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