Haploid & Diploid
Haploid - A cell with just one copy of every chromosome in its nucleus. (n)
Diploid- A cell with two copies of every chromosome in its nucleus. (2n)
Diploid organisms have two copies of each chromosome, one from their mother (maternal) and one from their father (paternal).
The two copies are considered ‘Homologous Chromosomes’ because they have the:
Same GENES but (potentially) different ALLELES
‘Homologous Chromosomes’ also have the same: shape, size, centromere position and position of genes ‘loci’. An easy way to think about this is consider the ‘A’ gene codes for ‘Hair Colour’. Everyone has genes for hair colour,even if theyre bald, what differs between people is the versions (alleles) of the gene they posses.
Meiosis is a specialised form of cell division which only takes place in gonad tissue, tissue which produces gametes; testes produce sperm and ovaries produce eggs by meiosis!
Meiosis works to produce as much genetic variety as possible in the gamete cells it produces. The purpose of meiosis is to convert diploid cells into haploid gametes with as much genetic variation as possible.
Meiosis proceeds through the same 5 basic stages as mitosis:
The only difference in meiosis is that when the cell arrives at Telophase it goes back to the beginning and completes the whole sequence a second time, giving us a division into Meiosis I and Meiosis II.
The cells are converted from Diploid to Haploid in the first stage (Meiosis I) when the homologous chromosomes are pulled to opposite poles. This creates two cells with one copy of each chromosome (Haploid).
The genetic variation in the gametes is created in three ways:
- Crossing Over of homologous chromosomes in Prophase I.
- Independent Assortment of chromosomes in Metaphase I.
- Independent Assortment of recombinant chromatids in Metaphase II.
Once these haploid, genetically variable, gametes have been created there is even more variation produce by the process of sexual reproduction:
- Random mating (there are no rules as to who mates with whom!)
- Random fertilisation (males release around 15 million (genetically variable/unique) gametes, which one fertilises the egg is largely random)
Interphase 1 & Prophase 1
- DNA doubles as chromatids replicated by semi-conservative replication.
- Individual chromosomes are not visible.
- Centrioles and organelles repliacte and protein synthesis occurs.
- Respiration rate increases to produce more ATP.
- Cell increases in size.
- Chromosomes condense and become visable. Homologous chromosomes join together to form a bivalent. This occurs during a process called synapsis.
- Crossing over occurs which results in variation in the final cells. The crossing over point is called the chiasmata.
- Centrioles move to opposite poles of the cell
- spindle fibres form
- nuclear envelope disappears
Metaphase, Anaphase, Telopase & Cytokinesis 1
- Bivalents line up along equator
- Centromeres attach to centiroles via spindle fibres
- Independent assortment occurs which results in variation in the final cells.
- homologous chromosomes seperate
- spindle fibres contract and shorten
- Chromosomes reach poles of the cell
- nuclear envelope may reform around the two new nuclei
- Individual chromosomes remain visable.
- Cytoplasm constricts. Two new cells are formed. Daughter cells are haploid and genetically different.
Interphase, Prophase & Metaphase 11
- Chromosomes short
- no DNA replication
- Chromosomes remian visable
- Centrioles replicate and move to opposite poles of the cell but at right angles to the first.
- Spindle fibres form
- Nuclear envelope disapears
- Chromosomes align at equator of cell
- Independant assortment occurs which results in variation in the final cells
Anaphase, Telophase & Cytokinesis 11
- Centromere splits
- Cromatids are pulled to opposite poles of the cell
- Spindle fibres contract and shorten
- Cromatids reach poles of the cell and become chromosomes
- Chromosomes de-condense
- Nuclear envelope reforms around the four new nuclei
- Individual chromosomes are visable
- Cytoplasm constricts
- Four new cells are formed
- Daughter cells are haploid and genetically different
Independant assortment and Crossing Over
Independant assortment (metaphase 1 and 11) - One source of variation in meiosis is created by the random way in which maternal and paternal copies of homologous chromosomes line up along the metaphase plate. As the chromosomes are pulled to opposite poles in Anaphase the different alignments of chromosomes will create genetically variable daughter cells.
Crossing over (prophase 1 )- In meiosis, to generate genetic variation, homologous chromosomes cross over and exchange sections of DNA.This crossing over creates ‘Recombinant Chromatids’ which are composed of sections of the maternal AND paternal copies of the chromosome.The value of crossing over is that it creates “new allele combinations”. In this example we are no longer restricted to ‘AB’ or ‘ab’ but now the new combinations of ‘Ab’ and ‘aB’ in the gametes.