Human Genetics

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Human Genetics

• (Chapter 3 and some of 4)

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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

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Figure 2.21
The structure of DNA
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Figure 2.22a
DNA Replication
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
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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.

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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.

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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

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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!

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Figure 3.3 (1)
Transcription DNA to RNA
(RNA polymerase)
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Figure 3.3 (2)
Transcription DNA to RNA
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Figure 3.3 (3)
Transcription DNA to RNA
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Figure 3.3 (4)
Transcription DNA to RNA
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A close-up view of transcription
RNA nucleotides
RNA polymerase
Newly made RNA
Direction of transcription
Template strand of DNA
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• 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
RNA
AUGGACUUGCAUCGAACGUACUGA
Met-Asp-Leu-His-Arg-Thr-Tyr-STOP
protein
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Translation

• Translation requires
• Amino acids (AAs)
• Transfer RNA (tRNA) Appropriate to its time,
  transfers AAs to ribosomes. The AAs 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.

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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

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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 AAs from outgoing tRNA.
  Ribosome moves to allow this to continue
• Termintion Stop codon--complex falls apart

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Figure 3.5 (1)
Translation
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Figure 3.5 (2)
Translation
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Figure 3.5 (3)
Translation
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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

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Figure 3.6a
Mutations
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Mutations
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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.

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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

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Remember Thalidomide?
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Genes can lead to inherited diseases

• A gene which doesnt 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

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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 dont work, build up in brain

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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

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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

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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

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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

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Incomplete Dominant traits

• Sickle Cell Anemia
• Controlled by intermediate phenotypes at a ratio
  of 121
• Red blood cells are not concave
• Normal Hemoglobin (HbA). Sickle cell (HbS)
• HbA-HbA-normal Hbs-Hbs sickle cell
• HbA-Hbs- have the trait

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Mutations
- any change in the nucleotide sequence of DNA
Normal hemoglobin DNA
Mutant hemoglobin DNA
mRNA
mRNA
Sickle-cell hemoglobin
Normal hemoglobin
Glu
Val
Figure 10.21
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Sickle Cell Anemia
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Individual homozygousfor 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
Accumulation of sickled cells in spleen
Damage to other organs
Pain and fever
Physical weakness
Heart failure
Brain damage
Spleen damage
Anemia
Impaired mental function
Pneumonia and other infections
Kidney failure
Paralysis
Rheumatism
Figure 9.21
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Genetic engineering
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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.

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Figure 4.1
Genetic engineering

• 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

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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

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Figure 4.3 (1)
Genetically engineered insulin
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Figure 4.3 (2)
Genetically engineered insulin
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Figure 4.3 (3)
Genetically engineered insulin
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Figure 4.3 (4)
Genetically engineered insulin
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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!
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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!!!!!!!!!!!!

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Figure 4.4 (1)
Gene therapy
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Figure 4.4 (2)
Gene therapy
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Figure 4.4 (3)
Gene therapy
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Figure 4.4 (4)
Gene therapy
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Figure 4.4 (5)
Gene therapy
Virus has genetic defect to prevent viral
reproduction and spreading to other cells
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Figure 4.4 (6)
Gene therapy
Virus vector must get the gene into the nucleus
of the patients lymphocyte
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Figure 4.4 (7)
Gene therapy
Gene has to be incorporated into cells DNA where
it will be transcribed Also inserted gene must
not break up some other necessary gene sequence
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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!)

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Genetic Profiling

• We could screen everyones DNA for mutations.
• How would this affect insurance?
• How would this affect health care?
• What about reproductive control?
• What do you think?

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The end!

• Any questions?