost Large segments fuse together

38 Familial Down Syndrome Heritable form of Down syndrome Fig. 7-17

39 Fragile Sites Regions of chromosomes susceptible to breakage May be due to regions of loosely coiled chromatin Linked to types of mental retardation and cancer Ex., Fragile X syndrome (Martin-Bell Syndrome) Most common form of inherited mental retardation Fig. 7-18

40 Fragile X Due to trinucleotide repeats (CGG) in gene FMR-1 Normally 6-50 repeats Syndrome expressed with >230 repeats Repeats may result in the inactivation of the gene FMR-1 Produces RNA binding protein involved with transport of mrna s Prominent in developing brain cells