Session
2
Medical Genetics
The Cellular and
Molecular Basis of
Inheritance
J a v a d
F a s a
J a m s h i d i
U n i v e r s i t y
o f
M e d i c a l
S c i e n c e s
Components of Inheritance
Cell
Nucleus
Cytoplasm
DNA
Composition
Structure
Replication
Transcription
Chromosomes
2
Chromosome Structure
3
Image from: Emery's Elements of Medical Genetics, 14th Edition, by Peter D. Turnpenny and Sian Ellard, (2012)
Cell Cycle / Mitosis
4
Image from: An Introduction To Human Molecular Genetics Second Edition by Jack J. Pasternak, Published by John Wiley & Sons, Inc., Hoboken,
New Jersey 2005.
Meiosis
5
Image from: An Introduction To Human Molecular Genetics Second Edition by Jack J. Pasternak, Published by John Wiley & Sons, Inc., Hoboken,
New Jersey 2005.
The Human Genome
The nuclear genome
Approximately 3 200 000 000 nucleotides of DNA
Either 23 or 24 different types of chromosomal DNA Molecules
About 23 000 genes
The mitochondrial genome
16 569 nucleotides
A small circular DNA
37 genes
6
The Human Genome
7
Image from: Human Molecular Genetics 4th Edition Tom Strachan, Andrew Read, Published by Garland Science (2011).
Types of DNA Sequence
Nuclear Genes
Unique single copy
Multigene families
Classic gene families
Gene superfamilies
Pseudogenes
Extragenic DNA
Tandem repeat
Satellite
Minisatellite
Telomeric
Hypervariable
Microsatellite
Interspersed
Short interspersed nuclear elements
Long interspersed nuclear elements
Mitochondrial
8
Nuclear Genes
/ Unique Single Copy
Most human genes are unique single-copy genes coding for
polypeptides
Include enzymes, hormones, receptors, and structural and
regulatory proteins.
9
Image from: Emery's Elements of Medical Genetics, 14th Edition, by Peter D. Turnpenny and Sian Ellard, (2012)
Nuclear Genes
/ Multigene Families
Have similar functions, arisen through gene duplication
events
Some are found physically close together in clusters; for
example, the α and β globin gene clusters on chromosomes
16 and 11.
16p13
11p15
10
Image from: Human Molecular Genetics 4th Edition Tom Strachan, Andrew Read, Published by Garland Science (2010).
Nuclear Genes
/ Multigene Families
Others are widely dispersed throughout the genome
occurring on different chromosomes, such as the HOX
home box gene family
11
Image from: Emery's Elements of Medical Genetics, 14th Edition, by Peter D. Turnpenny and Sian Ellard, (2012)
Multigene Families
Classic gene families
ribosomal RNAs
transfer RNA
Gene superfamilies
T-cell receptor genes
Human Leukocyte Antigen (HLA)
12
Pseudogenes
Closely resemble known structural genes but, in general,
are not functionally expressed.
Thought to have arisen in two main ways:
By genes undergoing duplication events
As the result of the insertion of complementary DNA
sequences
13
Tandem Repeated DNA Sequences
Satellite DNA (20 kb to many hundreds of kilobases)
Approximately 10% to 15% of the repetitive DNA sequences of the
human genome
located at centromeres and some other heteochromatic regions
Minisatellite DNA (100 bp to 20 kb)
Telomeric DNA: contains 10 to 15 kb of tandem repeats of a 6-base pair (bp)
DNA sequence
Hypervariable minisatellite DNA: made up of highly polymorphic DNA
sequences consisting of short tandem repeats of a common core sequence
Microsatellite DNA (fewer than 100 bp)
Consists of tandem single, di-, tri-,and tetra-nucleotide repeat base-pair
sequences located throughout the genome
14
Highly Repeated Interspersed Repetitive DNA Sequences
Approximately one-third of the human genome is made up
of two main classes of short and long repetitive DNA
Short Interspersed Nuclear Elements (SINEs)
About 5% of the human genome consists of some 750,000 copies
The most common are DNA sequences of approximately 300 bp (Alu)
Long Interspersed Nuclear Elements (LINEs)
About 5% of the DNA of the human genome
The most common, LINE-l , consists of more than 100,000 copies of a DNA sequence of up
to 6000 bp that encodes a reverse transcriptase
15
Mitochondrial DNA
The mtDNA genome is very compact, containing
little repetitive DNA
Codes for 37 genes, which include
Two types of ribosomal RNA
22 transfer RNAs
13 proteins
Inherited almost exclusively from the oocyte leading to the
maternal pattern of inheritance
16
Mitochondrial DNA
17
Image from: Human Molecular Genetics 4th Edition Tom Strachan, Andrew Read, Published by Garland Science (2011).
From DNA to Protein
18
Transcription
The process whereby genetic information is transmitted from DNA to RNA
messenger RNA, by RNA polymerase II
The template strand, antisense and sense strand
mRNA splicing
Capping
Polyadenylation
19
20
Alternative Splicing
21
Capping
After 20 to 30 nucleotides have been
transcribed, the nascent mRNA is modified by
the addition of a guanine nucleotide to the 5'
end of the molecule by an unusual 5' to 5'
triphosphate linkage. A methyltransferase
enzyme then methylates the N7 position of
the guanine, giving the final 5' cap.
Protect the RNA transcript from degradation
by endogenous cellular exonucleases
Facilitate transport of the mRNA to the
cytoplasm and attachment to the ribosomes
22
Polyadenylation
Approximately 200 adenylate residues-the so-called poly(A) tail-are added
to the mRNA,
Facilitates nuclear export and translation
23
Regulation of Gene Expression
Housekeeping genes
Many other genes only express in specific cells or stage of cell
development
Control of gene expression
Control of Transcription
Post- Transcriptional Control
24
Control of Transcription
Can be affected permanently or reversibly by a variety of
factors, both environmental (e.g., hormones) and genetic (cell
signaling).
All mechanisms ultimately affect transcription through the
binding of the transcription factors to short specific DNA
promoter elements
25
Transcription Factors
Proteins involved in the regulation of gene expression.
have a transcriptional activation domain and a DNA-binding
domain.
26
Control of Transcription
27
Post- Transcriptional Control of Gene Expression
Regulation of expression of most genes occurs at the level of
transcription but can also occur at the levels of RNA
processing, RNA transport, mRNA degradation and translation.
RNA-Mediated Control of Gene Expression
Small interfering RNAs (siRNAs) were discovered in 1998 and are the effector
molecules of the RNA interference pathway (RNAi)
MicroRNAs (miRNAs) also bind to mRNAs in a sequence-specific manner.
28
Mutations
Heritable alteration or change in the genetic material
Mutations can arise through exposure to mutagenic agents, but
the vast majority occur spontaneously through errors in DNA
replication and repair.
Somatic mutations may cause adult-onset disease, such as
cancer, but cannot be transmitted to offspring.
It is estimated that each individual carries up to six lethal or
semilethal recessive mutant alleles
29
Types of Mutation
Substitution
Deletion
Insertion
30
Substitution
Group
Type
Effect on Protein Product
Synonymous
Silent
Same amino acid
Missense
Altered amino acid-may affect protein function or
stability
Nonsense
Stop codon-loss of function or expression due to
degradation of mRNA
Splice site
Aberrant splicing-exon skipping or intron retention
Promoter
Altered gene expression
Nonsynonymous
31
Substitution/Missense
32
Deletion
Group
Type
In-frame deletion of one or more amino acid(s)-may
affect protein function or stability
Multiple of 3
(codon)
Not multiple of 3
Effect on Protein Product
Frameshift
Likely to result in premature termination with loss
of function or expression
Partial gene
deletion
May result in premature termination with loss of
function or expression
Whole gene
deletion
Loss of expression
Large deletion
33
Insertion
Group
Type
In-frame insertion of one or more amino
acid(s)-may affect protein function or
stability
Multiple of 3
(codon)
Not multiple of 3
Effect on Protein Product
Frameshift
Likely to result in premature termination
with loss of function or expression
Partial gene
duplication
May result in premature termination with
loss of function or expression
Whole gene
duplication
May have an effect because of increased
gene dosage
Dynamic mutation
Altered gene expression or altered protein
stability or function
Large insertion
Expansion of
trinucleotide
repeat
34
Frameshift Mutation
35
Mutation Nomenclature
36
Type of Mutation
Nucleotide
Protein Designation
Consequence
Description
Missense
c.482G>A
p.Argll7His
Arginine to histidine
Nonsense
c.1756G> T
p.Gly542X
Glycine to stop
Splicing
c.621 + 1G>T
Deletion (1 bp)
c.1078T
p.Va1358TyrfsX11
Frameshift mutation
Deletion (3 bp)
c.1 652_1654de1CTT
p.Phe508del
In-frame deletion of
phenylalanine
Insertion
c.3905_3906insT
p.Leu1258PhefsX7
Frameshift mutation
Splice donor site
mutation
Functional Effects of Mutations on the Protein
Loss-of-Function Mutations:
Either reduced activity or complete loss of the gene product
Usually inherited in an recessive manner
Haplo-insufficiency
Gain-of-Function Mutations:
Either increased levels of gene expression or the development of a new
function(s) of the gene product.
Dominant-Negative Mutations
37
DNA Repair
Base excision repair (BER)
Nucleotide excision repair (NER)
Post-replication repair
Mismatch repair (MMR)
38