Sec 3 BiologyChapter 20

Learning outcomes 1 - 4Outline the relationship between DNA, genes and

chromosomesState that DNA is made up of nucleotidesState that nucleotides consist of bases, sugars and

phosphate groupsState the rule of complimentary base pairing

Who discovered the structure of DNA?Watson and Crick were awarded the Nobel Prize in 1962 for discovering the double helix structure of DNA, but work was started long before by others like Rosalind Franklin.

20.1 What is DNA?A molecule carrying genetic

information

A small segment of DNA makes a gene

consists of two parallel strands twisted together to form a double helix

A DNA molecule is wrapped around proteins to form a chromatin thread.

During cell division, chromatin threads coil tightly into structures called chromosomes inside the cell nucleus.

proteins called histones

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Basic Unit of DNA: nucleotide

P O

O

OCH2

H

OH

H HH

H

OH

HO

phosphate

base

sugar

C

N CH

NC

C

N

HC

N

NH2

Any one of these 4 bases + 1 sugar + 1 phosphate =

1 nucleotide

Basic units of DNA

Nucleotides can join together to form long chains called polynucleotides

Each gene is made up of a sequence of nucleotides and the varying sequence results in different genes. For a gene made up of n nucleotides, there are 4n different combinations of nucleotides (why?)

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The Watson-Crick Model of DNA Structure

Double Helix

DNA is a double helix, so two strands of polynucleotides must come together. The rule of base pairing states that adenine will bind to thymine (A-T) while guanine will bind to cytosine (G-C). A and T are known as complementary bases, and so are G and C.

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Nitrogenous Base Pairing in DNA

N H O CH3

N

N

O

N

N

N

N H

Sugar

Sugar

Adenine (A) Thymine (T)

N

N

N

N

Sugar

O H N

H

NH

N OH

H

N

Sugar

Guanine (G) Cytosine (C)

H

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Purine:-Adenine-Guanine

Pyrimidine:-Thymine-Cytosine

Nitrogenous Base Pairing in DNA

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Nitrogenous Base Pairing in DNA

Learning outcomes 5 - 7State that DNA is used to carry the genetic code,

which is used to synthesize polypeptidesState that each gene is a sequence of nucleotides on a

DNA moleculeState that each gene controls the production of one

specific polypeptide

20.2 GenesA gene is a small segment of DNA which controls the formation of a

single protein.Each gene stores a message that determines how a protein should be

made in the cell.Each protein then determines a certain characteristic in your body.

Structure of a geneOnly one of the polynucleotide chains determines the protein to be

made. This is known as the template.

Every three nucleotides in the template code for one amino acid. This is known as the triplet code or codon.

Stop and recall: Many amino acids make up a polypeptide, and one or more polypeptides form a protein.

A gene carries the message for making a polypeptide. If a protein consists of many polypeptides, many genes will contribute to the making of this protein.

Synthesis of proteins can be broken down into two steps – transcription and translation

The Central Dogma of Molecular Genetics The information in

genes flows from DNA to RNA to polypeptides

DNA RNA → →polypeptide

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The DNA on your 23 pairs of chromosomes is the blueprint for life, while the over 10,000 different proteins in your cells determine traits and functions.

Gene expression Transcription

The synthesis of RNA (mRNA) under the direction of DNA (template strand)

Translation The process through which nucleotide sequence on

mRNA is translated into amino acid sequence of a polypeptide.

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Transcription and Translation

1. The message in the DNA template has to be copied into a mRNA molecule first.

2. The mRNA carries the message to the cytoplasm, where a ribosome translates the message into a protein.

DNA vs RNADNA

Deoxyribonucleic acidRNA

Ribonucleic acid

Bases: A T G C Bases: A U(uracil) G C

Large and insoluble Small and soluble

Permanent molecule in the nucleus

Temporary molecule made only when needed

Sugar unit is deoxyribose Sugar unit is ribose

First, the gene unzips.

1 part of a gene

Transcription and Translation

template

mRNA molecule is made

One of the strands in the gene is used as the template to make mRNA. This is transcription. The mRNA molecule copies the genetic code in the DNA template, following the rule of base pairing.

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Note that mRNA does not contain T (thymine). A (adenine) in DNA pairs with U (uracil) in mRNA.

Transcription and Translation

mRNA molecule is made

ribosome

mRNA

nuclear envelope

The mRNA leaves the nucleus and attaches to a ribosome in the cytoplasm.

2

nuclear pore

Transcription and Translation

The Genetic code 43 = 64 codons

20 amino acids

Redundancy, but no ambiguity

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Fill in the blanks!During protein synthesis, one strand of DNA is used

as a _________ for the synthesis of ______. This process is called __________. The mRNA moves from the ________ to the _________, where it attaches itself to a _________ for the process of __________. During this process, the ________ moves along the mRNA. In this way, __________ are linked to form a polypeptide.

Fill in the blanks!During protein synthesis, one strand of DNA is used

as a _________ for the synthesis of ______. This process is called __________. The mRNA moves from the ________ to the _________, where it attaches itself to a _________ for the process of __________. During this process, the ________ moves along the mRNA. In this way, __________ are linked to form a polypeptide.

Control of genesEach cell in the body contains a complete set of

genes, but many of them are switched off / not expressed, so they do not produce the corresponding protein.

E.g., the genes to make insulin are found in both the liver and islets of Langerhans cells in the pancreas, but liver cells do not produce insulin, so the insulin making genes in the liver cells are not expressed. The islets of Langerhans cells however can control when to produce insulin, so they can control when to switch on or off the insulin making gene.

In short, different cells express different genes.

When things go wrong - mutations

Genetic Basis of Sickle-Cell DiseaseGlu Val

Learning outcomes 8 – 10 Explain that genes may be transferred between cellsBriefly explain how a gene that controls human

insulin production can be inserted into bacterial DNA to produce human insulin

Outline process of large scale production of insulin using fermenters

20.3 Transferring genes between organismsGenetic engineering – a technique used to transfer genes from one organism to another.

A vector is a DNA molecule that is used to carry the genes of one organism into the other. Plasmids (circular DNA from bacteria) can be used to transfer genes a plasmid is an example of a vector

20.3 a) Inserting the human insulin gene into a bacteriaInsulin injections are needed to treat diabetics

who cannot control their blood glucose level.Insulin used to be harvested from the pancreas of

animals but prolonged treatment caused the patients to develop antibodies against the animal insulin and there was also the fear of transmitting disease from animal to human.

Inserting the human insulin gene into a bacteria will result in the bacteria expressing the human gene and insulin can be mass produced and harvested.

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insulin gene

• Obtain the human chromosome containing the insulin gene.

• Cut the gene using a restriction enzyme. This enzyme cuts the two ends of the gene to produce ‘sticky ends’.

• Each ‘sticky end’ is a single strand sequence of DNA bases. These bases can pair with complementary bases to form a double strand.

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cut by restriction enzyme

fragment of DNA containing the insulin gene

sticky end

How the human insulin gene is inserted into bacterial DNA

Genetic Engineering

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insulin gene

• Obtain a plasmid from a bacterium.

• Cut the plasmid with the same restriction enzyme. This produces complementary sticky ends.

2

cut by restriction enzyme

fragment of DNA containing the insulin gene

sticky end

plasmid

cut by same restriction enzyme

sticky ends

How the human insulin gene is inserted into bacterial DNA

Genetic Engineering

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insulin gene

• Mix the plasmid with the DNA fragment containing the insulin gene.

• Add the enzyme DNA ligase to join the insulin gene to the plasmid.

3

cut by restriction enzyme

fragment of DNA containing the insulin gene

sticky end

plasmid

cut by same restriction enzyme

sticky ends

insulin gene inserted into plasmid

How the human insulin gene is inserted into bacterial DNA

DNA ligase

Genetic Engineering

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Genetic Engineeringinsulin gene

• Mix the plasmid with E. coli bacteria.

• Apply temporary heat or electric shock. This opens up pores in the cell surface membrane of each bacterium for the plasmid to enter.

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cut by restriction enzyme

fragment of DNA containing the insulin gene

sticky end

plasmid

cut by same restriction enzyme

sticky ends

insulin gene inserted into plasmid

plasmid

bacterial DNA

plasmid enters the bacterium

trangenic bacterium

E. coli

bacterial DNA

How the human insulin gene is inserted into bacterial DNA

DNA ligase

The organism that receives a new gene is known as a transgenic organism.

Large-scale fermenters

As the transgenic bacteria multiples, it will use the new gene to produce insulin. These bacteria can be isolated and grown in fermenters for mass production of insulin.

*Insulin production by transgenic bacteria is not a fermentation process!

A fermenter is designed to keep the internal environment favourable (optimum pH, acidity, oxygen, temperature, nutrition) for the biological process occurring inside.

Do you think the production of insulin using transgenic bacteria requires an aerobic or anaerobic fermenter?

Characteristics of a fermenter1. Cooling system – Heat from bacteria growth is removed by

pumping water in through the base of a cooling jacket (recall the Liebig condenser)

2. Aeration system – Adequate aeration promotes growth and two devices can help. Sterile air is forced through the tiny holes of a sparger and the numerous bubbles dissolve in the nutrient broth. An impeller spreads the oxygen and nutrients out evenly and also ensures that bacteria does not clump together.

3. pH controller – A pH probe measures the pH of the broth and makes the adjustments accordingly.

4. Nutrients – The nutrient broth should contain a carbon and energy source e.g. glucose, a nitrogen source e.g. amino acids or nitrates and essential mineral salts.

The bacteria harvested from the broth are then burst open to release the insulin, which has to undergo purification before it can be administered,

20.3 b) Transferring foreign genes into plantsGenetic modification of plants generally aims to provide

transgenic plants with increased resistance to pests and pathogenincreased heat and drought toleranceincreased salt tolerancea better balance of proteins, carbohydrates, lipids,

vitamins and minerals, resulting in more nutritious crops

e.g. A weak solution of cyanamide kills weeds but damages tobacco plants too. A soil fungus, Myrothecium verrucaria, has a gene which produces cyanamide hydratase, an enzyme which converts cyanamide to urea, which is harmless to tobacco plants. When this gene is inserted into the tobacco plant, the plant becomes resistant to the herbicide and the urea is also a nitrogen source for the plant.

Just for your info

20.3 c) Transferring genes within the same speciesIncorporating resistant genes from wild species

into crop plants e.g. incorporating genes from wild species wheat into common wheat confers resistance to the Hessian fly, a major wheat pest.

Genes can also be transferred between people. Cystic fibrosis (bronchial tubes produce mucus) may be treated by replacing defective genes in the damaged airway cells with healthy genes – gene therapy

Selective breeding vs Genetic engineering

Factors to consider

Selective breeding Genetic engineering

Species of organisms

Only between closely related species

Genes can be transferred across non related / different species

Which genes are transferred?

Defective genes may be inherited along with healthy genes

Only the desired (beneficial) genes are transferred, so there is less chance of genetic defects

Speed?Efficiency?

Slow process involving breeding over generations, and may require large amounts of land.Less efficient – organisms grow slowly and may require more food

Targets individual cells which reproduce quickly in lab conditions

More efficient – e.g. transgenic salmon grows to harvesting size much faster than ordinary salmon

Learning outcome 10 Discuss the social and ethical implications of genetic

engineering using a named example

20.4 Genetic Engineering and Medical Biotechnology

While genetic engineering may seem highly beneficial, there are many social and ethical issues involved. Let’s look at the environmental, economic, health, social and ethical hazards…

20.4 Genetic Engineering and Medical Biotechnology

The ‘golden rice’ has had three genes added to its normal DNA content. Two come from daffodils and one from a bacterium. Together, these genes allow the rice to make beta-carotene, the chemical that makes carrots orange. More importantly, the beta-carotene is converted to vitamin A, which is essential for good eyesight, and could save children in very poor countries’ from going blind.

Bt (Bacillus thurigensis), is a bacterium that produces a toxin that can kill larvae. The Bt toxin has been sprayed onto plants so that larvae on the leaves get killed when the toxin digests their gut. In the 1980s. The gene for Bt toxin that killed the European corn borer was isolated and introduced into corn. The corn expressed the toxin and was able to kill the corn borer. Moreover, the toxin is harmless to humans, fish, wildlife and most insects. => Genetically engineered plants can be environmentally friendly by reducing pesticide use.

While genetic engineering can improve the quality of our lives, it can also potentially Disrupt the environmentAffect the economics of societyHarm human healthAffect the way an individual is looked upon in society

Issues of genetic engineering – 1. Environmental hazardsCrop plants have been genetically engineered to

produce insect toxins or be resistant to herbicides, resulting in

Loss of biodiversity from insect deaths (long term)Insects that feed on GM crops may adapt and develop

resistance to the toxins.Herbicide resistant plants and weeds could cross-breed

to create superweeds. Although sterile male plants could be created to prevent this, more problems are inadvertly created.

2. Economic hazardsThe company that first engineered the GM seed can

patent their seeds to prevent others from planting such seeds without their permission. Other biotechnology companies also cannot produce such seeds – competition from farmers and biotech companies is eliminated.

Some companies produce plants that produce non-germinating seeds. This terminator technology means that farmers have to spend money each year to buy new plants.

3. Health HazardsGenetic engineering could introduce allergens in food.

( Allergens cause a reaction from your immune system.) E.g. Lectin found in beans is an effective pest control against aphids, but lectin can be transferred to potatoes, and people allergic to lectin may unknowingly eat those GM potatoes.

Modifying a single gene in plants could alter metabolic processes within the plant and result in the production of toxins.

Genes that code for antibiotic resistance may be accidentally incorporated into bacteria, making antibiotics ineffective in treating these diseases.

People may deliberately create new combinations of genes to use in chemical or biological warfare.

4. Social and ethical hazardsIn gene therapy, a gene inserted in the body cells may find

its way into the gametes. Should there be a mutation, the offspring may be affected.

Genetic engineering is expensive. Only the rich can afford it.

Some religions are against genetic engineering as scientists are altering the natural genetic make-up of the organism.