Some common aneuploid conditions resulting from non-disjunction: - Down syndrome (trisomy 21) - Edwards syndrome (trisomy 18) - Patau syndrome (trisomy 13) - Turner syndrome (monosomy X) - Klinefelter syndrome (XXY) These conditions often cause intellectual and physical disabilities due to improper gene dosage. Non-disjunction increases with maternal age.

1. CELL DIVISION

2. CELL DIVISION
Why do cells need to
divide?
-To allow an organism
to grow.
-To pass on genetic
material.
-To assist an organism's
survival.

3. CELL DIVISION
2 types of cell division
Mitosis Meiosis
-daughter -daughter
cells are cells are
genetically genetically
identical different

4. CELL DIVISION
2 types of cell division
Mitosis Meiosis
-daughter cells are -daughter cells are
genetically identical genetically different
-performed by somatic - performed to create sex
body cells (ex. bone cells (gametes: sperm
nerve, or tissues) and egg cells)
- parts of the body with
high rates of attrition
must perform mitosis
more often (ex.
fingertips vs. liver cells)

5. CELL DIVISION
6 stages of
cell division:
1) Interphase
2) Prophase Mitosis
3) Metaphase - The division of
the cell's
4) Anaphase nucleus and all
genetic
5) Telophase material.
6) Cytokinesis

6. CELL DIVISION
Interphase
- Cell is not dividing but
also not dormant
- Cell is active,
growing, and
preparing for division
- Chromosomes are not
visible (chromatin
present).

7. CELL DIVISION
G0
Interphase:
4 stages
Gap 0 (G0):
- Cell rest cycle
- Some cells
spend their entire
lives in G0 (they
are functional but
no new cells are
produced (i.e.
neuron)

8. CELL DIVISION
G0
Interphase:
4 stages
Gap 1 (G1):
- Rapid growth
(4hrs)
- Cell takes in
nutrients for
energy, growth,
repair
- Cells either go
into S or G0 after
G1

9. CELL DIVISION
G0
Interphase:
4 stages
S phase (S):
- ~10 hrs
- DNA synthesis/
replication
- Cell duplicates its
genetic material to
make identical
copies of its DNA.

10. CELL DIVISION
G0
Interphase:
4 stages
Gap 2 phase (G2):
- ~4 hrs
- Second period of
growth in
preparation for cell
division.
-Cell grows larger
in size.

11. CELL DIVISION
S phase (S): Let’s take
a closer look
- The cell duplicates its
DNA.
- What is DNA?
- Deoxyribonucleic acid.
- Contains genetic
information in coded
form.

12. CELL DIVISION
Deoxyribonucleic acid
2 Strands of attached
nucleotides:

13. CELL DIVISION
Deoxyribonucleic acid
2 Strands of
nucleotides:
- Phosphate (PO4)
- Nucleobase
- Sugar (deoxyribose)

14. CELL DIVISION
Nucleotide bases enable
the coding of genetic
information.
A – Adenine
T – Thymine
G – Guanine
C – Cytosine

15. CELL DIVISION
- The "coded" information
is the sequence of
nucleotide bases
A – Adenine
T – Thymine
G – Guanine
C – Cytosine
Every 3 nucleotide
bases (called a codon)
code for 1 specific
amino acid.

16. CELL DIVISION
- This is the genetic
language.
For example,
A CG
makes
Threonine

17. CELL DIVISION
On opposing strands:
- A attracts only T
- G attracts only C
The bases attract
through HYDROGEN
BONDING.
This is called
complimentary base
pairing.

18. CELL DIVISION
-
adenine

19. CELL DIVISION
- C and G bond together with 3 hydrogen bonds.
- A and T bond together with 2 hydrogen bonds.

20. CELL DIVISION
- C and G bond together with 3 hydrogen bonds.
- A and T bond together with 2 hydrogen bonds.

21. CELL DIVISION
A&T
C&G
Which ones are A&T? C&G
A&T
Which ones are C&G?

22. CELL DIVISION
- How does DNA replicate?
STEP 1: Separation of the two DNA strands
H-Bonds

23. CELL DIVISION
- How does DNA replicate?
STEP 2: Each parent strand attracts
matching nucleotide bases to form
a new chain (enzymes help).
New chain
Nucleotide bases

24. CELL DIVISION
But DNA is a very long molecule.
Does replication start from one end?
Not usually. It can start at various points in
"bubbles".

25. CELL DIVISION
There are 3 models of
replication:

26. CELL DIVISION
…Back to interphase where DNA duplication occurs.
DNA
Replication
Normally After duplication

27. CELL DIVISION
But wait, how does all of the DNA remain
organized in the nucleus?
Why does it not tangle?

28. CELL DIVISION
DNA strands are wrapped around histone
proteins which act as “spools”.

29. CELL DIVISION
Histone proteins that have been wrapped
with DNA form “nucleosomes”.

30. CELL DIVISION
Moving into mitosis…
After interphase, the cell
enters mitotic (M) phase:
-Cell division occurs through:
A) Mitosis –
Process of dividing
nuclear material.
B) Cytokinesis - Process
of separating the
cytoplasm and its
contents into equal parts.
M phase takes ~2 hours in
humans.

31. CELL DIVISION
Prophase
- Nuclear
membrane and
nucleolus break
down.
- Centrosome
divides into 2
centrioles which
begin forming the
spindle fibres.

32. CELL DIVISION
Prophase
- Nuclear
membrane and
nucleolus break
down.
- Centrosome
divides into 2
centrioles which
begin forming the
spindle fibres.

33. CELL DIVISION
Prophase
chromatin
condenses

34. CELL DIVISION
Prophase
- Chromosomes
shorten and
thicken into X-
shaped structures
called dyads.

35. CELL DIVISION
Prophase
- Remember, every chromosome has been duplicated
during the S phase of interphase.
2 copies of each
chromosome:
Father’s Mother’s Father’s Mother’s
side side side Father’s side Mother’s
Mitosis side side
*Colors are not accurate

36. CELL DIVISION
Prophase
- A centromere holds two copies of the same
chromosome together.
- Each identical chromosome is called a sister
chromatid.

37. CELL DIVISION
Metaphase
- Proteins attach to the
centromeres creating
the kinetochores.
- Microtubules (spindle
fibres) attach at the
kinetochores and the
chromosomes move
to the centre (the
metaphase plate)

38. CELL DIVISION
Metaphase

39. CELL DIVISION
Anaphase
- Paired chromosomes
separate at the
kinetochores and move
to opposite sides of the
cell.
- Motion results from
kinetochore movement
along the spindle
microtubules.

40. CELL DIVISION
Telophase
- Chromatids arrive at
opposite poles of cell.
- New daughter nuclei
form.
- Chromosomes disperse
and become chromatin.
- Spindle fibres disperse.

41. CELL DIVISION
Cytokinesis
- Animal cells: the cell membrane pinches (forming a
cleavage furrow) into two daughter cells, each with
one nucleus

42. CELL DIVISION
Cytokinesis
- In plant cells, a cell plate (made of rigid cellulose)
is synthesized between the two daughter cells.

43. CELL DIVISION
Mitosis and cytokinesis: Review

44. CELL DIVISION
Mitosis and cytokinesis: Haploid and Diploid
Haploid = 1 set of chromosomes (ex. n=23 chromosomes)
Diploid = 2 sets of chromosomes (ex. 2n= 46 chromosomes)
Tetraploid = 4 sets of chromosomes (ex. 4n = 92 chromosomes)
2n
2n
2n 4n
4n

45. CELL DIVISION
Karyotype: Photograph of chromosomes taken at the
metaphase stage of mitosis. Homologous pairs are
identified and placed together.
daddy mommy
Homologous pair

46. CELL DIVISION
Karyotype: Technique
1) Treat the cells with a solution that
will make them swell-up, which
spreads the chromosomes.
2) Use a chemical to stop mitosis in
metaphase.
3) Compress the cells on a slide and
take a high resolution photograph.
5) Cutting up a photomicrograph and
arranging the result into a
karyogram.

47. CELL DIVISION
Karyotype: Technique
But during mitosis
(metaphase),
chromosomes are in
duplicated pairs.
So each
homologous pair
consists of 2
dyads.
Scientists can cut
the photographs
so that only one
chromatid of
every dyad is
displayed.

48. CELL DIVISION
Homologous pairs
Identified by:
-chromosome length
-centromere position
-banding (staining)
pattern
-satellites (tips) and
any other physical
characteristics

49. CELL DIVISION
Recall: 2 types of cell division
Mitosis Meiosis
-daughter -daughter
cells are cells are
genetically genetically
identical different

50. CELL DIVISION
Meiosis: The production of gametes
-2 cell divisions (meiosis I and meiosis II)
-Daughter cells are genetically unique
-4 daughter cells are produced
-Daughter cells are haploid

51. CELL DIVISION
Meiosis I:
Prophase I:
-start with 2x the normal
amount of chromosomes
-chromosomes condense
-nuclear membrane and Same as mitosis
nucleolus dissolve
-centrioles split

52. CELL DIVISION
Meiosis I:
Prophase I:
-homologous dyads come
together, forming tetrads
-they overlap in a process
called synapsis
-crossing-over of genetic
material, causing exchange of
segments of DNA, producing
recombinant chromosomes

53. CELL DIVISION
Meiosis I:
Prophase I:
-dyads come together
in homologous pairs,
forming tetrads
-they overlap in a
process called synapsis
-crossing-over occurs,
causing exchange of tetrad
segments of DNA

54. CELL DIVISION
Meiosis I:
Prophase I:
Chiasma:
crossover point

55. CELL DIVISION
Meiosis I:
Metaphase I
-chromosomes
attach themselves Same as mitosis,
to spindle fibres except the
chromosomes are
no longer identical
-chromosomes line (they are
up at equatorial recombinant).
plate

56. CELL DIVISION
Meiosis I:
Anaphase I
Same as mitosis,
- Homologous pairs except the
separate and move chromosomes are
to opposite poles of no longer identical
(they are
the cell recombinant).
(tetrads are pulled
apart, dyads move
to the poles)

57. CELL DIVISION
Meiosis I:
Telophase I
-cleavage furrow forms
Same as mitosis
-nuclear membranes
and nucleoli reform
-chromosomes DO NOT
un-condense

58. CELL DIVISION
Meiosis II:
Second cell division (both Meiosis I daughter cells
perform meiosis II)
Phase Meiosis
Prophase II -nuclear membranes and nucleoli
dissolve
-more spindle fibres form
-chromosomes attach to spindle
Metaphase II -chromosomes line up at equatorial
plate
Anaphase II -sister chromatids from each dyad
separate and move to opposite poles
Telophase II -spindle fibres dissipate
-nuclei and nucleoli reform

59. CELL DIVISION

60. CELL DIVISION

61. CELL DIVISION

62. CELL DIVISION
Mitosis vs. Meiosis
Diploid (2n)
46 46 Diploid (2n)
Diploid (2n) Diploid (2n)
46 46 46 46
Diploid (2n) Diploid (2n)
23 23 23 23
Haploid Haploid Haploid Haploid
(1n) (1n) (1n) (1n)

63. CELL DIVISION
Maternal Chromosome A Paternal Chromosome A
46
DNA Replication
DNA Replication
92
92
Sister chromatids
Division #1 (Meiosis I)
Synapsis and crossing over
Synapsis and crossing over
92
92
Metaphase I, anaphase I, & telophase I
Metaphase I, anaphase I, & telophase I
46 46
46 46
Division #2 (Meiosis II) Meiosis II
23
23
23 23
23 23
23

64. CELL DIVISION
Why is meiosis important?
1. allows continuity
2. ensures genetic diversity
within population
Diversity is generated through…
1) crossing-over (makes diverse
chromosomes)
2) random assortment (each
gamete is different)
3) random fertilization (random
sperm meets with random
mature egg)

65. CELL DIVISION
How does this relate to Mendel? Monohybrid cross
Before the process of meiosis
was discovered, Mendel’s
observations led him to make 2
conclusions:
1)Law of Segregation:
2 alleles for each trait separate
during gamete formation
(i.e. parents can only pass 1 of
their 2 alleles for any trait to
their offspring).

66. CELL DIVISION
How does this relate to Mendel?
2)Law of Independent
Assortment:
Alleles from different
chromosomes pass to
gametes randomly – alleles
are not connected to or
passed on with any other
alleles.
For example, the allele for
pea shape and the allele for
pea color are passed onto
offspring without relation to
one-another.

67. CELL DIVISION
How does this relate to Mendel?
Made possible by
the final division in
Meiosis II Made possible by
different chromosomes
having different alleles
All daughter cells are different

68. CELL DIVISION
How does this relate to Mendel?

69. CELL DIVISION
Mistakes during meiosis: Errors during Independent Assortment
Non-disjunction Normal Meiosis
Non-disjunction:
46 Failure of dyads or 46
tetrads to separate
during anaphase I
or anaphase II
44 48 46 46
22 22 24 24 23 23 23 23

70. CELL DIVISION
Mistakes during meiosis: Errors during Independent Assortment
Aneuploidy: A condition that results when there is a missing
or extra chromosome, resulting in changes to the normal
chromosome number

71. CELL DIVISION
What are the possible zygotes when a normal gamete is
joined with a non-disjunct gamete?
Non-disjunct egg sperm egg Non-disjunct sperm
22 23 23 24
45 47
Monosomy Trisomy

72. CELL DIVISION
Mistakes during meiosis: Errors during Independent Assortment
Monosomy: A condition in which an individual has only one
homologue of a specific pair of homologues

73. CELL DIVISION
Mistakes during meiosis: Errors during Independent Assortment
Trisomy: A condition in which an individual has three
homologues of a specific chromosome.

74. CELL DIVISION
Mistakes during meiosis: Errors during Independent Assortment
Polysomy: A condition in which an individual has more than
the normal number of a specific chromosome.

75. CELL DIVISION
Mistakes during meiosis: Errors during Independent Assortment
Haploid (n): having one copy of an entire chromosome
set (e.g. sex cells)
Diploid (2n): having two copies of an entire
chromosome set (e.g. somatic cells)
Triploid (3n): having three copies of an entire
chromosome set
Polyploidy: A condition in which an individual has three
or more copies of an entire chromosome set

76. CELL DIVISION
Mistakes during meiosis: Errors during Independent Assortment
How is it possible
to get 3 or more
copies of a
chromosome?

77. CELL DIVISION
Condition Final # of Affected Characteristic
chromosomes chromosome
Down 47 #21 Broad flat face, slanting eyes,
syndrome short, growth failure, mental
disabilities, congenital heart
disease.

78. CELL DIVISION
Condition Final # of Affected Characteristic
chromosomes chromosome
Turner’s 45 X chromosome Short, facial hair, undeveloped
syndrome (sex breast, degenerate ovaries,
chromosome) infertile.

79. CELL DIVISION
Condition Final # of Affected Characteristic
chromosomes chromosome
Klinefelter’s 47 or more X chromosome Tall, longer limbs, poor beard
syndrome growth, feminized physique,
loose chest hairs, underdeveloped
penis and testes, infertile

80. CELL DIVISION
Condition Final # of Affected Characteristic
chromosomes chromosome
Jacob’s 47 or more Y chromosome Normal male, tall, “aggressive”
syndrome (often unnoticed, 1/1000 males)

81. CELL DIVISION
Condition Final # of Affected Characteristic
chromosomes chromosome
Super 47 or more X chromosome Normal female, “aggressive”,
female taller, delayed motor skills, often
unnoticed, 1/1000 females