chromosome, cell division, meiosis and chromosome abnormalities

Question 1

Describe the basic structure and packaging of chromosomes.

Answer 1

Chromosome structure:
-Telomere: at chromosome ends. 5’-3’ TAAGGG repeated sequences.
-Chromatin: heterochromatin- silenced DNA-condensed
euchromatin- active DNA-open structure
-Centromere: holds together two chromatid sisters.
Site of kinetochore: where spindle fibres attach during replication

1-Chromosomes wrap around proteins called histones
2-146 DNA base pairs wrap around 6 histones to form a nucleosome- beads on string
3- 6 nucleosomes per turn
1 structure: nucleosome
2-chromatin fibre-30nm
3-fibre-scaffold complex-300nm
4-Chromosome-700nm

Question 2

Describe the laboratory diagnosis of genetic disease due to abnormalities in chromosome number.

Answer 2

types of DNA probe:
-centromeric- used to determine number of chromosomes 
-telomeric- used to determine subtelomeric rearrangements
-unique processes
-whole chromosomes: cocktail of different probes
determine rearrangements
KAROYOTYPE: 
Sample of venues blood-filter off RBC
Add medium to WBC and incubate for 3 days at 37 degrees
add colchine
separate off WBC
add hypotonic saline
place on plate
take photographs
FLUORESCENT IN SITU HYBRIDISATION:
label DNA probe with fluorescent label
denature and hybridise

Question 3

mitosis

Answer 3

Interphase:
G1- cell grows
S- replication of DNA
G2- cell gets ready to divide
Prophase: nuclear membrane disappear, spindle fibres form, chromosomes condense
Metaphase: chromosome align along equator and spindle fibres bind to centrioles
Anaphase: spindle fibres contract and pull the chromatid sisters to opposite sides of the cell
Telophase: Chromatid sisters reach opposite sides of the cell and nuclear membrane reforms

Question 4

Describe the types of structural chromosomal abnormalities.

Answer 4

1-Balanced: 2 homologous chromosomes undergo reorganisation but amount and type of DNA- genome-stays the same
2-Unbalanced: occur when one parent has normal chromosomes in gamete, other parent has balanced chromosomes
3- Deletion/insertion
4-Inversion:
-paracentric- not at centromere
-pericentric- at centromere
5- Translocations: can be balanced or unbalanced.
-Reciprocal : 2 chromosomes broken to form 2 new chromosomes.
-Robertsonian: two acrocentric-centromere far from centre- are fused together

Question 5

Meiosis and gametogenesis

Answer 5

Meiosis:

• from 2 diploid cells- 4 haploid cells
• In meiosis 1- non-sister chromatids cross over to form chiasmata which allows transfer of parts of chromosomes (DNA)- allows variation. Homologous pairs separate
• In meiosis 2- sister chromatids separate

Spermatogenesis- gives rise to 4 spermatids. Begins in puberty. Lasts 60-65 days.

Oogenesis: 1 ovum and 3 polar Bodies. Begins when embryo develops. Lasts 10-50 years

Question 6

Fertilisation

Answer 6

2 Haploid cells fuse to form a diploid cell (zygote)
Initially this cell has both X chromosomes activated.
In early embryo one of the X chromosomes is switched off.