Download - DNA for Grade 12
DNA – The Molecule of DNA – The Molecule of LifeLife
Hidden in all life is a Hidden in all life is a unique code unique code
This code is in the This code is in the form of a chemical, form of a chemical,
called DNA called DNA DDeoxyriboeoxyriboNNucleic ucleic AAcidcid
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Chromosome Chromosome uncoiled to uncoiled to show DNAshow DNA
Genes are Genes are pieces of pieces of DNA that DNA that code for a code for a
characteristicharacteristic e.g. Eye c e.g. Eye
colourcolour
ChromosomeChromosomes in the cell s in the cell
nucleusnucleus
One One chromosomechromosome
s s enlargedenlarged
Where is DNA found in the cell?Where is DNA found in the cell?22
DNA – The Molecule of LifeDNA – The Molecule of Life
What does DNA look like?What does DNA look like?
A photo of DNA from an electron A photo of DNA from an electron microscopemicroscope
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DNA – The Molecule of LifeDNA – The Molecule of Life
We can’t see too much from this but if We can’t see too much from this but if we look at the molecules that make up we look at the molecules that make up
DNA we see a a truly amazing molecule – DNA we see a a truly amazing molecule – one that contains the hidden code of all one that contains the hidden code of all
life on earthlife on earth
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DNA – The Molecule of LifeDNA – The Molecule of Life
What is DNA made up of ?What is DNA made up of ?
What results if a DNA molecule like What results if a DNA molecule like this is broken down?this is broken down?
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DNA – The Molecule of LifeDNA – The Molecule of Life
DNA is found to be made of three DNA is found to be made of three types of molecules:types of molecules:
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DNA – The Molecule of LifeDNA – The Molecule of Life
SugarSugar
This is a special This is a special type of sugar type of sugar
calledcalled deoxyribosedeoxyribose
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DNA – The Molecule of LifeDNA – The Molecule of Life
PhosphatePhosphate
Similar to the chemicals Similar to the chemicals used as fertilisersused as fertilisers to grow to grow
plantsplants
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DNA – The Molecule of LifeDNA – The Molecule of Life
BaseBase
DOOM
Similar to the Similar to the chemicals used in chemicals used in
insecticides – insecticides – NITROGENOUS BASENITROGENOUS BASE
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DNA – The Molecule of LifeDNA – The Molecule of Life
SugarSugar
PhosphatePhosphate
Nitrogenous BaseNitrogenous Base
SUMMARY: DNA is made of SUMMARY: DNA is made of three types of moleculesthree types of molecules
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THE NUCLEUS• Most plant and animal cells have a well-defined nucleus
surrounded by a double, permeable nuclear membrane made up of proteins and phospholipids
• Ribosomes occur on the outer membrane and these are concerned with protein synthesis.
• The nucleoplasm is the matrix, in which various substances are suspended.
• There may be one or more nueleoli [singular = nueleolus] in the nucleus. These contain proteins and nucleic acids found within the nucleus and they manufacture RNA and are concerned with protein synthesis.
• Chromosomes are long threads made of DNA and some protein. They are the carriers of GENES which are responsible for our hereditary traits. When a cell Is not dividing the chromosomes are not visible but are in a very tangled mass, the chromatin network
FUNCTIONS OF THE NUCLEUS
• Controls metabolic activities of the cell by dictating the type of proteins to be manufactured.
• Carries hereditary traits and allows the transmission of these from parent to offspring.
During cell division, the chromatin is visible as chromosomes. Chromosomes are the control
centres of the nucleus
• A DNA profile of a person can be made from these sources!
• Police and forensic scientists use this information in criminal investigations!
• This is known as DNA fingerprinting. No two people except identical twins have the same DNA
DNA STRUCTURE
• DNA [deoxyribose nucleic acid] is known as the key to life. It is the hereditary material found in the chromosomes as well as other parts of the cell e.g. mitochondria.
• It is a very long giant molecule made up of smaller units called NUCLEOTIDES. Watson & Crick were awarded the Nobel Prize for working out its structure [although a South African woman was just pipped at the post (1953-1962)].
• Each side is made up of alternating SUGARS [deoxyribose] and PHOSPHATES.
• Forming the 'rungs' are PAIRED NITROGENOUS BASES;
• ADENINE pairs with THYMINE A T• GUANINE pairs with CTYOSINE G C• These pairs are linked by weak hydrogen bonds.
• ONE sugar + ONE PHOSPHATE + ONE BASE = A NUCLEOTIDE.
• Many nucleotides = one gene• Thousands of genes may occur on one DNA (chromosome)
strand.
• The DNA is twisted into a double helix
These three These three molecules are molecules are bonded in a bonded in a
special way to special way to form the units of form the units of
the DNA the DNA molecule calledmolecule called NUCLEOTIDESNUCLEOTIDES
A SINGLE A SINGLE NUCLEOTIDENUCLEOTIDE
Consists of a Consists of a SugarSugar
and a base and a base joined to the joined to the
sidesidea Phosphatea Phosphatejoined tojoined to
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In the DNA In the DNA molecule, molecule,
nucleotides link nucleotides link together to form together to form long chains calledlong chains called
polynucleotide polynucleotide chains. chains.
(poly=many)(poly=many)
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The sugar and The sugar and phosphates are the phosphates are the
same throughout the same throughout the DNA molecule but …DNA molecule but …
There are four There are four different bases.different bases.
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The four bases are either The four bases are either single or doubled ringedsingle or doubled ringed
A A singlesingle ringed ringed base base
A A doubledouble ringed ringed basebase
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There are two types of There are two types of singlesingle ringed ringed basesbases
ThymineThymine
CytosineCytosine
TT
CC
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AA
GuanineGuanine
AdenineAdenine
GG
There are two types of There are two types of doubledouble ringed ringed basesbases
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Four different nucleotides Four different nucleotides therefore occur in the DNA therefore occur in the DNA
molecule molecule
A
GC
T
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The The polynucleotidpolynucleotide chain now e chain now with the four with the four
different different basesbases
A
G
C
T
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The DNA The DNA molecule molecule
consists of consists of TWO chainsTWO chains
The sugar The sugar and and
phosphates phosphates form the form the
backbone backbone
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It is It is arranged in arranged in the opposite the opposite direction to direction to
the first the first chainchain
What do you What do you notice about notice about the second the second
chain?chain?
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?
But how are But how are the bases the bases
arranged to arranged to link the two link the two
chains?chains?
?
?
?
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When bases combine ..When bases combine ..
always links always links to a one to a one
ringed baseringed base
A two ringed A two ringed basebase
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AA
GG
TT
CC
AdenineAdenine ThymineThymine
GuanineGuanine CytosineCytosine
Adenine always joins to Thymine
Guanine always joins to Cytosine
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A
G
C
T
C
A
T
G
Now look at how the
bases link with each
other to join the two
chains of the DNA molecule
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DNADNA
UNIT 3 UNIT 3 DNA ReplicationDNA Replication
• The DNA molecule has many
nucleotides•To see this clearly we need to
simplify a nucleotide
A
SugaSugarr
PhosphatePhosphate
BaseBase
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C
C
G
T
A
A
A
A
T
T
T
GG
G
C
A
T
T
T
T
A
A
A
C
Try to determine
which nucleotide will link to
the form the second DNA
chainWhich Which
nucleotide will nucleotide will link to A ?link to A ?
Remember A joins to T and
G to C
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As the bases link to each other, the
DNA molecule is twisted
into a double helix
Two Sugar-Two Sugar-Phosphate Phosphate backbonesbackbones
Inter-Inter-linkinlinkin
g g basesbases
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DNA DNA replicationreplication
How DNA is copiedHow DNA is copied
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When a cell divides, the
DNA needs to be copied so
that each new cell has the
same DNA as the parent cell
DNA replicationDNA replication55
An enzyme An enzyme breaks the breaks the hydrogen hydrogen
bondsbondslinking the linking the two chainstwo chains
The two chains The two chains separateseparate
New strandsNew strands form form
complimentary complimentary to parent strandsto parent strands
DNA replicationDNA replication66
C
C
G
T
A
A
A
A
T
T
T
GG
G
C
A
T
T
T
T
A
A
A
C
A closer view ofA closer view of DNA replication DNA replication looking at partlooking at part
of a strandof a strand
An enzyme unzipsAn enzyme unzips the DNA moleculethe DNA molecule
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C
C
G
T
A
A
A
A
T
T
T
GG
G
C
A
T
T
T
T
A
A
A
C
DNA DNA StrandsStrandsseparateseparate
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C
C
G
T
A
A
A
A
T
T
T
GG
G
C
A
T
T
T
T
A
A
A
CC
C
G
T
A
A
A
A
T
T
T
GG
G
C
A
T
T
T
T
A
A
A
C
FreeFreeNucleotidesNucleotides
link to link to form form newnew
chainschains and andtwotwo
identical identical DNADNA
moleculesmolecules
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DNA REPLICATION• The weak hydrogen bonds holding the
complementary base pairs together break as a result of enzyme action.
• Two single strands now result.• Each single strand picks up free nucleotides from the
nucleotide pool in the nucleoplasm and becomes double again.
• Since Thymine can only combine with Adenine, and Guanine with Cytosine, the two new double strands are identical to each other as well as to the original strand.
The drawing below summarises the functions of the nucleotides: D.N.A R.N.A D.N.A. mR.N.A. tR.N.A.
Proteins
Transcription
Translation