4 Science 1 RPK Meiosis
Transcript of 4 Science 1 RPK Meiosis
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MEIOSISMEIOSIS
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MeiosisMeiosis The word "meiosis" comes from
the Greek verb meioun, meaning"to make small,
The form ofcell divisioncell divisionby
which gametes,gametes, with halfhalfthenumber of chromosomes (chromosomes (
homologous chromosome)homologous chromosome) are
produced.
Diploid (2n)Diploid (2n) pp haploid (n)haploid (n)
MeiosisMeiosis is sexual reproduction.sexual reproduction.
Two divisionsTwo divisions (meiosis Imeiosis I and
meiosis IImeiosis II).
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MeiosisMeiosis
Sex cellsSex cells divide to produce gametesgametes (sperm(sperm
or egg)or egg).
GametesGametes have halfhalf ofchromosomeschromosomes.
Occurs only in gonads (testes or ovaries).Occurs only in gonads (testes or ovaries).
Male: spermatogenesisMale: spermatogenesis
Female:Female: oogenesisoogenesis
MeiosisMeiosis is similar to mitosismitosis with some
chromosomal differences.
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SpermatogenesisSpermatogenesis
2n=46
human
sex cell
diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
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meiosis ismade up oftwo phasesof celldivision
- meiosis I:-the reductiondivision
- meiosis II:-
separatesthe sisterchromatids
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InterphaseInterphase Similar to mitosismitosis
interphase.
ChromosomesChromosomes replicate
(S phase).(S phase).
Each duplicated
chromosomechromosome consist of
two identical sister
chromatidschromatids attached attheircentromerescentromeres.
CentrioleCentriole pairs also
replicate.
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InterphaseInterphase NucleusNucleus and nucleolusnucleolus visible.
nuclear
membrane
nucleolus
cell membrane
chromatin
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Meiosis I (four phases)Meiosis I (four phases) Cell divisionCell division that reduces the chromosomechromosome
number by oneone--half.half.
four phasesfour phases:
a.a. prophase Iprophase I
b.b. metaphase Imetaphase I
c.c. anaphase Ianaphase I
d.d. telophase Itelophase I
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Prophase IProphase I
Longest and most complex phase (90%).Longest and most complex phase (90%).
ChromosomesChromosomes condense. SynapsisSynapsis occurs: homologous chromosomeshomologous chromosomes
come together to form a tetradtetrad.
TetradTetrad is two chromosomeschromosomes or fourchromatidschromatids (sister and nonsister chromatids).
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Prophase IProphase I -- SynapsisSynapsisHomologous chromosomes
sister chromatids sister chromatids
Tetrad
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Homologous ChromosomesHomologous Chromosomes
Pair ofchromosomeschromosomes (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 ofhomologous chromosomes.homologous chromosomes.
a. 22 pairs ofautosomesautosomes
b. 01 pair of sex chromosomessex chromosomes
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Homologous ChromosomesHomologous Chromosomes
Paternal Maternal
eye color
locus
eye color
locus
hair color
locushair color
locus
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Crossing OverCrossing Over
Crossing over (variation)Crossing over (variation)may occur between nonsister
chromatidschromatids at the chiasmatachiasmata.
Crossing overCrossing over: segments ofnonsisterchromatidschromatids break
and reattach to the other
chromatidchromatid.
ChiasmataChiasmata ((chiasmachiasma)) are the
sites ofcrossing overcrossing over.
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Crossing OverCrossing Over -- variationvariation
nonsister chromatids
chiasmata: siteof crossing over variation
Tetrad
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Sex ChromosomesSex Chromosomes
XX chromosome - female XY chromosome - male
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Prophase IP
rophase I
centriolesspindle fiber
aster
fibers
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Metaphase IMetaphase I Shortest phaseShortest phase
TetradsTetrads align on the metaphase platemetaphase plate.
INDEP
ENDENT ASSORTMENT OCCURS:INDEP
ENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example:Example: 2n = 42n = 4thenthen n = 2n = 2
thusthus 2222 = 4 combinations= 4 combinations
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Metaphase IMetaphase I
metaphase plate
OR
metaphase plate
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Question:
Question:
In terms ofIn terms ofIndependent AssortmentIndependent Assortment --
how many different combinations ofhow many different combinations of
sperm could asperm could a humanhuman malemale produce?produce?
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AnswerAnswer Formula: 2Formula: 2nn
Human chromosomes:Human chromosomes: 2n = 462n = 46
n = 23n = 23
222323
= ~8 million combinations= ~8 million combinations
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Anaphase IAnaphase I Homologous chromosomesHomologous chromosomes separate and
move towards the poles.
Sister chromatidsSister chromatids remain attached at their
centromerescentromeres.
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Anaphase IAnaphase I
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Telophase ITelophase I
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Meiosis IIMeiosis II No interphase IINo interphase II
(or very short - no more DNA replicationDNA replication)
Remember:Remember: Meiosis IIMeiosis II is similar to mitosismitosis
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Prophase IIP
rophase 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 cellsfour haploid daughter cells
produced.produced.
gametes = sperm or egggametes = sperm or egg
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Telophase IITelophase II
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MeiosisMeiosis
2n=4
sex cell
diploid (2n)
n=2
n=2
meiosis I
n=2
n=2
n=2
n=2
sperm
haploid (n)
meiosis II
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VariationV
ariation Important to population as the rawImportant to population as the raw
material formaterial fornatural selectionnatural selection..
Question:Question:
What are the three sexual sources ofWhat are the three sexual sources of
genetic variation?genetic variation?
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Answer:Answer:1. crossing over (prophase I)1. crossing over (prophase I)
2. independent assortment (metaphase I)2. independent assortment (metaphase I)
3. random fertilization3. random fertilization
Remember: variation is good!
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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 howmany chromosomeschromosomes?
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Answer:Answer: 10 chromosomes (haploid)10 chromosomes (haploid)
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KaryotypeKaryotype A method of organizing theA method of organizing the chromosomeschromosomes of aof a
cell in relation to number, size, and type.cell in relation to number, size, and type.
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FertilizationFertilization The fusion of a spermsperm and eggegg to form a zygotezygote.
A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
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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 manychromosomeschromosomes?
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Answer:Answer: 10 chromosomes10 chromosomes
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Comparing Meiosis and Mitosis
Chromosome behavior
Mitosis: Homologous chromosomes independent
Meiosis: Homologous chromosomes pair forming bivalents
until anaphase I
Chromosome number
Mitosis- daughter cells diploid
Meiosis- daughter cells haploid
Genetic identity of progeny: Mitosis: identical to parent cell
Meiosis: not identical to parents cell
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Appreciating the movement of
chromosomes during Mitosis and Meiosis
To ensure continuity of life on Earth
Mitosis and meiosis are regulated in precise
manner.
For example:
- If the sister chromatids do not separate during
anaphase, the number of chromosome in
each cell would not be the same as the
parent cell.
- The gametes produced would have an
abnormal number of chromosome
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Downs syndrome:
The result of an additional
chromosome, causing
each somatic cell to have
a total of 47 chromosomes
instead of normal 46
chromosomes.- Shanted eyes, short
stature, mental
retardation.
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Klinefelter