Numerical chromosomal abnormalities Flashcards
What structure is most important in forming the tetrads?
Synaptonemal complex
At which stage of meiosis are sister chromatids separated from each other?
anaphase II
Which part of meiosis is similar to mitosis?
meiosis II
Which part of meiosis is similar to mitosis?
meiosis II
In a comparison of the stages of meiosis to the stages of mitosis, which stages are unique to meiosis and which stages have the same events in both meiosis and mitosis?
→ All of the stages of meiosis I, except possibly telophase I, are unique because homologous chromosomes are separated, not sister chromatids.
→ In some species, the chromosomes do not decondense and the nuclear envelopes do not form in telophase I.
→ All of the stages of meiosis II have the same events as the stages of mitosis, with the possible exception of prophase II.
→ In some species, the chromosomes are still condensed and there is no nuclear envelope.
→ Other than this, all processes are the same.
What is segregation? (in terms of mitosis and meiosis)
The process of separation of chromatids or pairs of homologous chromosomes.
What can ‘segregation gone wrong’ look like? What is it called? How does this impact on fertilisation of gametes?
→ Both chromatids going to same pole (rather than splitting and going to opposite poles) so a pole has no chromatids- known as NON DISJUCTION- the daughter cells have too many or too few chromosomes.
→ This causes issues in fertilisation of gametes as chromosomes are unbalanced so the resulting foetus has too many or too few chromosomes.
→ Non disjunction in mitosis may affect just a few cells (so only a few cells affected by the aneuploidy in mitosis- see flashcard on mosaicism!) but in meiosis would affect all cells of the gametes!
What is aneuploidy?
What are some examples?
→ Aneuploidy is any chromosomal condition that results from having either a missing (monosomy) or extra chromosome (trisomy).
→ The most common type of aneuploidy is trisomy, which means there is an extra chromosome (e.g. trisomy 21 is Down’s syndrome).
→ A common monosomy, in which a chromosome is missing, is Turner syndrome.
eg female with only one X chromosome has turners syndrome
→ someone with Patau’s syndrome has an extra chromosome has an extra 13 (trisomy 13)
→ someone with Edward’s syndrome has trisomy 18
→ someone with Down’s syndrome has trisomy 21
What are the different types of chromosomes based on where the centromeres are?
Humans have 23 pairs of chromosomes
22 pairs autosomes, 1 pair sex chromosomes XX or XY
Metacentric
→ p & q arms even length
→ 1-3, 16-18
Submetacentric
→ p arm shorter than q
→ 4-12, 19-20, X
Acrocentric
→ Long q, small p
→ p contains no unique DNA
→ 13-15, 21-22, Y
What is the meaning of haploid, diploid, polyploidy and aneuploid?
HAPLOID: one set of chromosomes (n=23) as in a normal gamete.
DIPLOID: cell contains two sets of chromosomes (2n=46; normal in human)
POLYPLOID: multiple of the haploid number (e.g. 4n=92)
ANEUPLOID: chromosome number which is not an exact multiple of haploid number - due to extra or missing chromosome(s) (e.g. 2n+1=47) (trisomy, monosomy)
What happens in recombination? (in meiosis I)
→ homologous chromosomes align
→ a bivalent structure is formed
→ genetic material is exchanged (recombination)
Name some autosomal aneuploidies and some sex chromosome aneuploidies and their common-ness in the population.
→ Autosomal aneuploidies:
Trisomy 13 (Patau’s, 2 in 10,000 births)
Trisomy 18 (Edward’s, 3 in 10,000)
Trisomy 21 (Down’s, 15 in 10,000)
→ Sex chromosome aneuploidies:
Turner’s (45,X) (1 in 5000 female births)
Triple X syndrome (47,XXX) (1 in 1000 female births)
Klinefelter’s (47,XXY) (1 in 1000 male births)
47,XYY (1 in 1000 male births)
What is mosaicism?
The presence of two or more genetically different cell lines derived from a single zygote
