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Flashcards in Genetics-Meiosis in Female Deck (28):

Which genes on a chromosome are gene poor and which are gene rich?

Dark bands (G bands) are rich is GC and are gene poor. Light bands are rich in AT and are gene rich. Note that 13 and 18 are dark and are somewhat gene wastelands.


What typically happens in a cell that has an abnormality in one of the phases of mitosis?

It usually makes the cell inviable and initiates apoptosis. If apoptosis doesn’t occur this is a common initiating mechanism of cancer.


What happens in the two phases of meiosis?

Meiosis I: Division of cell from diploid (46) to haploid (23): Interphase I (duplication) -> Prophase I (homologues pair) -> Metaphase/Anaphase I (homologues separate) -> Telophase I (cell division) Meiosis II: Cell divides and remains haploid: Prophase II (chromosomes line up) -> Metaphase/Anaphase II (sister chromatids are pulled apart) -> Telophase II (cell division).


What protein is responsible for holding together homologous pairs in meiosis I?

Synaptonemal protein


What critical event of meiosis ensures that you will not be an exact duplicate of your parents’ genes?

Chiasmata formation (crossing over between tetrads from double-stranded DNA breaks). Also note that if crossing over does not occur the chromosomes do not separate because the synaptonemal protein is not interrupted.


Where on the chromosome region do you typically see crossing over?

Only occurs in stretched out euchromatin. Exchange is lowest near the telomere and highest near the centromere. Note that there is typically one exchange per chromosomal arm.


What is the mechanism of most translocations?

Chromosomes are misaligned with a different chromosome, crossing over occurs between the two and you get a translocation.


How does male meiosis differ from female meiosis?

Male starts at puberty, female completed at birth. Male take 74 days to complete, female egg arrests at Metaphase I.


What is dictyotene?

A primary oocyte. It is the arrested egg in Metaphase I where the chiasmata has formed and crossing over occurs. The egg may be in this state for 12-50 years.


What causes the dictyotene to continue and finish meiosis? 

The LH surge in the menstrual cycle triggers unequal cytokinesis, which results in extruding of one large egg (secondary oocyte) and one small polar body to complete meiosis I. Contact with sperm is what actually pushes the secondary oocyte into meiosis II, which results in extrusion of yet another polar body. 


What accounts for one large ovum and 3 polar bodies?

After each meiosis, a polar body is extruded and then one of the polar bodies also separates to form a third polar body.


Why are women at higher risk for abnormalities in meiosis as they get older?

The longer the egg spends in Metaphase one the more risk for impairment of segregation machinery (spindle apparatus defect, less nuclear membrane, mitochondrial energy deficit for spindle apparatus). 


Why are women at higher risk for having a Down syndrome baby as they get older?

There is an overall reduction of crossing over as the egg gets older. Crossing over also happens more at the telomeres as you age, decreasing the loosening of the chiasmata and chromatids are less likely to separate.


50-75% of early miscarriages are due to what type of aneuploidy?

Trisomy, especially trisomy 16. The most common single specific abnormality is a monosomy X.


What is the cause of the most common live born aneuploidy?

Down syndrome: trisomy 21. 93% of the time it is due to a complete extra chromosome 21 (95% of time from maternal meiosis I abnormality). 5% of the time is due to chromosomal rearrangement. Rarely it can be a mosaic. Trisomy 18 and 13 can also be born alive as well as Turner syndrome (45 X).


Characteristics of a Down syndrome baby?

Upslanting palpebral fissures, small ears, flat face, small mouth, protruding tongue, short fingers, transverse palmar crease and hypoplastic middle phalanx of 5th finger.


Characteristics of a baby with trisomy 18.

Clenched hands, overlapping digits, windswept ears, narrow delicate face and hammer toe.


What other health risks are kids with Down syndrome at risk for?

CVD, leukemia and early onset Alzheimer’s.


Characteristics of a baby with trisomy 13.

Median facial cleft and prosencephaly. Note that 90% die within the 1st year of life.


What causes Cri du chat? What characteristics will these kids have?

5p chromosome deletion. Kids have a round face, small head and characteristic cry that sounds like a meow.


What causes Williams syndrome? What characteristics will they present with?

7q chromosome deletion. Kids will have upslanting eyes, large full lips and a coarsening and thickened face as they age.


What causes Angelman syndrome? What characteristics will they present with?

Maternal deletion of 15q11. Small, non-verbal, wide-spaced angular mouth, frequent seizures and paroxysmal bursts of laughter.


What causes Prader-Willi syndrome? What characteristics do they present with?

Paternal deletion of 15q11. Fair skin, small hands, small feet, small genitalia, obesity and hyperphagia. There is also a risk for paroxysmal anger bursts as they get older.


What causes velocardiofacial syndrome? What characteristics do they present with?

Chromosome deletion of 22q11. Long slender nose, mouth and ear. This is also associated with Tetrology of Fallot.


What causes Klinefelter syndrome? When do they present?

47 XXY. They present with infertility.


What causes Turner syndrome? How do they present?

45 X. Edema in hands and feet, webbing of the neck.


What do the colors mean in this image of microarray testing?

Red: reference DNA. Green: specimen DNA. Yellow: equal hybrid DNA of reference and specimen. Note that most duplications in DNA are benign.


Massively parallel sequencing

Finds free fetal DNA within the mother’s plasma (5% from placenta). This can help you pick up trisomy 21, 18 and 13. Also it can pick up fetal Rh and sex.