Meiosis Cornell notes. Meiosis animation video PMp0U0HOA PMp0U0HOA.
MEIOSIS
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Meiosis
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CHROMOSOMES ARE MATCHED IN HOMOLOGOUS PAIRS
In humans, somatic cells (body cells) have:
• 23 pairs of homologous chromosomes and
• one member of each pair from each parent. The human sex chromosomes (Gonosomes)
X and Y differ in size and genetic composition.
The other 22 pairs of chromosomes are autosomes with the same size and genetic composition.
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HOMOLOGOUS CHROMOSOMES ARE MATCHED IN:
• length,
• centromere position, and
• gene locations (locus). A locus (plural, loci) is the position of a gene. Different versions (alleles) of a gene may be found
at the same locus on maternal and paternal chromosomes.
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HOMOLOGOUS CHROMOSOME PAIR
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GAMETES HAVE A SINGLE SET OF CHROMOSOMES
• All organisms have different numbers of chromosomes.
• A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent.
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•These two sets of chromosomes are homologous.
•Each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent.
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• A cell that contains both sets of homologous chromosomes is said to be diploid.
• The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.
• For Drosophila, the diploid number is 8, which can be written as 2N=8.
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DIPLOID
• Di= two sets• Cell that contains both sets of homologus chromosomes• Cell contains
• 2 complete sets of chromosome• 2 complete sets of genes
• Number of chrms in diploid cell represented by 2N• For Drosophilia (fruit fly) 2N=8• Mendel said:• Each adult cell contains two copies of each gene
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• The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes.
• These cells are haploid. Haploid cells are represented by the symbol N.
• For Drosophila, the haploid number is 4, which can be written as N=4.
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HAPLOID
• Means “one set”
• Refers to cells that contain only one set of chromosomes
• Gametes (sex cells)
• Represented by N
• Drosophilia fruit fly• N=4
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WHAT IS MEIOSIS?
• Process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell
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WHAT HAPENS DURING MEIOSIS?
• Each organism must inherit a single copy of every gene from each of its “parents.”
• Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set.
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SUMMARY OF MEIOSIS
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MEIOSIS HAS TWO STAGES
MEIOSIS I consisting of 5 phases: Interphase I, Prophase I, Metaphase I,
Anaphase I, Telophase I.MEIOSIS II consisting of 4 phases
Prophase II, Metaphase II, Anaphase II, Telophase II.
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MEIOSIS I
Interphase I Prophase I Metaphase I Anaphase I Telophase I and
Cytokinesis
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INTERPHASE I
Cell build up energyDNA Replication (to make duplicated chromosomes
Cell doesn’t change structurally.
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PROPHASE I
• Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
• There are 4 chromatids in a tetrad.
• Non-sister chromatids exchange genetic material through the process of crossing over to ensure genetic variation.
• Centrioli move to opposite poles with spindle fibres between them.
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CROSSING OVER
• When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over.
• Crossing-over produces new combinations of alleles.
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METAPHASE I
Centrioli has reached the poles.
Homologous pairs align at the cell equator.
The two chromosomes attach to one spindle fiber by means of the kinetochore of the centromere.
.
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ANAPHASE I
• The spindles pull homologous chromosomes apart to opposite poles/ends
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TELOPHASE I AND CYTOKINESIS
• Duplicated chromosomes have reached the poles.
• A nuclear envelope and nucleolus re-forms around chromosomes.
• Each nucleus now has the haploid number of chromosomes.
• Cell invaginates forming a cleavage furrow, which extends to for 2 separate haploid cells.
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MEIOSIS II
Meiosis II• The two cells produced by meiosis I now enter a
second meiotic division.
• Unlike meiosis I, neither cell goes through chromosome replication.
• Each of the cell’s chromosomes has 2 chromatids.
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MEIOSIS II
Telophase I and Cytokinesis I Prophase II Metaphase II Anaphase II Telophase II
and Cytokinesis
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PROPHASE II
• Chromosomes coil and become compact (if uncoiled after telophase I).
• Nuclear envelope and nucleolus, if re-formed, disappears again.
• Centrioli move to opposite poles, forming spindle fibers between them.
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METAPHASE II
• Individual duplicated chromosomes align on the equator.
• One chromosome per spindle fiber attached by means of kinetochore of centromere.
• Centrioli has reached the poles.
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ANAPHASE II
• Spindle fibers contract.
• Duplicated chromosomes split in half (centromere dividing in 2)
• Daughter chromosomes move to opposite poles.
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TELOPHASE II AND CYTOKINESIS
• Daughter chromosomes has reached the poles.
• Two cells invaginate and form 4 daughter haploid cells (gametes)
• They uncoil and form chromatin.
• Nuclear envelope and nucleolus for around chromatin again.
• Centrioli for centrosome.
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GAMETE FORMATION
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DIFFERENCES BETWEEN MITOSIS ANS MEIOSIS
MITOSIS MEIOSIS• Cells produced by mitosis have
the same number of chromosomes and alleles as the original cell.
• Mitosis allows an organism to grow and replace cells.
• Some organisms reproduce asexually by mitosis.
ex: coral
• Cells produced by meiosis have half the number of chromosomes as the parent cell.
• These cells are genetically different from the diploid cell and from each other.
• Meiosis is how sexually-reproducing organisms produce gametes.
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ALTERATION IN CHROMOSOME NUMBER
An extra copy of chromosome 21 causes Down syndrome or also known as TRISOMY 21.
A. Trisomy 21
• involves the inheritance of three copies of chromosome 21 and
• is the most common human chromosome abnormality.
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DOWN SYNDROME
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Trisomy 21 produces a characteristic set of symptoms, which include:
• mental retardation,
• characteristic facial features,
• short stature,
• heart defects,
• susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and
• shortened life span. The incidence increases with the age of
the mother.
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Accidents during meiosis can alter chromosome number
Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during:
• meiosis I, if both members of a homologous pair go to one pole or
• meiosis II if both sister chromatids go to one pole.
Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes.
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Alterations of chromosome structure
Chromosome breakage can lead to rearrangements that can produce:
• genetic disorders or,
• if changes occur in somatic cells, cancer.
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These rearrangements may include:
• a deletion, the loss of a chromosome segment,
• a duplication, the repeat of a chromosome segment,
• an inversion, the reversal of a chromosome segment, or
• a translocation, the attachment of a segment to a non-homologous chromosome that can be reciprocal.
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THANK YOU!!!
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REFERENCES
• http://www.slideshare.net/MissMabena/savedfiles?s_title=meiosis-9964891&user_login=anesh95
• Anesh Jeyakumar
• http://www.slideshare.net/MissMabena/savedfiles?s_title=meiosis-31958724&user_login=mikeu74
• mikeu74
• http://www.slideshare.net/MissMabena/savedfiles?s_title=meiosis-422302&user_login=itamarita1984
• itamarita1984
• http://www.slideshare.net/MissMabena/savedfiles?s_title=10-1-meiosis&user_login=cspavao
• cspavao•