Numerical chromosomal abnormalities Flashcards
What is mitosis ?
The cell division process which produces somatic cells
What is meiosis?
The cell division process which produces gametes
What are the 3 stages of the cell cycle ?
- Interphase (G1,S,G2) - Normal cell growth, metabolic activity, DNA replication (to give 2 identical copies of each chromosome)
- Mitosis - Replicated DNA (in nucleus) separates (all body cells except sperm + egg). forms 2 identical daughter cells. each daughter cell has 46 chromosomes (like parent cell)
- Cytokinesis - Cell separates into two cells
What are the stages of Mitosis ?
Prophase:
Chromosomes condense + become visible.
Chromosome = 2 identical parts (sister chromatids) joined at centromere.
Spindle fibres start to form at opposite poles of cell.
Metaphase:
Mitotic Spindle pulls chromosomes to line up at cell equator
Anaphase:
Spindle fibres contract, pulling apart the sister chromatids to opposite poles of cell.
Telophase:
A nuclear membrane forms around each set of chromosomes. The chromosomes decondense + become less visible.
Cytokinesis:
Final stage of cell division
The cytoplasm divides, forming 2 genetically identical daughter cells.
Each daughter cell contains 46 chromosomes
Describe Meiosis
Meiosis = 2-step process which produces gametes.
Meiosis 1 and Meiosis 2
The reduction from 46 chromosomes (diploid) to 23 chromosomes (haploid number) occurs in Meiosis 1.
The chromosomes will first replicate, then recombination takes place.
- Interphase - chromosomes replicate.
- then Meiosis - homologous chromosome pair align along the spindle + swap genetic material (recombination). then spindle pulls chromosome pairs to opposite poles of cell + forms daughter cells. meiosis II separates the sister chromatids.
-meiosis forms 4 haploid daughter cells (23 chromosomes each, genetically non-identical) from 1 single diploid progenitor
Which errors occur in segregation?
Segregation:
- Separation of sister chromatids takes place in Mitosis and Meiosis II
- Separation of homologous chromosomes takes place in Meiosis 1
The process is called segregation. When segregation goes wrong e.g. both chromosomes/chromatids go to one pole (and none to the other) = non-disjunction.
In Meiosis nondisjunction, the resulting gametes are chromosomally unbalanced which can lead to errors of ploidy when gametes fertilise = foetus has too many/too few chromosomes
Triploidy = 69 chromosomes in zygote
What are aneuploidies and give examples
Aneuploidy = Abnormal number of chromosomes in a cell - 1 extra/missing chromosome
-Turner’s syndrome - XO (Missing X chromosome)
-Patau’s syndrome - Extra chromosome 13 = Trisomy 13
-Edwards syndrome = Trisomy 18
-Down’s syndrome = Trisomy 21
(congenital heart problems, digestive tract disorders, vision/hearing disorders, 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 - Cell contains 1 set of chromosomes (n=23) like in a normal gamete
Diploid - Cell contains 2 sets of chromosomes (2n=46, normal in human somatic cell)
Polyploid - A Multiple of the haploid number (4n=92)
Aneuploid - Chromosome number which is not exact multiple of haploid number - due to extra/missing chromosomes (2n+1=47) -trisomy, monosomy
How does Aneuploidy arise?
Aneuploidy arises 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 Metaphase
Non-disjunction in Meiosis 2 is when homologous chromosomes initially separate in meiosis I, but then both sister chromatids go into 1 single cell.
Describe the zygotes in meiotic non-disjunction (Meiosis 1)
Zygote = 2 Trisomy + 2 Monosomy
Describe the zygotes in meiotic non-disjunction
Meiosis 2
Trisomy
Monosomy
Normal -Diploid
Normal -Diploid
What is Recombination ?
Recombination = Process by which homologous chromosomes align in Meiosis I Metaphase and exchange genetic material. Generates genetic variation.
Describe Recombination
Recombination - Meiosis I Metaphase:
Homologous chromosomes align, form a bivalent structure and exchange genetic material (recombine)
What is a bivalent structure?
Bivalent structure = Synaptonemal complex binds 2 replicated homologous chromosomes together, allowing them to physically exchange genetic material.
This forms a Chiasma (point at which paired chromosomes remain in contact during the Meiosis I Metaphase)
Product = recombinant chromosomes
Describe Meiosis 1
homologous chromosomes
- Homologous chromosomes align as 23 bivalents
- Allows for chiasma formation
- Pulls apart homologues from one another
- Daughter cells have 23 chromosomes
Describe Meiosis II
- Align as independent chromosomes
- Sister chromatids are pulled apart
- Daughter cells have 23 chromosomes (1 sister chromatid each)
- Daughter cells are genetically unique
- No Recombination
What is independent assortment ?
Independent assortment = Maternal and Paternal chromosomes are sorted independently from one another during Meiosis 1.
= Further genetic variation
What are the autosomal aneuploidies?
Patau’s - Trisomy 13
Edward’s - Trisomy 18
Down’s - Trisomy 21
What are the sex chromosome aneuploidies?
Turners (45,XO)
Triple X Syndrome (47,XXX)
Klinefelter’s (47,XXY)
What is Mosaicism ?
The presence of 2 or more genetically different cell lines derived from a single zygote.
Mitosis (post-zygotic) nondisjunction – affects certain cells of individual = mosaicism = individual has a mixture of cell types
slide 30
How does Mosaicism occour ?
Mosaicism occurs when non-disjunction takes place in mitosis.
The cell does not split evenly.
1 of the daughter cells becomes trisomic and the other monsomic.
The monosomic cell will die.
Describe the difference b/w Full Monosomy and Partial Monosomy/ Trisomy
Full monosomy - Arises by Nondisjunction
Partial monosomy/trisomy - Microdeletion/Duplication syndromes - different mechanism - individual has deleted/additional parts of a specific chromosome - more common.
Example of an aneuploidy
Down’s syndrome (trisomy of chromosome 21)
numerical chromosomal abnormalities
numerical chromosomal abnormalities = having abnormal number of chromosomes
structural chromosomal abnormalities
part of chromosome is deleted/duplicated/translocated
mitosis =
meiosis =
mitosis = cell division that produces somatic cells meiosis = cell division that produces gamete cells (sperm, egg)
Turner’s Syndrome Symptoms
Fail to mestruate
Short stature
Puffy feet
Webbed neck
Most trisomy occurs due to ……….
Meiosis I Nondisjunction in the mother
-this stage is v long in women - starts before birth + ends at Ovulation
= aneuploidy risk increases w maternal age
= miscarriage risk increases in older women
nondisjunction is an error of ………. and can lead to ………………………………..
segregation
numerical chromosomal abnormalities
