Chromosomal
Disorders
WHAT HAPPENS WHEN MEIOSIS GOES
WRONG?
What is a mutation?

Mutations involve a change in the number of
chromosomes, or the structure of a
chromosomes or a nucleotide in the DNA

Can lead to disorders OR be beneficial to the
organism.
Karyotype
Human Chromosomes

Humans have 46 chromosomes
arranged in 23 pairs

Two sex chromosomes- determine an
individual’s sex

Females have two copies of the large X
chromosome

Males have one X and a smaller Y
chromosome
Human
Chromosomes

The remaining 44 chromosomes are known as autosomal
chromosomes or autosomes

Males and females are born in a roughly 50:50 ratio

All human egg cells carry a single X

Half of all sperm carry an X and the other half carry a Y
Male or Female???
Types of Mutations
 Mutations
effects vary. One
factor is the type of cell if
affects…read about the two types
now…
 GERM CELLS
Cells that undergo meiosis to produce egg and sperm cells (in
ovaries and testes)
 SOMATIC

CELLS-
All the “other” body cells that only do mitosis
How would this affect the person
differently?
Gene/DNA Mutations
 Gene/DNA
mutations involve
changes in ONE nucleotide
 Ex’s- Point and Frameshift
Mutations
Types of Mutations

Chromosomal mutations involve changes
in either the number chromosomes or
structure of chromosomes (A “chunk” of a
chromosome is mutated)
Chromosomal
Mutations
 Examples:
 Entire
chromosome
mutation =
nondisjunction
 “chunk”
of a
chromosome
mutation= listed to the
right in diagram
(insertion, deletion…)
Chromosomal Mutations

Entire chromosomes encounter mutations as well

There are two main types

Nondisjunction

Structural

A
four types:
1.
Deletion
2.
Inversion
3.
Insertion
4.
Translocation
B
Gene
C
D
E
Chromosome
Deletion
Before mutation
A
B
C
After mutation
D
E
A
C
D
Occurs when a single break causes a
chromosome to lose an end piece or
when two simultaneous breaks lead to
loss of internal segment
E
Inversion
Before mutation
After mutation
A
B
C
D
E
A
B
C
Occurs when a part of the
chromosome breaks off and is
reinserted backwards
E
D
Insertion (aka duplication or repeat)
Before mutation
A
B
D
C
E
B
A
After mutation
C
B
C
B
C
D
Occurs when a part of the chromosome
breaks off & re-attaches to the same
chromosome resulting in a duplication of
genes
E
Translocation
Before mutation
A
B
F
C
G
D
E
After mutation
H
F
A
B
C
G
D
E
H
Occurs when a part of one chromosome
breaks off & attaches to a nonhomologous chromosome
Nondisjunctions create…
Remember: Normal diploid cells
have 2 copies of each chromosome
creating homologous pairs…
After a nondisjunction , zygote cells will
have:
Trisomy: when a cell has 3 copies
of a chromosome
or
Monosomy: when a cell has only 1
copy of a chromosome
Nondisjunction
Creates gametes with the wrong number of
chromosomes!
Two ways it can occur:

Homologous chromosomes fail
to separate during meiosis I
OR

Sister chromatids fail to
separate during meiosis II
DNA/GENE
MUTATIONS
DNA/Gene Mutations

Protein synthesis does encounter errors

Errors in final sequence of amino acids can be
traced to errors in the original strand of DNA

Can effect the primary protein structure and
ultimately the final shape and function of the
protein

Two types: point mutations and frameshift
mutations
Lets see what happens!

Look at A on the handout and locate the
substitution

Transcribe that part of the gene into the codon it
will code for.

What amino acid should be there normally?
What amino acid is there now?
Point Mutations
Involve the substitution of one nucleotide for
another


Three types of substitution:
1.
Silent mutation
2.
Nonsense mutation
3.
Missense mutation
Effects vary from going unnoticed to
completely changing shape of final protein
Silent Mutation

NORMAL STRAND OF DNA:
TAC TTC GAC GTG ACT
• SUBSTITUTED BASE PAIR:
TAC TTT GAC GTG ACT
DNA
 In normal strand: TTC 
mRNA
Amino Acid
AAG = lysine
DNA
mRNA
Amino Acid
AAA = lysine

In substituted strand: TTT 

Same amino acid, so there is no change in protein structure
Nonsense Mutation

NORMAL STRAND OF DNA:
TAC TTC GAC GTG ACT

SUBSTITUTED BASE PAIR:
TAC ATC GAC GTG ACT
DNA
mRNA
Amino Acid
• In normal strand: TTC  AAG = lysine
DNA


mRNA
Amino Acid
In substituted strand: ATC  UAG = stop
Substituted base codes for a stop codon which
can have disasterous effects because the protein
isn’t finished
Missense Mutation

NORMAL STRAND OF DNA:
TAC TTC GAC GTG ACT
• SUBSTITUTED BASE PAIR:
TAC TCC GAC GTG ACT
DNA
 In normal strand: TTC 
mRNA
Amino Acid
AAG = lysine
DNA
mRNA
Amino Acid
AGG = arginine

In substituted strand: TCC 

Different amino acid, so there is a change in protein structure
Frameshift Mutations
Caused by either the addition or removal of one or
more nucleotides in the original strand of DNA


Two types:
1.
Insertion
2.
Deletion
Called frameshift because the addition or
removal shifts the “frame” that is read for the
amino acid codes
Insertion

NORMAL STRAND OF DNA:
TAC TTC GAC GTG ACT
C

INSERTION:
T A C TT C C G A C G T G A C T

By inserting one base, the entire DNA strand is
read differently and can result in a different
final protein shape & function
Deletion

NORMAL STRAND OF DNA:
TAC
TTC GAC GTG ACT
C

DELETION:
TAC

T T C G AG TG A C T
By deleting one base, the entire DNA
strand is read differently and can result in a
different final protein shape & function