revision powerpoint b2 topic 1

Additional Science

Topic 1

Lesson 1: Animal or plant cells

Cell Membrane

Cytoplasm

Nucleus

Mitochondria

How can we tell if this is an animal or plant cell?

Cell Membrane

Cytoplasm

Cell wall

Mitochondria

Nucleus

Vacuole

Chloroplast

Structure Function

Cell membrane

Separates the contents of the cell from its surroundings. Controls the movement of substances like oxygen, glucose and carbon dioxide into and out of the cell.

Cytoplasm A liquid gel where many of the chemical reactions needed to carry out life processes take place. It also contains organelles (tiny structures that carry out specific jobs).

Nucleus Organelle that contains DNA, which is the genetic material. The nucleus controls all the activities of the cell.

Mitochondria Organelles in which respiration occurs (oxygen and glucose react to release the energy needed by the cell).

Cell wall Made of tough cellulose to support the plant cell and allow it to keep its shape.

Large vacuole

A space in the plant cell cytoplasm that is filled with cell sap and helps to support the plant by keeping the cells rigid.

Chloroplast Organelles found in some plant cells that contain chlorophyll, a green substance that absorbs energy from light for photosynthesis

Lesson 2 ‐ Bacteria

• Tail / whip used to move themselves along.• Protects the cell and maintains its shape. NOT made of cellulose as in plants, and

is more flexible.• Rings of extra genetic material. They can carry genes for things like antibiotic

resistance.• Inside the cell wall and is similar to that found in plants.• Giant loop of DNA containing most of the genetic material. Is not contained in a

nucleus.• Some bacteria have a slime layer that can protect from antibiotics.

Chromosomal DNA

Plasmid DNA

Cell membrane

Cell wall

Flagellum

Jobs of each structure.

Long, whip‐like structures that bacteria can use to move themselves along. (Flagellum)

Protects the cell and maintains its shape. Is not made out of cellulose as it is in plants, and is more flexible. (Cell wall)Rings of extra genetic material. They can carry genes for things like antibiotic resistance. (Plasmids)Inside the cell wall and is similar to that found in plants (cell membrane)Giant loop of DNA containing most of the genetic material. Is not contained in a nucleus. (chromosomal DNA)

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Explain the similarities and differences between an animal cell ,plant cells and bacterial cells? (LEVEL 7 / Grade A)

All three types of cell contain a cell membrane to allow substances in and out of the cell. Plant and animal cells have a nucleus but bacterial cells do not have a nucleus. Bacterial cells have 2 types of DNA, plant and animal cells only have one type. Both animal and plant cells need mitochondria. Bacterial cells do not have mitochondria.

Plant and bacterial cells have cell walls. Plant cells have large vacuoles and chloroplasts, animal and bacterial cells don’t. Some bacterial cells have a flagellum so they can move. Plant and animal cells (apart from sperm cells) don’t have this.

Microscope calculations

Objective lens Eyepiece lensTotal

magnification×4 ×10

×15 ×1500×20 ×200×50 ×750

Example 1

A specimen appears 1mm under a light microscope at a magnification of x1500. What is its actual length?

Actual length of object = length of mag objectthe magnification

= actual length of person = 1mm = 0.0007mm1500

Lesson 3: DNA Structure

Cells: All of our cells contain a nucleus. Nucleus –This contains the chromosomesChromosomes –Tightly coiled and can be

made up of millions of different genes –contain the genetic material

Genes – Made up of lengths of DNA and contain the instructions for making proteins. These proteins give you your features.

DNA – Made up of complimentary paired bases. Genes can contain millions of paired bases.

DNA stands for Deoxyribonucleic acid

DNA is a molecule that is two stranded and like a twisted ladder or spiral ( double helix) shape. The two strands are linked by 4 bases. Adenine always pairs with Thymine.Cytosine always pairs with GuanineThe bases are what joins the bases together? Weak hydrogen bondsThe bases are linked to the deoxyribosesugars.The sugars alternate with the phosphates to form the backbone of the molecule.

It is the order of the bases that gives the coded instructions to make proteins.

Lesson 4: Extracting DNAProcedure WhySquashed with fingers (not blender)

so that... you don’t destroy the DNA

Put in soapy water and salt

so that... salt (NaCl) puts holes in the nuclear and cell membranes so DNA can be released.The soapy solution also helps removes proteins.

Filter / sieve the mixture so that... This separates the cell walls from the DNA.

Adding protease enzyme so that... To remove protein.

Pour iced ethanol VERY SLOWLY AND CAREFULLY into the filtrate

so that... Add slowly to form two separate layers. The DNA will appear as a white precipitate once the alcohol is added

Lesson 5: DNA Discovery

• In Cambridge, Watson and Crick used stick‐and‐ball models to develop the possible structure of DNA. They used data from other scientists and it was based on theory.

• Other scientists used experiments. Rosalind Franklin and Maurice Wilkins, working in London, used X‐ray diffraction.

• When you shine X‐rays on DNA the invisible rays bounce off the sample and then create patterns on photographic film. By looking at the patterns, it is possible to figure out important clues about the structure.

• It was Rosalind Franklin's famous "photograph 51" that finally revealed the double helix structure of DNA to Watson and Crick in 1953. However, it was her colleague, Maurice Wilkins whom she didn’t really get on with, who showed her images to Watson and Crick without her permission. It was the detail in these images (such as the one overleaf) that gave them all the clues they needed to build their double helix model.

Can you think of any benefits of knowing the sequence of the human genome? Read

the information.

Human genome – the complete list of bases in order for a human being.

Think about possible benefits for science and medicine?

Did you get these? What are the ethical, cultural and socio‐economic issues of the following points?

• Discoveries leads to more discoveries – our knowledge and technology improves.

• May lead to improved testing for genetic disorders, to discover if people are carrying a faulty allele

• New ways of finding genes that may increase the risk of certain diseases –such as Alzheimer’s disease, heart disease

• May lead to new treatments and cures for disorder eg. gene therapy, where scientists try to replace or mend faulty genes that cause the disorder (eg cystic fibrosis).

• Can look at how the genome changes over time – this shows us how humans have evolved and the evolutionary relationships between different species

• Personalised medicines – medicines that work with a particular genotype and target diseases more effectively and with less side effects.

“Discoveries leads to more discoveries – our knowledge and technology improves.”

Can you think of any ethical issues surrounding this?

It costs a lot of money to keep doing all the research.

Improving knowledge and technology is supposed to help improve our lives – sometimes it doesn’t. (Has it made us greedy, lazy).

• May lead to improved testing for genetic disorders, to discover if people are carrying a faulty allele

• New ways of finding genes that may increase the risk of certain diseases – such as Alzheimer’s disease, heart disease

Ethical issues : Would you want to know? Would insurance companies charge you more money? Would employers discriminate against you?

• May lead to new treatments and cures for disorder eg. gene therapy, where scientists try to replace or mend faulty genes that cause the disorder (eg cystic fibrosis).

Ethical issues : How much will it cost? Will everyone get the treatment? Are there any dangers?

• Personalised medicines – medicines that work with a particular genotype and target diseases more effectively and with less side effects.

Ethical issues : Should we develop personalised medicines when there are people around the world still dying of diseases that can already be cured easily and cheaply? What about drugs needed for people with genes found mainly in developing countries, which don’t have much money to pay for drugs?

Lesson 6: Genetic engineering

What is genetic engineering?Removing a gene from one organism and inserting it into the DNA of another organism. This is then known as a genetically modified organism (GMO).

So… What is Genetic Engineering?

Removing a gene from one organism and inserting it into the DNA of another organism. This is then known as a genetically modified organism (GMO).

What example was shown?

ALL: Define what genetic engineering means. Illustrate the process of genetic engineering to produce insulin (Grade C)MOST: Examine the advantages and disadvantages of genetic engineering of insulin. (Grade B)SOME: Discuss the advantages and disadvantages of a number of genetic engineering uses , give your opinion and justify why you think that way. (Grade A‐A*)

Recap: Producing insulin with bacteria


Advantages of making human insulin this way.

• Insulin used to be extracted from dead cattle and pigs – vegans may be against this and some religions are against using pig products

• Supply of insulin not affected by animal diseases / number of animals slaughtered for meat.

• Using bacteria in fermenters, lots of insulin can be made quickly and cheaply.

• HOWEVER: There are slight differences and it does not suit everyone.

Using what you know and the following information try to work out how genetic engineering is used to make golden rice and

how it helps to reduce Vitamin A deficiency in humans.

• Wild rice is normally white.• Beta‐carotene is a pigment that gives many vegetables their colour.• Two genes carry the instructions for making beta‐carotene. These can

be removed from an organism (eg a carrot) and inserted into the rice plants.

• The rice plants then make beta carotene – golden rice. • Beta‐carotene is converted in the human body to vitamin A.• A lack of vitamin A in humans can cause blindness and also death as

the immune system does not work properly.


Can you summarise the process of genetic engineering in 6 points?

• 1. Identify the desired characteristic (eg making beta carotene)

• 2. Removal of the gene for this characteristic from the chromosome of an organism using restriction enzymes.

• 3. Insert this gene into bacterial plasmid for cloning.• 4. Insert the gene/s into the host organism ‐‐ plasmid sticks gene into the plant / animal chromosomes using ligase.

• 5. The organism now has the gene for the desired characteristic.

• 6. Replication of the genetically modified organisms.ALL: Define what genetic engineering means. Illustrate the process of genetic engineering to produce insulin (Grade C)MOST: Examine the advantages and disadvantages of genetic engineering of insulin. (Grade B)SOME: Discuss the advantages and disadvantages of a number of genetic engineering uses , give your opinion and justify why you think that way. (Grade A‐A*)

6 Table to show the advantages and disadvantages of GM golden rice.

Advantages of golden rice Disadvantages of golden rice

Contains beta-carotene, which is used in the body to produce vitamin A Could save millions of lives because no-one need have a lack of vitamin A in their diet any more Prevents blindness caused by a lack of vitamin A

The people who most need this source of vitamin A might not be able to afford to buy golden rice seed Might crossbreed with wild plants and contaminate their DNAThere are concerns that eating GM foods might harm people (but no valid evidence)

Advantages of Genetic Engineering

Disadvantages of Genetic Engineering

Genetically modified golden rice can help to reduce Vitamin A deficiency in humans.

There is no evidence that GM food harms people.

Could we genetically engineer crops to grow in places where there is little water (eg Africa) and solve the problem of food shortages.

You can make herbicide resistant crops so that a stronger dose can be used to kill weeds instead of using several smaller doses.

But... The beta‐carotene levels in GM rice is not enough to make a difference

Also..if the GM rice crossbreeds with the wild rice this will contaminate the wild rice’s DNA! (hybrids usually have infertile offspring!)

I think eating GM food might harm people. We don’tknow the long term effects.

Genetically modified Organisms (GMO’s) can be expensive and some do not produce fertile seeds. Farmers buy the seeds every year. Who will pay for this –countries with food shortages are usually very poor.

The weeds may develop resistance to the herbicide. Ormore weeds might be killed meaning less biodiversity –this leads to loss of food and shelter for animals.

FARMING: Weed growth forces crops to compete for sunlight and nutrients, often leading to losses. Because herbicides cannot differentiate between plants that are crops and plants that are weeds, farmers use 'selective' herbicides. Such herbicides do not harm the crop, but are not effective at removing all types of weeds. If farmers use herbicide resistant crops, 'non‐selective' herbicides can be used to remove all weeds in a single, quick application. This means less spraying, less traffic on the field, and lower operating costs.

HOWEVER: May lead to herbicide resistant weeds,Leads to decrease in biodiversity as fewer weeds survive – less

food and shelter for animals.

Can you summarise the process of genetic engineering in 6 points?

• 1. Identify the desired characteristic (eg making beta carotene)

• 2. Removal of the gene for this characteristic from the chromosome of an organism using restriction enzymes.

• 3. Insert this gene into bacterial plasmid for cloning.• 4. Insert the gene/s into the host organism ‐‐ plasmid sticks gene into the plant / animal chromosomes using ligase.

• 5. The organism now has the gene for the desired characteristic.

• 6. Replication of the genetically modified organisms.ALL: Define what genetic engineering means. Illustrate the process of genetic engineering to produce insulin (Grade C)MOST: Examine the advantages and disadvantages of genetic engineering of insulin. (Grade B)SOME: Discuss the advantages and disadvantages of a number of genetic engineering uses , give your opinion and justify why you think that way. (Grade A‐A*)

Advantages and disadvantages of Genetic engineeringAdvantages Disadvantages

Insulin used to be extracted from dead cattle (pigs etc) and some people couldn’t take it (vegans). The use of GMO’s to make insulin means more people can take it.

Insulin supply not affected by cattle disease or number of animals slaughtered for meat.

Lots of insulin can be made cheaply and quickly in fermenters.

Normal rice plants have gene inserted to make beta carotene. When eaten, humans make Vitamin A which prevents blindness and strengthens immunity.

Some genes are added to plants to make them resistant to herbicides (weedkiller). The farmer uses less herbicide which is cheaper.

Insulin produced by GMO’s is slightly different and does not suit everyone.

GM rice could crossbreed with wild rice and contaminate wild rice’s DNA.

Some people thing GMO’s are harmful, although there is no evidence of this. (Yet).

GMO’s can be expensive, and some do not produce fertile seeds and so farmers have to buy seeds every year.

Herbicide resistant weeds may develop.

Biodiversity reduced, weeds killed so less food and shelter for animals.

Lesson 7: Mitosis and meiosisWhy do cells divide by mitosis?

For growth ‐ organisms would only ever exist as single cells – fine for bacteria but not so good for plants and animals!

To replace old and damaged cells.

For asexual rproduction ‐ Eg. Bacteria reproduce by asexual reproduction (only one parent, no fertilisation)‐ they simply divide by mitosis.

What is fertilization?Fertilization is the stage of sexual reproduction when gametes fuse. This is the first step in the creation of a new life.

When an egg cell is fertilized, it becomes a zygote.

This zygote divides by mitosis many times and becomes an embryo.

The embryo continues to grow and develop into a foetus.

Mitosis steps

Diploid cell – 2 sets of chromosomes.

ALL: State the name of the process whereby body cells divide. Outline the sequence of mitosis and explain why cells need to divide. (Grade C)MOST: Discuss how cells divide in mitosis using most of the keywords and how it is involved with reproduction (Grade B)SOME: Define the words “haploid and diploid” and consider the difference between haploid and diploid cells and use these words in your explanation of mitosis (Grade A)


Each chromosome replicates – makes a copy of itself.

Each copied chromosome lines up on the equator.


The cell divides in two – each cell gets one copy of each chromosome.


What is meiosis?

Unlike mitosis, meiosis produces four unique daughter cells.

Why is it important to produce genetically unique gametes?

The number of chromosomes is halved in meiosis (Produces haploid cells with only 1 set of chromosomes)

Gametes are produced by a type of cell division called meiosis.

Mitosis Meiosisnumber of cells formed from one cell

2 4

number of sets of chromosomes in daughter cells

2 1

processes division is needed for

growth, repair, asexual reproduction and division of a zygote

producing gametes

are daughter cells identical or not?

Yes No

Haploid or diploid daughter cells?

Diploid Haploid

ALL: State the name of the process whereby gametes are produced. Outline the sequence of meiosis. (Grade C)MOST: Discuss how cells divide in meiosis using most of the keywords and how it is involved with reproduction (Grade B)SOME: Compare and contrast mitosis and meiosis. Explain fully how meiosis produces genetically unique daughter cells (Grade A)

MeiosisCell division that produces

gametes (sperm and egg cells)

Starts with a diploid cell –2 sets of chromosomes

23 chromosomes are from mum (one set)

23 chromosomes are from dad (the second set)

Altogether there are 46 chromosomes (23 pairs)


Each of the 46 chromosomes

replicate So each of these crosses is 2 chromosomes – the original and the replicate.


They line up side by side in the middle of the cell

ready for the first division


• Chromosomes line up in the middle • Each chromosome cross gets pulled apart into a new cell.

4 Egg cells with half the number of chromosomes to begin with (haploid)

• In a man, obviously, 4 sperm would be produced with half the number of chromosomes to begin with (haploid). All 4 daughter cells are genetically different, which gives rise to genetic variation.

• A haploid sperm cell will then fertilise a haploid egg cell to fuse the two sets of chromosomes together to make a diploid cell – a zygote. This then divides by mitosis to get bigger and bigger.

A fertilised egg – this will then divide by mitosis to get bigger.


What is a clone?

A clone is genetically identical to its “parent”

Cloning facts

Clones are genetically identical individuals.Produced by mitosis.Its an example of asexual reproduction. Asexual reproduction involves 1 parent cell dividing by mitosis to produce clones (egbacteria, some plants). No gametes are involved. Offspring are genetically identical.

1

2

3

4

5

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Cloning Process

Advantages of cloningUseful for cloning organisms with desirable characteristics eg bulls whose sperm produce high quality calves.

Cloning can also be used to clone organisms with a genetically engineered trait. Their offspring will then have this trait.

Can be used to clone organisms close to extinction.

Organisms that are slow to breed can be reproduced quickly.

Cloning of stem cells could in the future treat or cure diseases such as Parkinson’s disease, diabetes or even cure paralysis caused by nerve damage.

Disadvantages and Risks of cloning

• 6. CC (for “carbon copy”) the cat was born in 2001. She was the first household pet to be cloned.

• There are several stages in the cloning process.• Describe the stages in the process of cloning CC the cat. (CC was the clone that was born).

• ......................................................................................................................................• ......................................................................................................................................• ......................................................................................................................................• ......................................................................................................................................• ......................................................................................................................................• ......................................................................................................................................• ......................................................................................................................................• ......................................................................................................................................• ......................................................................................................................................• ......................................................................................................................................• (6)

Mark scheme

Body cell taken from the cat to be cloned. Nucleus removed.

Egg cell taken from surrogate cat. Egg cell enucleated – diploid nucleus from body cell from cat to be cloned transferred into the egg cell of surrogate (nuclear transfer).

Electric current used to stimulate the egg to start to divide into an embryo.

Embryo implanted into surrogate cat uterus and allowed to develop.

Clone born

Why use a surrogate?Why cant an animal to be cloned carry her own clone?

The genetic mother can carry her own clone if the embryo is implanted in her.

This is usually not done because the genetic donor is valuable and researchers want to collect additional eggs from her, so she can't be pregnant during this process.

Also if they produced multiple clones from her, each would need a separate birth mother. . The other reason could be if the genetic donor was male.

Advantages of cloning Disadvantages of cloning Risks of cloning

Cloning could help to save endangered species.

Could be used to create clones of the very best farm animals. These are worth a lot of money.

Genetically modified organisms (such as cows modified to produce milk with insulin) can also be cloned. (Like the bacteria with the insulin plasmid)

Could be used to make cells that could treat diseases such as Parkinson’s disease, diabetes or nerve damage.

Some people think that making clones like this is like 'playing God' and could produce dangerous organisms that cannot be controlled like natural ones.

Some people think that meat or milk from cloned animals is 'Frankenstein food', and is not as safe to eat or drink as food from normal animals.

Success rates for cloning animals are low. It took 237 attempts before Dolly was born. Only 29 embryos were created, 26 died during pregnancy and Dolly was the only lamb to survive beyond birth.

Dolly also seemed to grow older quicker, and died young. She suffered health problems. Coincidence or due to cloning?

Lesson 8 Cloning

A clone is an identical genetic copy of another organism. Eg an identical twin.

1. Diploid body cell removed from donor. Nucleus is removed from cell (enucleated).

2. Egg cell removed from surrogate. Nucleus from enucleated body cell inserted into egg cell.

3. Electric shock applied to egg cell to stimulate cell division.

4. The embryo is implanted into the uterus of the surrogate mother.

5. The embryo divided further, differentiates and develops in the uterus of the surrogate mother.

6. The clone is born – it is identical to the donor.

Modelled answer – What are stem cells?

• A stem cell is an unspecialised cell that can divide to make more stem cells or different kinds of specialised cells.

• How can they be used to create new tissue?• The stem cells will replicate / divide by mitosis and then specialise into a new type of cell (eg nerve cells.). Many of these cells working together make a tissue.

ALL: Explain where different types of stem cells are found (Grade C)MOST: Discuss the uses, advantages, disadvantages and risks associated with stem cell research and highlight

which relate to adult and embryonic stem cells. (Grade B)SOME: Evaluate the advantages, disadvantages and risks associated with the research associated with the adult

and embryonic research. Give your justified opinion. (Grade A‐A*)

Where do stem cells come from?

• Early embryo (embronic stem cells). • Cloned embryos.• Umbilical cord of baby.• Adult stem cells from differentiated tissue.• Reprogrammed adult body cells. *



and embryonic research. Give your justified opinion. (Grade A‐A*)

What are stem cells used for?Adult stem cell Embryonic stem cell

Benefits No ethical issues. If taken from the person who is being treated then no problem with rejection.

Easier to remove from tissue. Can produce a wide range of specialised cells.

Drawbacks Difficult to identify and remove from tissue (they are mixed with differentiated cells in the tissue). Can only produce a limited range of specialised cells.

Ethical issues with destroying embryo to get stem cells.

Risks There is a risk of rejection if the stem cells are taken from another person.

Differentiated cells may be rejected when used in treatment, as the cells come from a different person rather than the patient.



and embryonic research. Give your justified opinion.

Where and what are your genes?

The nucleus of the cell contains the chromosomes.

The chromosomes can be made up of thousands of different genes.

Each gene is a section of DNA which contains bases which give the instructions for making proteins. These proteins give you your characteristics or perform important functions in your body. Proteins are made up of amino acids.

What do all of these have in common?

• Haemoglobin in red blood cells• Enzymes (such as amylase in saliva – breaks down carbohydrates)

• Hormones (such as insulin)• Melanin (gives you skin, hair and eye colour)

Establish how the order of the DNA basesdecides the order of the amino acids in a protein. (Grade C)

Explain the process of transcription and translation. (Grade B)

Theorise how mutations occur. (Grade A‐A*)

They are all




A section of DNA is a gene. A gene contains the code formaking a specific protein.

Eg the protein that gives yourhair colour.

Remember proteins are made up of amino acids.




What are proteins made of?

Proteins are long molecules made of amino acids. There are 20 different amino acids.

protein molecule

amino acidWhat happens if amino acids are combined in a different order?

Different orders of amino acids make different proteins.




A gene is a section of DNA DNA is made up of base pairs

It is the order of the base pairs on one strand on the DNA that codes for the protein.

Each amino acid is coded for by three bases called a triplet.

The amino acids join together by peptide bonds to form a protein.

Establish how the order of the DNA bases decides the order of the amino acids in a protein. (Grade C)


Theorisehow mutations occur. (Grade A‐A*)

DNA mutations and proteins

A mutation is a change in the sequence of bases in DNA.

Will the mutated version of DNA make the same protein?

mutation




A problem for you!•Important information you need to produce an essay is in a reference book in the special section in the library.

You cannot take the library book out.

What should you do???




Solution!!!• Use the photocopy machine to copy the part of the book you need that contains the info for you to write your essay.

• You then take the info home and use it to write your essay.

• This is similar to how the DNA is copied, the instructions are taken somewhere else and the code interpreted to build a protein molecule.




Ribosomes –an organelle in the cytoplasm of the cell.




Any part of a cell that has a particular function is called an

ORGANELLE!




Ribosomes –an organelle in the cytoplasm of the cell.




Remember the DNA is in the nucleus of the cell – a copy of the DNAs instructions has to be taken to the ribosome in the cytoplasm – this is where the protein is assembled.

The DNA unzips – weak hydrogen bonds between the bases are broken and the strand separates.

Only one side of the strand is copied.




Watch this!!!

• http://www.youtube.com/watch?v=ztPkv7wc3yU&feature=related




Protein synthesis is in two stages: Transcription (copying the DNA strand) and translation (joining the amino acids together to make a protein).

Transcription happens in the nucleus.Translation happens at the ribosome. The copied DNA code needs to be taken out of the cell nucleus to the ribosomes in order to make a protein.

This is done by mRNA. Amino acids must be brought to the ribosomes so they can be built into a protein.

This is done by tRNA!!Establish how the order of the DNA basesdecides the order of the amino acids in a protein. (Grade C)



Protein synthesis

mRNA = messenger ribonucleic acid

It has a similar structure to DNA except:

• Is single stranded• Replaces Thymine with Uracil




Sequence the sentences to give a list of what happens during transcription.

5. An mRNA molecule forms alongside the coding strand2. The DNA double helix unzips by breaking the weak hydrogen bonds between bases and the strands separate.6. mRNA molecule breaks away from the DNA and leaves the nucleus through a nuclear pore to the ribosomes in the cytoplasm.1. A molecule of RNA binds to the DNA.4.Bases of the mRNA pair up with the corresponding bases on the coding strand of the DNA. U is used instead of T on the mRNA3. Bases on the coding strand of the DNA are exposed

7. DNA molecule ‘zips up’ again.




83

mRNA Transcript

•mRNA leaves the nucleus through its pores and goes to the ribosomes

copyright cmassengale




The mRNA arrives at the ribosome.

copyright cmassengale 84




The mRNA attaches to a ribosome at one end. tRNA brings the first corresponding amino acid.

Ribosome85

mRNA

tRNA

anticodon

Amino acid

At one end of the tRNA is an exposed triplet of bases (anti‐codon) which matches up with a particular codon

(triplet) on the mRNA.

86




At the other end of the tRNA is where the

amino acid attaches.





This first amino acid is now held in place by the tRNA, ready to start a chain.




The ribosome moves along the mRNA strand to read the next triplet code. Another tRNA molecule then brings the second corresponding amino acid. This tRNA has an anti‐codon

matching the next triplet on the mRNA.




A peptide bond now forms between the first amino acid

and the second.The first tRNA is now released to collect a new amino acid.

90




A third tRNA now arrives, carrying another amino acid. The anti‐codon

matches (is complementary) to the

next triplet on the mRNA. A peptide bond now forms between the second and

third amino acid.





This carries on until a stop triplet / codon is reached.





Eventually a long chain of tens or hundreds of amino acids is made. This called a polypeptide.





Mark it. In green pen add in the bits that you missed out. Re‐ grade your work.

• Transcription: RNA binds to the DNA which unzips, breaking the weak hydrogen bonds between bases. Bases on the coding strand of the DNA are exposed. Complimentary bases on the mRNA pair up with the corresponding bases on the coding strand of the DNA as the mRNA is formed.

U is used instead of T on the mRNA. When finished mRNA molecule breaks away from the DNA and leaves the nucleus through a nuclear pore to the ribosomes in the cytoplasm. DNA molecule ‘zips up’ again.3 bases are called a triplet and code for a specific amino acid.

Compare and contrast different proteins. (Grade C)

Discuss how the base sequence leads to proteins having a particular shape. (Grade B)

Justify why the shape of proteins is so important. Explain how mutations occur and how this can lead to protein shape and function being altered. (Grade A – A*)

Mark it. In green pen add in the bits that you missed out. Re‐grade your work.

Translation: The mRNA arrives and attaches to the ribosome. The ribosome moves along the mRNA and decodes the triplets. tRNA brings the first corresponding amino acid, which is now held in place by the tRNA, ready to start a chain. The ribosome moves along the mRNA strand to read the next triplet code. Another tRNA molecule then brings the second corresponding amino acid. This tRNA has anti‐codons matching the triplets on the mRNA. A peptide bond forms between the first amino acid and the second.The first tRNA is now released to collect a new amino acid. A third tRNA now arrives, carrying another amino acid. The anti‐codon matches (is complementary) to the next triplet on the mRNA. A peptide bond now forms between the second and third amino acid. This carries on until a stop triplet / codon is reached. This forms a chain of amino acids called a polypeptide. This can twist / fold / link with othere polypeptides to become a protein.

Compare and contrast different proteins. (Grade C)



A GradeProteins are made of a chain of amino acids. Each amino acid is coded for by

a triplet (codon) on the DNA. Each protein has its own specific number and sequence of amino acids.The number and order of amino acids gives the protein a particular shape,

which determines the proteins function. Some proteins are globular in shape and others are like long chains.

If there is an error in the base sequence of the triplet, this will code for a different amino acid.

This would mean the protein would be made with the wrong amino acid, wrong number or in the wrong order.

This gives the protein a different structure and consequently the red blood cell a different shape. This affects its function ‐ the red blood cell cannot carry oxygen as efficiently and travel around the blood vessels as well as it should do.

.Compare and contrast different proteins. (Grade C)



TemperatureAs temp increases the enzyme becomes more active and so the reaction rate increases – gets faster.

At the optimum temperature the enzyme is the most active and efficient at forming enzyme – substrate complexes.

Increasing the temperature will make the reaction slow down as the active site changes shape and the substrate can no longer fit into the active site. The enzyme is denatured and no longer works.

C GradeEnzymes are catalysts because the speed up they rate of chemical reactions, they are called biological catalysts because they do this in the body. Enzymes are made of proteins.B GradeEnzymes are catalysts because the speed up the rate of chemical reactions, they are called biological catalysts because they do this in the body. Enzymes are made of proteins and remain unchanged after they have speeded up the rate of reaction. They can build substances up or break them down.

Establish what an enzyme is by explaining why it is called a biological catalyst. (Grade C)

Compare the uses of enzymes inside and outside of cells. (Grade B)

Justify why enzymes are needed in the body by explaining the consequences if we didn’t have them. (Grade A)

A GradeAs B grade but also……………… if we didn’t have enzymes in the body then the chemical reactions would still occur but at a much slower rate and not quick enough for the body to function. Digestion would be too slow to provide glucose for respiration. Proteins would not be built quick enough.

This would eventually lead to death.







MODELLED ANSWER: In mitosis or meiosis the DNA replicates. One enzyme is needed to catalyse the splitting apart of the DNA (like unzipping) by separating the weak hydrogen bonds. New bases line up along each half so that complimentary bases match. A different enzyme joins the complimentary bases together. Two identical strands of DNA are formed. The enzyme is unchanged so can repeat this action again and again. This happens inside the cells.




MODELLED ANSWER: Enzymes are involved in protein synthesis. For example, the joining of peptide bonds is catalysed by a specific enzyme. Another enzyme releases tRNA so it can go and get another amino acid. This is happening in cytoplasm – so inside the cell.




MODELLED ANSWER: Different enzymes are released into the mouth, stomach and small intestine to help break up large insoluble food molecules into small soluble food molecules that can be absorbed through the small intestine wall into the blood.

This occurs outside of cells.

OUTSIDE OF CELLS: Mould is a fungus ‐ growing on the strawberries on the right. Fungi are saprophytic and absorb nutrients from decaying matter. Fungi release digestive enzymes onto the food. It breaks it down into small molecules which then get absorbed into the fungus.




Enzymes and wash powder

Biological Washpowders contain enzymes as they digest food and other large molecules that stain clothes.

Enzymes have an optimum temperature – they work best / most efficiently at this temperature.

If the temperature gets too high – enzymes are denatured. They change shape and no longer work. Establish what an enzyme is by explaining why it is called a biological catalyst. (Grade C)



Optimum Temp

Enzymes are denatured, substrate cannot fit into the active site, no reaction

Enzymes reaction slow, lack of heat energy therefore substrates do not collide and bind to active site.

ALL: Analyse graphs to describe the factors that affect enzymes. (Grade C)

MOST: Discuss why enzymes become denatured (Grade B)

SOME: Determine how the lock and key hypothesis explains the specificity of enzymes for their substrates (Grade A)

Enzymes work most efficiently at this temperature.

Enzymes work at an optimum pH.This is usually 7 in most cells.This can change depending on the job of the enzyme




Low rate of reaction as there is a low concentration of substrate, less chance that the substrate will bind to active site

Substrate concentration increases, more chance of collision with the enzyme

Substrate concentration increases but the enzymes cannot work any faster at this time. Increase in substrate concentration will not increase rate of reaction.




Factors that affect enzyme activity

TemperaturepHSubstrate concentration

Enzyme action• What kind of shape are enzymes? Enzymes are …a 3D shape.• What is the active site on an enzyme? Enzymes are highly specific for their substrate and will only work on that substrate. The active site is …site on the enzyme molecule that has a specific shape that the substrate fits into during the reaction.

• What happens to the active site when the pH or temperature changes too much? If the temperature of pH change too much then the …active site starts to change shape.

• What does this change lead to? This change leads to …the substrate not fitting as well. Too much change will break the bonds within the enzyme. This can change the shape of the active site so much that the active site is destroys and the enzyme is denatured.

• zWhat is the lock and key hypothesis? The lock and key hypothesis is …the model that refers to enzyme and substrate and how they fit together and how they work.

revision powerpoint b2 topic 1

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