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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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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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 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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SugarSugar

This is a special This is a special type of sugar type of sugar

calledcalled deoxyribosedeoxyribose

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PhosphatePhosphate

Similar to the chemicals Similar to the chemicals used as fertilisersused as fertilisers to grow to grow

plantsplants

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BaseBase

DOOM

Similar to the Similar to the chemicals used in chemicals used in

insecticides – insecticides – NITROGENOUS BASENITROGENOUS BASE

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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

Education

DNA for Grade 12