Download - DNA MUTATIONS
DNA MUTATIONS
TypesMutagens
Diseases
Click on an area for more information
Mutations are any change in the genetic make up of an organism and can occur naturally or can be increased by mutagens.
TYPES OF MUTATIONS
POINT
CHROMOSOMAL ABBERATIONS
BLOCK
Click on an area for more information
Mutations can be spontaneous or natural as a result of errors in DNA replication or gamete production.
Error rate is 1 in 1,000,000 replications.
Point mutations•Involves a change to one of the DNA bases that makes up a
gene.
•Also known as a MISSENSE mutation.
•Consequence varies depending on type of change.
Change DNA Change amino acid sequence
Change protein
Change organism
Click to find out more
Three types of point mutation
deletioninsertionsubstitution
T A C T T C A A A C C G C G TDNA
Base Substitution
•Involves one base in the DNA being substituted by another.
•This mutation only affects one codon in the gene.
T A C T T C A A A T C G C G T
C is replaced by T in the DNA.
•End result is a different amino acid in protein.
•Generally minor effect on protein/organism.
•Known as NONSENSE mutation•Totally different protein made•Effect on organism can be dramatic
But…ATA
UAU
Tyr
mRNA
DNA ATT
UAA
STOP
•Serine is still added•Known as a SILENT mutation
AlsoAGA
UCU
Ser
mRNA
DNA AGG
UCC
Ser
Base Insertion/Deletion
•Involves one base either being inserted or removed from the DNA sequence.
•Both these mutations affect all codons after the insertion or deletion.
•The earlier the mutation occurs in the sequence the greater the effect.
T A C T T C A A A C C G C G TDNA
T A C T T C A A A A C C G C G
Trp Arg
BASE INSERTION INSERTION
T A C T T C A A C C G C G T A
Known as frame shift mutations.
Dramatic effect on
protein made.
Block mutations•Involves large scale changes in DNA segments on chromosome.
•Only occur during meiosis, formation of gametes (sex cells).
•Can affect the functioning of many genes
•Severe effect on functioning and characteristics of organism.
Change DNA Change amino acid sequence
Change protein
Change organism
Four types of block mutation
inversions
translocations
deletions
Click to find out more
duplications
Deletion
•Section of chromosome is lost, anywhere from three bases to large segments.
•The DNA breaks due to the effect of mutagens.
break
A DB EC F HGA DB EC F HG
A DB EC F HG
segment is lost
STEP 2
A D E F HG
DNA ligase rejoins chromosome
STEP 3
A DB EC F HG STEP 1
genes
Translocation
•Section of chromosome breaks and is joined to another chromosome.
•Problem when gametes formed-some receive extra genes, some deficient.
break
STEP 1
A DB EC F HG
3 42 51
segment removed
STEP 23 42 51
D E F HG
A DB EC F HGA DB EC F HG
D E F HG
DNA ligase joins segment
STEP 33 42 51A DB EC F HGA DB EC F HG
Duplication•Segment of one chromosome is added to the homologous chromosome.
•Problem when gametes formed-some have double the genes, some have no genes.
STEP 1
A DB EC F HG
A DB EC F HG
break
D E F HG
STEP 3A DB EC F HGA DB EC F HGA DB EC F HG
D E F HG
A DB EC F HGA DB EC F HGA DB EC F HGSTEP 2
segment removed
Inversion•Section of chromosome falls out and rotates through 180o.
•DNA ligase rejoins section but the genes are in reverse order.
•There is no loss of genetic information.
break
A DB EC F HG STEP 1
A DB EC F HGSTEP 2
segment rotates 180oA
D
B
E
C
F
HG
A
D
B
E
C
F
HG
segment rejoins
STEP 3A D B EC F HG
Chromosomal abberations
•Loss or gain of entire chromosomes- called aneuploidy.
•Main cause is non-disjunction of chromosomes during meiosis.
•Gametes end up with extra chromosome(s) or fewer than 23 chromosomes.
2 x 23Normal diploid (2N) human cell containing 2 sets of chromosomes.
Through meiosis haploid (N) gametes are produced, containing only one set of chromosomes.
23
23
<23, >23
<23, >23
Mainly occurs with sex chromosomes.
Turner syndrome
Normal female
Metafemale
Normal male
Jacob syndrome
XO
XX
XXX
XY
XYY
Examples
Mutagens
•Chemicals or radiation that increase the likelihood of a mutation occurring.
•Rate of mutation induced by mutagen is proportional to the dose of mutagen.
•Mutagens cause a decrease in the stability of the DNA molecule.
•Thus DNA more susceptible to error during replication or meiosis.
Types
Alcohol & dietary factors
•High fat, alcohol•Trigger cancers
Environmental poisons/irritants
•Benzene, asbestos•Tobacco tar, formaldehyde
Ionising radiation•Nuclear•UV rays•X rays
Viruses & micro-organisms
•Incorporate into genome•Trigger cancers
Diseases
Sickle cell disease
Cystic fibrosis
Click on a disease to find out more about how it is caused by a mutation.
Down syndrome
Sickle cell disease
haemoglobin
•Haemoglobin is made up of 4 polypeptide chains (2 alpha & 2 beta chains).
•Gene for beta chain is found on chromosome 11 and consists of 438 bases.
•A mutation occurs in the gene coding for the beta chain.
•The mutation is a substitution where adenine replace thymine on the DNA template strand.
•As a result the amino acid valine replaces glutamic acid.
•This change the properties of the haemoglobin and results in distorted red blood cells.
normal red blood cell
sickle cell
This single point mutation has a dramatic effect. Individuals have many health problems, eg weakness, jaundice, anaemia, heart & kidney defects, brain damage, skin lesions and inflamed spleen.
HBB gene on chromosome 11
•Inherited disorder and occurs when an individual has both copies of the faulty gene.
•The gene (CFTR) is found on chromosome 7 and normally contains 1480 amino acids.
•A mutation occurs whereby the 508th triplet (AAA) is deleted resulting in a mutated CFTR protein of 1479 amino acids.
•The abnormal protein is unable to control chloride ion balance in cells.
•The build up of chloride ions in cells results in severe effects.
• Individuals have reduced life expectancy, high salt levels, blocked airways, pancreatic deficiencies and males are infertile.
•Cystic fibrosis is relatively common and occurs 1 in every 2500 individuals.
Cystic Fibrosis
CFTR gene on chromosome 7
AUU AUC UUU GGU GGUmRNA
ile ile phe gly valamino acids
TAA TAG AAA CCA CAADNA
506th 507th 508th 509th 510th
AUU AUC GGU GGU
ile ile gly val
mRNA
amino acids
TAA TAG AAA CCA CAADNA
506th 507th 508th 509th 510th
AAA
triplet deleted
Down Syndrome•Individuals have characteristic facial features, are relatively short,
slowed intellectual and physical development.
•Two main types of Down syndrome-97% non-familial, 3% familial.
Non-familial Familial24 23
One of the parent’s sex cells contains two no. 21 chromosomes, due to non-disjunction during meiosis.
One of the parents has only 45 chromosomes. A translocation has occurred where one of no.15 and one of no. 21 have joined together. One of the sex cells contain 23 but one of the 23 is the 15/21 chromosome.
•Arise from having 3 copies of chromosome 21
•known as trisomy-21.
15 21 15/21
Parent 1
Parent 2
Down child 3 no.21