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MONWABISI MKHIZE201110525
MEIOSIS SLIDE PRESENTATION
Cell DivisionMEIOSIS
Meiosis: Important Vocabulary
Diploid- contains 2 sets of chromosomes (2n = 46) Example: human
somatic cells
Haploid- contains 1 set of chromosomes (n = 23) Example: human
gametes
Meiosis: Important Vocabulary
Fertilization- fusion of two haploid gametes forms a diploid zygote
Zygote- fertilized egg cell The first cell of a new individual
What would happen if gametes were diploid?
Meiosis Meiosis occurs in sexual reproduction when a diploid
germ cell produces four haploid daughter cells that can mature to become gametes (sperm or egg).
Meiosis
Goal: reduce genetic material by half
Why? n (mom) + n (dad) = 2n (offspring)
from mom from dad child
meiosis reducesgenetic content
toomuch!
Just right!
Meiosis: cell division in two parts
Chromatidsseparate
Sister chromatidsseparate
Result: one copy of each chromosome in a gamete.
Diploid 2n
Diploid
Meiosis I(reductiondivision)
Meiosis II
Haploid
(equationaldivision)
2n
n
Meiosis I : the reduction division
Prophase I(early)
(diploid)
Prophase I(late)
(diploid)
Metaphase I(diploid)
Anaphase I(diploid)
Telophase I(diploid)
Nucleus Spindlefibers
Nuclearenvelope
Prophase I
Early prophase
Chromatids pair. Crossing over
occurs.
Late prophase
Chromosomes condense.
Spindle forms.Nuclear
envelope fragments.
Metaphase I
Chromatid pairs alignalong the equator of the cell.
Anaphase I
Chromosomes separate andmove to opposite poles.
Sister chromatids remain Attached at their centromeres.
Telophase I
Nuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into two.
Meiosis II
Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene.
Sister chromatids carry identical genetic
information.
Gene X
Meiosis II : the equational division
Prophase II(diploid)
Metaphase II Anaphase II Telophase II Four Non-identical
haploid daughter cells
Prophase II
Nuclear envelope fragments.
Spindle forms.
Metaphase II
Chromosomes align along equator of cell.
Anaphase II
Sister chromatids separateand move to opposite poles.
Telophase II
Nuclear envelope assembles.
Chromosomes unravel.
Spindle disappears.
Cytokinesis divides cell into two.
Results of meiosis
• Four haploid cells
• One copy of each chromosome
What Meiosis is About
Meiosis allows the creation of unique individuals through sexual reproduction.
Mitosis Meiosis
Number of divisions 1 2
Number of daughter cells
2 4
Genetically identical?
Yes No
Chromosome # Same as parent Half of parent
Where Somatic cells Sex cells
When Throughout life At sexual maturity
Role Growth and repair Sexual reproduction
MITOSIS MEIOSIS
Occurs in somatic (body) cells Occurs only in gonads (sex organs: ovary/testes)
Produces cells for repair,maintenance, growth, asexual reproduction
Only produces gametes (sex cells: egg/sperm)
Results in identical diploid (2n) daughter cells
Reduction division results in 4 haploid (n) cells
KARYOTYPE
1. Photograph a cell in metaphase.
2. Cut out the chromosomes from the picture.
3. Paste homologous pairs together according to size, banding pattern and centromere.
4. Lay pairs out largest to smallest, sex chromosomes at end.
Karyotype is used to determine
1.If there is an abnormality in number or structure of the chromosomes
(eg. Down’s syndrome)
2. The gender
•Karyotypes of embryos are obtained from:
CVS (chorionic villus sampling) and amniocentesis.
•Karyotypes of adults can be obtained from any growing cells.
•Karyotypes can not tell if there are mutations of a gene.
Products of MITOSIS: 2 x 2n cells.
2 diploid cells
Products of MEIOSIS: 4 x n cells
(4 monoploid cells)
2n 2n 2n
2n n n n n
MEIOSIS is two mitotic divisions in a
row, (meiosis I and II) except you will see the homologous chromosomes coming
together.- -- in prophase I the double homologous chromosomes come together to form a tetrad
- Crossing-over (synapsis) occurs in the tetrad in prophase to increase the variability in gametes.
- There is no dna replication in the interphase
between the first and second divisions
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. 1 pair of sex chromosomessex chromosomes
Homologous Homologous ChromosomesChromosomes
Paternal Maternal
eye color locus
eye color locus
hair color locus
hair color locus
Crossing Over Crossing Over (SYNAPSIS) Crossing over Crossing over (variation) (variation) may occur
between nonsister chromatidschromatids at the chiasmatachiasmata.
Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.
ChiasmataChiasmata (chiasma) (chiasma) are the sites of crossing overcrossing over.
Synapsis increases genetic variability – and that’s a good thing…
Crossing Over - variation Crossing Over - variation
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
Sex ChromosomesSex Chromosomes
XX chromosome - female XY chromosome - male
Cytokinesis: cytoplasm divides. Total product is 4 cells, each with only a mom or Dad’s chromosome, in other words, half the normal chromosome number, or 4 monoploid (n) cells, or 4 gametes.
Uneven cytokinesis in females: In oogenesis, get one big egg and 3 non-functional polar bodies:
( who die off) and 1 big functional gamete
Spermatogenesis
There are equal divisions producing four equal sized sperm.
= Non-separation of chromosomes resulting in diploid or empty gametes.
Non-disjunction
Downs syndrome = Trisomy 21 - 3 copies of chromosomes 21 equalling a total of 47 chromosomes. = 2n + 1(chance of occurring in oogenesis increases with maternal age)
Turners syndrome = Monosomy X - has only oneone XX chromosomes totalling only 45 chromosomes in her body cells. (monosomy X) = 2n-1
Klinefelter’s syndrome = XXY - male which has an extra Xextra X chromosome = 2n + 1
Conditions caused bynon-disjunction
A simple check for the presence of a bone in the nose could more accurately test unborn babies for Down's syndrome, scientists say. Researchers say combining the nose test with existing screening methods (amniocentesis, CVS) could lead to a five-fold reduction in the number of miscarriages linked to an invasive procedure used to confirm Down's syndrome.
Down’s syndrome – trisomy 21
Normal boy who develops some female secondary sex character-istics at puberty.
-slightly lower IQ, infertile, delayed motor, speech, maturation
-treated with testosterone.
Klinefelter’s syndrome XXY
REFERENCES
Farrellw (2012) MEIO. ACCESS ON 7 MARCH 2014
http://www.slideshare.net/farrellw/meio-11234603
Docsawyer (2010) MEIOSIS. ACCESS ON 07 MARCH 2014
http://www.slideshare.net/docsawyer/meiosis-notes-3075315