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Class notes Biology Oxford IB Diploma Programme: Biology Course Companion
IBDP HL Biology - 2.2 Water Notes
Summary of the entire Biology book from the IB diploma
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10.1 Meiosis
Essential idea
Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells.
Understandings
Chromosomes replicate in interphase before meiosis.
Homologous chromosomes separate in meiosis I.
Sister chromatids separate in meiosis II.
Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes
in meiosis I.
Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
Crossing over is the exchange of DNA material between non-sister homologous chromatids.
Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
Skills
Drawing diagrams to show chiasmata formed by crossing over.
INTERPHASE
Chromosomes replicate in interphase before meiosis.
DNA is replicated during the S phase of interphase.
results in chromosomes that contain two identical DNA strands called sister chromatids.
sister chromatids are held together by a central region called the centromere.
these chromatids separate during meiosis II, becoming independent chromosomes each made of a single DNA
strand.
, If DNA replication did not occur prior to meiosis there would be no need for a meiosis II.
meiosis I = diploid → haploid.
basically chromosomes double then halve twice, could simply just halve once.
DNA replication suggests that meiosis evolved from mitosis (initial DNA replication is necessary).
benefit: duplication of chromatids increases potential genetic recombination (more variation).
MEIOSIS I
Homologous chromosomes separate in meiosis I.
Meiosis I is a reduction division (diploid → haploid) in which homologous chromosomes are separated.
Prophase-I
chromosomes condense.
nuclear membrane dissolves.
homologous chromosomes form bivalents.
crossing over occurs.
Metaphase-I
spindle fibres from opposing centrosomes connect to bivalents (at centromeres).
spindle fibres align bivalents along the middle of the cell.
Anaphase-I
spindle fibres contract and split the bivalent.
homologous chromosomes move to opposite poles of the cell.
Telophase-I
chromosomes decondense.
nuclear membrane may reform.
cell divides (cytokinesis) to form two haploid daughter cells.
Independent assortment
Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I.
Independent assortment describes how pairs of alleles separate independently from one another during gamete
formation.
the inheritance of one gene/trait is independent to the inheritance of any other gene/trait.
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