MEIOSIS
Formation of gametes, division of the
sex cell (egg and sperm)
(Readapted from Slideshare):
1. Jay Swan
2. Karl Po...
Chromosome Matching
 In humans, somatic cells (body cells) have:
• 23 pairs of homologous chromosomes
and
• one member of...
• Homologous chromosomes are matched in:
• Similar length,
• Centromere (attaches sister chromatids together)
position
• g...
MEIOSIS

 The process to make cells with half the number of
chromosomes for sexual reproduction
 Usually humans and most...
 Meiosis is a process that converts diploid
nuclei to haploid nuclei.
• Diploid cells have 2 sets of chromosomes.
• Haplo...
Why do we need
Meiosis?
It is the fundamental basis of sexual reproduction
Two haploid (n) gametes are brought together
...
Meiosis must reduce the chromosome number by half
(n)
Fertilization then restores the 2n number
Summary of the Meiotic
process
MEIOSIS has two distinct
stages
MEIOSIS I consisting of 5
phases:

 Interphase I, Prophase I,
Metaphase I, Anaphase I,
T...
MEIOSIS 1:
Interphase
 Cell build up
energy
 DNA Replication
(to make
duplicated
chromosomes
 Cell doesn’t
change
struc...
MEIOSIS 1:
Prophase 1
Prophase 1
in detail

 Events occurring in the nucleus:
• Chromosomes coil and become individual
chromosomes, nucleolus a...
PROPHASE 1

• Early prophase 1
 Homologous pair.
 Crossing over occurs.

Late Prophase 1
 Chromosomes condense.
 Spind...
Prophase 1: Crossing over
• Synapsis – the pairing of homologous
chromosomes
• Group of 4 chromatids
Homologous chromosome...
Prophase 1: Crossing over
Homologous
chromosomes in
a tetrad cross
over each other
- Genes are
exchanged
Crossing over
Metaphase I
Spindle fibre
attached to a
kinetochore

Metaphase
plate

 Homologous pairs of
chromosomes align along
the eq...
Anaphase 1
Spindle fibers contact.
Homologous chromosomes separate and
move to opposite poles.

Sister chromatids remain
a...
Telophase 1
Nuclear envelopes reappear

Spindle fibres disappear.

Cytokinesis (when the cytoplasm divides)
divides cell i...
Telophase 1 and Cytokinesis
• Duplicated chromosomes have
reached the poles.
• A nuclear envelope and
nucleolus re-forms a...
Meiosis ii: prophase II
• Chromosomes coil and
become compact (if
uncoiled after telophase
I).
• Nuclear envelope and
nucl...
Meiosis ii: metaphase II
• Individual duplicated
chromosomes align on the
equator.
• One chromosome per
spindle fiber atta...
Anaphase 2
- Spindle fibers
contract.
- Duplicated
chromosomes
split in half
(centromere
dividing in 2)
Sister chromatids ...
Meiosis 2: Telophase 2
• Daughter chromosomes
has reached the poles.
• Two cells invaginate and
form 4 daughter haploid
ce...
MEOITIC DIVISION 2:
SUMMARY
Prophase II

Metaphase II

Anaphase II

Telophase II
and Cytokinesis
Results of Meiosis
 Gametes (egg & sperm) form
 Four haploid cells (n) with one
copy of each chromosome

 One allele of...
When Chromosome number is
altered
 An extra copy of chromosome 21
causes Down syndrome or also known
as TRISOMY 21.
 A. ...
 Trisomy 21 (Down Syndrome) produces a
characteristic set of symptoms, which include:
1. mental retardation,
characterist...
Bibliography
• Jackson, R., & Jackson, J. M. (2011). Henderson's
Dictionary of BIOLOGY (15th ed.). (E. Lawrence, Ed.)
(pp....
Meiosis 1 and 2
Meiosis 1 and 2
Meiosis 1 and 2
Meiosis 1 and 2
Meiosis 1 and 2
Meiosis 1 and 2
Meiosis 1 and 2
Meiosis 1 and 2
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Meiosis 1 and 2

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Meiosis and the stages involved in them. The powerpoint was readapted from various powerpoint presentations.

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Meiosis 1 and 2

  1. 1. MEIOSIS Formation of gametes, division of the sex cell (egg and sperm) (Readapted from Slideshare): 1. Jay Swan 2. Karl Pointer 3. Mbrown
  2. 2. Chromosome Matching  In humans, somatic cells (body cells) have: • 23 pairs of homologous chromosomes and • one member of each pair from each parent.  The human sex chromosomes (Gonosomes) X and Y differ in size and genetic composition.  The other 22 pairs of chromosomes are autosomes with the same size and genetic composition.
  3. 3. • Homologous chromosomes are matched in: • Similar length, • Centromere (attaches sister chromatids together) position • gene locations (locus). • A locus (plural, loci) is the position of a gene. • Different versions or variations (alleles) of a gene may be found at the same locus on maternal and paternal chromosomes.
  4. 4. MEIOSIS  The process to make cells with half the number of chromosomes for sexual reproduction  Usually humans and most animals and some plants have diploid (2n) body cells. Meaning that they have two sets of chromosomes (one from each parent)  Meiosis occurs in our germ cells that produce gametes (Sperm & egg) • Meiosis results in four cells which are genetically different from parent cell and from each other. • The end products of Meiosis are 4 Haploid (n) cells
  5. 5.  Meiosis is a process that converts diploid nuclei to haploid nuclei. • Diploid cells have 2 sets of chromosomes. • Haploid cells have 1 set of chromosomes. • Meiosis occurs in the sex organs, producing gametes—sperm and eggs.  Fertilization is the fusion of a sperm and egg cell.  The zygote has a diploid chromosome number, one set from each parent.
  6. 6. Why do we need Meiosis? It is the fundamental basis of sexual reproduction Two haploid (n) gametes are brought together through fertilization to form a diploid (2n) zygote If egg and sperm had the same number of chromosomes as other body cells then the offspring would have too many chromosomes.
  7. 7. Meiosis must reduce the chromosome number by half (n) Fertilization then restores the 2n number
  8. 8. Summary of the Meiotic process
  9. 9. MEIOSIS has two distinct stages MEIOSIS I consisting of 5 phases:  Interphase I, Prophase I, Metaphase I, Anaphase I, Telophase I.  MEIOSIS II consisting of 4 phases  Prophase II, Metaphase II, Anaphase II, Telophase II.
  10. 10. MEIOSIS 1: Interphase  Cell build up energy  DNA Replication (to make duplicated chromosomes  Cell doesn’t change structurally.
  11. 11. MEIOSIS 1: Prophase 1
  12. 12. Prophase 1 in detail  Events occurring in the nucleus: • Chromosomes coil and become individual chromosomes, nucleolus and nuclear envelope disappear. • Homologous chromosomes come together as pairs by synapsis forming a tetrad (Each pair, with four chromatids) • Non-sister chromatids exchange genetic material through the process of crossing over to ensure genetic variation. • Centrioli move to opposite poles with spindle fibers between them.
  13. 13. PROPHASE 1 • Early prophase 1  Homologous pair.  Crossing over occurs. Late Prophase 1  Chromosomes condense.  Spindle forms.  Nuclear envelope fragments.
  14. 14. Prophase 1: Crossing over • Synapsis – the pairing of homologous chromosomes • Group of 4 chromatids Homologous chromosomes (each with sister chromatids) Join to form a TETRAD
  15. 15. Prophase 1: Crossing over Homologous chromosomes in a tetrad cross over each other - Genes are exchanged
  16. 16. Crossing over
  17. 17. Metaphase I Spindle fibre attached to a kinetochore Metaphase plate  Homologous pairs of chromosomes align along the equator of the cell  The two chromosomes attach to one spindle fiber by means of the kinetochore of the centromere.
  18. 18. Anaphase 1 Spindle fibers contact. Homologous chromosomes separate and move to opposite poles. Sister chromatids remain attached at their centromeres.
  19. 19. Telophase 1 Nuclear envelopes reappear Spindle fibres disappear. Cytokinesis (when the cytoplasm divides) divides cell into two.
  20. 20. Telophase 1 and Cytokinesis • Duplicated chromosomes have reached the poles. • A nuclear envelope and nucleolus re-forms around chromosomes. • Each nucleus now has the haploid number of chromosomes. • Cell invaginates forming a cleavage furrow, which extends to for 2 separate haploid cells.
  21. 21. Meiosis ii: prophase II • Chromosomes coil and become compact (if uncoiled after telophase I). • Nuclear envelope and nucleolus, if re-formed, dissappears again. • Centrioli move to opposite poles, forming spindle fibers between them.
  22. 22. Meiosis ii: metaphase II • Individual duplicated chromosomes align on the equator. • One chromosome per spindle fiber attached by means of kinetochore of centromere. • Centrioli has reached the poles.
  23. 23. Anaphase 2 - Spindle fibers contract. - Duplicated chromosomes split in half (centromere dividing in 2) Sister chromatids separate and move to opposite poles.
  24. 24. Meiosis 2: Telophase 2 • Daughter chromosomes has reached the poles. • Two cells invaginate and form 4 daughter haploid cells (gametes) • They uncoil and form chromatin. • Nuclear envelope and nucleolus for around chromatin again. • Centrioli for centrosome.
  25. 25. MEOITIC DIVISION 2: SUMMARY Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis
  26. 26. Results of Meiosis  Gametes (egg & sperm) form  Four haploid cells (n) with one copy of each chromosome  One allele of each gene  Different combinations of alleles for different genes along the chromosome
  27. 27. When Chromosome number is altered  An extra copy of chromosome 21 causes Down syndrome or also known as TRISOMY 21.  A. Trisomy 21 • involves the inheritance of three copies of chromosome 21 and • is the most common human chromosome abnormality.
  28. 28.  Trisomy 21 (Down Syndrome) produces a characteristic set of symptoms, which include: 1. mental retardation, characteristic facial features, 2. short stature, 3. heart defects, 4. susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and 5. shortened life span.  The incidence increases with the age of the mother.
  29. 29. Bibliography • Jackson, R., & Jackson, J. M. (2011). Henderson's Dictionary of BIOLOGY (15th ed.). (E. Lawrence, Ed.) (pp. 181-183). London: Pearson Education Limited. • Livingstone, C. D., & Nobbe, M. E. (1998). The Molecules of Life. Oxford: Biochemistry Department, University of Oxford. • Reece, J., Urry, L. A., Cane, M. L., Wasserman, S. A., Minrsky, P. V., & Jackson, R. B. (2011). Campbell Biology (9th ed.). San Francisco: Pearson Benjamin Cummings. • Urry, L. A., & Cane, M. L. (2011). The Molecular Basis of Inheritance. In J. B. Reece, Campbell Biology (pp. 351370). San Francisco: Pearson Benjamin Cummings.
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