3.3 Meiosis Flashcards
What is meiosis?
A reduction division of the nucleus to form 4 genetically distinct haploid gametes.
Meiosis has 2 main phases, with first division separating pairs of homologous chromosomes (diploid»_space;> haploid). The second division separates sister chromatids.
Why do gametes have to be haploid?
To sexually reproduce, sperm fertilises an egg. Therefore, both must contain half the number of chromosomes to result in a diploid zygote (23 + 23 = 46)
why is DNA replicated (S phase of interphase) before meiosis?
So all chromosomes consist of two sister chromatids. Homologous chromosomes split in meiosis I, then split into sister chromatids in meiosis II.
What is crossing over?
The exchange of segments of DNA between homologous chromosomes in prophase I. Chromosomes cross over (chiasma) and form bivalents, results in genetic recombination, offspring all have unique gene combinations not present in either parent.
What is random orientation?
Metaphase I, when chromosomes line up in homologous pairs in completely random order. Increases variation in gametes.
What is random fertilisation?
When sperm and egg fuse to form diploid zygote. “Random” as specific sperm and egg are chosen randomly.
What is karyotyping?
A visual representation of chromosomes in specific order. Cells harvested -> chemically induced to undergo cell division -> stained and photographed
What is non-disjunction?
When chromosomes fail to separate correctly in meiosis, resulting in gametes with extra/missing chromosome. Occurs in separation of homologous chromosomes/sister chromatids.
What is down syndrome?
When there are 3 copies of chromosome 21 instead of two (trisomy 21). Results in stunted growth, low muscle tone, mental impairment.
What is a nuchal translucency scan?
Ultrasound to measure thickness of nuchal fold (fluid at back of baby neck). Thicker = increased risk of chromosomal abnormalities
What is chorionic villi sampling?
Suction tube removing foetal cells from placenta. Could risk miscarriage and infection
What is amniocentesis?
Needle withdraws foetal cells from amniotic fluid from sac surrounding foetus. Risks infection, miscarriage and injury from needle.
