Download Human Genetics

Human Genetics
(Chapter 3: and some of 4)
The structure of DNA
• Composed of 4 nucleotide
bases, 5 carbon sugar and
phosphate.
• Base pair = rungs of a ladder.
• Edges = sugar-phosphate
backbone.
• Double Helix
• Anti-Parallel
The structure
Figure 2.21of DNA
DNA
Replication
Figure
2.22a
Remember – the two strands run in
opposite directions
Synthesis of a new (daughter) strand
occurs in the opposite direction of the
old (parental) strand.
Complementary base-pairing occurs
A with T and G with C
G and C have three hydrogen bonds
A and T have two hydrogen bonds
DNA Replication
• Each new double helix is composed of an old
(parental) strand and a new (daughter) strand.
• As each strand acts as a template, process is called
Semi-conservative Replication.
• Replication errors can occur. Cell has repair
enzymes that usually fix problem. An error that
persists is a mutation.
• This is permanent, and alters the phenotype.
The structure of RNA
• Formed from 4
nucleotides, 5 carbon
sugar, phosphate.
• Uracil is used in RNA.
– It replaces Thymine
• The 5 carbon sugar has
an extra oxygen.
• RNA is single stranded.
Central Dogma of Molecular
Biology
• DNA holds the code
• DNA makes RNA
• RNA makes Protein
• DNA to DNA is called
REPLICATION
• DNA to RNA is called
TRANSCRIPTION
• RNA to Protein is called
TRANSLATION
Central Dogma of Molecular
Biology
• There are exceptions:
• Retroviruses
– Use RNA as the genetic code
– Must make DNA before making
protein product
– This new DNA makes RNA and
then a protein
• Also, one protein is not always the
product of a single gene – we will
talk about this later in the
course!
Transcription – DNA to RNA
Figure 3.3 (1)
(RNA polymerase)
Transcription – DNA to RNA
Figure 3.3 (2)
Transcription – DNA to RNA
Figure 3.3 (3)
Transcription – DNA to RNA
Figure 3.3 (4)
A close-up view of transcription
RNA nucleotides
RNA
polymerase
Newly made
RNA
Direction of
transcription
Template
strand of DNA
• How does the order or sequence of
nucleotides in a DNA and then a RNA molecule
determine the order of amino acids in a
protein? (Translation)
TACCTGAACGTACGTTGCATGACT
DNA
AUGGACUUGCAUCGAACGUACUGA
RNA
Met-Asp-Leu-His-Arg-Thr-Tyr-STOP
protein
Translation
• Translation requires:
– Amino acids (AAs)
– Transfer RNA: (tRNA) Appropriate to its time, transfers
AAs to ribosomes. The AA’s join in cytoplasm to form
proteins. 20 types. Loop structure
– Ribosomal RNA: (rRNA) Joins with proteins made in
cytoplasm to form the subunits of ribosomes. Linear
molecule.
– Messenger RNA: (mRNA) Carries genetic material from
DNA to ribosomes in cytoplasm. Linear molecule.
Translation
• The mRNA has a specific
“open reading frame” made
up of three base pairs –
codon.
• The tRNA has the
complementary base-pairing
fit to the codon –known as
an Anticodon
• Each of these codes for an
amino acid
Translation
• Initiation—
– mRNA binds to smaller of ribosome subunits, then,
small subunit binds to big subunit.
– AUG start codon--complex assembles
• Elongation—
– add AAs one at a time to form chain.
– Incoming tRNA receives AA’s from outgoing tRNA.
Ribosome moves to allow this to continue
• Termintion—
Stop codon--complex falls apart
Translation
Figure 3.5 (1)
Translation
Figure 3.5 (2)
Translation
Figure 3.5 (3)
What happens when it all
goes wrong?
– MUTATIONS!!!!!!!!!!
– two general categories
1.result in changes in the amino acids in proteins
A change in the genetic code
2.Change the reading frame of the genetic message
Insertions or deletions
Mutations
Figure 3.6a
Mutations
Remember Thalidomide?
• The structure of thalidomide is
similar to that of the DNA purine
bases adenine (A) and guanine
(G).
• In solution, thalidomide binds
more readily to guanine than to
adenine, and has almost no affinity
for the other nucleotides, cytosine
(C) and thymine (T).
• Furthermore, thalidomide can
intercalate into DNA, presumably
at G-rich sites.
Remember Thalidomide?
• Thalidomide or one of its
metabolites intercalates into these
G-rich promoter regions,
inhibiting the production of
proteins and blocking
development of the limb buds.
• This intercalation would not
significantly affect the over 90 per
cent of genes that rely primarily
on guanine sequences.
• Most other developing tissues in
the embryo rely on pathways
without guanine, and are therefore
not affected by thalidomide
Remember Thalidomide?
Genes can lead to inherited
diseases
• A gene which doesn’t function on an autosomal
chromosome can lead to devastating diseases
• Autosomal chromosomes are 22 pairs of
chromosomes which do not determine gender
• Such diseases can be caused by both a dominant or a
recessive trait
Autosomal Recessive Disorders
• Tay-Sachs Disease:
– Jewish people in USA (E. Euro descent)
– Not apparent at birth
– 4 to 8 months
• Neurological impairment evident
• Gradually becomes blind and helpless
• Develops uncontrollable seizures/paralyzed
• Allele is on Chromosome 15
– Lack of enzyme hexosaminidase A (Hex A)
• Lysosomes don’t work, build up in brain
Autosomal Recessive Disorders
• Cystic Fibrosis
– Most common in USA (Caucasian)
– 1 in 20 caucasians is a carrier
– Mucus in bronchial and pancreas thick/viscous
– Breathing and food digestion problems
• Allele is on chromosome 7
– Cl ions can not pass through plasma membrane
channels
• Cl ions pass –water goes with it. No water, thick
mucus
Autosomal Recessive Disorders
• Phenylketonuria (PKU)
– Affects in in 5,000 newborns
– Most common nervous system disorder
• Allele is on chromosome 12
– Lack the enzyme needed for the metabolism of the
amino acid phenylalanine
– A build up of abnormal breakdown pathway
• Phenylketone
• Accumulates in urine. If diet is not checked, can lead
to severe mental retardation
Autosomal Dominant Disorders
•
•
•
•
•
Neurofibromatosis
Very common genetic disorder
Tan spots on skin
Later tumors develop
some sufferers have large head and ear and eye
tumors.
• Allele is on chromosome 17
– Gene controls the production of a protein called
neurofibromin
– This naturally stops cell growth
Autosomal Dominant Disorders
•
•
•
•
•
Huntington Disease
Leads to degeneration of brain cells
Severe muscle spasms and personality disorders
Attacks in middle age
Allele is on chromosome 4
– Gene controls the production of a protein called
huntington
– Too much AA glutamine. Changes size and shape of
neurons
Incomplete Dominant traits
• Sickle Cell Anemia
• Controlled by intermediate phenotypes at a ratio of
1:2:1
• Red blood cells are not concave
• Normal Hemoglobin (HbA). Sickle cell (HbS)
• HbA-HbA-normal
Hbs-Hbs – sickle cell
• HbA-Hbs- have the trait
Mutations
- any change in the nucleotide sequence of DNA
Normal hemoglobin DNA
Mutant hemoglobin DNA
mRNA
mRNA
Normal hemoglobin
Glu
Sickle-cell hemoglobin
Val
Figure 10.21
Sickle Cell Anemia
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickled cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Physical
weakness
Impaired
mental
function
Anemia
Heart
failure
Paralysis
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other
organs
Rheumatism
Spleen
damage
Kidney
failure
Figure 9.21
Genetic engineering
Genetic engineering
• The direct alteration of a genotype
– Human genes can be inserted into human cells for
therapeutic purposes
– Genes can be moved from one species to another
• Moving genes from human to human or between
species requires the use of special enzymes known
as restriction enzymes.
– These cut DNA at very specific sites
– They restrict DNA from another species – isolated from
bacteria.
Genetic
engineering
Figure 4.1
•Each restriction enzyme cuts the DNA at a specific site, defined by the DNA sequence
•Enzymes which produce “sticky ends” are more useful
•Allows gene of interest to be inserted into a vector
•Also need a DNA probe
•Radioactive ssDNA that will bind to gene of interest so you can locate it
Genetic engineering
• Transferred DNA is denatured to give ssDNA
• The probe will bind to gene of interest by
Complementary base-pairing - A with T and G with C
Genetically engineered
Figure
4.3
(1)
insulin
Genetically engineered
Figure
4.3
(2)
insulin
Genetically engineered
Figure
4.3
(3)
insulin
Genetically engineered
Figure
4.3
(4)
insulin
Genetically engineered
insulin
• Why do some people not like the idea?
The plasmid also needs a “marker gene”
This is usually an antibiotic resistance gene
Some people fear that the insulin which is
extracted from the bacteria would also
contain a gene product to make anyone who
uses the insulin resistant to antibiotics!
Gene therapy
• Can treat human diseases
– eg – severe combined immune deficiency syndrome (SCIDS)
• Bubble- Boy/Girl syndrome
• The enzyme which causes this is on chromosome 20
– Called adenosine deaminase (ADA)
• Many problems
– Difficult to transfer large genes
– Insert in a way that the gene expresses to protein correctly
– TRANSLATION!!!!!!!!!!!!
Gene
therapy
Figure
4.4 (1)
Gene
therapy
Figure
4.4 (2)
Gene
therapy
Figure
4.4 (3)
Gene
therapy
Figure
4.4 (4)
Gene
therapy
Figure
4.4 (5)
Virus has genetic defect to
prevent viral reproduction
and spreading to other cells
Gene
therapy
Figure
4.4 (6)
Virus vector must get the
gene into the nucleus of the
patient’s lymphocyte
Gene
therapy
Figure
4.4 (7)
Gene has to be incorporated into
cell’s DNA where it will be
transcribed
Also inserted gene must not
break up some other necessary
gene sequence
Gene therapy
• The genetically engineered lymphocytes injected
into the patient should out grow the “natural”
(defective) lymphocytes
• As ADA-deficient cells to not divide as fact as those
with the active enzyme
• Not permanent - need repeat injections as injected
lymphocytes are mature and have limited life span
• Stem cells would get around this problem (later!)
Genetic Profiling
• We could screen everyone’s DNA for mutations.
• How would this affect insurance?
• How would this affect health care?
• What about “reproductive control”?
• What do you think?
The end!
Any questions?