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Genetics > Chromosomal alterations > Flashcards

Chromosomal alterations Flashcards

1
Q

Chromosomal territories

A

contain a single chromosome within an arbitrary region that is not bound by any membranes

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2
Q

karyotype

A

We can visualise our chromosomes by means of

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3
Q

autosomes

A

Chromosomes 1-22

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4
Q

sex chromosome

A

Chromosome 23 (X and Y, identified separately but together make the homologous 23rd )

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5
Q

How are chromosomes divided?

A

Chromosomes are divided at the centromere into chromosome arms

Usually unequal lengths

Short arm (p arm)

Long arm (q arm)

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6
Q

nondisjunction

A

When chromosomes and sister chromatids fail to separate properly

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7
Q

What are the effects of non disjunction?

A

Affects the numbers of chromosomes within cells

Adds or removes large amounts of genetic material

Usually alters the phenotype, affects development and/or fertility

Bc incorrect dosage compensation

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8
Q

euploid

A

Chromosome numbers that are a multiple of the haploid number (ie 2n, 3n, 4n, etc)

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9
Q

aneuploid

A

If we add or remove a chromosome, it alters the euploid number and generates an

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10
Q

aneuploidy

A

Caused by Nondisjunction in germ-line cells

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11
Q

Describe what happens if non disjuniction occurs in meiosis 1

A

In meiosis I, results in the failure of homologous chromosome separation

Gametes end up with one extra or one missing chromosome

Trisomic (2n+1): three of one chromosome instead of a homologous pair

Monosomic (2n-1): single copy of one of the chromosomes instead of a homologous pair

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12
Q

Describe what happens if non disjuniction occurs in meiosis 2

A

In meiosis II, results in the failure of sister chromatid separation

Typically follows normal meiosis I

Two resulting gametes produce normally

Two resulting gametes become (n+1) and (n-1

After fertilisation, we get: Trisomic (2n+1): three of one chromosome instead of a homologous pair

Monosomic (2n-1): single copy of one of the chromosomes instead of a homologous pair

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13
Q

What does aneuploid alter?

A

changes the gene dosage of all of the genes on the affected chromosome- if we have an extra cope of chromosome 1 we will have extra expression of it

In diploid organisms, gene dosage is 100%

Monosomic individuals, gene dosage is 50%

Trisomic individuals, gene dosage is 150%!

Leads to imbalance of gene products

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14
Q

How are plants and animals affected by aneuploidy?

A

Animals are usually more affected by gene dosage
Developmental delays and/or nervous system underdevelopment due to aneuploidy
Often lethal

Plants are usually a little bit more tolerant of changes in gene dosage
Different developmental programming than animals

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15
Q

How does aneuploid affect humans?

A

We have potentially 24 different kinds of trisomy (one for each autosome, one for X, and one for Y)

Only really see trisomy 13, 18, and 21 and rarely any monosomies in autosomes

Often see trisomies in sex chromosomes

Other trisomies/monosomies usually result in death of the embryo

Developmental abnormalities are so severe, the zygote cannot survive and aborts

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16
Q

Trisomy 21 (Down Syndrome)

A

The most common autosomal aneuploidy in humans

Linked to age of the mother

Meiosis begins in the fetal ovaries, where it reaches synapsis in prophase I and then stops

At puberty, monthly cycles reinitiate meiosis in a few follicles and the egg is released into the fallopian tubes after meiosis I

Meiosis II begins after fertilisation with a sperm

Meisos in female sex cells is very slow

90% of trisomy 21 cases were a result of meiosis I nondisjunction

Progeny have 2 copies of maternal C21, and one copy of paternal C21

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17
Q

Turner Syndrome

A

(X0 females)- monosomy missing an x

Although one X chromosome is normally inactivated to compensate for gene dosage, 2 X chromosomes are still needed for normal development in females

The genes on the single X chromosome in Turner syndrome are not enough for normal development

Results in no secondary sexual characteristics, infertility, short stature, webbed neck

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18
Q

Triple X Syndrome

A

(XXX females)

Tall stature, possible reduction in fertility, menstrual irregularity

May have speech delays

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19
Q

Mosaicism

A

X-inactivation mosaicism

One X chromosome in female somatic cells is randomly inactivated
- condition in which an individual is composed of 2 or more cell types having different genetic or chromosomal makeup

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20
Q

How can Mosaicism develop

A

Mitotic nondisjunction
Occurs during embryogenesis early in development

25-30% of cases of Turner syndrome can result in 45, X0 monosomy and others 46, XX.

Other individuals can also of mosaicism carrying 47, XXX cells

Derived from a 46, XX zygote undergoing nondisjunction

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21
Q

Uniparental disomy

A

both copies of the homologous chromosome pair are derived from the same parent- normally we have one from each parent

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22
Q

Polyploidy

A

s the presence of three or more sets of chromosomes

Very common in plants (3n, 4n 5n, 6n, 8n, 12n, etc)

23
Q

Autopolyploids

A

have chromosomes derived from a single species

24
Q

Allopolyploids

A

have chromosomes derived from multiple species

25
Q

What does polyploidy result from?

A

Results from

Meiotic nondisjunction

Ie a 2n egg fertilized with a 1n sperm -> 3n plant- polyploid

Mitotic nondisjunction

Doubles chromosome number (ie a 2n cell undergoing mitosis now becomes 4n)

26
Q

What can changes to chromosome structure result in?

A

can result in the loss or addition of genes
 Causes gene dosage imbalances
 Animals greatly affected by this abnormalities

27
Q

Why are Chromosomal mutations are important?

A

A common source of genetic variation
 A major cause of genetic disorders and conditions  A cause of infertility
 4 mains kinds of mutations
 Deletions, duplications, inversions, translocations

28
Q

When and why do chromosomes break?

A

may break during DNA synthesis, crossing over during cell meiosis, etc
 Breaks may also be due to
 External mutagens, chemicals, radiation  Transposable (ie mobile) bits of DNA
 Recombination errors
 Failure of the chiasma to disconnect puts strain on the chromosome

29
Q

What happens when a break happens?

A

segments of the chromosome can be lost

 The ends may be “sticky”, allowing them to rejoin the chromosome (or another) in anew configuration

30
Q

Partial chromosome deletion

A

Both strands of DNA are severed at the
chromosome break point
 The broken ends retain their chromatin structure
Can re-adhere to each other, other truncated chromosomes, or ends of intact chromosomes

Breakage can lead to partial deletion of genes on the chromosome
Size and number of genes involved in the break are significant factors in phenotypic abnormality

31
Q

Terminal deletion

A

the part of or the entire chromosomal arm breaks off
Contains termini, consisting of a telomere
and additional genetic material
 Leaves a chromosome fragment acentric (ie without a centromere)
Can be lost during cell division
Lacks a kinetochore for attaching to the spindle fibers

32
Q

partial deletion heterozygote

A

where one chromosome is wildtype and the homolog has the terminal deletion
 Can also have a normal recessive homolog and the homolog with the terminal deletion
 “pseudodominance”
 Whichever alleles are on the normal homolog will be phenotypically expressed

33
Q

Interstitial deletion

A

e can get 2 chromosomal breaks, resulting in the loss of an internal segment
internal broken ends are then joined back together, resulting in a short than normal chromosome

34
Q

Why do deletions matter?

A

Missing a segment of the chromosome
Affects gene dosage  Can be lethal
If the centromere is lost, chromosome cannot complete meiosis -> huge loss of genetic info
If the segment is deleted from both homologous chromosomes, it is a deletion homozygote

35
Q

Duplications

A

are repeated segments on a chromosome(s)  Can arise from unequal crossing over
can change the phenotype
 Increased gene dosage
 Extra genes can take on new functions in evolutionary time or develop into new gene families
 Ex. Globin genes that code for protein subunits  Expression can vary both before and after birth

36
Q

What occurs when unequal crossing over occurs

A

Partial duplication
 Partial deletion
 Individual with one wild type chromosome and one with duplicated material is a partial duplication heterozygote
 Individual with one wild type chromosome and one with deleted material is a partial deletion heterozygote

37
Q

Why does unequal crossover occur

A

Unequal crossover is rare and occurs when homologs misalign during prophase I

38
Q

Describe which chromosome is responsible for Williams-Beuren syndrome

A

ound in partial deletion heterozygotes for chromosome 7
 Wildtype chromosome 7 contains duplicate copies of gene PMS, with 17 genes between them
 Misalignment of the homologous chromosomes results in mispairing for PMSA and PMSB on the homologs
 This forces a copy of each PMS gene to loop out from the homolog during misalignment
 Unequal crossing-over occurs, resulting in one homolog having a partial deletion  Contains a hybrid PMSA-PMSB gene and is missing intact copies of the full genes
The shortened partial deletion chromosome causes the phenotypic effects in the disorder
 The lengthened partial duplication chromosome has no negative phenotypic effects
 Still has intact PMSA and PMSB genes, separated by the 17 genes in the middle

39
Q

chromosome inversion

A

Remember those “sticky ends”?
 If they join back to the broken chromosome, but in the wrong orientation (ie 180°), the chromosome produces….
Usually only affects one member of the homologous pair

40
Q

Paracentric inversions

A

Inversion does not include the centromere

 Occurs on a single arm

41
Q

Pericentric inversions:

A

Inversion includes the centromere

 Appears to “rotate” on the centromere

42
Q

Inversion homozygotes

A

still have all genes and a normal phenotype unless:
 Inversion break disrupts a coding region
 Position along the chromosome affects gene expression

43
Q

Inversion heterozygotes

A

have one normal chromosome and one inverted chromosome
still have all genes and a normal phenotype but:
 The inversion is mismatched with regular meiotic pairing
 Inversion loops form during meiosis
 Viability of gametes and fertility is reduced due to crossing over events within the inversion

44
Q

Describe crossing over in paracentric/ paracentric inversion

A

Crossing over occurs inside the
region of the inversion loop
 Results in duplications and deletions in recombinant chromosomes
 If crossing over occurs outside of the inversion loop, all is normal and good (ie reciprocal)
Result : 2 paranental- 1 nor mal 1 inverted- viable
2 recombinants- inviable

45
Q

i

A
  1. Theprobabilityofcrossoverwithintheinversionloopislinkedtothesizeofthe
    inversion loop
     Small inversions produce small loops, which have a low frequency of cross over
  2. Inversionsuppressestheproductionofrecombinantchromosomes
     Viable gametes produced by inversion heterozygotes contain either the
    normal-order chromosome, or the inverted chromosome
    No recombinant chromosomes in progeny! -> crossover suppression
  3. Fertility may be altered if an inversion heterozygote carries a very larger inversion
     If an inversion spans a great length of the chromosome, any crossover will produce 2 viable gametes and 2 nonviable recombinant gametes
46
Q

translocation

A

When a chromosome breaks and the fragment reattaches to a different chromosome (ie a nonhomologous chromosome) or to a new spot on the same chromosome,

47
Q

Translocation heterozygotes

A

have one normal chromosome and one altered chromosome

48
Q

 Unbalanced translocation

A

reattachment to a new chromosome (a one-way event)

49
Q

Reciprocal translocation:

A

two chromosomes swap fragments with each other (a two-way event)
A form of Down Syndrome is caused for the reciprocal translocation of chromosome 14 and 21

50
Q

Robertsonian translocation

A

(chromosome fusion): two nonhomologous chromosomes fuse together, with the loss of one of the centromeres

51
Q

Alternate segregation

A

will move chromosomes I and IV to opposite poles and II and III to opposite poles

52
Q

Adjacent segregation

A

will move chromosomes I and III to opposite poles and II and IV to opposite poles

53
Q

Why do translocations matter?

A 
Associated with many types of cancers
Leukemia (chronic myelogenous)
Interrupts a gene required in the cell cycle regulation (checkpoints)
 Genomic instabilities
Can lead to other cancers
Due to mutations in DNA repair genes

Genetics (12 decks)

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