In which we examine mitosis and
meiosis, as well as normal and
abnormal chromosome patterns.
2 of 44
A Look Ahead
Chromosomes & Chromosomal Abnormalities
Genes & Genetic Defects
Prenatal Stages & Congenital Defects
Part 1: Chromosomes & the Cell
4 of 44
Chromosomes & the Cell
The nucleus of every normal human cell (except
ovum and sperm) contain 46 chromosomes.
These 46 chromosomes occur in pairs:
22 pairs of autosomes
1 pair sex chromosomes
Autosomes: chromosomes on which males and
females do not differ
Sex Chromosomes: normal males have XY; normal
females have XX
5 of 44
The Human Karyotype
Karyotype = a photomicrograph of chromosomes
arranged according to a standard classification
6 of 44
Chromosomes and DNA
Chromosomes are made up of DNA
DNA molecule is held together by bases -
Adenine (A) bonds with thymine (T);
cytosine( C) bonds with guanine (G).
These letters form the "code of life“
There are about 2.9 billion base pairs in the
Human DNA contains about 30,000 genes,
which human cells use as starting
templates to make proteins; which then are
used to build and maintain our bodies.
Part 2: Cell Division
8 of 44
Mitosis vs. Meiosis
Mitosis refers to a process
by which 2 identical cells
are produced, each con-
taining 46 chromosomes
Meiosis refers to a process
in 4 cells are produced,
each containing only 23
9 of 44
Mitosis vs. Meiosis, continued
Mitosis is the process by which all cells in the
body replicate, except for production of
sperm and ova.
Meiosis is the process by which parent cells
divide into the gametes (sperm and ova)
Part 2A: Mitosis
11 of 44
Mitosis is the division of a cell's
nucleus. Along with cytokinesis
(the division of the rest of a cell),
mitosis results in a parent cell
dividing into two daughter cells.
The genetic information within
each of these daughter cells is
identical. In the illustration, only
4 of the 46 chromosomes are
12 of 44
Replication of Chromosomes
The first step is chromosome
replication: the strands of DNA
replicate, forming 2 chromosomes
(called chromatids) identical to the
original. At this point, the pairs of
chromatids are connected.
Two pair of centrioles lie just outside
the nucleus, next to each other.
13 of 44
DNA: deoxyribonucleic acid
DNA is shaped like a twisted
ladder. The sides of the ladder
are made of sugar & phosphate
The rungs of the ladder are made
of 4 bases: thymine, adenine,
cytosine, and guanine. Each rung
is made of a pair of bases: CG,
GC, AT or TA
14 of 44
Replication of DNA
Because cytosine only bonds with guanine and thymine
only bonds with adenine, the two new strands of DNA
are exact duplicates of the original strand.
15 of 44
Each original chromosome is now
a pair of chromatids.
The centrioles begin to grow
fibers, called spindles. The
centrioles move to opposite sides
of the nucleus.
16 of 44
The centrioles are now at
opposite ends of the cell.
The spindle fibers from both of
the centrioles attach to each
one of the chromosomes.
17 of 44
The chromosomes line up on
the metaphase plate, an
imaginary line that divides the
cell in two.
Also, the fibers begin to tug
each chromosome toward
opposite ends of the cell.
18 of 44
The fibers pull the chromatids
toward opposite ends of the
19 of 44
The chromatids (now also
arrive at the opposite ends of
the cell, and new nuclear
Mitosis, which describes only
the division of the nucleus, is
20 of 44
Mitosis: End of Cytokinesis
The rest of the cell divides.
Cytokinesis, the division of
the cell's cytoplasm, is
Part 2B: Meiosis
22 of 44
Process by which gametes (ova and sperm) are
The parent cell is a diploid cell, containing 46
The gametes are haploid cells, each containing 23
chromosomes, one from each pair.
If the number of chromosomes in the gametes was not
reduced by half, each sperm and egg would have 46
chromosomes, and after joining, the fertilized egg would
have 92 chromosomes!
23 of 44
During meiosis, the x-
shaped chromosomes line
up and intermix, yielding a
novel genetic product.
Thus, the chromosomes of
a child are not exact
duplicates of the parents’
24 of 44
Meiosis: Two Phases
In meiosis, each cell replicates twice, resulting in 4
gametes from each parent cell.
In males all four In females only one of
sperm survive. the four ova survive.
25 of 44
Chromosomes Pair Up
In Phase 1, the chromosomes
pair up with their corresponding
chromosome. This is the point at
which cross-over occurs.
26 of 44
Chromosome Pairs Split Up
The spindle fibers from the
centriole at one end of the nucleus
attach to one chromosome in each
of the 23 pairs.
The spindle fibers from the
centriole at the other end of the
nucleus attach to the other
chromosome in each pair.
27 of 44
End of Phase 1
As in mitosis, the chromosomes
arrive at opposite ends of the cell,
and new nuclear membranes form.
The cell shown here is a male's
sperm cell. With meiosis in a female,
most of the cell's cytoplasm will be
concentrated in one of the two
emerging cells, resulting in one large
cell and one small cell. The large cell
will go on to divide again; the small
cell will degenerate.
28 of 44
Meiosis Phase 2
At end of Phase 1, each cell has
only 23 chromosomes.
Phase 2 in meiosis is like mitosis:
each cell replicates into 2 exact
For males, both sperm cells
survive. For females, only one
egg cell will survive.
Part 3: Chromosomal Abnormalities
30 of 44
Abnormal number of one of the chromosomes
May occur at time of cell division or may occur later,
due to spontaneous splitting and replication of a
May occur in either the ovum or sperm
Three possible abnormalities:
Having an extra chromosome
Missing a chromosome
Missing part of a chromosome
31 of 44
Types of Abnormalities
Autosomal Abnormality: Having an
abnormal number of one of the autosomes.
Sex Chromosomal Abnormality: Having
an abnormal number of one of the sex
32 of 44
Autosomal Abnormality: Disorder caused by having too
many or too few of one of the autosomes.
Down’s Syndrome 1 in 700 births
Monosomy 21 extremely rare
Cri du Chat Syndrome 1 in 50,000 births
33 of 44
Trisomy 21 (Down’s Syndrome)
Have three 21st chromosomes.
small, low-set ears
problems & heart defects
34 of 44
Incidence of Down’s Syndrome &
Under 30 Less than 1 in 1,000
30 1 in 900
36 1 in 300
38 1 in 180
40 1 in 105
42 1 in 60
44 1 in 35
46 1 in 20
48 1 in 16
35 of 44
Monosomy 21 (Antimongoloid)
Have only one 21st chromosome
broad prominent nasal bridge
Only monosomics that survive until birth are
Monosomy 21 and Monosomy 22, the smallest
chromosomes. They both have severe retardation.
36 of 44
Cri du Chat Syndrome
Missing part of an arm of the 5th chromosome
catlike cry as newborn
low birth weight and slow growth
small head with wideset eyes
partial webbing of fingers/toes
37 of 44
Sex Chromosomal Abnormalities
Sex Chromosomal Abnormality: abnormality in
number of sex chromosomes.
Most common types:
XO Turner’s Syndrome 1 in 2500
XXX Trisomy X Syndrome 1 in 1000
XXY Klinefelter’s Syndrome 1 in 600
XYY Jacob’s Syndrome 1 in 1000
38 of 44
Missing one of the X chromosomes.
Physically female, but does not develop at puberty.
lack of ovarian development; infertility
may have webbed neck, arms that turn out
slightly at the elbow, low hairline in the back of
may have easier time with verbal learning than
with math and spatial learning.
characteristically more feminine than average
39 of 44
Most common X chromosome abnormality.
Have three X chromosomes.
taller than average for girls
may have speech and language delays and mental
some have infertility problems.
Many Trisomic X girls have no symptoms
40 of 44
Physically male, but does not mature at puberty.
In addition to XY, have extra X chromosome.
Do not develop testes, thus infertile
Generally very placid; can be shy
Has delays in motor development and language
development; latter causes problems in school
41 of 44
In addition to XY, have extra Y chromosome
taller than average male
otherwise, no physical symptoms
have tendency to be more active than normal
usually have some mental retardation
some suggestion that XYY males have higher than
normal levels of aggression
42 of 44
Number of X Chromosomes
chromosomes femininity height
Turner’s shorter than
Normal XX average average
low taller than
Trisomy X XXX
43 of 44
Number of Y Chromosomes
chromosomes aggression height
Klinefelter’s shorter than
Normal XY average average
Jacob’s XYY high
44 of 44
Abnormalities are caused by either having too many or
too few chromosomes.
Caused before conception (usually at time that parent cell
divides into gametes)
Reason for occurrence not known.
Each chromosomal abnormality is multi-symptomatic:
Each chromosome contains many genes for many traits
Having three chromosomes means having 3 genes for each
of those traits
Having one chromosome means having 1 gene for each trait
on that chromosome