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BS1050 Genes notes: The study of genes
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BASICS OF DNA​:
1
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2
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3
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(One
goes up the other goes down
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Each strand has a polarity, five
prime to three prime
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The strands have bases (AT,CG)
6
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Section 2: DNA in the cell (light microscopy)
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DNA can be stained in order for it to be seen and examined under a light
microscope
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When a cell divides, you can visually see chromosomes move to either side of
the cell during division
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Chromosomes are very tightly packaged DNA
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Each chromosome is one DNA molecule
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1
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Chromatin is the one responsible for packaging DNA into smaller volumes so that
they can fit inside the cell

3
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Chromatin also prevents damaging the DNA and ​controls​ the gene expression and
replication of the DNA
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There are two varieties of chromatin
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6
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(The
staining is due to the amount of packaging in a given area
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These two forms are distinguished in by dealing with how intensely each form is
stained
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This only indicates that
heterochromatin has tighter DNA packaging
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At
the lighter regions there is less packaging (euchromatin) so the genes can be
expressed
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 Darker staining indicates tighter DNA packaging
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 Heterochromatin’s are compactly coiled regions while euchromatins are loosely
coiled regions
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 Euchromatin is early replicative while heterochromatin is late replicative
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 Heterochromatin is only found in eukaryotes
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Section 4: Genes
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Gene’s code for proteins
2
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chromosomal location
1
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2
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Gene Expression:
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central dogma =

Gene Example:

● the protein haemoglobin has 4 subunits in total, 2 alpha and 2 beta
subunits
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● The HBA1 gene located on chromosome 16 codes for the alpha
chain
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What is a gene?
● 1 gene = 1 functional unit
● This ‘functional unit’ can be a sequence of amino acids in a
polypeptide Or… a sequence of nucleotides in an untranslated
RNA

Where is a gene located?
● Genes have a chromosomal location
How many?
● Humans have around 25,000 genes
● Fruit flies have around 15,000 genes
● E
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The amount of genes is not important, the regulation of their expression is
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A genome is the entire DNA sequence of an organism
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● The human genome contains 3,200,000,000 base pairs
● The fruit fly genome contains 140,000,000 base pairs
● E
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Section 6: Gene Expression:
Transcription in gene expression
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● The messenger RNA synthesis goes from 5 prime to 3 prime
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● The DNA is not read, the mRNA is read 5 prime to 3 prime
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● In gene expression the code is read in a non overlapping triplet code, it is
read in codons
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● You can read a DNA code in 3 different reading frames
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● You can also read it in frame 2 and 3 despite them not making sense
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The genetic code:

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●
●
●
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It’s a 3 letter code
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All code for the 20 amino acids found in nature
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There are start and stop codons included
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It is also…
● The process that produces the alteration
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A change in non-coding DNA it will not matter
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● Transposable element insertions:
● Transposable elements​ can ​cause​ deletions or inversions of DNA
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Single base substitution:

Transition​: A change to the same type of base:

Transversion: ​A change to a different type of base:

Section 8: Sickle cell anaemia:
An Example of what affect mutations have on the body can be seen through ‘sickle
cell anaemia’
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● This mutation causes the haemoglobin in red blood cells to distort to a sickle 
shape when deoxygenated
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Symptoms in terms of profession: 
 
A physician may pick up on:  
 
● Susceptible to infections
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A biochemist: 
● The haemoglobin of the patient precipitates in low oxygen levels 
 
 
 
Sickle cell mutation: 
 
● It is a base substitution mutation 
● There is a one letter change in the 7​th​ codon of the haemoglobin gene 
● It changes one amino acid, the 6​th​ amino acid
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● Genotype: 
● The genetic makeup
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Sickle cell anaemia is an example of: 
 
 

 
 
 
 
 
 
This is an example of a silent mutation:  
 
 

 
 
 

 
Mutations that change the amount of gene product: 
● This mutation may decrease or increase the amount of gene expression 
● They affect gene expression by affecting transcription and translation 
 
 
They work by doing the following: 
 

 
 

● If you change a single base it may be that the promoter activity is 
changed 
● If you make a single base change it may alter the translation initiation 
or mRNA splicing which may have an affect on gene product 
 

 
 
 
 
 
 
 
 
 
 
Another small scale mutation is: 
 
 

●

Amino acids may not be present due to deletion of bases: 

 
 

● Another way of changing the polypeptide length is by nonsense 
mutations (chain-termination mutations)
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● In the normal alpha chain you see UAA as the stop codon 
● In alpha chain Wayne instead of the first codon being AAA we 
have a base deletion and are left with AAU 
● The ribosome reads as AAU and ‘the frame shifts’ due to the 
deletion of a single base
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● One amino acid has now changed 
● It could also create a stop codon 
● The code in that case stops at “ THE BIG” 
● Missense and nonsense mutations are called synonymous 
mutations
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Minus 1 = moves up one 
 
In frame shift mutations the code still makes sense however it may code differently: 
 
 
 
 
 
 
 
 
 

 
 
 
 
 
 
However… 
● Where this frame shift occurs is important: 
 

 
 

● The following makes no sense
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● Deletion:​ ​a part of a ​chromosome​ or a sequence of ​DNA​ is lost during
DNA replication
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● Inversion​: a mutation where parts of a chromosome are flipped or
inverted
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Transposable element insertions:

● In the breakage of chromosome 9 and 22, and the last bits of the Q
arms can swap over

● This results into a changed chromosome 9 and 22
● This may result in a fusion gene, this means the gene cannot be turned
off
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Main points of transposable gene insertions:

● Dna is not fixed, it can move freely
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● Here the TE as a unit can jump out of its original
chromosome and move into another chromosome
● This may inactivate the gene causing disease
● It may also excite a gene, this will activate other genes
such as the red pigment on the flower below:



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