Numerical chromosomal abnormalities Flashcards
What is mitosis ?
The cell division process where somatic cells are produced
What is meiosis?
The cell division process where gametes are produced
What are the three stages of the cell cycle ?
Interphase -Normal cell growth and DNA replication
Mitosis -Replicated DNA separates
Cytokinesis -Cell separates into two
What are the stages of Mitosis ?
Prophase -
The chromosomes condense and become visible.
Two identical parts (sister chromatids joined at centromere)
Spindle fibres form and join at the centromere.
Metaphase -
The chromosomes will line up at the middle
Anaphase:
The spindle fibres contract pulling apart the sister chromatids to opposite poles.
Telophase-A nuclear membrane will form around each set of chromosomes, the chromosomes will coil and become less visible
Cytokinesis: The final stage , the cytoplasm will divide and two identical daughter cells will form
Describe Meiosis
This is a two step process which forms gametes.
Meiosis 1 and Meiosis 2
The reduction to 23 chromosomes takes place in meiosis 1.
The chromosomes will first replicate, then recombination takes place.
What are some errors which can occur in segregation?
Separation of sister chromatids takes place in Mitosis and Meiosis 2
Separation of homologous chromosomes takes place in Meiosis 1.
The process is called segregation. When it goes wrong for e.g. both chromosomes, chromatids goes to one pole its called non disjunction.
In Meiosis , the resulting gametes are chromosomally unbalanced which can lead to errors of ploidy.
Triploidy=69 chromosomes
What are aneuploidy’s and give examples
This is the presence of an abnormal number of chromosomes in a cell-single extra or missing.
Turners syndrome -XO (Missing X)
Patau’s syndrome -Extra 13 -Trisomy 13
Edwards syndrome- Trisomy 18
Downs syndrome -Trisomy 21
(congenital heart problems
digestive tract
leukaemia)
Describe a typical karyotype
-Humans have 23 pairs of chromosomes
(22 pairs autosomes, 1 pair sex chromosomes XX/XY)
-Metacentric
p and 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 are some numerical abnormalities of chromosomes?
Haploid- One set of chromosomes (n=23) like in a normal gamete
Diploid-Cell contains two sets of chromosomes (2n=46, normal in human)
Polyploid -Multiple of the haploid number (4n=92)
Aneuploid-chromosome number which is not exact multiple of haploid number-due to extra or missing chromosomes (2n+1=47) -trisomy, monosomy
How does Aneuploidy arise?
This is through Meiotic non-disjunction which is where an error occurs in the chromosome segregation.
This can occur in the segregation of homologous chromosomes -Meiosis 1
Non-disjunction in Meiosis 2 is when they initially separate but then both chromatids go into a single cell.
Describe the zygotes in meiotic non-disjunction (Meiosis 1)
2 Trisomy + 2 Monosomy
Describe the zygotes in meiotic non-disjunction
Meiosis 2
Trisomy
Monosomy
Normal -Diploid
Normal -Diploid
What is recombination ?
This is the process which causes the generation of genetic variation and is the reason that the chromosomes align in Meiosis 1.
It is the physical exchange of genetic material between homologous chromosomes.
It takes place in Metaphase of Meiosis 1
Describe recombination
- Homologous chromosomes align
- Form a bivalent structure
- Exchange genetic material(recombine)
What is a bivalent structure?
This is the association of two replicated homologous chromosomes.
They are stuck together by the synaptonemal complex which allows them to physically exchange genetic material.
This will form a Chiasma(
point at which paired chromosomes remain in contact during the first metaphase of meiosis
)
The product is recombinant chromosomes
