Download CH 8. DNA: The Universal Molecule of Life

DNA is universal
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The DNA in all organisms has the same basic
structure.
The nucleotides with their 4 bases Adenine
Thymine, Cytosine and Guanine are all the
same.
The only difference is the order in which they
are put together in different organisms.
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Watson and Crick model of DNA
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Watson and Crick model of DNA
Using these results
and other
accumulated evidence,
Watson and Crick
suggested that DNA
consisted of the now
familiar two chains
twisted around each
other to form a double
helix ladder, crosslinked by nitrogenous
bases.
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From DNA to Proteins
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DNA codes for the production of proteins.
Proteins (enzymes) determine the
reactions that occur in a cell and other
proteins determine some of its structural
characteristics.
Proteins differ from each other in the
sequence of the 20 amino acids and the
length of the polypeptide chain.
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From DNA to Proteins
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Since there are only 4 different
nucleotides in DNA and they need to code
for 20 amino acids, the code must be in
triplets (AGC, TGA, etc.) called codons.
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Transcription – formation of mRNA
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The code on the DNA has to
get from the nucleus to the
ribosomes in the cytoplasm.
The DNA itself does not leave
the nucleus.
The DNA is transcribed into
mRNA which then leaves the
nucleus and attaches to the
ribosomes.
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Comparison of DNA and RNA
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Comparison of DNA and RNA
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Transcription – formation of mRNA
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The DNA in the relevant region first unwinds, and then unzips.
Only one of these strands is used to direct the synthesis of
mRNA; this is called the template strand.
The other strand of DNA is called the non-template strand.
A particular nucleotide sequence at the beginning of the gene,
called a promoter, signals the start of a gene.
Proteins position RNA polymerase on to the DNA to bind with
the promoter.
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Transcription – formation of mRNA
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Complementary RNA nucleotides are progressively
joined together by RNA polymerase moving along the
length of DNA.
A base sequence at the end of the gene serves as a
stop signal.
The mRNA is released as a single strand.
The DNA zips up and twists itself back into a double
helix again once the mRNA has peeled off.
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Pre-mRNA to mRNA
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The mRNA strand at this stage is called pre-mRNA.
Before it leaves the nucleus it is modified by the addition of:
 a methylated cap at the 5’ end and
 about 100–200 adenine nucleotides at the 3’ end.
Most eukaryotic genes have regions of base sequences
(introns) that are not translated into proteins.
Exons contain the actual information for protein formation.
The introns are removed, and the mRNA leaves the nucleus.
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SRAM 208: “Transcription”
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Translation – mRNA into Proteins
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When mRNA moves into the cytoplasm,
it attaches itself to a ribosome, where it
causes amino acids to assemble in a
particular order.
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Translation – mRNA into Proteins
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Ribosomes are made up of
two subunits, one small
and one large. The nucleus
assembles them both from
ribosomal RNA (rRNA) and
proteins.
The subunits move from
the nucleus into the
cytoplasm, where they
combine to form the
functional units of
translation.
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Translation – mRNA into Proteins
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Another type of RNA, called transfer RNA (tRNA), is
also needed for protein synthesis.
tRNAs carry amino acids to ribosomes.
They do not have a linear arrangement of
nucleotide bases but are folded back on themselves
to form a compact three-dimensional structure
rather like a clover leaf.
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Translation – mRNA into Proteins
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The three nucleotide bases
at the bottom of the
molecule make up the
anticodon.
There is an amino acid
binding site at the top of the
molecule.
The symbols D, Ψ and T
represent unusual
nucleotides that are
characteristic of tRNAs.
Base pairing only occurs in
certain regions.
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Translation – mRNA into Proteins
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The anticodon of the t RNA is complementary to
the codon of the mRNA.
The type of amino acid picked up by tRNA is
related to the sequence of the anticodon.
For example, a tRNA molecule with the anticodon
ACG will pick up a cysteine amino acid on its
binding site by means of an enzyme.
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Translation – mRNA into Proteins
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The ribosome bonds to the methylated cap on the mRNA
and moves along it ‘scanning’ for an AUG start.
The ribosome passes along the mRNA strand and, as it
passes each codon in the mRNA, a tRNA, carrying the
appropriate amino acid, moves to the ribosome. The codon
in the mRNA bonds to the complementary anticodon in the
tRNA molecule.
The ribosome moves to the next codon of the mRNA
strand, another tRNA molecule with a corresponding
anticodon brings another amino acid into position, and so
on.
Once the job of the tRNAs is complete, they detach
themselves from the mRNA and return to the pool of tRNAs
in the cytoplasm, from where they can be drawn upon
again when required.
The amino acids are linked to form a polypeptide chain in
an order corresponding to the sequence of base triplets in
the mRNA, and therefore the DNA.
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Translation – mRNA into Proteins
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Meanwhile other ribosomes are carrying on the
same process, moving along the mRNA strand
simultaneously, each synthesising a polypeptide
chain as it goes.
(see Animation “Polyribosome”)
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Translation – mRNA into Proteins
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Summary
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The Genetic Code
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The genetic code shows the relationship between the
triplets of bases in mRNA (i.e. the codons) and the amino
acids that are translated from the mRNA code.
From this, it is possible to work out the relationship
between the bases in the original DNA and the amino acids
that result.
Most of the amino acids are coded for by more than one
codon. Thus, the code contains more information than is
actually acted on in the cell (the code is redundant).
Three of the codons do not actually code for an amino
acid, but they stop the polypeptide chain at that point,
acting as termination signals.
These stop codons play an essential role in the cell,
allowing polypeptides of precisely the right length to be
produced.
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The Genetic Code
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Also see SRAM 193
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Interactive site:
http://learn.genetics.utah.edu/units/basics/transcribe/
Dustbin game:
http://www.classtools.net/my/dustbin89919TESTER.htm
Whole Process:
http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect14.htm
SRAMs:
 “Translation”
 “Review of Protein Synthesis”
 “Genes to Proteins”
 “Analysing a DNA Sample”
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X
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Gene Regulation in E.coli
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E.coli can break down glucose or lactose to obtain
energy.
E.coli can turn lactose metabolism on or off by
turning the genes that code for the enzymes on or
off.
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The lac operon consists of an operator (the binding
site for the repressor protein), a promoter (binding
site for RNA polymerase) and three genes that code
for enzymes involved in lactose breakdown.
A repressor protein regulates the three genes by
binding to the operator and inhibiting transcription.
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Lactose is a signal molecule or inducer.
If lactose is present, it binds to the repressor,
altering its shape so that it cannot bind to the
operator.
The RNA polymerase is able to transcribe the
three genes which are then translated into the
three enzymes.
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If lactose is absent, or low, the repressor protein is
able to bind to the operator, covering part of the
promoter.
This means that RNA polymerase cannot bind to
the promoter and transcription of the genes is
blocked..
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Gene Regulation in Eukaryotes
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Gene regulation is far more complicated in
eukaryotes than in prokaryotes, particularly when
multicellular eukaryotes are considered.
For instance, enhancers are regions found in
eukaryotic DNA that act as binding sites for some
activator proteins.
They seem to act by increasing the number of RNA
polymerase molecules transcribing the associated
gene.
Chemical modification also seems to exert control
over gene expression.
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Watch video clip “Genetic switch” >>
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VARIATION
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Variation in genetic material can result from:
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Independent Assortment of chromosomes during
meiosis (metaphase 1)
Crossing over during meiosis (metaphase 1)
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Random fertilisation of ova by sperm
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Mutations
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MUTATIONS
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Mutations are changes to the DNA by:
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Addition
Deletion
Rearrangement
Mutations can occur on individual genes or
on chromosomes
Mutations can occur in somatic cells or in
germ-line (sex) cells
SRAM 259, 2012: “The Effect of Mutations”
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Gene Mutations
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Most gene mutations occur during DNA replication
and are usually repaired by enzymes, but
sometimes they are not.
When a single DNA nucleotide base changes, it is
referred to as a point mutation.
Some changes can involve more than one
nucleotide.
The changes usually have an effect on protein
synthesis.
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Gene Mutations
Substitution
 a nucleotide is replaced by another type (e.g.
adenine substituting for guanine)
 This has a number of possible effects:
• The new codon still codes for the same amino acid as
the original codon (e.g. GAG and GAT both specify for
the addition of a leucine amino acid in a polypeptide
chain). This is an example of a neutral (or silent) point
mutation.
• The new codon codes for a different amino acid, but a
functional protein is still produced, although it is slightly
different.
 The new codon codes for a different amino acid but the
resulting polypeptide is non-functional. The new codon
may even specify a codon for ‘stop’, which would shorten
the length of the whole protein, making it non-functional.
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Gene Mutations
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INSERTION and DELETION
The addition or deletion of one or two nucleotides
in a gene sequence normally has a major effect on
the polypeptide produced because it is a frameshift
mutation.
Every codon after the insertion or deletion changes
because nucleotides are read in groups of three.
This is illustrated by the addition of the letter S
before the word ‘PIE’ in the sentence:
KIM ATE THE SPI EFO RTE A
or the deletion of P from PIE:
KIM ATE THE IEF ORT EA.
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Gene Mutations
SRAMs:
 243, 2013 “Gene Mutations”
 244, 2013 “Sickle Cell Mutations”
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Chromosome Mutations
Chromosome mutation may result from:
 Deletion (sections of a chromosome are missing
and therefore some genes are missing)
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Chromosome Mutations
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Inversion (part of the chromosome breaks off,
rotates 180˚ and rejoins).
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Chromosome Mutations
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Translocation (part of one chromosome breaks off
and joins onto another non-homologous
chromosome)
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Chromosome Mutations
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Duplication (part of a chromosome is duplicated
and added onto the same chromosome)
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Chromosome Mutations
Aneuploidy
 The chromosome number is more or less than that
in the normal diploid or haploid cell.
 Results in the addition or loss of whole
chromosomes from a cell.
 Normally in meiosis, homologous chromosomes
come together and then segregate into separate
cells, so that the gametes finish up with only one of
each pair of chromosomes. However, on some
occasions the two homologous chromosomes,
instead of separating, go into the same cell.
 This phenomenon is known as non-disjunction.
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If the gametes with both copies fertilise a normal cell, the
zygote will have three copies of the chromosome.
This is called Trisomy.
Trisomy 21 results in Down Syndrome.
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Chromosome Mutations
Abnormalities in sex chromosomes
 Non-disjunction also causes various sex chromosome
abnormalities in humans.
 XXY – Klinefelter syndrome. This may result either from the
fusion of a Y sperm with an XX egg or from the fusion of an
XY sperm with an X egg. Although XXY individuals are
phenotypically men, they have very small genitals and are
infertile; in addition, they may develop breasts, but
testosterone therapy at puberty can often help alleviate the
symptoms.
 Turner syndrome is due to the absence of one of the sex
chromosomes.
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OY individuals never survive to birth.
XO individuals are infertile females.
XYY - Jacob syndrome males tend to be taller than average
and may be mildly mentally retarded.
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Chromosome Mutations
Polyploidy:
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When cell division fails altogether, half the gametes have
two of each type of chromosome (i.e. being diploid) and the
rest having none.
If a diploid gamete fuses with a normal haploid gamete, the
resulting individual is triploid – (it has three of each type of
chromosome).
If two diploid gametes fuse, a tetraploid individual results.
Polyploidy occurs when an organism has one or more
complete extra sets of chromosomes.
Polyploidy is rare in animals but common in plants.
Polyploidy is lethal in humans.
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Chromosome Mutations
Seedless grapes (and other fruit) are
the result of polyploidy and are
infertile.
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Chromosome Mutations
SRAMs:
 246, 2013 “Chromosome Mutations”
 248, 2013 “Non-disjunction in Meiosis”
 249, 2013 “Aneuploidy in Humans”
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Chromosome Mutations
Sites on Mutations:
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http://www.biozone.com.au/biolinks/GENETICS.html#D4
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The Selfish Gene
“We are survival machines - robot vehicles
blindly programmed to preserve the selfish
molecules known as genes. This is a truth
that still fills me with astonishment.”
The Selfish Gene
Richard Dawkins 1941- English biologist
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Heredity
“Heredity provides for the modification of
its own machinery.”
James Mark Baldwin, 1896
GEENOR's comment: now we have genetic engineering,
which can do the same.
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