Flashcards in 3.3 Meiosis Deck (14):
A reductive cell division process which produces four haploid gametes from one diploid somatic cell. (Meiosis I is reductive: the number of allele varieties per gene is reduced from 2 to 1).
What leads to genetic variations in meiosis and sexual reproduction in general?
Random orientation in metaphase I & II
Crossing over in prophase I
Random sperm fertilizing the egg (random combination of gametes)
Nucleus membrane dissolves
Supercoiling (condense) of chromosomes
Replicated homologous chromosomes form bivalents by synapsis
Crossing overs --> reshuffle the alleles between homologous pairs
Centrioles migrate to the poles of the cell
Bivalents line up at the equator
Spindle fibres attach to the centromeres from the centrioles at the poles
Random orientation: the centriole to which each sister chromatid pair faces, and eventually is pulled towards is random and independent for each bivalent
Segregation of alleles
Disjunction of chromosomes
Spindle fibres contract
Sister chromatid pairs for each chromosome are pulled to opposite poles
Where crossing over takes place
The separation of alleles into different nuclei.
Decondense of chromosomes
Nuclear membrane and nucleolus reform
Cytokinesis takes place: the cytoplasm begins to divide
When homologous chromosomes/sister chromatids fail to separate during anaphase I or anaphase II, causing the gametes produced to have one more (trisomy)/one fewer (monosomy) chromosome.
Ex) Down syndrome, caused by disjunction in 21st pair
Name the two sampling methods for karyotyping
1. Amniocentesis sampling
2. Chorionic villus sampling
Sampling 20 ml amniotic fluid
Using a syring, which is directed by ultrasound
The amniotic fluid contains cells from the fetus, which can be used for karyotyping
1% chance of miscarriage
Chorionic villus sampling
Sampling villus cells from the chorion membrane of the placenta
Using a needle guided by ultrasound
The cells sampled are then cultured until there is sufficient amount for karyotyping
2% chance of miscarriage
What test is taken before amniocentesis and chorionic villus sampling? And why?
Nuchal translucency scan: which involves examining the amount of nuchal fluid behind the neck of the fetus.
Large Q of nuchal fluid is correlated with Down syndrome
Not 100% accurate, but is non-invasive: mothers with a large amount of nuchal fluid will then take the invasive amniocentesis/chorionic villus samples.