This document defines key terms related to meiosis such as allele, autosome, chromosome, diploid, gamete, gene, haploid, homologous chromosomes, karyotype, monoploid, monosomy, polyploid, tetrad, and trisomy 21. It then provides details about the stages of meiosis (meiosis I and meiosis II), including prophase I, metaphase I, anaphase I, telophase I and prophase II, metaphase II, anaphase II, and telophase II. It explains that meiosis results in four haploid cells with one copy of each chromosome to produce gametes like eggs and sperm, and discusses nondisjunction and

1. CELL BIOLOGY
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2. MEIOSIS
• STAGES OF MITOSIS
• Terms
• Allele
• Autosome
• Chromosome
• Diploid
• Gametes
• Genes
• Haploid
• Heterozygous
• Homozygous
• Homologous chromosomes
• Monoploid
• Monosomy
• Polyploid
• Tetrads
• Trisomy 21
• Karyotype
• MEIOSIS READING ASSIGNMENT!!
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3. MEIOSIS
• Allele An alternative version of a gene
• Autosome A chromosome not directly involved in determining the sex
of an organism; in mammals, for example, any chromosome other than
X or Y
• Chromosome A threadlike, gene-carrying structure found in the
nucleus of a eukaryotic cell and most visible during mitosis and
meiosis;
• also, the main gene-carrying structure of a prokaryotic cell.
• A chromosomes consists of one very long piece of chromatin, a combination of
DNA and protein.
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4. •Diploid An organism that reproduces
sexually, a cell containing two homologous sets
of chromosomes (46), one set inherited from each
parent; a 2N cell
•Gamete A sex cell; a haploid egg or sperm.
The union of two gametes of opposite sex
(fertilization) produces a zygote
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5. • Genes A discrete unit of hereditary information consisting
of a specific nucleotide sequence in DNA (or RNA, in some
viruses). Most of the genes of a eukaryote are located in its
chromosomal DNA; a few are carried by the DNA of
mitochondria and chloroplasts
• Haploid A cell containing a single set of chromosomes (23);
an N cell
• Heterozygous Having two different alleles for a given gene
• Homozygous Having two identical alleles for a given gene
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6. • Homologous chromosomes-The two chromosomes
that make up a matched pair in a diploid cell.
• Are of the same length, centromere position, and staining
pattern and possess genes for the same characteristics at
corresponding loci.
• One homologous chromosome is inherited from the
organism’s father, the other from the mother
• Monoploid An organism having one set of
chromosomes
• Monosomy The cell is 2N -1
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7. • Polyploid An organism that has more than two complete sets of
chromosomes as a result of an accident of cell division
• Tetrads A paired set of homologous chromosomes, each composed
of two sister chromatids.
• Tetrads form during prophase I of meiosis, when crossing over may occur
• Trisomy 21 (The cell is 2N+1) or Down syndrome A human genetic
disorder resulting from the presence of an extra chromosome 21;
• Characterized by heart and respiratory defects and varying degrees of mental
retardation
• Karyotype-A display of micrographs of the metaphase chromosomes
of a cell, arranged by size and centromere position. Karyotypes may
be used to identify certain chromosomal abnormalities
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8. MEIOSIS
•The formation of gametes or a type of cell
division that produces haploid gametes in diploid
organisms.
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9. FACTS ABOUT MEIOSIS
•Preceded by interphase which includes
chromosome replication
•Like Mitosis, Meiosis is preceded by the duplication
of chromosomes which is followed by two
consecutive cell divisions, called meiosis I and
meiosis II.
•Because one duplication of the chromosomes is
followed by two divisions, each of the four daughter
cells resulting from meiosis has a haploid set of
chromosomes
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10. •Is a 2 part cell division.
• Two meiotic divisions -Meiosis I and Meiosis II
•Called Reduction-division; because it reduces the
chromosome number by half;
• It reduces the genetic content (fertilization the restores
the genetic content, 2N)
• Two haploid gametes (egg & sperm) combine
(fertilization) to restore the diploid state in the zygote
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12. FERTILIZATION
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13. • Original cell is diploid (2N)
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14. • Four daughter cells produced that are monoploid (1N)
• Daughter cells contain half the number of
chromosomes as the original cell
• Produces gametes (eggs & sperm)
• Occurs in the sex cells/germ cells that produce
gametes
• e.g. testes in males (Spermatogenesis) and
• Ovaries in females (Oogenesis)
• Start with 46 double stranded chromosomes (2N)
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15. • After 1 division -23 double stranded chromosomes (N)
• After 2nd division -23 single stranded chromosomes
(N)
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16. MEIOSIS I
• Called the reduction division
• PROPHASE I
• Early prophase
• Homologs pair
• Crossing over occurs
• Homologous chromosomes join to form a tetrad (A paired set of
homologous chromosomes, each composed of two sister chromatids) in a process called
synapsis: The pairing of homologous chromosomes
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18. LATE PROPHASE
• Chromosomes condense.
• Spindle forms.
• Nuclear envelope fragments
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19. METAPHASE I
• Homologous pairs of chromosomes align along the equator of the cell
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20. ANAPHASE I
• Homologs separate and move to opposite poles.
• Sister chromatids remain attached at their centromeres
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21. TELOPHASE I
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22. MEIOSIS II – REDUCING NUMBER OF
CHROMOSOMES
•Produces gametes with one copy of each
chromosome and thus one copy of each gene.
•Only one homolog of each chromosome is
present in the cell.
•Sister chromatids carry identical genetic
information
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24. •Prophase II
•Nuclear envelope fragments.
•Spindle forms
•Metaphase II
•Chromosomes align along equator of cell
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25. •Anaphase II
•Sister chromatids separate and move to opposite
poles
•Telophase II
•Nuclear envelope assembles.
•Chromosomes decondense
•Spindle disappears
•Cytokinesis divides cell into two
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26. RESULTS OF MEIOSIS
•Gametes (egg & sperm) form
•Four haploid cells with one copy of each
chromosome
•One allele of each gene
•Different combinations of alleles for different genes
along the chromosome
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27. SPERMATOGENESIS
•Occurs in the testes
•Two divisions produce 4 spermatids
•Spermatids mature into sperm
•Men produce about 250,000,000 sperm per
day
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29. OOGENESIS
• Occurs in the ovaries
• Two divisions produce 3 polar bodies that die and 1
egg
• Polar bodies die because of unequal division of
cytoplasm
• Immature egg called oocyte
• Starting at puberty, one oocyte matures into an ovum
(egg) every 28 days
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32. NONDISJUNCTION
• The normal separation of chromosomes in meiosis I or
sister chromatids in meiosis II is termed disjunction.
• When the separation is not normal, it is called
nondisjunction.
• This results in the production of gametes which have either
too many or too few of a particular chromosome, and is a
common mechanism for trisomy or monosomy.
• Can occur in the meiosis I or meiosis II, phases of
cellular reproduction, or during mitosis
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33. This is a cause of several medical conditions
in humans
•Down syndrome - trisomy of chromosome 21.
• Heart and respiratory defects
• Varying degrees of mental retardation
• Difficulty in thinking and understanding
• Excess skin on the back of the neck
• Hearing loss
• Vision disorder
•Patau Syndrome - trisomy of chromosome 13
•Edward Syndrome - trisomy of chromosome 18
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34. Klinefelter Syndrome - extra X chromosomes in
males (only) - i.e. XXY, XXXY, XXXXY
•Happens when:
•An XX egg is fertilized by a normal Y sperm
•OR an normal X egg is fertilized by an XY sperm
•Resulting with chromosome complement 2n = 44 +
XXY
•Individuals are always male and possess male sex
organs
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35. • Underdeveloped sexual organs (testes).
• However they are infertile since their testes only develop
to half the normal size and fail to produce sperm
• Testes produce low levels of testosterone so facial hair,
deepening of voice are only weakly expressed.
• Some sufferers develop small breasts.
• Occurs in 1:1000 live male births
• Taller than average male
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37. TURNER SYNDROME
• Lacking of one X chromosome in females - i.e. XO
• If a gamete which possesses no sex chromosomes
meets and fuses with a normal X gamete, the zygote
formed has the following chromosome complement -
2n = 44 + XO
• Individuals are always females;
• short in stature (than average),
• Webbed neck, unusual fingers, and low neckline.
• Heart, hearing and visual problems can also occur
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38. •Their ovaries do not develop
•So they are infertile and fail to develop secondary
sexual characteristics e.g. breast development and
menstruation.
•99% of all 45,X embryos die before birth
•No major effects until puberty
•Happens 1:2500 live births
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39. •Treatment can help but the condition cannot be
cured
•Its chronic; can last for years or be lifelong
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40. •Triple X syndrome - an extra X chromosome in
each cell of a female
• Also known as trisomy X and 47
• Those affected are taller than average
• Increased distance between the eyes
• Learning difficulties
• Decreased muscle tone
• Seizures
• kidney problems
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41. •XYY Syndrome - an extra Y chromosome in
males
• A male having an extra copy of the Y chromosomes in
their cells (some cells)
• Also called Jacob’s Syndrome
• Hyptonia (weak muscle tone)
• Delayed motor skill development e.g. walking, crawling,
writing
• Delayed or difficult speech (autism)
• Emotional or behavioral issues
• Taller than average height
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