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Transcript of Meiosis
Homologous pairs associate
Crossing over can occur at chiasmas Metaphase I Crossing over also at prometaphase I
Bivalents (homologous pair) line up at the equator, two by two, random orientation of chromosomes
Spindle fibres attach to centromeres Anaphase I Telophase I Spindle fibres contract to pull chromosomes to opposite poles
Centromeres do not separate
One chromosome from each homologous pair moves to opposite ends of cell Cytokinesis Cleaving of the cytoplasm forms 2 diploid cells State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei.
Define homologous chromosomes.
Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells. Limit crossing over to the exchange of genetic material between non-sister chromatids during prophase I. Names of the stages are required.
Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21). The characteristics of Down syndrome are not required.
State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.
State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.
Analyse a human karyotype to determine gender and whether non-disjunction has occurred. 4.2.1
4.2.7 Describe the behaviour of the chromosomes in the phases of meiosis.
Outline the formation of chiasmata in the process of crossing over.
Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.
State Mendel’s law of independent assortment.
Explain the relationship between Mendel’s law of independent assortment and meiosis. 10.1.1
10.1.5 SL HL Prophase II No homologous pairs
No crossing over
Chromosomes condense again Metaphase II Chromosomes align singly & randomly
Spindle fibres attach to each centromere
Chromosomes are randomly orientated Anaphase II Telophase II New haploid nuclei form
Results in four haploid gamete cells Cytokinesis Cleaving of cells to form 4 daughter haploid cells
Haploid cells develop into gametes Interphase (Interkinesis) Organelles produced
No further DNA replication Mitosis Vs. Meiosis This is where mitosis and meiosis differ most significantly
Meiosis now undergoes a second nuclear division Crossing over/recombination: Chromosomes from your parents are randomly distributed into your gametes
Each new gamete can have all your maternal or all your paternal chromosomes or any combination of the two.
Crossing over in Prophase I
There are 2^23 possible orientations, a huge number of maternal and paternal chromosome combinations. Bibliography:
"Brent Cornell." 4.2 Meiosis. N.p., n.d. Web. 03 Mar. 2013. <http://ib.bioninja.com.au/standard-level/topic-4-genetics/42-meiosis.html>.
"TOPIC 3 The Voice of the Genome." Edexcel AS Biology. Harlow: Pearson Education, 2008. 158-59. Print.
Nature.com. Nature Publishing Group, n.d. Web. 03 Mar. 2013.
McDarby, Michael. "Introduction to Biology Molecules and Cells Chapter 7 - DNA, Genetics, and Reproduction." N.p., n.d. Web. 3 Mar. 2013.
"Cell Division, SEM Poster." Cell Division, SEM Poster. N.p., n.d. Web. 03 Mar. 2013.
Taylor, Steven. "4.2Â Meiosis." IBiology. N.p., n.d. Web. 03 Mar. 2013. <http://i-biology.net/ibdpbio/04-genetics-and-genetic-engineering/meiosis-inc-ahl/>. Chromatids break at chiasmatas
Exchange genetic information by breaking and recombining
Errors lead to mutation Non-disjunction Spindle fibres pull both chromosomes in homologous pair to one side
Results in two triseme cells, two monoseme cells Non-disjunction Spindle fibres pull both chromatids in chromosome to one side
Results in a triseme cell, a monoseme cell, and two normal cells Sister chromatids at poles
Nuclear membrane reforms
Cytoplasm begins to divide by cytokensis
Nuclei are no longer diploid
Each contain one pair of sister chromatids Spindle fibres contract and the centromeres are broken
Spindles pull sister chromatids to opposite poles
Cells remain haploid
If non-disjunction occurs, two gametes containing the wrong number of chromosomes will remain Learning Objectives SL HL Synapsis: Fusion of the chromosomes MEIOSIS I MEIOSIS II Genetic Variation By Michael and Julien