IB TOPIC 10 | GENETICS AND EVOLUTION
2016 | SYJ0014
Topic 10.1: Genetics and evolution – Meiosis
Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells.
• Understanding: Chromosomes replicate in interphase before meiosis.
DNA is replicated in the interphase before meiosis
This produces chromosomes, each made of two chromatids joined at the centromere
• Understanding: Crossing over is the exchange of DNA material between non-sister homologous chromatids.
Crossing over: exchange of genetic material between non-sister homologous chromatids
Location: occurs in prophase of meiosis I (prophase I)
Process of crossing over:
Paired synapsis: pairing of homologous chromosomes
Breaks in DNA: breaks occur at corresponding point in non-sister chromatids
Exchange of DNA: non-sister chromatids exchange gene
Chiasma formation: non-sister chromatids continue to adhere at the chiasma
Stabilization of bivalent: connection via chiasmata is essential for stabilising the structure for meiosis
• Understanding: Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
Chiasma: the point at which crossing over and exchange of genetic material occur between the strands during synapsis
Consequences of chiasmata formation: result in the exchange of alleles for a chromosome that leads to recombination
• Understanding: Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
Chiasma and different combination of alleles: chromosomes can have different combination of alleles
Crossing over can decouple linked combination allowing for independent assortment
Exchange of portion of DNA can result in the alleles in to chromosomes being “mixed up”
• Understanding: Homologous chromosomes separate in meiosis I.
Meiosis I: homologous chromosomes separate
Chromosome number: meiosis I reduces chromosome number into half
Sister chromatids: sister chromatids remain associated with each other
• Understanding: Sister chromatids separate in meiosis II.
Meiosis II: sister chromatids separate
Chromosome number: chromosome number in meiosis II does not change
Sister chromatids: sister chromatids are likely non-identical due to crossing over
• Understanding: Independent assortment of genes is due to the random orientation of pairs of homologous chromosome in meiosis I
Mendel’s second law (law of independent assortment):
Statement: homologous pair of each chromosome is segregated without regard to how other pairs segregate
Implication: there are no order or pattern how homologous chromosomes orientate before segregation
Random orientation: homologous pairs will randomly orientate (50% chance either direction) during meiosis I
Increasing variety: this further gives new combination of chromosome in haploid cells
• Skill: Drawing diagrams to show chiasmata formed by crossing over.
Drawing of chiasmata formation:
Close alignment: all chromatids are closely aligned and elongated
Indication of breaks: position at which the crossing over is going to occur can be shown
with breaks at the same point in each chromatid
Formation of chiasma: each break of one chromatid is connected to the break in non-sister
chromatid and vice versa
Chiasma: X-shaped knot-like structure formed in crossing over
Breakdown of chiasma: chiasma side to the end after a while to allow division of
homologous pairs in meiosis I
• Nature of science: Making careful observations—careful observation and record keeping turned up anomalous data that Mendel’s law of
independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies.
• Guidance: Diagrams of chiasmata should show sister chromatids still closely aligned, except at the point where crossing over occurred and a chiasma
was formed.
LAST EDITED 2017-03-17 | 1
2016 | SYJ0014
Topic 10.1: Genetics and evolution – Meiosis
Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells.
• Understanding: Chromosomes replicate in interphase before meiosis.
DNA is replicated in the interphase before meiosis
This produces chromosomes, each made of two chromatids joined at the centromere
• Understanding: Crossing over is the exchange of DNA material between non-sister homologous chromatids.
Crossing over: exchange of genetic material between non-sister homologous chromatids
Location: occurs in prophase of meiosis I (prophase I)
Process of crossing over:
Paired synapsis: pairing of homologous chromosomes
Breaks in DNA: breaks occur at corresponding point in non-sister chromatids
Exchange of DNA: non-sister chromatids exchange gene
Chiasma formation: non-sister chromatids continue to adhere at the chiasma
Stabilization of bivalent: connection via chiasmata is essential for stabilising the structure for meiosis
• Understanding: Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
Chiasma: the point at which crossing over and exchange of genetic material occur between the strands during synapsis
Consequences of chiasmata formation: result in the exchange of alleles for a chromosome that leads to recombination
• Understanding: Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
Chiasma and different combination of alleles: chromosomes can have different combination of alleles
Crossing over can decouple linked combination allowing for independent assortment
Exchange of portion of DNA can result in the alleles in to chromosomes being “mixed up”
• Understanding: Homologous chromosomes separate in meiosis I.
Meiosis I: homologous chromosomes separate
Chromosome number: meiosis I reduces chromosome number into half
Sister chromatids: sister chromatids remain associated with each other
• Understanding: Sister chromatids separate in meiosis II.
Meiosis II: sister chromatids separate
Chromosome number: chromosome number in meiosis II does not change
Sister chromatids: sister chromatids are likely non-identical due to crossing over
• Understanding: Independent assortment of genes is due to the random orientation of pairs of homologous chromosome in meiosis I
Mendel’s second law (law of independent assortment):
Statement: homologous pair of each chromosome is segregated without regard to how other pairs segregate
Implication: there are no order or pattern how homologous chromosomes orientate before segregation
Random orientation: homologous pairs will randomly orientate (50% chance either direction) during meiosis I
Increasing variety: this further gives new combination of chromosome in haploid cells
• Skill: Drawing diagrams to show chiasmata formed by crossing over.
Drawing of chiasmata formation:
Close alignment: all chromatids are closely aligned and elongated
Indication of breaks: position at which the crossing over is going to occur can be shown
with breaks at the same point in each chromatid
Formation of chiasma: each break of one chromatid is connected to the break in non-sister
chromatid and vice versa
Chiasma: X-shaped knot-like structure formed in crossing over
Breakdown of chiasma: chiasma side to the end after a while to allow division of
homologous pairs in meiosis I
• Nature of science: Making careful observations—careful observation and record keeping turned up anomalous data that Mendel’s law of
independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies.
• Guidance: Diagrams of chiasmata should show sister chromatids still closely aligned, except at the point where crossing over occurred and a chiasma
was formed.
LAST EDITED 2017-03-17 | 1
