3.3 Meiosis
3.3.U3 DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
The homologous chromosomes associate with each other to form bivalents (synapsis).
Sister Chromatids
Synapsis
Centromere
Homologous Chromosomes
Kinetochore
The micrographs above show the formation of bivalents (left) and the segregation caused by both anaphase I and II (right). These possesses combined with crossing over and random orientation ensure a near infinite variation of genetic information between gametes.
3.3 Meiosis
Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.
Essential idea:
| Statement | Guidance |
3.3.U1 | One diploid nucleus divides by meiosis to produce four haploid nuclei. | |
3.3.U2 | The halving of the chromosome number allows a sexual life cycle with fusion of gametes. | |
3.3.U3 | DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids. | |
3.3.U4 | The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation. | The process of chiasmata formation need not be explained. |
3.3.U5 | Orientation of pairs of homologous chromosomes prior to separation is random. | |
3.3.U6 | Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number. | |
3.3.U7 | Crossing over and random orientation promotes genetic variation. | |
3.3.U8 | Fusion of gametes from different parents promotes genetic variation. | |
3.3 Meiosis
Understandings
| Statement | Guidance |
3.3.A1 | Non-disjunction can cause Down syndrome and other chromosome abnormalities. | |
3.3.A2 | Studies showing age of parents influences chances of non-disjunction. | |
3.3.A3 | Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks. | |
3.3.S1 | Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells. | Drawings of the stages of meiosis do not need to include chiasmata. Preparation of microscope slides showing meiosis is challenging and permanent slides should be available in case no cells in meiosis are visible in temporary mounts. |
3.3 Meiosis
Applications and Skills
3.3 Meiosis
Vocabulary
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3.3 Meiosis
Review: 1.6.U1 Mitosis is division of the nucleus into two genetically identical daughter nuclei.
Use the animated tutorials to learn about mitosis
3.3 Meiosis
Review: 1.6.U1 Mitosis is division of the nucleus into two genetically identical daughter nuclei.
1876 - German biologist Oscar Hertwig recognized the role of the cell nucleus during inheritance and chromosome reduction during meiosis from work on Sea Urchins.
1883 - Belgian zoologist Edouard Van Beneden discovered in the roundworm Ascaris how chromosomes organized meiosis (the production of gametes).
1890 - The significance of meiosis for reproduction and inheritance was first described by German biologist August Weismann who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells.
In the 19th century was very difficult to observe the behaviour of chromosomes in cell: the choice of organism and tissue, slide preparation and interpreting microscope images are all difficult to do successfully. It therefore it took years of careful examination by Scientists to discover and fully understand meiosis.
3.3 Meiosis
The History of Meiosis
Nature of Science: Making careful observations—meiosis was discovered by microscope examination of dividing germ-line cells. (1.8)
3.3 Meiosis
Meiosis is a reduction division of the nucleus to form haploid gametes
3.3.U1 One diploid nucleus divides by meiosis to produce four haploid nuclei.
3.3 Meiosis
Meiosis is a reduction division of the nucleus to form haploid gametes
3.3.U1 One diploid nucleus divides by meiosis to produce four haploid nuclei.
Meiosis is a reduction division of the nucleus to form haploid gametes
Second division of the nucleus
3.3 Meiosis
What is meiosis?
3.3.U1 One diploid nucleus divides by meiosis to produce four haploid nuclei.
It results in cells with half the number of chromosomes ( hence reducing)
Occurs in sexual reproduction in the synthesis of gametes
The animations are a great way to visualise the process – watch and take notes.
3.3 Meiosis
Meosis is a reduction division of the nucleus to form haploid gametes
3.3.U1 One diploid nucleus divides by meiosis to produce four haploid nuclei.
3.3 Meiosis
Outline the differences between the behaviour of chromosomes in Mitosis and Meiosis (5 marks)
Mitosis | Meiosis |
One division | Two divisions |
Diploid cells produced | Haploid gametes produced |
No crossing-over in prophase | Crossing-over in prophase I |
No chiasmata formation | Chiasmata form |
Homologous pairs do not associate and line up at the equator in metaphase | Homologous pairs associate as bivalents and lined up at the equator in metaphase I |
Sister chromatids separate in anaphase | Homologous pairs separate in anaphase I Sister chromatids separate in anaphase II |
3.3 Meiosis
3.3.U3 DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
The homologous chromosomes associate with each other to form bivalents (synapsis).
Sister Chromatids
Synapsis
Centromere
Homologous Chromosomes
Kinetochore
Crossing-over between non-sister chromatids can take place. This results in recombination of alleles and is a source of genetic variation in gametes.
3.3 Meiosis
Prophase I
3.3.S1 Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells. AND 3.3.U4 The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
3.3 Meiosis
Prophase I
3.3.S1 Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells. AND 3.3.U4 The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
Plural: Chiasmata
courtesy of: http://www.flickr.com/carolinabio
Random assortment occurs when each bivalent aligns independently and hence the daughter nuclei get a different mix of chromosomes.
The bivalents line up at the equator.
Edited from: http://www.slideshare.net/gurustip/meiosis-ahl
3.3 Meiosis
Metaphase I
3.3.S1 Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells. AND 3.3.U5 Orientation of pairs of homologous chromosomes prior to separation is random.
This is a significant source of genetic variation: there are 2n possible orientations in metaphase I and II. That is 223 in humans – or 8,388,068 different combinations in gametes!
3.3 Meiosis
Prophase II
3.3.S1 Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
Spindle fibres form and attach at the centromeres.
Pairs of sister chromatids align at the equator.
3.3 Meiosis
Metaphase II
3.3.S1 Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
Four new haploid nuclei are formed.
3.3 Meiosis
Telophase II
3.3.S1 Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
3.3 Meiosis
Telophase II
3.3.S1 Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
Prophase I
Metaphase I
Random orientation of the homologous chromosomes means there are 2n possible orientations in metaphase I and II. That is 223 in humans – or 8,388,068 different combinations in gametes!
Metaphase II
3.3 Meiosis
Because both crossing-over and random orientation occur during meiosis the result is is effectively infinite genetic variation in the haploid gamete.
3.3.U7 Crossing over and random orientation promotes genetic variation.
Crossing-over between non-sister chromatids results in recombination of alleles
Increased genetic variation produces a more resilient population that is more likely to withstand environmental change such as a disease. Genetic variation is essential for successful change by evolution.
Meiosis in a single individual produces near infinite variation, but genetic variation is further increased by:
3.3 Meiosis
For a new organism to arise sexually meiosis must occur in both parents followed by fusion of the gametes (fertilisation)
3.3.U8 Fusion of gametes from different parents promotes genetic variation.
3.3 Meiosis
Sexual reproduction involves fertilisation, the fusion of gametes (sex cells), one from each parent.
3.3.U2 The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
Metaphase I
Anaphase I
Anaphase II
Metaphase II
*Disjunction is the term used to describe the seperation of chromosomes
For example homologous chromosomes can fail to separate at anaphase. This is called non-disjunction.
Normal division
Disjunction in Meiosis I
Disjunction in Meiosis II
3.3 Meiosis
Meiosis like all processes is sometimes subject to mistakes.
3.3.A1 Non-disjunction can cause Down syndrome and other chromosome abnormalities.
3.3 Meiosis
3.3.A1 Non-disjunction can cause Down syndrome and other chromosome abnormalities.
The result of non-disjunction is gametes that either have one chromosome too many or one too few.