BIOL 4003 Week 5 Notes
BIOL 4003 Week 5 Notes 4003
U of M
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This 7 page Class Notes was uploaded by Rachel Heuer on Sunday February 21, 2016. The Class Notes belongs to 4003 at University of Minnesota taught by Robert Brooker in Spring 2016. Since its upload, it has received 12 views. For similar materials see Principles of Genetics in Biology at University of Minnesota.
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Date Created: 02/21/16
Chapter 8: Lecture 1 - Microscopic Examination o Cytogenetics: field of genetics that involves the examination of chromosomes microscopically o Cytogeneticist: typically examines the chromosomal composition of a particular cell or organism § Views the composition of chromosomes § Allows detection of individuals with abnormal chromosome number or structures § Also provides a way to distinguish between species that look morphologically similar - Humans have 46 chromosomes in somatic cells o Vary in size and number compared to other species chromosomes - Cytogeneticists use three main features to identify and classify chromosomes o Location of centromere o Size o Banding patterns - These features are all seen in a karyotype o Micrograph of all the chromosomes in a single cell arranged in a standard fashion o Procedure for making karyotype was discussed in chapter 3.2 o Larger chromosomes have a smaller number o X chromosome is much larger than Y § 22 pairs of autosomes, 1 pair of sex chromosome o Centromere location § Metacentric: Center § Submetacentric: barely off center § Acrocentric: much off center but not at end § Telocentric: all the way at the end of chromosome o Centromere location causes a long and short arm § Long arm = q § Short arm = p o Since different chromosomes can be the same size with similar centromere location, cytogeneticists will stain chromosomes to create banding patterns (G-banding à Giesma dye) § Some regions bind the dye heavily and some regions do not, producing a characteristic banding pattern o In humans roughly 300 G bands in metaphase, 800 in prometaphase (elongation à chromosomes are more tightly packed) § Can distinguish between multiple chromosomes due to banding pattern § Can detect changes in chromosome structure § Reveals evolutionary relationship between chromosomes of closely related species o 2 primary ways in which chromosome structure can be altered § Total Amount of genetic material is changed: • Deletion/deficiency (taken out) • Duplication (added in) § Total amount of genetic material stays the same but is rearranged • Inversions (flipped/reversed) • Translocations (exchanging) o Deletions (Fig 8.3) § Can happen two ways • Chromosome breaks into two pieces o Causes terminal deletion of one end of the chromosome • Chromosomes breaks in two places o Middle segment lost and degraded (interstitial deletion) o Ends of chromosomes have telomeres and will not rejoin together o Middle segment has “sticky” ends that are reactive § Phenotypic consequences of deficiencies depend on: • Size of deletion • Chromosome material deleted o i.e. are lost genes vital to function? § Consequence is normally detrimental (large deletions) o Duplications § Normally caused by abnormal crossing over event during recombination § Repetitive sequences can cause misalignment between homologous chromosomes • Such as misaligned chromosomes during crossover o Called nonallelic homologous recombination because the alleles of the homologous chromosomes are not lining up correctly and the crossover occurs o Occurs because of repetitive sequences in chromosome o This cross causes one duplicated chromosome and one deleted chromosome (and 2 normal chromosomes) § Duplications lead to the formation of gene families § Phenotypic effects of duplications tend to be correlated with size • More harm if larger deletions § Tend to be less harmful effects than deletions of comparable size § Relatively few syndromes that are caused by small duplications o Copy number variation (CNV) à relatively common § Segment of DNA that varies in copy number among members of the same species • May be missing a gene • May be a duplication • See figure 8.8 § .1-10% of genome may show CNV § Human rate is around .4% § Associated with some diseases • Schizophrenia, autism, infectious agent susceptibility, cancer § Might affect phenotype Genetic Hybridization is used to identify deletions and duplications associated with cancer (experiment 8A à comparative genomic hybridization) **may be difficult to detect with karyotype analysis REVIEW!!! Chapter 8 Lecture 2: - Inversion o A situation where a segment of chromosome is flipped in the opposite direction § Pericentric inversion: Includes centromere (I = includes centromere) § Paracentric inversion: Does not include centromere o Inversion heterozygote § Has one copy of normal chromosome and one copy of inverted chromosome § Usually are phenotypically normal • But have high probability of producing gametes abnormal in genetic content • Abnormality is due to crossing over within the inverted segment § During meiosis 1, pairs of homologous sister chromatids must synapse • For normal and inverted chromosome to synapse, inversion loop must form • If crossover occurs within the loop, highly abnormal chromosomes result § Pericentric looping creates normal chromosome that contain crossover • Loop contains centromere • Inner two chromatids crossover, outer do not • Produces two recombinant chromosomes and two normal chromosomes § Paracentric chromosome: • Loop does not contain centromere • Results in 2 normal chromosomes • Acentric fragment (no centromere) o Will be degraded and lost because no centromere • Chromosome with Dicentric bridge (2 centromeres) o Centromeres will move to outsides and break in half in the dicentric bridge, forming two chromosomes with deletions and duplications § Look at Figure 8.11 o Translocations: § Piece of chromosome becomes attached to another chromosome • Reciprocal translocations o Two non-homologous chromosomes exchange genetic material § Two mechanisms: • Chromosomal breakage/DNA repair o 2 break, leaving reactive ends, mismatch occurs § Telomeres normally prevent chromosomal DNA from sticking to each other § Breaks that don’t have telomeres at the end create reactive ends that are sticky to readhesion • Abnormal Crossover o Occurs between non homologous chromosomes due to repetitive sequences or transposable elements o Chromosomes will bind to eachother and crossover o Lead to rearrangement in genetic material (no change in total amount of genetic material) § “Balanced Translocation” o Often occur without phenotypic consequences o In a few cases, they can result in position effects • People who have balanced translocations have a high probability of producing gametes with unbalanced combinations of chromosomes o Depends on the segregation pattern during meiosis I à homologous chromosomes synapse with eachother • For translocation to synapse properly, a translocation cross must form o Centromeres usually separate propertly § Can separate across or diagonal o Figure 8.14 o Alternate segregation are the chromosomes across from eachother (result in normal offspring) o Adjacent 1 and 2 segregations produce unbalanced gametes with too much of one type of DNA o Alternate and adjacent-1 segregations are likely outcomes (same frequency in fact) when individual carries ar reciprocal translocation o Adjacent-2 is very rare • Individual with reciprocal translocation produces four types of gametes • 2 are viable, 2 are nonviable o “Semi-sterile” Chapter 8 Lecture 3: Changes in chromosome numbers can vary in two main ways - Euploidy o Variation in the number of complete sets of chromosome § Haploid (n) vs Diploid (2n) vs triploid (3n) v tetraploid (4n) for all sets § N=complete set o Less common in animals put very common in plants o Most animals and plants are diploid o Polyploidy = organisms that have three or more sets of chromosomes o Most changes in euploidy are not tolerated § Polyploidy is normally lethal in animals o Some euploidy changes are naturally occurring § Female bee is diploid while male is haploid o Few vertebrates are tetraploid o Some diploid animals produce tissues that are polyploid § Called endopolyploidy • Liver cells do this (can be triploid, tetraploid or even octaploid) • Hypothesized to be used when lots of gene product is needed o Polytene chromosomes of insects provide an unusual example of natural variation in ploidy o Plants commonly exhibit polyploidy § 30-35% of ferns and flowering plants are polyploid § Polyploid plants display outstanding agricultural characteristics • Often larger and more robust than diploid relatives § Polyploids with an odd number of sets of chromosomes are usually sterile (3n) • Can’t get tetrads because there are three chromosomes • When the chromosomes segregate, two chromosomes go to one cell and one goes to another cell § Plants that are odd in chromosome number (3n) do not produce seeds (are seedless) • Bananas, watermelon § These plants produce highly aneuploidy gametes o Sterility is normally a detrimental trait § Except for agriculture • May produce seedless fruits (bananas and watermelons) • May produce Seedless flowers o Plants waste less energy producing seeds and bloom very large and with more flowers - Aneuploidy (Figure 8.16) o Variations in number of particular chromosomes within a set o Considered ‘abnormal’ in any organism o Commonly causes abnormal phenotype o Leads to an imbalance in the amount of gene products o Occurs in only few chromosomes or just 1 o Phenotype of eukaryotic species is influenced by thousands of genes § These gene expressions are balanced § Aneuploidy results in imbalance of gene products • 3 chromosome copies = 150% production of gene product (trisomy à 2n+1) • 1 chromosome copy = 50% production of gene product (monosamy à 2n-1) o Often detrimental or lethal phenotypes o In humans, trisomies of chromosome 13, 18 , 21 are most likely to be aneuploidy § Compatible with survival § Chromosomes are relatively Small and carry fewer genes than larger chromosomes o Involving sex chromosomes are less severe in effect because of X inactivation à barr bodies § Less severe effects than those of autosomes o Most aneuploidy are actually lethal during pregnancy (miscarriage)
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