L14- Mitosis and meiosis review Flashcards
missegregation in meiosis (non-disjunction)
• A third of all identified miscarriages • Infertility • Leading cause of learning disabilities
in a metaphase spread
chromosomes are stained when they are actively dividing - can then be grouped according to size and shape (A-G)
karyotyping
A way of organising/ numbering chromosomes in order of size and pattern
- How are karyotypes described (separated by commas, no space)
o Chromosome number o Sex complement o Structural changes E.g. 46,XY, Normal male karyotype
describe a male with trisomy 21
47, XY, +21
during proper disjunction of chromosomes
one chromatid goes to either pole
mosaicism
Presence of two or more cell lines in an individual
mosaicism can be
Throughout the body or tissue limited
in non disjunction
both chromatids go to one pole-causes aneuploidy
if mitotic nondisjunction occurs at the first post zygotic division
leads to non-mosaic karyotype - monosomy line usually lost unless involves X chromosome
if mitotic nondisjunction occurs in later cell divisions
leads to mosaic karyotype
degree of mosaicism
depends on when the nondisjunction occurs (i.e. during the first post-zygotic division or in later mitotic divisions)
mitotic nondisjunction can occur in both
meiosis I and II
the later the non-disjunction occurs in meiosis
the more likely you will have more normal cells
nondisjunction in the sex chromosomes - X chromosomes
• Living with one X is still viable i.e. in women one X chromosome is always silenced through imprinting • Living with 3 X is viable (2 silenced)
nondisjunction in the sex chromosomes- Y
XYY- though male may have more masculine traits e.g. over production of testosterone (criminal thing)
what stage is G0
- Separate stage outside the cell cycle directly after M - Quiescent stage Can be destination for the cell- e.g. nerve or cardiac cell which doesn’t need to divide anymore. Could enter cell cycle (temporary rest) again if correct signal are there
is random assortment or crossing over more important in eugenic variation
both equally important
how many chromosomes does every human somatic cells contain in G2, just before mitosis
47 chromosomes (92 chromatics)
during which phase of mitotic are the homologues chromosomes of each pair lined up
ever in mitosis- homologous pairs do not line up in mitosis (only in meiosis when maternal and paternal chromosomes line up)
- During mitosis the nucleolus disappears during prophase. - What is the nucleolus?
Region of 5 diff chromosomes where ribosomal RNA genes are expressed Heavily stained area in the nucleus
Why is it that the nucleolus is present in the nucleus during interphase, but absent during most of mitosis?
Nucleolus is absent during mitosis: Chromosomes condense in prophase Genes on chromosome will not be transcribed anymore Join other chromosomes to be separated in mitosis
How do the sex chromosomes ‘find each other’?
X and X have very similar sequences X and Y (different sequences) find each other due to having a small region of identical sequence
- Consider the consequences of a two crossing over events happening, one chiasma in PAR1 and one chiasma in AMELX/AMELY.
- Swap over between the 2 cross over events- any material between cross over will swap from non-sister chromatid to another
describe anaphase lag
Anaphase lag- chromosome either stays behind or is too slow and will stay behind and become degraded- diff to nondisjunction- unequal distribution
