Cytogentics

Question 1

What is cytogenetics?

Answer 1

• study of chromosomes and cell division

Question 2

What does cytogenetics include the analysis of?

Answer 2

• analysis of G banding and other chromosome banding techniques
• also modern molecular cytogenetics techniques (e.g fluorescent in situ hybridisation (FISH))

Question 3

What are the specific staining techniques?

Answer 3

• Q banding
• G banding
• C banding
• R banding

Question 4

What is Q banding?

Answer 4

• the chromosomes are stained with a fluorescent dye e.g quinacrine (adenine-thymine rich portions stains with quinacrine)

Question 5

What is G banding?

Answer 5

• produced by staining with Giemsa after digesting the chromosomes with trypsin (adenine-thymine rich portions stained with Giemsa)

Question 6

What is C banding?

Answer 6

• chromosomes are treated with acid and base, then stained with Giesma stain

Question 7

What is R banding?

Answer 7

• is a reverse technique which stains the guanine-cytosine rich portions

Question 8

What does the location do on a chromosome?

Answer 8

• it describes the position of a particular band in a stained chromosome e.g 17q12

Question 9

What does the first letter or number describe about a genes location?

Answer 9

• represents the chromosome (1-22, the autosomes)
• the letter designated the arm of the chromosome on which the gene is found ( q is the long arm of xsome and p is short arm)
• followed by a number which is to indicate the position of the gene on the p or q arm. The position is usually designated by two digits ( representing a region and a band )

Question 10

What are the positions of the centromere?

Answer 10

• Metacentric : middle
• Submetacebtric : near the top
• Acrocentric : close to the top
• Telocentric : right at the top

Question 11

Comparing Karyotypes?

Answer 11

• differences in size and no. of chromosomes
• differences in position of centromeres - brought about by translocation
• differences in basic number of chromosomes
• differences in no. / position of satellites when they occur
• differences in degree and distribution of heterochromic regions (heterochromatin stains darker than euchromatin) indicating tighter packing and mainly consist of genetically inactive repetitive DNA sequences

Question 12

What is aneuploidy?

Answer 12

• the occurrence of one or more extra or missing chromosomes leading to an unbalanced chromosome complement or any chromosome no. that is not an exact multiple of the haploid number e.g monosomy

Question 13

What is monosomy?

Answer 13

• results in the presence of only one chromosome from the normal homologous pair

Question 14

Trisomy?

Answer 14

• results in the presence of 3 copies instead of the normal 2 homologous chromosomes

Question 15

What happens in meiotic non disjunction?

Answer 15

1. Meiosis starts normally
   : tetrads line up in the middle of cell
2. Then one set of chromosomes does not separate (ND)
3. Meiosis ll occurs normally
4. All gametes have an abnormal no. of chromosomes - either too many of one too few

Question 16

Downs Syndrome?

Answer 16

• 3 copies of chromosome 21

Question 17

What is Edwards syndrome?

Answer 17

• trisomy 18 aka Edwards syndrome is a genetic disorder caused by the presence of all or part an extra 18th chromosome

Question 18

Klinefelters syndrome? Gentotype XXY

Answer 18

• individuals exhibit a karyotype containing 47 chromosomes
• even though they are males, the extra X chromosome is evident
• they have underdeveloped internal and external sex organs
• they exhibit varying degrees of female secondary sexual characteristics including breast development, increased production of follicle stimulation hormone and white adipose tissue and body hair

Question 19

Turners Syndrome? Genotype: XO

Answer 19

• they have a karyotype containing 45 chromosomes
• the absent chromosome is an X resulting in an XO genotype
• characteristics : short stature sterility due to lack of gonads formation, wide neck, underdeveloped rests, failure to menstruate, lack of pubes

Question 20

Structural aberrations

Answer 20

• breakage of chromosomes can occur due to direct physical or chemical damage
• fragments of chromosomes may be lost during cell division or may join onto other chromosomes
• fragments may join to the homologous chromosomes causing duplication of some genes
• fragments may join to the original chromosome but in the reverse orientation ( inversion )
• fragments may join to a non-homologous chromosome ( translocation )

Question 21

What is translocation?

Answer 21

• a fragment of a chromosome is moved from one chromosome to another

Question 22

What is polyploidy?

Answer 22

• refers to a numerical change in a while set of chromosomes
• it occurs in cells and organisms when there are more than two homologous sets of chromosomes
• occurs in some animals e.g salmon
• it can occur in human tissues, especially liver