Meiosis Lesson
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Transcript of Meiosis Lesson
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Meiosis
NEREA LÓPEZ PEIRONA 4ºB
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Significance of Meiosis
(1) Crossing over in bivalent- Produces new combinations of
genes in both chromosomes
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Chiasma and Crossing Over
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Significance of Meiosis
(2) Reduction and fusion of gametes- Meiosis produces haploid gametes- In sexual reproduction, a male
gamete fertilizes a haploid female gamete to produce a normal diploid zygote
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Significance in Mitosis
(3) Independent (random assortment)- During metaphase I, homologous
pairs of chromosomes align at the equator
- It is by chance which “way round” each pair lies, before these homologous pairs of chromosomes separate into two different daughter cells.
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Cell Cycle
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Stages of Meiosis
Meiosis I
Prophase IMetaphase IAnaphase ITelophase I
• Meiosis II
Prophase IIMetaphase IIAnaphase IITelophase II
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Interphase
Interphase is an important stage preceding meiosis. Without this stage
meiosis would not occur. During this stage, each individual
chromatid replicates, similar to mitosis. B replicates B and b replicates b
At this stage, the chromosomes are long and stringy and are not visible.
**Remember: All somatic cells are diploid in number (2n), therefore for each
chromatid there also exists its homolog, which also replicates during interphase.
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Early prophase I
During early prophase it is the same as in mitosis
Chromosomes
Nuclear envelope
Spindle pole
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Prophase I
Now, during prophase, also occurs the crossing over, where the homologous chromosomes
exchange information.
Spindle fiber
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Metaphase I
The chromosomes align in the middle
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Telophase I
Each chromosome separates from the
other, making two news.
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Meiosis I Flowchart
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After telophase I After telophase I
the second meiotic division the second meiotic division occurs occurs (without DNA duplication (without DNA duplication before), before),
as a final result we have 4 as a final result we have 4 haploid cells)haploid cells)
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Prophase II
The nuclear membranes of the daughter cells disintegrate again. The spindle fibres re-form in each daughter cell
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Metaphase II
The chromosomes, each still made up of sister chromatids, are positioned randomly on the metaphase plate with the sister chromatids of each chromosome pointing towards the opposite poles.
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Metaphase II
Each sister chromatid is attached to the spindle fibres at the centromere
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Anaphase II
The centromeres of the sister chromatids finally separate, and the sister chromatids of each chromosome are now individual chromosomes.
The chromosomes move towards the opposite poles of the cell.
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Telophase II
Finally, the nucleoli and nuclear membranes re-form. The spindle fibres break down.
Cytokinesis follows and four haploid daughter cells are formed, each containing half the number of chromosomes and is genetically different from the parent diploid cell.
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Telophase II
These haploid cells will develop into gametes.
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Males: all 4 hapoid cells become sperm
Females: in oogenesis, only 1 of the haploid cells becomes an egg, and the other 3 are reabsorbed by the body.
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Human genetics = 46 chromosome (2n)23 pairs of chromosome
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Mutation
Mutation is a change in structure, arrangement or quantity of the DNA in the chromosome
May be caused by: Mistakes in the replication of DNA Damage to the DNA by radioactive
and carcinogenic substance Disruption to the orderly movement
of chromosomes during cell division
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In Mitosis
If the functions of these genes are disrupted due to mutation, cancers may form.
Somatic mutations are not transmitted to the offspring, but may cause body cells to malfunction
Cancers are caused by somatic mutation
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In Meiosis
Meiosis involves an orderly movement and reduction (in meiosis I) of a diploid cell to two haploid cells that subsequently divide (in meiosis II) to form four haploid gametes
Since these are gametes, so any mistakes – caused by disorderly movement of chromosomes during meiosis --- are inherited by the offspring.
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Example: non-disjunction or improper segregation (separation) of chromosome
During anaphase I, certain homologous chromosomes fail to segregate, resulting in the production of gametes with either an extra chromosome (n+1) or a missing chromosome (n-1)
If this abnormal gametes unites with a normal gamete, an abnormal zygote will be produced.
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Down’s syndrome
3 copies of chromosomes number 21, instead of the normal 2 chromosomes
This means a down syndrome patient has (2n+1 = 47) 47 chromosomes instead of the normal (2n=46) chromosomes
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Trisomy 21/ Down Syndrome
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References
TRabajo de clase para Biología MEC-BC. IES Pedro de Luna- 2011- Zaragoza SPAIN
by imkaelah on Jul 24, 2013
http://www.slideshare.net/imkaelah/meiosis-lesson?qid=95dae3f0-5bd5-4b9a-9ffa-9f69c1d878a2&v=default&b=&from_search=1
Rebecca Choong
http://www.slideshare.net/rchoong/mitosis-meiosis-lesson-3?qid=5905ba13-12a1-4b46-ae5f-969f78f84434&v=default&b=&from_search=3
by Anesh Jeyakumar on Oct 31, 2011
Slides abducte.d from slideshare.