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meiosis - Coggle Diagram
meiosis
Meiosis 1 involves the first meiotic division- first, all DNA (chromosomes) in the parent cell are replicated during interphase (S phase) producing two copies of each chromosome
In prophase 1, the chromosomes tightly coil and condense, and each individual chromosome can be seen to consist of two sister chromatids (identical copies of DNA) joined at the centromere
The chromosomes are arranged into homologous pairs called bivalents- consisting of one maternal and one paternal chromosome that are the same length, and contain the same gene loci, but not necessarily the same alleles of the gene
During prophase, the homologous chromosomes are within close proximity allowing a process called crossing over to occur
Non-sister chromatids from homologous partner chromosomes become entangled and wind around one another at a specific point called the chiasmata
This creates tensions that weaken bonds and break off sections of the chromatids- these sections are then rejoined to the non-sister chromatids of the homologous partner chromosome
This is called recombination, and produces genetic variation since it produces different combinations of maternal and paternal alleles- swaps alleles between the homologous partner chromosomes
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Crossing over normally occurs at least once for each homologous pair- and occurs further down the chromosomes away from the centromere
The nuclear membrane disintegrates, and the nucleoli dissappears, as spindle fibres begin to form at opposite poles of the cell
In metaphase 1, the homologous pairs of chromosomes line up in their pairs along the spindle equator. the organisation of the pairs is completely random- either the maternal or paternal chromosome could be on either side of the equator and therefore pulled to either pole of the cell. This random process is called independent segregation.
Independent segregation is another mechanism that is used in meiosis to produce genetic variation- the random sorting of the homologous partners into haploid daughter cells produces random combinations of male and female alleles in the daughter cells
Only one chromosome from each homologous pair is sorted into each daughter cell- so there is one allele for each gene characteristic in each cell
Therefore the random combination of maternal and paternal alleles in each daughter cell is determined by the random alignment of the homologs maternal and paternal chromosomes either side of the spindle equator
Anaphase 1- this is where spindle fibres extending from opposite poles of the cell that are attached to the centromeres, pull homologous partner chromosomes to opposite poles of the cell.
It is completely random what partner goes to what pole due to independent segregation.
The centromeres do not divide
Telophase 1- a new nuclear envelope forms around each set of chromosomes at either pole of the cell
A new nucleoli forms and the spindle fibres that are remaining will disintegrate- the chromosomes decondense
Then cytokinesis- division of the cytoplasm occurs
Cytokinesis is the division of the cytoplasm to form two seperate genetically different haploid cells and complete cell division
The first stage of meiosis produces two haploid daughter cells each with 23 chromosomes in humans
Cytokinesis in animal cells- a cleavage furrow is formed around the cell in line with the original spindle equator- pinning of the cell surface membrane cuts the cell into two, dividing the cytoplasm and the cell contents
In plant cells, a new cell plate is formed called a phagoplast, in between the two nuclei, which gives rise to the middle lamella on which magnesium and calcium pectates are deposited forming cellulose cell wall that cuts the cytoplasm into two
Meiosis 2 involves the second meiotic division to produce four genetically varied haploid daughter cells- the four stages are repeated once more but there is no interphase before prophase (since DNA was already replicated
Prophase 2- in each haploid daughter cell produced in meiosis 1, the chromosomes condense once more to become visible, and the nuclear envelopes break down as the nucleoli disappears. Spindle fibres begin to form the new spindle apparatus at right angles to the old one- once more from opposite poles of the cell
In metaphase 2, spindle fibres extend from the opposite poles of the cell and attach to the centromeres of the chromosomes. this pulls them along the spindle apparatus to line up along the spindle equator equidistant to both poles of the cell
In anaphase 2, the centromeres divide and the spindle fibres contract, and pull sister chromatids to opposite poles of the cell that they originate from. The chromatids are now considered chromosomes (single copies of genetic material once again)
Finally in telophase 2, new nuclear envelopes form around each set of chromosomes at either pole of the cell, with new nucleoli also. Remaining spindle fibres disintegrate and chromosomes decondense becoming invisible again.
Cytokinesis follows to produce in total, four daughter haploid cells with half the number of chromosomes as the original parent cell
All gametes are genetically different and therefore the random pairing of gametes as they fuse during fertilisation will further increase genetic variation, producing different combinations of maternal and paternal alleles in offspring
In humans, these are called gametes and include egg and sperm cells each containing 23 chromosomes
When the gametes fuse together during fertilisation, their recombination restores the original diploid number and produces a zygote that can go on to divide rapidly by mitosis forming a cline and an embryo.
A type of nuclear division that produces four haploid daughter cells all genetically varied from one another, from a single parent cell
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Meiosis is important for the production of genetic variation within an organism population, producing different species
It does this through different mechanisms such as crossing over of chromosomes, and independent segregation of homologous chromosomes
Genetic variation is hugely important for the process of natural selection, as it produces adaptations that can have survival value for a species
To calculate the number of possible combinations of chromosomes during meiosis use: 2 to the power of n where n= the number of homogous pairs of chromosomes in the organism species
To calculate the number of possible chromosome combinations after the random pairing of gametes produced in meiosis: (2 to the power of n) squared