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Molecular and Experimental Genetics > Chromosome and their abnormalities > Flashcards

Chromosome and their abnormalities Flashcards

1
Q

How long is DNA?

A

3x10^9 base pairs long (3bn)

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2
Q

What is the normal human karyotype?

A

46 chromosomes
(46XY or 46XX)
22 pairs of autosomes and 2 sex chromosomes

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3
Q

How can we prepare chromosomes to look at?

A 
From:
blood lymphocytes
Bone marrow cells
Skin fibroblasts
Amniotic fluid cells
Chorionic villus
Fetal Blood
Giemsa banding
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4
Q

What are G banding chromosomes?

A

G - Giesma (stain for chromosomes)

chromosome pairs identical in banding pattern

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5
Q

What may vary when looking at G banded chromosomes?

A

Size of heterochromatic blocks at the centromere
satellite regions
Y chromosome

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6
Q

what determines the banding pattern of chromosomes?

A

The banding pattern may vary depending on the length of the chromosome

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7
Q

What is the purpose of mitosis?

A

cell division - produces genetically identical full complement of DNA
Normal growth of organism and in wound healing

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8
Q

Why are microtubule targeting drugs important in cancer?

A

As they target the microtubules forming during tumour proliferation and hence prevent the cells from dividing

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9
Q

What is the function of meiosis?

A

Reduction division from 46 chromosomes to only 23 per gamete

Reassortment of genes

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10
Q

What is the basic mechanism of meiosis?

A

Each homologue replicates to give two chromatids
homologous pair
exchange of material between non-sister chromatids (crossing over / recombination)

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11
Q

What are Chiasmata?

A

sites of crossing over

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12
Q

Where does variation come from during meiosis?

A

Independent assortment of maternal and paternal chromosomes

Presence of crossing over exchanges between homologous chromosomes (recombination)

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13
Q

what happens during normal meiosis for a female?

A

the primary cell divides into 2 which splits into 2 cells each of 23 chromosomes where one has the mitochondria and will go one to form the oocyte but the other cell has little mitochondria and forms a polar body which will eventually be lost.

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14
Q

What are some sex differences in meiosis?

A
  1. In males all the cell division products develop into mature sperm
    In females one division product gives the ovum. The remainder are lost as polar bodies.
  2. In the male, meiosis, follows a long series of mitotic divisions which is completed when the spermatids develop into sperm.
    In the female, meiosis starts very early in development- it is then arrested- and finishes only after fertilisation
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15
Q

What is chromosomal non-disjunctions?

A

where there is uneven distribution of chromosomes into the new cells resulting from a chromosome pair failing to separate.

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16
Q

What is the result of a non-disjunction of sex chromosomes at the 1st and 2nd meiotic division of a female cell

A

1st:
The cell splits into 2 cells where one cell has no chromosomes (1st polar body) and the other cell has the rest of the chromosomes.
2nd:
This will produce a 2nd polar body and a cell with 3 chromosomes in giving the karyotype XXX or XXY

17
Q

What is klinefelters syndrome?

A

Sex chromosome non disunction disease

47XXY producing a normal but infertile male

18
Q

What is Turners syndrome?

A

Sex chromosome non disjunction disease

45X0 producing a short stature, webbed neck infertile female

19
Q

what are some other possible results from non disjunction of sex chromosomes

A

47 XXX - outwardly normal female with some fertile

47 XYY - tall or very tall male, relatively infertile

20
Q

give an example of an autosomal non-disjunctional mutation and its syndrome it causes

A

Trisomy of 21
leads to down syndrome
(47 XX+21 or 47XY+21)

21
Q

how does the extra chromosome 21 arise in down syndrome?

A

Either non-disjunction at meiosis or mitotic non-disjunction.
80% of cases the extra 21 is maternal in origin
<1% of cases it is mitotic
Most cases arise from a non-disjunction in the first mitotic division

22
Q

What is translocation down syndrome

A

Where there are 3 chromosome 21 however they are not all separate as at least one of the chromosomes 21 is attached to another autosome such as 14.

23
Q

At what age is there the greatest individual risk of giving birth to a child with down syndrome

A

Older age >50 years old.

24
Q

What is the result of a non-disjunction of sex chromosomes at the 1st and 2nd meiotic division of a male cell

A

1st:
Produces 2 cells 1 with no chromosomes and the other with the sex linked chromosomes. This will then undergoes the second meiotic division to form 2 cells with both an X and Y chromosome.
2nd:
Non- disjunction at the second mitotic division would produce 4 chromosomes where two are polar cell bodies , 1 cell has 2 X chromosomes and the other cell has 2 Y chromosomes

25
Q

Why are trisomy X individuals normal intellectually but not trisomy 21 patients?

A

Due to X chromosome inactivation. In early embryonic development one of the X chromosomes is inactivated and therefore where there are individuals with a trisomy of X chromosomes, there is a mosaic effect and hence 2 out of the 3 X chromosomes are now inactivated and therefore there is still only ever 1 active chromosome meaning that these individuals are normal.

26
Q

What is Triploidy?

A

Where there is an abnormality in chromosome number and hence rather than having 46 chromosomes, the individual has 69.
These fetuses although can be born, they do not survive.

27
Q

What is Aneuploidy

A

Either the loss of one autosome (monosomy) or the gain of a chromosome (trisomy)

28
Q

What does several chiasmata between non sister chromatids per bivalent mean?

A

Leads to an increasing reassortment of genes

29
Q

What are synaptonemal complexes?

A

Hold chromatids together for recombination to occur.

30
Q

What is prader willi syndrome?

A

Deletion of chromosome 15

31
Q

What is a translocation chromosome mutation?

A

Breakage of 2 chromosomes and exchange between them.

32
Q

What is bloom syndrome?

A

unusual sensitivity to DNA crosslinking agents

due to chromosome breakage disorder

Molecular and Experimental Genetics (10 decks)

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