Human Chromosomes and chromosomal abnormalties Flashcards
What are somatic chromosomes?
Contain two copies of each chromosomes (homologous pairs), except the sex chromosomes.
How many chromosomes are there in human cells?
How many in a gamete?
46
23
What is the karyotype?
A full set of chromosomes in a cell.
How many chromosomes are there on each chromosome type?
2 (one maternal and one paternal)
Same genes in the same order on the homologous genes in each pair.
How is the order of chromosomes determined?
Except for what?
Size of the chromosome
Except for chromosome 21 which is smaller than 22.
Chromosome anatomy: G band? Telomeres? rDNA? Centromere?
G band = region that takes up Giema stain, giving characteristic pattern (dark band takes up lots of stain).
Telomeres = specialised repeated DNA sequence, protect ends of chromosomes.
rDNA = DNA encoding rRNA.
Centromere = where the spindle attaches to the chromosome.
Which chromosome arm size is smaller and which is larger?
p is smaller and q is larger.
What are telomeres and why are they kept this length?
Every time we replicate our chromosome, it gets a bit shorter so we have repeated sequences of DNA telomeres.
This means we can lose this DNA and it doesnt matter.
What is non-centromeric heterochromatin?
Non-coding genes present here.
Give information on fluorescence in situ hyrbidisation
What can chromosome painting detect?
- Specific cloned DNA sequences tagged with fluorescent chemicals are hybridisation (base-pairee) to the chromosome DNA sequence
- Chromosomes viewed under fluorescence microscope: image computer enhanced
Can detect small structural abnormalities in the chromosomes. This technique depends on FISH.
What is an inversion on a chromosome?
Inversion abnormality - piece of the chromosome has come out and flipped over and attached back into the chromosome the wrong way around.
Which sex chromosome carries fewer genes?
Y chromosome
Which gene encodes for testis determining factor?
SRY gene
What do Leydig cells of testis secrete?
They secrete testosterone, an androgenic steroid hormone, which activates male-specific gene expression.
Explain chromosome X inactivation
Explain how this persists
Explain what a Barr Body is
One X chromosome means that it will get permanently unactivated.
This inactivation choice is random.
All descendants of these cells will have the same X chromosome inactivated.
X inactivation persists through cell division (so the adult female is a genetic mosaic).
Barr body - the inactive X chromosome once it has condensed.
Explain polyploidy, triploidy, tetraploidy?
Polyploidy - multiple sets of chromosomes in humans
Triploidy - 3n
Tetraploidy - 4n
What is aneuploidy?
What does this arise due to? What is this?
Having one extra or one less number of chromosomes.
Failure of homologous chromosomes to separate at anaphase. This occurs by chance. Cells have double the number of chromosomes.
One cell ends up with two copies and one ends up with no copies when disjunction happens and at second division.
Give the 4 subtopics under aneuploidy
- Gene dosage: important for normal development, so aneuploidy causes abnormalities.
- Nullisomy (2n-2). Loss of one homologous pair of chromosomes. No viable human examples
- Monosomy (2n-1). Loss of a single chromosome. One viable human example (Turner syndrome)
- Trisomy (2n+1). One extra chromosome. Several viable human syndromes.
What is Klinefelter syndrome?
Extra sex chromosome (2 X and 1 Y).
Male.
Common cause of male fertility.
Tall stature, some learning difficulties, underdeveloped testes, breast development.
Testosterone can improve secondary sexual characteristics.
What is Turner syndrome?
X and no other sex chromosome.
Female.
Few noticeable defects until puberty: do not develop secondary sexual characteristics.
Mainly infertile.
Explain down syndrome
All autosomal monosomies and most autosomal trisomies can survive birth.
Trisomy chromosome 21
Many developmental abnormalities, low IQ
Only autosomal trisomy to survive to adulthood, but life expectancy low.
Why is trisomy 21 the only human autosomal trisomy to survive to adulthood?
This is the shortest autosome.
Extra copy of 21 - not as many genes as if you had an extra number of chromosome 1 meaning that survival is more likely.
Why are numerical sex chromosome abnormalities better tolerated than numerical autosomal abnormalities?
Few genes on Y chromosome. All but one X chromosome inactivated. Not as many genes on Y so not missing as many chromosomes if it is not there.
So why do Turner syndrome and Klinefelter syndrome individuals have any abnormalities?
Pseudoautosomal region on X chromosome is not included in X inactivation: gene dosage of genes in this region is abnormal.
What do structural chromosomal abnormalities arise from?
What are they induced by?
Arises from breaks in chromosomes.
Induced by radiation, viruses, chemicals, transposable elements, errors in crossing over.
Inversions:
What is pericentric?
What is paracentric?
Why may it not be a problem?
Pericentric - inversion includes centromere
Paracentric - one chromosome arm
No loss of material, but breaks may occur within genes or regulatory regions.
In heterozygous, duplication and deletions can arise when —- takes place within an inversion during —-.
What does this mean for individuals with inversions?
Recombination
Meiosis
Means that the individual may be normal but may have children with severe chromosomal abnormalities.
What is a translocation and what are the three types?
Translocation - change in position of chromosome segment.
Reciprocal and non-reciprocal, intrachromosomal.
Reciprocal is swapping bits between chromosomes, non-reciprocal is moving bits from one chromosome into another and intrachromosomal is a translocation in one chromosome.
In translocation, heterozygous segregation at meiosis can lead to what …?
Can lead to genetically unbalanced gametes.