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AOS 1- Heredity
Gene Function: Genes in Action
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Beta Thalassemia is the inherited disease also known as ‘sea in the blood’ disease
The blood cells of people with this disorder are smaller than usual, contain less haemoglobins
than normal RBCs and live less than the usual four months.
 Haemoglobins are oxygen carrying molecules.
 They are made out of 4 protein chains and iron containing haem molecules.
 There are 3 types of Haemoglobin: A(two alpha chains and two beta), A2 (two alpha
chains and two delta) and F (fetal) ( two alpha and two gamma chains)
 People with Beta Thalassemia cannot produce the Beta protein chains and so have less of
Haemoglobin A and high amounts of Haemoglobin A2.
Symptoms include:
 Anaemia because of the short life of RBC
 Tiring easily because of the low amounts of oxygen RBC carry lead to less getting to
muscles causing fatigue.
 Enlarged liver and spleen
 Enlargement of Bone marrow (in attempt to produce more RBC) can lead to deformed
skull bones.
 Break down products of Haemoglobin overload body as RBC die rapidly so abnormal
depositation of iron in the liver, kidneys and brain
Treatment includes the drug Desferal to bind to excess iron and get rid of it.
The Gene for Beta Thalassemia is the HBB gene located on chromosome 11.
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Genes are decoded and expressed as a phenotype.
Gene action involves two processes: Transcription and Translation
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TRANSCRIPTION
1. The DNA cannot leave the nucleus and so must be copied: The enzyme RNA Polymerase binds
to the 5` end of the template strand of the DNA as the required section unwinds from its double
helix.
2. The enzyme moves along the length of the DNA
towards the 3` end and as it does, it matches free
nucleotides to the ones on the template strand and
joins them together to form a messenger RNA
molecule (mRNA). This molecule is called the
Primary Transcript at this stage because it is an
exact complementary copy of the DNA template
strand.
3. In a process called Post-transcription modification, the introns are cut out of mRNA and it is
capped with a Poly A tail.
4. This genetic information can now leave the nucleus and start protein synthesis.
TRANSLATION
1. In the Cytoplasm, the mRNA moves to attaches to the small unit of a
ribosome.
2. Free amino acids in the cytoplasm attach to transfer RNA (tRNA) molecules.
The bottom of tRNA molecules is an anti codon complementary to that on the
mRNA molecule for the amino acid it carries.
3. The Ribosome reads the START code and a tRNA carrying a methionine
amino acid matches its anticodon to the codon on the mRNA at the P site. The
code is accepted and the amino acid attaches to the P side on the larger
ribosome subunit.
4. The ribosome moves along the mRNA to the next codon and the next tRNA comes and brings its
amino acid to the A site. If the anticodon is correct, the amino acid bonds to the met amino acid.
5. The ribosome again moves to the next codon and the last amino acid is moved to the P site to
allow the next tRNA to bring its amino acid and the cycle continues.
6. Once the end of the code is reached, the Ribosome reads the STOP codon and translation is
terminated.
7. The protein may now move to another part of the cell to be further processed or packaged to
leave the cell and be utilised.
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Research has now shown that 30% of genes are regulated in this way:
 One gene could produce one protein at one stage of development and another protein at
another stage of development
 One gene could produce a particular protein in one tissue and another in another tissue.
The above could occur in the following ways:
 Retention of Introns
 Juggling of introns
Alternative Splicing is where the number of outputs of the genetic instructions in a genome is
far greater than the number of genes.
DIFFERENCES BETWEEN EUKARYOTES AND PROKARYOTES
SIMILARITIES
Transcription of DNA to mRNA
Translation on Ribosomes to Protein
chains
DIFFERENCES
Eukaryotes
DNA found in nucleus
DNA bound in histones forming
Chromosomes
Many repeated nucleotide
sequences
mRNA lasts a few days
Coding region interrupted by
introns
Post transcription modification
Transcription must complete
before Translation can start
Prokaryotes
No nucleus- found in
cytoplasm
DNA naked in a single circular
chromosome called plasmid
All nucleotide sequences are
unique.
mRNA lasts a few hours
Coding region not interrupted
No modification required.
Translation can start even
before Transcription is
complete (same compartment)
GENES IN THALASSEMIA
 In a Heterozygous person (does not have condition), genotype Tt, the Gene T enables the
production of proper Beta chains so the Haemoglobin A can be formed.
 In a Homozygous person with the condition, genotype tt, they have the genes for the alternative,
inactive protein to be made. The protein formed as a single nucleotide changed in the sequence
and causes a STOP codon many codons before the protein is finished. Hence, no Haemoglobin A
can be formed. (Beta-0 Thalassemia)
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Genes can have many different alleles, in Thalassemia, the allele ‘T’ means that Haemoglobin A
can be produced and ‘t’ indicates that it cannot be produced.
In reality, there are many types of the ‘t’ allele all of them producing very little or no beta chains.
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NOTE: The trait is dominant no the allele. The allele controls the dominant trait.
DIFFERENT FUNCTIONS OF GENES
 Genes that produce proteins that contribute to the structure and functioning of an organism are
called structural genes
 Genes that produce proteins that control the functioning of other genes are called regulator
proteins. They can determine whether genes are active or not (on or off) and the rate of protein
synthesis.
 They can produce DNA binding proteins which bind directly onto the gene and switch it
on or off.
 Produce signalling proteins that bind to cells of the particular tissue and trigger a series
of intracellular reactions that switch genes on or off.
 Homeotic Genes are master embryonic genes that control structural genes ensuring the
various body parts are built in the correct locations.
 In mammals, the genes are known as the HOX genes, form four clusters or
four different chromosomes.
 They produce DNA binding proteins with a positive charge so that they can
bind to the negatively charged DNA and switch genes on or off.
DNA REPLICATION
 DNA has a remarkable ability to copy itself through DNA replication.
 DNA replication must occur so that during mitosis, the daughter cells each carry the diploid
number of chromosomes. It is also important in the early stages of meiosis.
GENE ACTIVITY
 Genes vary in their time of action: some throughout the whole life and others only at specific
times.
 Some genes are only active during the embryonic period whilst others such as Huntington’s
disease are only expressed in the phenotype only when the individual is well into adulthood.
 Some genes are only active in certain tissues (eg. Genes that produce insulin are only active in
the pancreas).
 Some genes are active in all tissues eg. Those involved in cellular respiration.
 Gene action refers to the processes of gene transcription and translation.
 The final phenotype is more complex than just the proteins produced by genes.
 At any given moment in any cell only a few of the many genes are ‘switched on’
 A ‘switched on’ gene is one that is being transcribed into mRNA at the time.
 A ‘switched off’ gene is one that is not.
 Pattern of gene activity differs between cells depending on their environment and type.
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Microarray Technology
 Also called DNA arrays or gene chips
 Many molecules of DNA are placed on different spots on the glass slide.
 Each piece of DNA represents one part of a particular gene. These can then act as probes
for a particular gene.
 They are placed in precise locations by a robotic instrument.
 Used to
 Identify which genes are active and which are switched off
 Compare gene expression in different cell types
 Compare active genes in the same cells under different conditions.
SWITCHING GENES OFF
 Advantageous in the case of a mutant allele- if it could be switched off, diseases could be
prevented eg. Parkinson’s or Huntington’s Diseases that are caused by the over production of
particular proteins.
 Double stranded RNA (dsRNA) can be used to
silence genes if one of strands is identical to the
mRNA strand that is produced by that gene. This is
called RNA interference (RNAi)
 This works by not acting on the gene
directly, but by destroying the mRNA
produced.