Chromosomes, cell division, meiosis and chromosome abnormalities

Question 1

Basic structure and packing of chromosomes

Answer 1

At the centre there is a centromere, branching from this are telomeres with to sections; euchromatin (active genes) and heterochromatin (silenced genes). The top section with two telomeres is known as a chromatid. Chromatin is DNA packaged with histone and proteins packaged into nucleosomes.

Question 2

Mitosis

Answer 2

Cell division. In the prophase chromosomes condense and the nuclear membrane disappears. The spindle fibres form from the centriole. In the metaphase; chromosomes align at the cell equator, attached by fibres to each centriole. Anaphase; sister chromatids separate longitudinally at the centromere. Telophase; new nuclear membranes form and each cell contains 46 chromosomes. The cytoplasm then separates forming two new daughter cells.

Question 3

Purpose of packaging DNA

Answer 3

Negatively charged DNA is neutralised by positively charged histone. DNA takes up less space and inactive DNA remains folded until required.

Question 4

Meiosis

Answer 4

Cell division in germ cells. Diploid cells become haploid cells.

Question 5

Gametogenesis

Answer 5

Oogenesis- egg formation and spermatogenesis - sperm formation.

Question 6

Karyotype

Answer 6

The number and visual appearance of the chromosomes in the cell nuclei of an organism or species.

Question 7

Fluorescent In Situ Hybridisation (FISH)

Answer 7

A way to visualise and map the genetic material in individual cells. A probe is a short sequence of single stranded DNA that matches a portion of the gene the researchers are looking for. These probes are labelled by attaching coloured fluorescent dye. As the probes are single stranded they can bind to the complementary strand and its fluorescent tag allows researchers to see its location.
Unique sequence probes, centromeric probes useful to determine chromosome number, telomeric probes detect subtelomeric rearrangements which are often present in kids with unexplained mental retardation, whole chromosome probes used to see translocation and rearrangements.

Question 8

Structural Chromosome Abnormalities

Answer 8

1) Too few/too many; incorrect distribution of chromosomes is called nondisjunction. Trisomy - 3 copies of a chromosome e.g. Downs syndrome. Monosomy - missing a chromosome e.g. Turner syndrome.
2) Chromosome deletions; sometimes pieces of chromosome are lost or rearranged during meiosis. Terminal deletions are deletions of the tips of chromosomes and interstitial deletions mean the chromosome has broken, lost material and rejoined.
3) Translocations and rearrangements; reciprocal - swap between 2 chromosomes. Robertsonian - long arms of 2 acrocentric chromosomes fuse at a centromere losing the 2 short arms.