Chromosome and their abnormalities Flashcards
How long is DNA?
3x10^9 base pairs long (3bn)
What is the normal human karyotype?
46 chromosomes
(46XY or 46XX)
22 pairs of autosomes and 2 sex chromosomes
How can we prepare chromosomes to look at?
From: blood lymphocytes Bone marrow cells Skin fibroblasts Amniotic fluid cells Chorionic villus Fetal Blood Giemsa banding
What are G banding chromosomes?
G - Giesma (stain for chromosomes)
chromosome pairs identical in banding pattern
What may vary when looking at G banded chromosomes?
Size of heterochromatic blocks at the centromere
satellite regions
Y chromosome
what determines the banding pattern of chromosomes?
The banding pattern may vary depending on the length of the chromosome
What is the purpose of mitosis?
cell division - produces genetically identical full complement of DNA
Normal growth of organism and in wound healing
Why are microtubule targeting drugs important in cancer?
As they target the microtubules forming during tumour proliferation and hence prevent the cells from dividing
What is the function of meiosis?
Reduction division from 46 chromosomes to only 23 per gamete
Reassortment of genes
What is the basic mechanism of meiosis?
Each homologue replicates to give two chromatids
homologous pair
exchange of material between non-sister chromatids (crossing over / recombination)
What are Chiasmata?
sites of crossing over
Where does variation come from during meiosis?
Independent assortment of maternal and paternal chromosomes
Presence of crossing over exchanges between homologous chromosomes (recombination)
what happens during normal meiosis for a female?
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.
What are some sex differences in meiosis?
- 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. - 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
What is chromosomal non-disjunctions?
where there is uneven distribution of chromosomes into the new cells resulting from a chromosome pair failing to separate.
What is the result of a non-disjunction of sex chromosomes at the 1st and 2nd meiotic division of a female cell
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
What is klinefelters syndrome?
Sex chromosome non disunction disease
47XXY producing a normal but infertile male
What is Turners syndrome?
Sex chromosome non disjunction disease
45X0 producing a short stature, webbed neck infertile female
what are some other possible results from non disjunction of sex chromosomes
47 XXX - outwardly normal female with some fertile
47 XYY - tall or very tall male, relatively infertile
give an example of an autosomal non-disjunctional mutation and its syndrome it causes
Trisomy of 21
leads to down syndrome
(47 XX+21 or 47XY+21)
how does the extra chromosome 21 arise in down syndrome?
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
What is translocation down syndrome
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.
At what age is there the greatest individual risk of giving birth to a child with down syndrome
Older age >50 years old.
What is the result of a non-disjunction of sex chromosomes at the 1st and 2nd meiotic division of a male cell
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
