When two gametes fuse to make one cell the chromosomes are combined into one nucleus. Therefore, the chromosome number in each gamete needs to be haploid (half).
Meiosis is the type of nuclear division where the chromosome number is halved.
It involves 2 divisions: Meiosis I and Meiosis II
Each division has four stages: Prophase, Metaphase, Anaphase, Telophase.
Interphase occurs briefly before Prophase I and the DNA and organelles are replicated.
Each chromosome consists of two identical sister chromatids joined at the centromere.
The cell contains 4 instead of 2 copies of each chromosome.
- The chromatin condenses amd undergoes supercoiling so that the chromosomes shorten and thicken.
- The chromosomes come together in homologous pairs to form a bivalent. Each member of the pair has the same genes at the same loci and consists of one paternal and maternal chromosome.
- Non-sister chromatids wrap round eachother and attach at points called chiasmata
- Crossing over may occur
- The nucleolus dissappears and the nuclear envelope disintergrates
- A spindle forms (microtubules).
- Bivalents line up along the equator of the spindle attached to the spindle fibres at the centromeres.
- Bivalents are arranged randomly (random assortment) with each member of a homologous pair facing opposite poles.
- Allows chromosomes to independently segregate in anaphase I.
- The homologous chromosomes in each bivalent are pulled by spindle fibres to opposite poles.
- The centromeres do not divide
- The chiasmata separate and lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached.
- A new nuclear envelope forms, one around each set of chromosomes at each pole .
- The cell divides by cytokinesis.
- There is a brief interphase
- In most plant cells the cell goes straight from anaphase I to meiosis II.
This division is in a plane at right angles to meiosis I.
- Nuclear envelope breaks down again
- The nucleolus disappears, chromosomes condense and spindles form.
- The chromosomes arrange themselves on the equator of the spindle, attached to spindle fibres at the centromere.
- Chromatids of each chromosome are randomly assorted.
- The centromeres divide and the chromatids are pulled to opposite poles by spindle fibres.
- The chromatids randomly segregate
- Nuclear envelopes form around the haploid daughter nuclei
- In animals the two cells now divide to give four haploid cells
- In plants a tetrad of four haploid cells is formed.
The significance of Meiosis
- Sexual reproduction increases genetic variation as the genetic material of two unrelated organisms combines.
- Genetic variation increases the chances of evolution as natural selection can favour the organisms that are best adapted to a changing environment.
Meiosis increases genetic variation by:
- crossing over during prophase I to shuffle alleles.
- genetic reassortment due to random distribution and subsequent segregation of the maternal and paternal chromosomes in the homologous pairs during meiosis I
- genetic reassortment and segregation of sister chromatids at meiosis II
- Random mutation
- Fertilisation- randomly combines two sets of chromosomes from two unrelated individuals
- occurs during Prophase I
- homologous chromosomes come together to form a bivalent
- non sister chromatids wrap around eachother, attaching at points called chiasmata
- Chromosomes may break at these points, the broken ends rejoin to the ends of the non sister chromatids in the same bivalent (similar sections of the non sister chromatids are swapped over: same genes but different alleles)
Produces new combinations of alleles on the chromatids
Chiasmata remain in place during metaphase holding the paternal and maternal chromosomes together, this ensures that when segregation occurs one member goes to each pole.
Reassortment and Segregation
- reassortment is the consequence of random distribution of maternal and paternal chromosomes on the spindle equator during metaphase I and the segregation in anaphase I.
Each gamete aquires a different mixture of maternal and paternal chromosomes.
- the reassortment of sister chromatids is the result of random distriubtion during metaphase II.
- Due to crossing over they are no longer identical
- How they align at metaphase II determines how they segregate at anaphase II.
Mutation and Key Terms
- DNA mutation may occur during interphase
- chromosome mutations may also occur
- increases genetic variation.
Allele - is a version of a gene (a gene is a length of DNA that codes for a polypeptide)
Locus- is the position of a gene on a chromosome
Crossing Over - is when lengths of DNA are swapped from one chromatid to another.