This document contains notes and summaries made during class which summarizes: The process of Meiosis I & Meiosis II; chromosomes; disorders associated with meiosis and the difference between mitosis and meiosis.
A chromosome consist of a chromatid
joined by a centromere.
A set of chromosomes from the nucleus
are sorted into pairs according to features
such as their size, shape and position of
the centromere.
Chromosome pairs 1-22 are called
autosomes.
The last pair of sex chromosomes are
called gonosomes.
After interphase, a chromosome consist
of two chromatids joined by a single
centromere.
Each chromosome of a homologous pair
comes from one of the parents.
Chromosome pairs coming from father
called paternal chromosome.
Chromosome coming from mother is
called maternal chromosome.
➢ Two nuclear divisions.
➢ Meiosis 1 = Occurs after DNA replication and results in two identical
daughter nuclei.
➢ Meiosis 2 = Results in 4 daughter nuclei.
• Crossing- over during Prophase Ⅰ
➢ Pairs of homologous chromosomes exchange small parts of the
chromatids with each other to form bivalents.
➢ The point where crossing-over occurs is called a chiasma.
➢ There can be multiple chiasma along a chromosomes.
• Meiosis I
➢ Interphase:
- DNA replication results in 1 chromosome consisting of 2
chromatids joined at the centromere.
- When homologous chromosomes come together there are
4 chromatids.
➢ Prophase Ⅰ:
- Chromosomes condense and become visible. [ 2 identical chromatids joined
by a centromere.]
- Homologous chromosomes pair up to form bivalents.
- Crossing-over occurs between non-sister chromatids.
- Centrioles and spindle.
- Nuclear membrane and nucleolus disappear.
Possible exam answer:
= Crossing-over occurs at points called
chiasmata between chromatids of pair of
homologous chromosomes during
prophase Ⅰ
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