Meiosis Flashcards
Intro
Normal cells have two chromosomes of each type (diploid)-one inherited from each parent. During mitosis the nucleus divides once following DNA replication. This results in two genetically identical diploid daughter cells.
In sexual reproduction two sex cells (gametes), one from each parent, fuse to produce a fertilised egg. The fertilised egg (zygote) is the origin of all cells that the organism develops. Gametes must therefore only contain half of the standard (diploid) number of chromosomes in a cell or the chromosome number of an organism would double with every round of reproduction.
Gametes are formed by another form of cell division known as meiosis. Unlike in mitosis, the nucleus divides twice to produce four daughter cells-the gametes. Each gamete contains half of the chromosome number of the parent cell-it is haploid. Meiosis is therefore known as reduction division.
Homologous chromosomes
Each characteristic of an organism is coded for by two copies of each gene, one from each parent. Each nucleus of the organisms cells contains two full sets of genes, a pair of genes for each characteristic. Therefore each nucleus contains a matching set of chromosomes, called homologous chromosomes, and is termed diploid. Each chromosome in a homologous pair has the same genes at the same loci.
Homologous chromosomes-Alleles
Genes for a particular characteristic may vary, leading to differences in the characteristic, for example blue eyes and brown eyes. The genes are still the same type they both code for eye colour but the colour is different, meaning they are different versions of the same gene. Different versions of the same gene are called alleles. The different alleles of a gene will all have the same locus (position on a particular chromosome).
As homologous chromosomes have the same genes in the same positions, they will be the same length and size when they are visible in prophase. The centromeres will also be in the same positions.
The stages of meiosis
-Meiosis 1-The first division is the reduction division when the pairs of homologous chromosomes are separated into two cells. Each intermediate cell will only contain one full set of genes instead of two, so the cells are haploid. Prophase-metaphase-anaphase-telophase
-Meiosis 2- the second division is similar to mitosis, and the pairs of chromatids present in each daughter cell are separated, forming two more cells. Four haploid daughter cells are produced in total.-prophase-metaphase-anaphase-telophase
Meiosis 1-prophase 1
Chromosomes condense, the nuclear envelope disintegrates, the nucleolus disappears and spindle formation begins, as in prophase of mitosis.
The difference in prophase 1 is that the homologous chromosomes pair up, forming bivalents. Chromosomes are large molecules of DNA and moving them through the liquid cytoplasm as they are brought together results in the chromatids entangling. This is called crossing over.
Meiosis 1-metaphase 1
Metaphase 1 is the same as metaphase in mitosis except that the homologous pairs of chromosomes assemble along the metaphase plate instead of the individual chromosomes.
The orientation of each homologous pair in the metaphase plate is random and independent of any other homologous pair. The maternal or paternal chromosomes can end up facing either pole. This is called independent assortment, and can result in many different combinations of alleles facing the poles. Independent assortment of chromosomes in metaphase 1 results in genetic variation.
Meiosis 1-anaphase 1
Anaphase 1 is different from anaphase of mitosis as the homologous chromosomes are pulled to the opposite poles and the chromatids stay joined to each other.
Sections of DNA on ‘sister’ chromatids, which became entangled during crossing over, now break off and rejoin-sometimes resulting in an exchange of DNA. The points at which the chromatids break and rejoin are called chiasmata.
When exchange occurs this forms recombinant chromatids, with genes being exchanged between chromatids. The genes being exchanged may be different alleles of the same gene, meaning the combination of alleles on the recombinant chromatids will be different from the allele combination in either the original chromatids. Genetic variation arises from this new combination of alleles-the sister chromatids are no longer identical.
Meiosis 1-telophase 1
Telophase 1 is essentially the same as telophase in mitosis. The chromosomes assemble at each pole and the nuclear membrane reforms. Chromosomes uncoil.
The cell undergoes cytokinesis and divides into two cells. The reduction of chromosome number from diploid to haploid is complete.
Meiosis 2-prophase 2
In prophase 2 the chromosomes, which still consist of two chromatids, condense and become visible again. The nuclear envelope breaks down and spindle formation begins.
Meiosis 2-metaphase 2
Metaphase 2 differs from metaphase 1, as the individual chromosomes assemble in the metaphase plate, as in metaphase in mitosis.
Due to crossing over, the chromatids are no longer identical so there is independent assortment again and more genetic variation produced in metaphase 2.
Meiosis 2-anaphase 2
Unlike anaphase 1, anaphase 2 results in the chromatids of the individual chromosomes being pulled to opposite poles after division of the centromeres-the same as in anaphase of mitosis.
Meiosis 2-telophase 2
The chromatids assemble at the poles at telophase 2 as in telophase of mitosis. The chromosomes uncoil and form chromatic again. The nuclear envelope reforms and the nucleolus becomes visible.
Cytokinesis results in division of the cells forming 4 daughter cells in total. The cells will be haploid due to the reduction division. They will also be genetically different from each other, and from the parent cell, due to the processes of crossing over and independent assortment.
