Chapter 10 Flashcards
What does chromosome breakage (deletion) cause?
mutation by loss, gain and rearrangement of chromosomes
-can have dramatic (negative) consequences
-sometimes no effect on health of individual
chromosome break point
the location at which both DNA strands are severed (when a chromosome breaks)
terminal deletion
when an entire chromosome arm, or part of it severs - takes telomere with it
(ex: cri-du-chat: syndrome resulting from terminal deletion)
acentric fragement
if a broken chromosome fragment is acentric (lacks a centromere), its is usually lost during cell division , whole chromosome is lost
interstitial deletions
internal chromosomal deletions, smaller insertions or deletions result from cell trying to repair itself (indel) huge can become small deletion
chromosome inversion
after breaking a chromosome it may reattach.
When reattachment of the wrong end occurs
chromosome translocation
when reattachment to a non-homologous chromosome occurs
(If no critical genes or regulatory regions are mutated, there may be no phenotypic consequence)
2 types of chromosome inversion
- Paracentric inversion: centromere is outside of inversion (breakage is downstream from centromere and reattaches in flipped manner)
- Pericentric inversion: centromere is inside inversion (within centromere, short and long arm can be switched)
Does inversion generally suppress recombination in heterozygotes?
Yes!
- recombination requires chromosomes to have regions of homology that can be exchanged
-an inversion heterozygote has one normal chromosome and one inverted chromosome that lacks homology
-often leads to more chromosome breaks that leads to more inviability
recombination can happen but not successfully
Inversion loop
sometimes, rarely recombination can still occur in inverted heterozygotes.
an inversion loop is produced between the 2 chromosomes, crossover can lead to chromosomal breakage, this results in gametes with large deletions that result in inviable offspring when fertilised with a normal gamete
-if crossover occurs in inversion area, deletions occur and break off (reliable gametes that don’t participate in recombination are left)
unequal crossover
can occur between two homologs, results in partial duplication on one homolog and partial deletion on another. Rare phenomenon.
Williams-Beuren syndrome
occurs from partial duplication of PMS gene from unequal crossovers in chromosome 7. both chromosomes make loops and then combine, makes 3 copies of one gene (duplicates) and results in one deletion in other gene.
Naive, overly trusting/outgoing personality, mild intellectual disabilities and heart problems.
Deletion mapping - typically used in what organism
mapping a gene based on comparing a phenotype of interest in mutants with known deletions
usually done when you already know relative location of gene of interest
(typically used in drosophila, have dosage compensation organisms w/many deletion mutants with known mapped deletions - deficiency lines)
Deletion mapping relies on:
deficiency lines
mutation to be recessive, relies of pseudodominance (if an organism has a recessive allele on one chromosome, and a deletion on the other, the recessive allele is expressed - same as being hemizygous)
Deletion mapping on Drosophila Notch Gene
notch is a developmental gene
common notch mutants: altered wing phenotypes
-deletion mapping narrowed it down to: region on X chromosome
Konopka and Benzer
applied deletion mapping to narrow down the first discovered circadian clock gene (period gene) to a locu within the X-chromosome
chromosome number in selected animal species vary because
more chromosomes does not mean more genetic content/more complex (some may have large base pairs, others have many tiny ones)
Thomas Cremer and Christoph Cremer - chromosome territories
chromosomes are partitioned into specific chromosome regions/territories during interphase, they do not occupy the exact same territory in each nucleus, once situated a chromosome does not stray from its territory until mitosis occurs
Karyotype
an organised display of chromosomes (done by taking microscopy images during mitosis/meiosis and having a trained eye)
-can identify abnormal numbers of chromosomes in a cell or abnormal structure
-arranged in descending order of size w/autosomes first
- multiple slides may be needed to get perfect image
- staining done to identify specific chromosomes
centromere and its arms
the centre of the chromosome; divides chromosomes into 2 arms which often have unequal length
short arm: p arm
long arm: q arm
4 types of chromosome shape (meta):
metacentric: equal p and q length
submetacentric: disproportional (bottom q longer)
acrocentric: even shorter p length w/satellites (pinched off arm portion for p)
telocentric: no p arm
Fluorescent in situ hybridization (FISH)
- uses molecular probes to detect a target sequence
- probes are labeled with compounds that emit fluorescence
- various probes that emit diff wavelengths of light and target diff genetic regions can be used simultaneously
can be used to identify each chromosome in a cell
chromosome banding techniques
older method than FISH
- identifies chromosomes in a cell based on size, shape, and banding patterns when dyes and stains are used
involves: stop cell cycle in metaphase using chemical, dye/stain microscope slide (to burst the cell and enable examination/photography in isolation), may requires multiple rounds for clean images of all chromosomes
Standard for human chromosome banding is: _ banding
euchromatin and heterochromatin
G banding
- uses letters and numbers to identify major/minor band regions
- numbering begins at centromere and goes outward along each arm
-each alternating light/dark band is labeled in a hierarchical numbering system (ex: 5q2.3.1:
5 = chromosome number, q = q arm (long arm), 2 = region 2 on chromosome 5 q arm, 3.1 = specific dark band)
euchromatin: less stained, light regions because chromatin in less compact. regions w/ higher gene expression
heterochromatin: dark regions, more stained, chromatin is compact, lower gene expression
