MEIOSIS
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Transcript of MEIOSIS
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MEIOSISMEIOSIS
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Organisms that reproduce Sexually are made up of two different types of cells.
1. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc.
2. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova are gametes.
n = number of chromosomes in the set… so….2n means 2 chromosomes in the set…. Polyploid cells have more than two chromosomes per set… example: 3n (3 chromosomes per set)
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Gametes• The Male Gamete is the Sperm and is produced in the male gonad the Testes.
• The Female Gamete is the Ovum (ova = pl.) and is produced in the female gonad the
Ovaries.
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During Ovulation the ovum is released from the ovary and transported to an area where fertilization, the joining of the
sperm and ovum, can occur…… fertilization, in Humans, occurs in the Fallopian tube. Fertilization results in the formation of the
Zygote. (fertilized egg)
Sperm + Ovum (egg) Zygotefertilization
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FertilizationFertilization
• The fusion of a spermsperm and eggegg to form a zygotezygote.• A zygote is a fertilized egg
n=23egg
sperm n=23
2n=46zygote
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Chromosomes• If an organism has the Diploid number (2n) it has
two matching homologues per set. One of the homologues comes from the mother (and has the
mother’s DNA).… the other homologue comes from the father (and has the father’s DNA).
• Most organisms are diploid. Humans have 23 sets of chromosomes… therefore humans have 46 total chromosomes….. The diploid number for
humans is 46 (46 chromosomes per cell).
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Homologous ChromosomesHomologous Chromosomes• Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) that are
similar in shape and size.
• Homologous pairs (tetrads) (tetrads) carry genes controlling the same inherited traits.
• Each locuslocus (position of a gene) (position of a gene) is in the same position on homologues.
• Humans have 23 pairs of homologous chromosomes.homologous chromosomes.
22 pairs of autosomesautosomes 1 pair of sex chromosomessex chromosomes
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Homologous ChromosomesHomologous Chromosomes((because a homologous pair consists of 4 chromatids it is called a because a homologous pair consists of 4 chromatids it is called a ““TetradTetrad””))
Paternal Maternal
eye color locus
eye color locus
hair color locus
hair color locus
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Humans have 23 Sets of Homologous ChromosomesEach Homologous set is made up of 2 Homologues.
Homologue
Homologue
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AutosomesAutosomes(The Autosomes code for most of the offspring(The Autosomes code for most of the offspring’’s traits)s traits)
In Humans the “Autosomes” are sets 1 - 22
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Sex ChromosomesSex ChromosomesThe Sex Chromosomes code for the sex of the offspring.The Sex Chromosomes code for the sex of the offspring.
** If the offspring has two ** If the offspring has two ““XX”” chromosomes it will be a chromosomes it will be a femalefemale.. ** If the offspring has one ** If the offspring has one ““XX”” chromosome and one chromosome and one ““YY””
chromosome it will be a chromosome it will be a malemale..
XX chromosome - female XY chromosome - male
In Humans the “Sex Chromosomes” are
the 23rd set
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Sex ChromosomesSex Chromosomes
“Sex Chromosomes” …….the 23rd set
23
This person has 2 “X” chromosomes…
and is a female.
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Meiosis is the process by which is the process by which ””gametesgametes”” (sex cells) , (sex cells) , with halfhalf
the number of chromosomes,chromosomes, are produced.
During Meiosis diploid cells are reduced to haploid cells
Diploid (2n) Diploid (2n) Haploid (n)Haploid (n)
If Meiosis did not occur the chromosome If Meiosis did not occur the chromosome number in each new generation would number in each new generation would
double…. The offspring would die.double…. The offspring would die.
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MeiosisMeiosis is Two cell divisionsMeiosis is Two cell divisions
(called meiosis Imeiosis I and meiosis IImeiosis II)
with only one duplication of chromosomes.with only one duplication of chromosomes.
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Meiosis in males is called Meiosis in males is called spermatogenesisspermatogenesis and produces sperm. and produces sperm.
Meiosis in females is called Meiosis in females is called oogenesisoogenesis and produces ova.and produces ova.
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SpermatogenesisSpermatogenesis
2n=46
humansex cell
diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
4 sperm cells are produced from each
primary spermatocyte.
Primary Spermatocyte
Secondary Spermatocyte
Secondary Spermatocyte
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Oogenesis
*** The polar bodies die… only one ovum (egg) is produced from each primary oocyte.
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Interphase IInterphase I• Similar to mitosismitosis interphase.
• ChromosomesChromosomes replicate (S phase).(S phase).
• Each duplicated chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres.
• CentrioleCentriole pairs also replicate.
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Interphase IInterphase I• NucleusNucleus and nucleolusnucleolus visible.
nuclear membrane
nucleolus
cell membrane
chromatin
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Meiosis I (four phases)Meiosis I (four phases)• Cell division Cell division that reduces the chromosomechromosome
number by one-half.one-half.
• four phasesfour phases:a.a. prophase Iprophase Ib.b. metaphase Imetaphase Ic.c. anaphase Ianaphase Id.d. telophase Itelophase I
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Prophase IProphase I
• Longest and most complex phase.Longest and most complex phase.• 90% of the meiotic process is spent in Prophase I90% of the meiotic process is spent in Prophase I
• ChromosomesChromosomes condense.
• SynapsisSynapsis occurs: homologous chromosomeshomologous chromosomes come together to form a tetradtetrad.
• TetradTetrad is two chromosomeschromosomes or four chromatidschromatids (sister and nonsister chromatids).
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Prophase IProphase I - - SynapsisSynapsis
Homologous chromosomes
sister chromatids sister chromatidsTetrad
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During Prophase I During Prophase I ““Crossing OverCrossing Over”” occurs. occurs.
• During Crossing overDuring Crossing over segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid. The Chiasmata Chiasmata
(chiasma) (chiasma) are the sites of crossing overcrossing over.
Crossing Over is one of the Two major occurrences of Meiosis
(The other is Non-disjunction)
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Crossing OverCrossing Over creates variation (diversity) in the offspringcreates variation (diversity) in the offspring’’s traits.s traits.
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
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Question:Question:
• A cell containing 20 chromosomes20 chromosomes (diploid)(diploid) at the beginning of meiosis would, at its
completion, produce cells containing how many chromosomeschromosomes?
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Answer:Answer:• 10 chromosomes (haploid)10 chromosomes (haploid)
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Question:Question:
• A cell containing 40 chromatids40 chromatids at the beginning of meiosis would, at its completion,
produce cells containing how many chromosomeschromosomes?
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Answer:Answer:• 10 chromosomes10 chromosomes
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Prophase IProphase I
centriolesspindle fiber
asterfibers
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Metaphase IMetaphase I• Shortest phaseShortest phase
• TetradsTetrads align on the metaphase platemetaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n
Example:Example: 2n = 42n = 4then then n = 2 n = 2 thusthus 2 22 2 = 4 combinations= 4 combinations
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Metaphase IMetaphase I
metaphase plate
OR
metaphase plate
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Anaphase IAnaphase I• Homologous chromosomesHomologous chromosomes separate and
move towards the poles.
• Sister chromatids Sister chromatids remain attached at their centromerescentromeres.
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Anaphase IAnaphase I
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Telophase ITelophase I
• Each pole now has haploidhaploid set of chromosomeschromosomes.
• CytokinesisCytokinesis occurs and two haploid daughter cells are formed.
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Telophase ITelophase I
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Meiosis IIMeiosis II• No interphase II No interphase II
(or very short - no more DNA replicationDNA replication)
• Remember:Remember: Meiosis IIMeiosis II is similar to mitosismitosis
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Prophase IIProphase II• same as prophaseprophase in mitosismitosis
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Metaphase IIMetaphase II• same as metaphasemetaphase in mitosismitosis
metaphase platemetaphase plate
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Anaphase IIAnaphase II• same as anaphaseanaphase in mitosismitosis• sister chromatids separatesister chromatids separate
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Telophase IITelophase II
• Same as telophasetelophase in mitosismitosis.
• Nuclei form.
• CytokinesisCytokinesis occurs.
• Remember:Remember: four haploid daughter cells four haploid daughter cells produced.produced.
gametes = sperm or egggametes = sperm or egg
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Telophase IITelophase II
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Non-disjunction
• Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to
separate during meiosis.• Non-disjunction results with the production
of zygotes with abnormal chromosome numbers…… remember…. An abnormal
chromosome number (abnormal amount of DNA) is damaging to the offspring.
disjunction is one of the Two major occurrences of Meiosis
(The other is Crossing Over)
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Non-disjunctions usually occur in one of two fashions.
• The first is called Monosomy, the second is called Trisomy. If an organism has
Trisomy 18 it has three chromosomes in the 18th set, Trisomy 21…. Three chromosomes in the 21st set. If an
organism has Monosomy 23 it has only one chromosome in the 23rd set.
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Common Non-disjunction Disorders
• Down’s Syndrome – Trisomy 21• Turner’s Syndrome – Monosomy 23 (X)• Kleinfelter’s Syndrome – Trisomy 23 (XXY)• Edward’s Syndrome – Trisomy 18
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Amniocentesis• An Amniocentesis is a prrocedure a pregnant woman can have in order to
detect some genetics disorders…..such as non-disjunction.
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Amniocentesis
Amniotic fluid withdrawn
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Karyotype(picture of an individual’s chromosomes)
One of the ways to analyze the
amniocentesis is to make a Karyotype
What genetic disorder does this karyotype show?
Trisomy 21….Down’s Syndrome