o level chapter 20 molecular genetics
TRANSCRIPT
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.
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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.
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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
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.
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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
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.