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Chromosomes and the Genetic Code
Chromosomes contain DNA (which is tightly packed) and transmits information from
one generation to the next
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DNA combines with groups of 8 histones forming nucleosomes
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During cell division; chromosomes are sorted out
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Before the nucleus divides, a copy
of DNA is produced
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Karyotypes:
Chromosomes: human (46), fruit flies (8), cats (38), dogs (78)
Karyogram: diagrammatic representation of all chromosomes in an organism
Karyotype: set of chromosomes

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-

Non-reproductive cells: identical pair of chromosomes (same length and stain
the same) and similar genetic information – homologous chromosomes
(diploids – 2n)

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Reproductive cells: (gametes) called haploid (n) – 23 chromosomes
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Male: XY and the Female: XX

Advantages of having two sets of chromosomes:
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Homologous pairs of chromosomes have genes for the same characteristics
...
Therefore
having two chromosomes prevents this as one serves as a ‘back up’

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Genetic variation

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Plants have 3+ sets of chromosomes = polyploidy

Genes and Alleles
Genes are located on chromosomes and it is the unit of inheritance
...
Humans have 19,000
different genes
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They occupy the same loci on homologous chromosomes
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When the virus attacks a bacterium, part of the virus enters the cell and takes over
its functioning, making it procedure new viral particles
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They grew bacteria in the lab and infected them with viral particles
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On one culture viruses protein coats were labelled with
radioactive sulphur, in another, DNA was labelled with radioactive phosphorus
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(Bacterial cells – pellet, viral material – supernatant)
Analysis of supernatant:
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Concentration of

32

S was high, indicating that the protein coat did not enter

bacterial cells
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Concentration of

32

P was low, indicating that DNA entered

Analysis of pellet:
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No radioactive protein coat was present

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Radioactive DNA detected

Conclusion: T2 bacteriophage injected the DNA from its head into the bacterium
while the external protein coat remained outside
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DNA replication
Chromosomes duplicate so each daughter cell inherits an exact copy of generic
information
...

1
...


2
...


3
...

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Proof for semi-conservative replication (The Meselson and Stahl experiment):
They used the ‘heavy’ isotope

15

N, making molecules denser
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Coli
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Then, they grew another

15

N culture and transferred it to

14

N
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Each strand acted as a template for a
second strand (less dense)
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(15-15 à 15-14, 15-14 à 15-14, 14-14, 15-14, 14-14à 15-14, 14-14, 14-14, 14-14,
15-14, 14-14, 14-14, 14-14)

Semi-Conservative DNA replication
1
...
When
they unwind, the bases of the two polynucleotide chains are exposed; therefore
single-strand binding proteins bind to them to prevent reassociation
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2
...
Two DNA
polymerase III bind to the unwound strands
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3
...


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Primase/DNA polymerase synthesises the primer
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Two phosphates are removed when the nucleotide attaches to DNA, releasing
energy therefore the remaining phosphate can form a phosphodiester bond with the
sugar molecule
...
They
two DNA polymerase III, (one on each parental strand) move in opposite directions
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-

It moves away from the helicase when it reaches the place where the first
has started; the two short DNA are joined to make a continuous DNA strand
by DNA ligase
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4
...
Proof-reading and repair
H bonding between bases makes DNA accurate
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Mismatch repair mechanism:
Proteins survey the replicated molecules and looks for mistakes
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If it is bound wrongly, this will not happen, and the error
will be detected
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DNA polymerase
and DNA ligase synthesis and seal up a new base sequence and replace it
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Enzymes
inspect the DNA and cut the defective strand
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Summary of the use of enzymes:
•

DNA helicase:
i) unwind and separates

•

DNA polymerase III:
i)
ii)

Matches complementary bases to bases on template strand

iii)

Forms polydiester bonds

iv)
•

Recognises bases on parent/template strand

Proof-reading

Primase/DNA polymerase:
i)

•

Synthesises the primer

DNA polymerase I:
i)

Replaces primer with DNA nucleotides
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•

DNA ligase:
i)

Forms phosphodiester bonds between the Okazaki fragments

Protein synthesis
-

Central dogma of molecular biology

polypeptide (make up proteins e
...
enzymes, structural proteins and hormones)
...
The
information stored in DNA specifies the structure and function of each cell and
organism
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These are made on
messenger RNA (mRNA) during transcription
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The other strand will have a complementary but
different base sequence that is unlikely to code for a protein
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Elongation
RNA polymerase travels along the template strand from 3’ to 5’ direction
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Only one strand is used as a template
2
...
RNA links nucleotides together (not DNA polymerase)
4
...
No primers needed
6
...
RNA polymerases do not proof-read their work
When mRNA has the genes, it leaves the nucleus through pores in the nuclear
envelope and enters the cytoplasm and binds to ribosomes


Post-Transcriptional processing
Modifications occur to the mRNA by RNA polymerase in the nucleus (primary mRNA
or pre-mRNA), before it is exported to the cytoplasm
...

5’cap: G gap, nucleotide with guanine as a base and 3 phosphate groups
3’tail: adenine nucleotides


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Splicing
Certain sections of the pre-mRNA are cut out before it is used in translation
...
Remaining sections of the gene code are called
exons
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The Genetic Code
Relation between the base sequence of RNA and amino acid sequences in proteins
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A triplet code
DNA and RNA have 4 different bases; proteins are made up of 20 amino acids,
therefore one base cannot code for just 1 amino acid
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It can use
3 bases to specify each amino acid (codons)
Degenerate
Each amino acid can be coded for by more than one codon, therefore degenerate
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Punctuated
AUG (methionine) is a start codon – all amino acids start with this
UAG, UAA and UGA are stop codons – mark the end of the mRNA
Universal
Genes are transferred from one organism to another and successfully expressed
...
No base, of a particular
codon can be shared with an adjacent codon

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tRNA
Translation: sequences of bases in mRNA converted into a sequence of amino acids
in a polypeptide chain on ribosomes
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tRNA binds and
transfers amino acids present in the cytoplasm to the ribosome
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tRNA charging reaction
Amino acids are attached to tRNA molecules by activating enzymes known as
aminoacyl-tRNA synthestases
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Enzyme + ATP + a
...
a + PPi
Enzyme then catalyses the transfer of amino acid from the AMP to the 3’ terminal
Enzyme-AMP-a
...
a
The activating enzyme finally releases the charged tRNA and can charge another
tRNA molecule
...


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Ribosomes
Made up of ribosomal RNA (rRNA) and proteins
...

-

Small subunit: recognises and binds mRNA and tRNA

-

Large subunit: contains enzymatic (catalytic) site that catalyses the formation
of peptide bonds

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P and A sites

Translation
Initiation
Initiation complex (consists of tRNA
bearing methionine and ribosomal
unit) AUG is the start codon (assisted
by initiation factors) and AUG and
UAC pair by complementary base
pairing
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All these are assisted with protein factors
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Protein releasing factors cut the finised protein and releases it
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Be released to the cytoplasm once translation is complete
ii
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These serve as signal sequence which attach to
signal recognition particle
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Protein synthesis begins on free ribosomes
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The polypeptide chain binds to the signal recognition particle and
both bind to a receptor protein in the membrane of the ER
v
...
The signal sequence
passes through a channel in the membrane
vi
...
The polypeptide continues to elongate
viii
...
Reobisome is released

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•

Post – translational modification

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Cleavage of the polypeptide allowing fragments to fold into different

-

Adding sugars for targeting and recognition

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Adding phosphate groups alters the shape of the protein

The nature of induction of enzyme synthesis in E
...

-

Synthesised continuously (constitutive enzymes)

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Synthesised only in the presence of an inducer (inducible enzymes) e
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βgalactosidase

When grown on lactose two enzymes are needed:
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Β- galactosidase (enzyme induction) (hydrolysis lactose into glucose and
galactose)

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Lactose permease (enables the cell to take up lactose)

High concentrations of tryptophan suppressed the production of enzyme tryptophan
synthestase used to make typtophan (enzyme repression)
Jacob – Monod hypothesis of gene control
Structural genes – determine the amino acid sequence of enzymes
Regulatory gene (allosteric enzyme) – controls the activity of structural genes, this
affects the operator gene (o), this is like a switch, it switches the structural genes on
and off simultaneously
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This contains the genes, for lactose-metabolising enzymes in E coli is the lac operon
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The operator gene becomes active and switches on the structural genes
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The structural genes are
not activated and no β-galactosidase and lactose permease are produced
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The structural genes become
active, mRNA is produced and proteins are synthesised
...


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Enzyme repression
If a co-repressor molecule binds with the allosteric site on the repressor molecule, it
reinforces the normal biding response of the repressor molecule to the operator
gene
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The product of a reaction can also act as a co-repressor of enzyme synthesis by
combining to the repressor molecule
...

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The initial substrate and product can act as inducers and co-repressor respectively
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The product interacts with the repressor
molecule to make it capable of binding to the operator, thus blocking transcription
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Changes in the sequence of bases in DNA are called mutations
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Some effect single genes, gene mutations
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A mutation is a change in the amount, arrangement or structure of the DNA of an
organism
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A chromosomal mutation is a large scale mutation involving more than a single
gene
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Substitution mutations are described as missense mutations
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g
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ü Protein function is destroyed by a stop codon: an inappropriate stop codon
will interrupt the translation of mRNA before the protein is finished
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Sickle-cell anaemia
It is inherited, however it is recessive therefore the person will only suffer from it if
they have homozygous chromosomes i
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both parents have to be carriers of it
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This changes its shape into a longer, sickle shape
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They tend to aggregate, clogging capillaries and reducing the rate by which tissues
receive oxygen or have their wastes removed
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Paralysing strokes can occur, they are prone to infections
and death in some cases, and they show poor growth and development
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Cause:
In HbS, the amino acid glutamic acid is replaced by valine, glutamine acid is polar
and hydrophilic whereas valine is non-polar and hydrophobic
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Heterozygous individuals have half their molecules HbS and the other half HbA
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These exhibit some resistance to the parasite that
causes malaria and are less susceptible to malaria because the malarian parasite
multiplies inside normal RBCs and is unable to multiply in sickle-shaped cells
(selective advantage)
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Mutations arising in non-reproductive cells (somatic mutations) inherited only by
those cells derived from the mutant cell
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Somatic mutations may produce cells with an increased rate of growth and division,
causing tumours, which live on healthy cells causing cancer
...

Mutations are essential for evolution, all genetic variations originates from random
changes in the base sequence of DNA
...
It just happens, whether that change is helpful or harmful, depends on the
environmental conditions over which the organism has little or no control

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