MEIOSIS. MEIOSIS cell divisiongametes, half chromosomes, The form of cell division by which...
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Transcript of MEIOSIS. MEIOSIS cell divisiongametes, half chromosomes, The form of cell division by which...
MEIOSIS
The form of cell divisioncell division by which gametes, gametes, with halfhalf the number of chromosomes,chromosomes, are produced.
Diploid (2n) Diploid (2n) haploid (n)haploid (n)
MeiosisMeiosis is sexual reproduction.sexual reproduction.
Two divisionsTwo divisions (meiosis I meiosis I and meiosis IImeiosis II).
MEIOSISMEIOSIS
Sex cells Sex cells divide to produce gametesgametes (sperm (sperm or egg)or egg).
GametesGametes have halfhalf the # of chromosomeschromosomes.
Occurs only in gonads (testes or ovaries).Occurs only in gonads (testes or ovaries).
Male: spermatogenesisMale: spermatogenesis
Female: oogenesisFemale: oogenesis
MeiosisMeiosis is similar to mitosismitosis with some chromosomal differences.
Produces two diploid daughter cells
Produces daughter cells that ARE genetically identical
Produce four haploid daughter cells
Produces daughter cells that are NOT genetically identical
SPERMATOGENESIS - SPERMATOGENESIS - FORMATION OF SPERM FORMATION OF SPERM
CELLSCELLS
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
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.
INTERPHASE IINTERPHASE I
NucleusNucleus and nucleolusnucleolus visible.
nuclear membrane
nucleolus
cell membrane
chromatin
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 I
b.b. metaphase Imetaphase I
c.c. anaphase Ianaphase I
d.d. telophase Itelophase I
PROPHASE IPROPHASE I
Longest and most complex phase (90%).Longest and most complex phase (90%).
ChromosomesChromosomes condense.
SynapsisSynapsis occurs: homologous homologous chromosomes chromosomes come together to form a tetradtetrad.
TetradTetrad is two chromosomeschromosomes or four chromatidschromatids (sister and nonsister chromatids).
PROPHASE IPROPHASE I - - SYNAPSISSYNAPSIS
Homologous chromosomes
sister chromatids sister chromatidsTetrad
HOMOLOGOUS HOMOLOGOUS CHROMOSOMESCHROMOSOMES
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 homologous chromosomes.chromosomes.
a. 22 pairs of autosomesautosomesb. 01 pair of sex chromosomessex chromosomes
HOMOLOGOUS CHROMOSOMESHOMOLOGOUS CHROMOSOMES
Paternal Maternal
eye color locus
eye color locus
hair color locus
hair color locus
CROSSING OVERCROSSING OVER
Crossing over (variation) Crossing over (variation) may occur between nonsister chromatidschromatids at the chiasmatachiasmata.
Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.
Chiasmata (chiasma) Chiasmata (chiasma) are the sites of crossing overcrossing over.
CROSSING OVER - VARIATION CROSSING OVER - VARIATION
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
SEX CHROMOSOMESSEX CHROMOSOMES
XX chromosome - female XY chromosome - male
PROPHASE IPROPHASE I
centriolesspindle fiber
asterfibers
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
METAPHASE IMETAPHASE I
metaphase plate
OR
metaphase plate
QUESTION:QUESTION:
In terms of In terms of Independent Independent AssortmentAssortment -how many different -how many different combinations of sperm could a combinations of sperm could a humanhuman malemale produce? produce?
ANSWERANSWER
Formula: 2Formula: 2nn
Human chromosomes:Human chromosomes: 2n = 462n = 46
n = 23n = 23
222323 = ~8 million = ~8 million combinationscombinations
ANAPHASE IANAPHASE I
Homologous chromosomes Homologous chromosomes separate and move towards the poles.
Sister chromatids Sister chromatids remain attached at their centromerescentromeres.
ANAPHASE IANAPHASE I
TELOPHASE ITELOPHASE I
Each pole now has haploidhaploid set of chromosomeschromosomes.
CytokinesisCytokinesis occurs and two haploid daughter cells are formed.
TELOPHASE ITELOPHASE I
Figure 2.3
Interkinesis
A short interphase between the two meiotic divisions
Chromosomes unfold into very thin threads
Proteins are manufactured
However, DNA is NOT replicated a second time
MEIOSIS IIMEIOSIS II
No interphase II No interphase II (or very short - no more DNA replicationDNA replication)
Remember:Remember: Meiosis II Meiosis II is similar to mitosismitosis
PROPHASE IIPROPHASE II
same as prophaseprophase in mitosismitosis
METAPHASE IIMETAPHASE II
same as metaphasemetaphase in mitosismitosis
metaphase platemetaphase plate
ANAPHASE IIANAPHASE II
same as anaphaseanaphase in mitosismitosis sister chromatids separatesister chromatids separate
TELOPHASE IITELOPHASE II
Same as telophasetelophase in mitosismitosis.
Nuclei form.
CytokinesisCytokinesis occurs.
Remember:Remember: four haploid daughter four haploid daughter cells cells produced.produced.
gametes = sperm or egggametes = sperm or egg
TELOPHASE IITELOPHASE II
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
VARIATIONVARIATION
Important to population as the raw Important to population as the raw material for material for natural selectionnatural selection..
Question:Question:
What are the three sexual sources What are the three sexual sources ofof
genetic variation? genetic variation?
ANSWER:ANSWER:
1. crossing over (prophase I)1. crossing over (prophase I)
2. independent assortment 2. independent assortment (metaphase I)(metaphase I)
3. random fertilization3. random fertilization
Remember: variation is good!
Table 3.1
Comparison of Mitosis and Meiosis
Interphase
CytokinesisTelophase Anapha
se
Metaphase
Prophase
What looks similar between mitosis and meiosis?What looks different?
Figure 2.3
Meiosis vs. Mitosis Animation
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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?
ANSWER:ANSWER:
10 chromosomes (haploid)10 chromosomes (haploid)
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
QUESTION:QUESTION:
A cell containing 40 chromatids40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?
ANSWER:ANSWER:
10 chromosomes10 chromosomes