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Cell biology and genetics - semester one > Karyotype analysis > Flashcards

Karyotype analysis Flashcards

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1
Q

Outline karyotype analysis

A
  • Karyotyping is a test to** examine chromosomes** in a sample of cells
  • This test can help identify genetic problems as the cause of a disorder or disease
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2
Q

What is a karyotype?

A
  • The general appearance of the complete set of chromosomes in the cells of a species or in an individual organism, mainly including their sizes, numbers, and shapes
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3
Q

How are chromosomes prepared for karyotype determination?

A
  • Metaphase spread
  • The cell membrane is ruptured to help visualize the chromosomes
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4
Q

Outline the process of metaphase spread

A
  1. An appropriate cell sample is taken (usually blood or amniocentesis)
  2. Culture and arrest cells at metaphase (using colchicine to inhibit mitosis)
  3. “Spread” chromosomes
  4. Fix cells and stain chromosomes (C and G bands; fluorescent ‘paints’)
  5. Count and classify
  6. Assemble an ordered karyotype
  7. Chromosomes are arranged in homologous pairs for analysis
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5
Q

Describe the typical human karyotype

A
  • 46 chromosomes in total
  • 2 sex chromosomes, X and Y
  • XX : female
  • XY: male
  • 44 autosomes
  • 23 homologous pairs
  • (X is much longer than Y; hemizygous pairing)
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6
Q

How are chromosomes typically classified?

A
  • Initially based on gross morphology
  • Autosomal chromosomes are placed into** seven groups (A-G)**
  • Placed in order of decreasing length, A = longest and G = shortest
  • X = Indistinguishable from group C
  • Y = similar to group G
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7
Q

Describe chromosome banding

A
  • In any given group, A-G, individual chromosomes are difficult to distinguish as they are all approximately the same length
  • Identification of these within any given group is therefore achieved with band staining techniques
  • C bands (Centromere): selectively stain centromeres (C banding uses dye which just picks up just the centromeres to show different types)
  • G (uses Giemsa) bands: heterochromatin tends to stain more than euchromatin (specific binding patterns)
  • R bands (Reverse): approximately the reverse of G bands
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8
Q

How are chromosomes classified based on centromere position?

A
  • p arms (top part of chromosome) = short
  • q arms (bottom part, below centromere) = long
  • q/p = ratio characteristic (arm ratio)

Thus,
- Metacentric = (ratio) 1
- Submetacentric = (ratio) 1.5-2.5
- Acrocentric = (ratio) 2.5-10
- Telocentric = Typically absent in humans

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9
Q

What does FISH stand for?

A
  • Fluorescent
  • in
  • Situ
  • Hybridisation
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10
Q

What is FISH used for?

A
  • Uses fluorescently labelled probes (short lengths of synthetic DNA complimentary to chromosome specific sequences)
  • Probes are labelled with differently coloured fluorescent dyes
  • It provides researchers with a way to visualize and map the genetic material in an individual’s cells, including specific genes or portions of genes
  • This may be used for understanding a variety of chromosomal abnormalities and other genetic mutations
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11
Q

What is the tern used to describe the number of complete sets of chromosomes in a cell?

A

Ploidy

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12
Q

Outline a human diploid cell

A
  • It has two complete sets of chromosomes
  • Most cells in humans are diploid, comprising 23 chromosome pairs, so 46 chromosomes in total
  • This is 22 pairs of autosomes and a pair of sex chromosomes
  • The total number of chromosomes in diploid cells is described as 2n, which is twice the number of chromosomes in a haploid cell
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13
Q

Outline a common human haploid cell

A
  • Gametes, a sex cell containing only one set of different chromosomes, or half the genetic material necessary to form a complete organism (i.e., haploid)
  • Referred to as n
  • Due to meiosis
  • 2n regained upon zygote formation (n from sperm and n from egg = 2n zygote)
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14
Q

What is reductive division?

A
  • The first cell division in meiosis, the process by which gamete cells are formed
  • A unique event in which the chromosome number is reduced from diploid (46 chromosomes) to haploid (23 chromosomes)
  • Also called first meiotic division or first meiosis
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15
Q

Outline Polyploidy

A
  • The heritable condition of possessing more than two complete sets of chromosomes
  • Relatively common in plants
  • 3n = triploid, 4n = tetraploid, etc.
  • Rare in the animal kingdom
  • Usually lethal in humans, less than 1% of human conceptions are triploid (but most that are tend to be miscarried early into the pregnancy - 99%)
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16
Q

Outline aneuploidy

A
  • The condition of having an abnormal number of chromosomes in a haploid set
  • Changes in chromosome number involving LESS than the entire complement of chromosomes
  • Usually plus or minus one
  • Example in humans: 46 + 1 = 47 - Trisomy, 46 -1 = 45 - Monosomy
  • Arises due to non-disjunction of chromosomes and is the basis of most human chromosomal disorders
  • Resulting gametes are n+1 and n-1
  • After fertilization, offspring are either 2n+1 trisomic or 2n-1 monosomic
17
Q

What is nondisjunction of chromosomes?

A
  • Nondisjunction is the failure of the chromosomes to separate, which produces daughter cells with abnormal numbers of chromosomes
  • It can be either when homologous chromosomes do not separate during meiosis I, or when sister chromatids do not separate during meiosis II
18
Q

Outline Down syndrome

A
  • Flat facial profile, down slanting eyes with epicanthal skin folds, low set ears, protruding tongue, low nose bridge
  • Mental retardation
  • An IQ under 60
  • Generally a reduced life expectancy (but this has increased by around 20 years since 1990)
  • Over 80 associated physical and mental disorders - congenital heart defect occurs in around 60% of all cases
  • Down syndrome is due to trisomy at chromosome 21 (unknown cause)
19
Q

Describe Trisomy 21

A
  • Where chromosome 21 has an extra chromosome
  • Chr 21 contains around 1.5% DNA
  • Not many genes there, but those that are are ones linked with symptoms to the disorder
  • It was the first fully sequenced genome as a part of the Human Genome Project
20
Q

Outline other chromosomal caused disorders

A
  • Edward’s syndrome: Trisomy 18, few children live longer than 10 years, small birth weight and clenched fists at birth
  • Patau syndrome: Trisomy 13, cleft lip/palate, defects in digit development, 70% die within a year of birth
21
Q

What are the effects of aneuploidy of sex chromosomes?

A
  • A variety of conditions are produced by the non-disjunction of sex chromosomes
  • Genetic ‘balance’ is less disturbed than if it involved autosomes due to Y chromosomes contain around 1.5% genome and few genes, and X chromosomes containing around 5% genome and many genes
  • Extra X chromosome copies are inactivated; this mechanism exists to compensate for the potential difference in ‘dosage’ of X-linked genes
  • This is called X-inactivation
22
Q

What hypothesis did Mary Lyon propose?

A
  • The Lyon Hypothesis
  • First suggestion of X-inactivation
  • Suggested that in any given female cell, only one X functions
  • The other is thus inactive and can be observed as a ‘Barr body’
  • HOWEVER, not all are turned off; females are mosaics and composed of different cell lines- thus, whether it is the maternal one or paternal one that is inactivated is a random process
23
Q

Outline Klinefelter’s syndrome

A
  • XXY
  • Male external genitalia
  • Normal life span
  • Possible infertility
  • Barr bodies present in cells
  • Additional copy of an X chromosome may result in mental retardation (such as XXXY or XXXXY - and multiple Barr bodies)
24
Q

Outline “Super-female” syndrome

A
  • Xn
  • 47, XXX = female, healthy and indistinguishable from XX females except by karyotype (which reveals an extra Barr body)
  • 48, XXXX = female, usually mild mental retardation or learning difficulties
  • 49, XXXXX = female and typically results in severe disability
25
Q

Outline “Super-male” syndrome

A
  • XYY
  • Not well classified
  • Tall, muscled, but poorly coordinated
  • Increased hormone production means acne is often severe and may be problematic long term
26
Q

Outline Turner syndrome

A
  • X0
  • Only viable human monosomy
  • Short stature, webbed neck and shield shaped chest
  • Usually normal intelligence
  • NO Barr bodies
27
Q

State four types of chromosomal structural changes

A
  • Deletion
  • Duplication
  • Inversion
  • Translocation
28
Q

Outline DiGeorge/Velocardiofacial syndrome

A
  • 22q 11.2 del (deletion of band in long arm of chromosome 22)
  • Suspected by karyotype analysis, confirmed by FISH
  • Cleft palate and cardiac defects
  • Immune system deficiencies
  • Moderate mental retardation
29
Q

Outline Fragile X syndrome

A
  • Occurs in males
  • Major cause of male mental retardation
  • Characteristic facial phenotype (‘long’ face and large ears)
  • Hypogonadism (reduced functional activity of gonads)
  • Associated with fragile site on X chromosome
  • Break occurs at FMRI (Fragile X mental Retardation-1) gene
  • Functions of this gene are currently emerging
30
Q

Outline Robertsonian translocation

A
  • 45 XX t (9,22)
  • Originally thought to be a deletion on chromosome 22, but determined to be a translocation to chromosome 9
  • Occurs in around 90% of chronic myelogenous leukaemia (CML) cases
  • It is acquired, not congenital
31
Q

Outline chromosomal inversion

A
  • E.g., inversion 16 (45, XY, inv 16))
  • Inversions do not involve a lot nor gain of chromosomal material
  • If the breakpoints of the inversion do not disrupt the genes they may not have an associated phenotype
32
Q

Explain the XY system of sex determination

A
  • It was noted that female cells had 20 large chromosomes but an odd number and appearance of chromosomes in male cells (19 large and 1 small)
  • This showed that chromosomes paired (homologous) and x and y paired too (hemizygous)
  • Determined that unfertilised eggs contained an X chromosome, and sperm contained EITHER an X or Y chromosome
  • Thus, sex is determined by which sex chromosome is in the sperm cell
33
Q

Explain the X0 system of sex determination

A
  • Only one type of sex chromosome = X
  • Sex is determined on whether or not sperm contains an X chromosome
  • For example, in crickets, females are 22+XX
34
Q

Explain the ZW system of sex determination

A
  • Found in birds, fish, and butterflies
  • Sex chromosome is present in the ova, not the sperm
  • Designated Z and W
  • Male = ZZ, female = ZW
35
Q

Explain the Haplo-diplo system of sex determination

A
  • Found in most species of bee and ant
  • No sex chromosomes
  • Female develop from a fertilised ova and are diploid (2n)
  • Male are developed from an unfertilised ova and are haploid (n)
  • Unfertilised eggs form via parthenogenesis

Cell biology and genetics - semester one (9 decks)

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