L2 - meiosis

• Function of meiosis:

  -to produce gametes for sexual reproduction

  -the gametes produced during meiosis have half the number of chromosomes
• Haploid cells have 23 chromosomes
• Diploid cells have 46 chromosomes
• Meiosis summary:

  -occurs only in gonads; spermatogenesis in testes; oogenesis in ovaries

  -two stages of division (meiosis I and meiosis II)

  -produces four haploid cells from one diploid cells

  -each haploid cell is different from one another and from the diploid cell (this leads to variation)
• Importance of variation:

  -greater variation leads to a larger diversity of life

  -variation allows species to adapt to their environments and so to evolve

  -variation can lead to new species forming

  -therefore most species can reproduce sexually as this leads to variation
• Interphase:

  -when a cell is growing and carrying out its functions

  -DNA is uncoiled and not visible as chromosomes

  -DNA and cell organelles replicate
• Prophase I:

  -the chromosomes form from condensed chromatin

  -they consist of two chromatids attached with a centromere

  -the nucleolus disappears and the nuclear envelope begins to disintegrate

  -spindle begins to form

  -homologous chromosomes match up and form a bivalent
• Prophase I homologous chromosomes:

  -replication of chromosomes occurs prior to division

  -chromosomes are matched in pairs based on their size and shape. They contain genes that code for the same characteristics in the same position
• Homologous chromosomes in prophase I

  
• Prophase I crossing over:

  -once the homologous pairs form their chromatids wind round each other and become entangled

  -this is called a bivalent

  -the points at which the chromatids cross are called chiasmata

  -genetic information gets swapped at chiasmata, forming recombinant chromatids
• Crossing over in prophase I:

  
• Metaphase I independent assortment:

  -the nuclear envelope has now disintegrated

  -the centrioles move to opposite poles and set up the spindle fibres

  -the homologous pairs of chromosomes move to the equator and spindles attach to each pair’s centromere

  -the orientation of each homologous pair is random and independent

  -this is called independent assortment of chromosomes
• Anaphase I:

  -the spindle fibres contract and pull the chromosome pairs to opposite poles

  -the centromere does not divide

  -the cell membrane starts to pinch inwards
• Telophase I:

  -a nuclear envelope forms around each group of chromosomes at each pole

  -in animals cytokinesis follows telophase I, producing two diploid cells. A brief interphase comes after

  -in plants meiosis II follows on immediately from telophase I
• Prophase II:

  -the chromosomes reform from condensed chromatin

  -they consist of two attached with a centromere

  -the nucleolus disappears and the nuclear envelope disintegrates once more

  -the spindle begins to reform
• Metaphase II:

  -the chromosomes line up on the equator

  -the spindle fibres set up and attach to the centromeres

  -again the orientation of the chromosomes is random and independent. different chromatids will end up on each side
• Anaphase II:

  -the spindle fibres contract

  -the centromeres are divided and the chromatids are pulled to the opposite poles of the cell

  -the cell membrane starts to pinch inwards
• Telophase II:

  -new nuclear envelopes form

  -chromosomes uncoil to form chromatin

  -in animals four haploid cells are formed following cytokinesis

  -each haploid is genetically different from each other and the parent cell

  -in plants a tetrad of four haploid cells is formed
• How meiosis causes variation in organisms:

  -crossing over during prophase I

  -crossing over creates new combinations of alleles
• How meiosis causes variation in organisms:

  -independent assortment during anaphase I

  -independent assortment causes many different combinations of combinations

  -during anaphase II when centromeres split and chromatids randomly migrate to opposite poles
• Additional causes of variation:

  -mutation before meiosis

  -random fertilisation after meiosis
• Mutation before meiosis:

  -DNA mutation can occurs in interphase when DNA replicates

  -as can chromosome mutations
• Random fertilisation after meiosis:

  -as a result of crossing over and independent assortment each gamete is genetically different

  -it is a random process which sperm fertilises which egg