Meiosis 2.1.8 OCR biology

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Meiosis
Asexual reproduction can be achieved through mitosis in eukaryotes or by binary fission in prokaryotes.
Offspring produced are genetically identical to the parent and to each other
o Random mutation is the only way that variation can occur in asexual reproduction
Sexual reproduction occurs when a male and a female gamete fuse together at fertilisation. This produces a
zygote. The gametes involved in sexual reproduction are produced during meiosis in eukaryotes.
Offspring produced are genetically different from each other and different from the parents
Mitosis and Meiosis
Mitosis Meiosis
Definition A process of asexual reproduction in A type of cellular reproduction
which the cell divides in two in which the number of
producing a replica, with an equal chromosomes are reduced by
number of chromosomes in haploid half through the separation of
cell homologous chromosomes in
a diploid cell.
Number of divisions One Two (meiosis I and meiosis II)
Stages... Interphase, Prophase, Metaphase, Interphase, Prophase I,
Anaphase, Telophase and Metaphase I, Anaphase I,
Cytokinesis Telophase I, Prophase II,
Metaphase II, Anaphase II and
Telophase II
Number of daughter cells 2 genetically identical daughter cells 4 cells genetically different
produced... (diploid) (haploid)
Chromosome number 46 chromosomes (maintained) 23 chromosomes (halved)
Do bivalents form? No Yes
Does crossing over occur? No Yes
What are the daughter cells All cells other than sex cells Sex cells male and female
for? gametes
Fusing of two gametes ­ chromosome number must be halved
Two gametes fuse to make one cell. Prior to the fusion the number of chromosomes in the cells must be
halved (23 chromosomes). This produces a haploid cell.
After fertilisation the number of chromosomes in the zygote is returned to normal (46 chromosomes)
Before the division of the cells the DNA replicates. This stage is called interphase.
Each chromosome therefore consists of two identical sister chromatids
o Joint together at the centromere
The resulting cell therefore contains four copies of each chromosome. In the original cell there were just two
copies.
Meiosis reduction division
Meiosis is a form of nuclear division in which the number of chromosomes in a cell is halved. This produces
haploid cells which can be used in sexual reproduction.
Involves 2 separate divisions meiosis I and meiosis II
o Each division involves 4 stages
Prophase, metaphase, anaphase and telophase

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Meiosis I
Prophase I
Chromatin condenses and supercoils
o Chromosomes shorten and thicken
Stains can be added to enable the chromosomes to be seen under a light microscope
Chromosomes pair up in homologous pairs One maternal and one paternal chromosome
o Forms a bivalent
o Genes are located at the same loci in both chromosomes
Each pair of chromosomes wrap around each other at chiasmata
o The pairs may also swap sections of chromatid crossing over
Nucleolous disappears
o Nuclear envelope disintegrates
Spindle made of…read more

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Meiosis II ­ division on a right angle plane to meiosis I
Prophase II
Nuclear envelope if formed, breaks down again
o Nucleolus disappears
o Chromosomes condense and spindles form
Metaphase II
Chromosomes line up along the equator randomly assorted
o Attached by centromeres
Anaphase II
Centromeres divide
o Chromatids pulled to opposite poles by spindle fibres
Chromatids randomly segregate
Telophase II
Nuclear envelope forms around haploid daughter nuclei
o Animals two cells divide to give 4 haploid cells
o Plants tetrad of 4 haploid…read more

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