10.1 Meiosis
Essential Idea: Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells.
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10.1 U1 Chromosomes replicate in interphase before meiosis.
10.1 U1 Chromosomes replicate in interphase before meiosis.
Be able to:
Identify tetrad, bivalent, sister chromatids and non-sister chromatids in diagrams of replicated chromosomes.
During the S phase of the cell cycle, so that each chromosome has a copy of itself and consists of two sister chromatids. During meiosis I, chromosomes condense and synapse to form bivalents (homologous chromosomes are aligned next to each other).
10.1 U2 Crossing over is the exchange of DNA material between non-sister homologous chromatids.
10.1 U2 Crossing over is the exchange of DNA material between non-sister homologous chromatids.
Be able to:
State that crossing over occurs during prophase I
Define chiasmata
Crossing over involves the exchange of segments of DNA between homologous chromosomes during Prophase I of meiosis
The process of crossing over occurs as follows:
Homologous chromosomes become connected in a process called synapsis, forming a bivalent (or tetrad)
Non-sister chromatids break and recombine with their homologous partner, effectively exchanging genetic material (crossing over)
The non-sister chromatids remain connected in an X-shaped structure and the positions of attachment are called chiasmata
10.1 U3 Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
10.1 U3 Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
Be able to:
Draw a diagram to illustrate the formation of new allele combinations as a results of crossing over.
As a result of crossing over, chromatids may consist of a combination of DNA derived from both homologues - these are called recombinants (new combinations). The formation of recombinants basically makes the possible allele combinations unlimited. Crossing over results in new combinations of alleles in haploid cells and thus increases the genetic diversity of potential offspring
10.1 U4 Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
10.1 U4 Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
Be able to:
State two consequences of chiasmata formation between non-sister chromatids.
During prophase I of meiosis, homologous chromosomes become connected in a process known as synapsis. During synapsis, prophase I, homologous chromosomes pair forming bivalents in synapsis, non-sister chromatids may break and recombine with their homologous partner (crossing over). These non-sister chromatids remain physically connected at these points of exchange – regions called chiasmata.
Chiasmata are points where two homologous non-sister chromatids exchange genetic material during crossing over in meiosis. Chromosomes intertwine and break at the exact same positions in non-sister chromatids. The two chromosomes are now attached at the same corresponding position on the non-sister chromatid. Many chiasmata can form between the chromatids.
Once attached the non-attached portions of the chromatids actually repel each other. Chiasmata refer to the actual break of the phosphodiester bond during crossing over. The chiasmata are separated during anaphase 1 which can result in an exchange of alleles between the non-sister chromatids from the maternal and paternal chromosomes.
10.1 U5 Homologous chromosomes separate in meiosis I.
10.1 U5 Homologous chromosomes separate in meiosis I.
Be able to:
Contrast meiosis I with meiosis II.
Prophase I
Homologous chromosomes are attracted to (associate with) each other and pair up and become sister chromatids.
Crossing over (recombination) takes places between non-sister chromatids - chiasmata is the location at which crossing over takes place on chromosomes.
Nuclear membrane disintegrates.
Metaphase I
The bivalents (pairs of homologous chromosomes i.e. two sister chromatids) line up at/across the cell's equator.
Spindle fibres made from microtubules form.
Random orientation takes place.
Anaphase I
Spindle fibres from the poles at each end of the cell attach to one sister chromatid and pull them (contract) to opposite poles.
Non-disjunction (when pairs of chromosomes do not separate) can occur here and will affect the chromosome number of all four gametes.
Telophase I
Spindles and spindle fibres disintegrate.
Usually, the chromosomes uncoil and new nuclear membranes form.
10.1 U6 Sister chromatids separate in meiosis II.
10.1 U6 Sister chromatids separate in meiosis II.
Be able to:
Compare meiosis II with mitosis.
Prophase II
DNA condenses into visible chromosomes again.
No crossing over takes place.
Nuclear membranes disintegrate.
Metaphase II
Sister chromatids line up at the equator of each cell.
Random orientation takes place again.
Anaphase II
Spindle fibres form and break the centromeres of each sister chromatid, releasing each as individual chromosomes.
Spindle fibres contract and pull chromosomes to opposite poles of the cells.
Because of random orientation, the chromosomes can be pulled towards either of the newly forming daughter cells.
Non-disjunction can occur here as well.
Telophase II
Chromosomes unwind their strands of DNA.
Nuclear envelopes form around each of the four haploid cells, preparing them for cytokinesis (cytoplasmic division of each of the two cells).
10.1 U7 Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I.
10.1 U7 Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I.
Be able to:
Describe random orientation and independent assortment.
Given a parent cell genotype, determine the allele combinations that are possible in the gametes due to independent assortment and random orientation.
When Mendel first did his experiments on pea plants, he looked at the traits that were passed on from generation to generation. He did not know how the traits were inherited in terms of meiosis. We now know that independent assortment is an essential component in explaining how chromosomes align themselves during meiosis law of independent assortment applies only to traits carried on different chromosomes, i.e. unlinked genes
independent assortment occurs as a result of the alignment of homologs during metaphase I, determining which maternal and paternal chromosomes assort to each daughter cell
each pair of alleles separates independently of every other pair of unlinked allele
variety produced by recombination of maternal and paternal chromosomes
for each pair of homologous chromosomes, maternal and paternal chromosomes assort to daughter cells randomly
possible arrangements of chromosomes in haploid daughter cells = (2)nth, where n = number of homologous pairs
in humans, n = 23, and possible arrangements = (2)23 = about 8 million
10.1 S1 Drawing diagrams to show chiasmata formed by crossing over.
10.1 S1 Drawing diagrams to show chiasmata formed by crossing over.
Be able to:
Draw a diagram to illustrate the process and result of crossing over.
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