Lecture 18: Large Scale Chromosomal Changes (Part 2) Flashcards
(40 cards)
how does chromosome rearrangement represent a major feature of evolution?
- rearrangement breakpoint may acquire new patterns of gene expression and create new gene functions by fusion of two separate genes
- some rearrangements contribute to the process of speciation
- duplications provide extra gene copies that can acquire new functions
state and draw the four classes of chromosomal rearrangements resulting from chromosome breakage and subsequent DNA repair
- deletion
- inversion (180 degree rotation of a piece of DNA)
- deletion in one chromosome, duplication in another
- translocation of a piece of DNA into another chromosome
state and draw the four types of chromosomal rearrangements resulting from aberrant crossing over at repeated sequences
- deletion
- inversion
- deletion in one chromosome, duplication in another
- reciprocal translocation of a piece of DNA into another chromosome
two types of deletions
- intragenic: small deletion within a single gene
- multigenic: many genes deleted
Del (Df) homozygotes
- short for deletion (deficiency) homozygote, is an individual that has both copies of the same chromosomal region deleted
- usually inviable
Del (Df) heterozygotes
- gene imbalance
- might result in haploinsufficiency
deletion loop
a DNA loop formed during meiosis when one homologous chromosome has a segment deleted. The extra DNA on the normal homolog that has nothing to pair with loops out.
pseudodominance
when a recessive allele is expressed in a heterozygous individual because the dominant allele has been deleted or is missing.
what is a practical application of deletions?
- deletions may uncover recessive mutations
- they can be used to locate genes for mapping
deletion mapping: complementation
- Start with a mutant strain that has a recessive mutation (e.g. mut) causing a known phenotype.
- Cross it with a strain that carries a known deletion of part of the chromosome (e.g. Df1).
- Examine the phenotype of the offspring (heterozygotes: mut / Df1):
- If the offspring show the mutant phenotype, then the deletion likely removes the same gene as the mutation → No complementation.
- If the offspring show the wild-type phenotype, the mutation must lie outside the deleted region → Complementation occurs.
two main types of duplications
tandem duplications: the duplicated segment is inserted right next to the original.
non tandem (dispersed) duplications: the duplicated segment is inserted elsewhere in the genome, not adjacent to the original.
impact of duplications
- less likely to affect phenotype
- in some cases causes a dosage effect/genetic imbalance
- genes may be placed in a new location that modifies their expression
how do duplications arise?
- X-ray breaks/any other cause of breaks:
- X-rays break one chromosome in two places
- X-rays break homologous chromosome in one place
- during repair, the freed segment from the first chromosome is mistakenly inserted at the break site on the homolog -> non tandem duplication - Unequal crossing over:
- Homologous chromosomes misalign during meiosis.
- Crossing over occurs at these misaligned points.
- One chromosome gains extra DNA (duplication), the other loses it (deletion).
duplications can result in
unequal crossing over, causing increase and reciprocal decrease in the number of copies (eg Bar-eye in drosophila)
potential impacts of inversions
- most inversions to not alter phenotype unless breakpoints occur within genes
- but genes may be placed in a new location that modifies their expression (eg Antennapedia)
two main types of inversion
- pericentric inversion - includes the centromere
- paracentric inversion - does not include the centromere
breakpoints between genes
- Genes remain intact.
- Order of genes is reversed in the inverted segment.
- Usually no gene disruption or loss of function.
- May affect gene expression if regulatory regions are affected.
breakpoints within ONE gene
- The gene is disrupted (split) and mutated
- Usually causes loss of gene function or creates a truncated protein.
- Can lead to a nonfunctional or altered gene product.
breakpoints within TWO genes
- Both genes are disrupted at the breakpoints.
- May create fusion genes by joining parts of two genes.
- Can produce novel or dysfunctional proteins.
- Often causes loss of function or gain of abnormal function.
inversion loops
- form in inversion heterozygotes
- enables pairing of homologous regions despite the reversed gene order.
- produces abnormal recombinant chromosomes.
paracentric inversion loop
Normal chromosome + Inversion chromosome. Inversion loop forms outside the centromere:
- if crossing over occurs inside this loop, it produces one dicentric chromosome (with two centromeres) and one acentric fragment (without a centromere)
- the acentric fragment is lost
- there is a random break in the dicentric bridge of the dicentric fragment
Results in: one normal product, two deletion products, and one inversion product with all genes present. Reduced number of viable gametes
pericentric inversion loop
Normal chromosome + inversion chromosome. Inversion loop includes the centromere:
- if crossing over occurs inside this loop, it results in gene imbalance
- one normal product, two different inviable deletion/duplication products, one viable inversion product (all genes present). Reduced number of viable gametes
look over how to find the possible gametes arising from a paracentric and pericentric inversion
What are balancer chromosomes and why are they useful?
- engineered chromosomes used in genetics that carry multiple inversions and sometimes other rearrangements.
- these prevent crossing over from happening during meiosis
