Chapter 8 Flashcards
Modifications occur at the level of…
the chromosome
Phenotypic variations result from changes of
individual genes
What are some chromosome mutations or aberrations that could occur?
-Total number of chromosomes vary
-Deletions of genes or segments
-Duplications of genes or segments
-Rearrangements of genetic material within or among chromosome
Aneuploidy
-Variations in chromosome number
-Organism gains or loses one or more chromosomes (not the entire set)
Aberration
The chromosomal number is altered
Mutation
an alteration in the nucleic acid sequence of the genome of an organism, virus, or extra chromosomal DNA.
Monosomy
Loss of single chromosome in diploid genome
2n - 1
Trisomy
Gain of a single chromosome
2n + 1
Euploidy
Complete haploid sets of chromsomes are present
MULTIPLES OF N
Polyploidy
More than two sets of chromosomes are present
3n, 4n, 5n….
Triploid
Three sets of chromosomes
3n…
Nondisjunction
-Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division.
-Gives rise to chromosomal variation
-paired homologs fail to disjoin during segregation
-Nondisjunction occurs during meiosis I or II
Monosomy
-Loss of one chromosome
-Produces 2n-1 complement
-Although one copy remains, if the copy is lethal, organism is not viable
-Monosomy unmasks recessive alleles
-Haploinsuffciency may occur
Haploinsufficiency
When one copy is not sufficient for organism to survive
Trisomy
2n + 1 chromosomes
-Addition of chromosome produces more viable organisms
-Trisomies for autosomes are often LETHAL
-Plant trisomies are viable:
-Phenotype is altered
-Example: Datura stramonium with altered
phenotype
Trisomy 21: _______ Syndrome
Down Syndrome, trisomy of chromsome 21
-Results in an extra chromsome (three #21’s)
-On average affected individuals express 6 to 8 characterisics
What is DSCR?
Down Syndrome Critical Region
-Critical region of chromosome 21
-Genes are dosage sensitive
-Responsible for many phenotypic-associated syndromes
Origin of Extra 21st Chromosome?
-Nondisjunction of chromsome 21 during meiosis
-Homologs do not disjoin (detach) during anaphase I or II
-Leads to n +1 gametes
-Ovum is the source of 95% of trisomy cases
-Increased incidence with increasing maternal age
Diagnostic Testing Methods
1.Amniocentesis or chorionic villus sampling (CVS)
- NIPGD: Noninvasive prenatal genetic diagnosis
Explain the CVS method of Diagnostic Testing
Fetal cells are obtained from the amniotic fluid or chorion of the placenta
Explain the NIPGD method of Diagnostic Testing
Fetal cells and DNA obtained from maternal circulation
Familial Down Syndrome
-The translocation of chromosome 21, and occasionally runs in families
-Remember, involves translocation of chromosome 21
Patua syndrome
Trisomy 13
Edwards syndrome
-Both trisomies survive to term
-Manifest severe malformations and early lethality
Trisomies are often found in ….
-Spontaneously aborted fetuses (they make up for 20% of conceptions)
What doe autosomal monosomies suggest?
-That monosomc gamets may be functionally imparied
-they are seldom found
Polyploidy
More than two multiples of haploid chromosomes are found
Triploid
3n chromosomes
Tetraploid
4n chromosomes
Pentaploid
Has 5n chromosomes
Aneuploidy
2n +- x chromosomes
Disomy
2n
Euploidy
Multiples of n
Autopolyploidy
Addition of one or more sets of chromosomes identical to the haploid complement of same species
-Multiples of the same genome
Allopolyploidy
Combination of chromosome sets from different species as a consequence of hybridization
In autopolyploidy…
How does it arise? etc.
each identical set of chromomsomes is identical to the parents species
-Arises when:
-diploid gamete is
produced
-Two sperm fertilize on
ovum (rare)
More prevenlant in the natural population; produce balanced gametes
Autotetraploid
-Chromosomes have replicated, but the parent cell never divides and instead reenters into interphase, and the chromosome number becomes doubled
-experimentally induced by colchicine
Allopolyploidy
Results from hybrodizing tow closely related species
-Hybrid plant may be sterile (cannot produce vaible gametes)
-Chromosomes are not homologous
(cannot synapse in meiosis
Allotetraploid
Polyploid contains equivalent of four haploid genomes derived from separate species
Original species is unknoen
Amohidiploid
An allotetraploid wjere both original species are known
-These kinds of plants are found in nature
Example: Gossypium plant (cotton)
Endopolyploidy
Condition where only certain cells in diploid organism are polyploid
-Set of chromosomes replicates repeatedly without nuclear division
-Can occur in cancer cells
List the possible chromosome rearrangements:
- Deletions
- Duplications
- Inversions
- Nonreciprocal translocations
- Reciprocal translocations
What do deletions and duplications specifically do to a chromosome?
Total amount of genetic information in chromosome changes
What do inversions and translocations specifically do to a chromosome?
Genetic material remains the same but rearranged
What are Deletions, or deficiency? Explain.
-missing regions of chromsome
-chromosome breaks in one or more places
-portion is lost
location of deletion can vary
-terminal deletion (near one end)
-intercalary deletion (interior of the chromosome)
What is a compensation loop (Buckling out)?
-Synapsis between chromosome with large intercalary deletion and normal complete homolog
-Requires unpaired region of normal homolog to loop out of linear structure into deletion or compensation loop
Cri du chat Syndrome
-Deletion of small terminal part of chromosome 5
-Segmental deletion
-infant exhibit anatomical malformations
What is duplication? What does it arise from?
-Repeated segment of a chromosome
-a single locus is present more than once in the genome
-can produce compensation loop
-Arises from unequal crossing over between synapsed chromosome during meiosis.
gene redundancy
Example: rRNA
Multiple genes perform the samejob
Multiple copies of genes encode for rRNA genes
This DNA is called rDNA
-E. Coli’s haploid genome is .7 percent rDNA (senven copies)
-Drosophila: .3 percent rDNA
Gene amplification
-Another mechanims that increases rRNA
-the number of copies of a gene is increased “without a proportional increase in other genes”.
-oocytes store nutrients including ribosomes
-used by embryo in early development for rRNA synthesis
Bar mutation in Drosophila
-Duplications caise phenotypic variations
-Bar-eye phenotype in Drosophila results from duplication
-Bar-eyed flies have narrow, slit-like eyes
-inhereted phenotype
-Mutation is inherited
Gene duplication “may play a role in evolution”
Gene duplication is hypothesized to be a major source of new genes
Hypothesis supported by discovery of genes with substantial amount of DNA sequence in common, but distinct gene products
Ex. Genes encoding digestive enzyme trypsin and chymotrypsin
CNVs: Copy number variants
Represent quantitative differences in number of large DNA sequences
-Found in coding and noncoding regions of the genome
-Play a crucial role in the expression of traits
sections of the genome are repeated and the number of repeats in the genome varies between individuals.
Inversion
Rearrangement of linear gene sequence
-No loss of genetic information
-Segment of chromosome turned 180 degrees within chromosoe
-Two breaks in chromosome, and reinsertion inverted segment
-May arise from chromosomal looping
Paracentric inversion
-Does not change the lengths of two arms of chromosome
-centromere is of part of inverted segment
Pericanetric inversion
Centromere is part of the inverted segment
-DOES change lengths of the two arms of the chromosome
Inversion loops
inverted and noninverted chromosomes in meisois paired only if they form and inversion loop
Inversion heterozygotes
Organisms with one inverted chromosome and one non-inverted homolog
-can be peri or paracentric
Paracentric inversion crossover
One recombinant chromatid is dicentric
-two centromeres
One is acentric
-Lacking a centromere
Percentric inversion crossover
Crossover between pericentric inversion and noninverted homolog
-Recombinant chromatids have duplications and deletions
Translocation
Movement of chromosomal segment to new location in genome
Reciprocal translocation
Involves the exchange of segments between two nonhomologous chromosomes
-Genetic information is lost or gained
-Has unusual synapsis configuration during meiosis
Reciprocal Translocation has two possible segregation patterns:
Alternate segregation:
-Segregation pattern at
first meiotic division
- Has complete
complement of genetic
information
Adjacent segregation:
-Leads to gametes containing duplications and deficiencies
if these participate in fertilization in animals, the resultant offspring do not usually survive
Parent is said to have semisterility
Robertsonian translocation (centric fusion)
Involves breaks at extreme ends of short arms of two nonhomologous acrocentric chromosomes
Small segments are lost
Large submetacentric or metacentric chromosome are produces
Example: familial down syndrome
Fragile sites
More susceptible to chromosome breakage when cultured in the absence of folic acid or other chemicals
-Sites indicate regions of nontightly coiled chromatin
Fragile -X syndrome (martin-bell syndrome)
-Folate-sensitve site on X chromosome exhibits FXS
-Most common form of inherited mental retardation; dominant trait
-Caused by trinucleotide repeats in FMR1 gene
Trunucleotide repeats
A sequences of three nucleotides in repeated many times
-Expands the size of the gene
Genetic anticipation
The number of trinucleotide repeats increases with every generation
Carrier status: 55 to 230 repeats
Syndrome status: over 230 repeats
Link between fragile site and ling cancer
-Defect associated with formation of tumors
-FHIT gene on fragile site location FRA3B altered or missing in lung cancer cells
-Normal protein product of this gene absent in other types of cancer cells
