Chromosomes are found inside the nucleus of cells and consist of long strands of DNA. Each human cell has 46 chromosomes (23 pairs), apart from gametes (sex cells) which have only 23.
You can think of chromosomes as a set of books. Each book (chromosome) contains a set of instructions (genes). All of the books together contain all the instructions needed to make a certain organism (a living thing), for example a human or a flower.
There are always two copies of the same gene in an organism because chromosomes come in pairs. These two versions of the same gene are called alleles.
The two alleles for one characteristic do not have to be the same. They can be different. If both alleles for one gene are the same, they are homozygous (from homo = the same). If the alleles are different, they are heterozygous (from hetero = different).
For example, everyone has two alleles of the gene that decides eye colour. If someone has brown eyes and is homozygous for that gene, they will have two alleles for brown eyes. We can see this for individual B in the middle of the image..
A person could also have brown eyes and be heterozygous for that gene. They have one allele for brown eyes and another allele for blue eyes. We can see this for individual A to the left in the image. The reason that this person has brown eyes is that alleles can be either dominant or recessive.
The gene for brown eyes is dominant and we will always see this characteristic, no matter what other gene is present. The gene for blue eyes is recessive and we only see it when the dominant gene for brown eyes is not present. We can see this for individual C to the right in the image.
Characteristics that we see on the outside are called the phenotype. The characteristics in our genes are called the genotype. Phenotype and genotype can be slightly different. In our example we have seen that someone can have brown eyes as their phenotype, but both brown and blue eyes in their genotype.
- What are chromosomes and where are they found?
- What are alleles?
- What is meant by homozygous?
- What is meant by heterozygous?
- What is meant by dominant?
- What is meant by recessive?
- Why does a person have brown eyes even when they have genes for both brown and blue eyes?
- What is meant by phenotype?
- What is meant by genotype?
- What is the phenotype (= the colour seen on the outside) of a rose that is homozygous and has two dominant alleles for red colour?
- What is the phenotype of a rose that is homozygous and has two recessive alleles for white colour?
- What is the phenotype of a rose that is heterozygous and has one dominant allele for red colour and one recessive allele for white colour?
DNA is found inside the nucleus of each cell. One very long, coiled up molecule of DNA is called a chromosome. Human cells have 46 chromosomes in total.
Each DNA molecule contains two strands that are connected by a pair of substances called bases. It looks like a ladder, where the bases form the rungs. In addition, the ladder is wound and looks a bit like a spiral staircase. We call this wound-ladder structure of DNA a “double helix”.
This double helix structure of DNA was discovered by the British scientists James Watson and Francis Crick who received the Nobel Prize for their work in 1962.
There are four bases in DNA, adenine, thymine, cytosine and guanine, normally just called A, T, C and G. When forming pairs to make the rungs of the ladder, A always pairs with T and C with G. We call this complementary base pairs. The base pairs are held together by a weak attraction called hydrogen bonding.
Furthermore, each base is attached to a sugar which in turn bonds to a phosphate group. The sugars and phosphate form the backbone of the DNA strands.
A gene is one section of DNA that codes for one single characteristic or protein. We all have very small differences in our genes caused by slightly different orders of the bases in our DNA. This means that everyone’s DNA is unique. It allows scientists to match DNA from cells to specific people. For example, it helps scientists to find out how people are related or it can be used by forensic scientists to identify criminals.
- Where is DNA found?
- What is a chromosome? How many chromosomes do humans have?
- What does the structure of DNA look like? What do we call it?
- Who discovered the structure of DNA?
- Name the four bases in DNA.
- What is complimentary base pairing?
- What holds the base pairs together?
- What is a gene?
- Why is everyone’s DNA unique?
- How can the knowledge that everyone’s DNA is unique help scientists?
Image credit: Copyright by Kenneth Allen. CC BY-SA 2.0. The image shows stinging nettle on the left and dock leaf on the right.
It is the 2020 Corona virus lockdown and I am teaching online from home. Trying to keep things a bit interesting I am putting together small experiments to do at home. While thinking about possible experiments for the topic ”Acids and Alkalis”, I remember something I was taught myself in primary school. The sap of dock leaves is supposed to relieve the symptoms of nettle stings.
Me and many other children around the world were taught that stinging nettles sting because their poison contains acids. The sap of dock leaves is supposed to help because it is alkaline and neutralizes (cancels out) the nettle’s acid.
Perfect! I thought and was very excited to have found a great activity for the children. Finding stinging nettles and dock leaves and investigating their properties at home.
However, I did some further reading and quickly realized that I would not be able to use this activity.
The leaves of stinging nettles are covered in tiny hairs. When you brush against them their tip breaks of and they turn into tiny needles injecting the venom into your skin. It is true that the venom contains acids like formic acid oxalic acid and tartaric acid. Nevertheless, scientists argue that their concentrations are too low to cause any pain.
Today the bad guys of nettle stings are believed to be three compounds that are found in our own bodies as well. Serotonin, acetylcholine and histamine. Serotonin and acetylcholine are produced by our nervous system where they carry messages between nerve cells. But when injected directly into our skin, they cause irritation and pain. Histamine is probably the worst of the trio, causing inflammation and allergic reactions to the skin. The effect or nettle stings is most likely due to a nasty combination of all three. However, in some nettle species tartaric acid and oxalic acid are thought to at least contribute to a longer duration of the pain.
Now we know that the pain and itching of nettle stings is not really caused by acids at all. But what about the alkali part? Is dock leaf sap really alkaline? The answer is no. It has also been suggested that dock leaves may contain antihistamines to cancel out the effect of the histamines, but there is no evidence for this either.
The effect of dock leaves might simply by attributed to the sap cooling the irritated skin or a placebo effect. However, there is some evidence that dock leaves could contain a chemical that reduces the effect of serotonin in the nettle venom.
Image: Osmosis. The process plant roots use to take in water.
Plants use their roots to absorb (= take up) water and mineral ions from the soil. The water absorbed by plant roots is used for:
- Transporting dissolved mineral ions
- Keeping cells rigid (= stiff), so the plant stays upright
- Cooling the leaves (when the water evaporates from them)
Root hair cells
The outside of roots is covered with root hair cells. The “root hair” is an extension of the cell and provides a larger surface area, so that water and mineral ions can be absorbed faster. In addition, root hair cells have very thin cell walls to improve the flow of water into the cells.
Diffusion and osmosis
Diffusion is the movement of particles from a place of high concentration to a place of low concentration. We say that the particles move along a concentration gradient.
Osmosis is the diffusion of water. Water particles move from a place with a high water concentration (lots of water) to a place with a low water concentration (less water). This happens across a semi-permeable membrane which lets small molecules like water pass, but keeps back larger molecules like sugar. The semi-permeable membrane acts like a sieve.
Root hair cells take in water from the soil using osmosis. The water moves from the soil with lots of water into the root with less water. Cell wall and cell membrane act as the semi-permeable membrane.
Mineral ions are ionic compounds or salts that are found naturally in soil. Plants need these ions to build important substances like proteins.
The concentration of mineral ions inside the root is higher than outside. Mineral ions cannot diffuse from a lower concentration outside to a higher concentration inside the root. Therefore, the cell membrane needs to actively pump mineral ions inside. This process requires energy and is called active transport.
- What is the water absorbed by plant roots used for?
- What process do plants need carbon dioxide, water and sun light for?
- Give two adaptations (= special features) that root hair cells have to let water enter the cells faster.
- What is the difference between diffusion and osmosis?
- How does water enter root hair cells? What is the name of the process?
- What is a semi-permeable membrane?
- What are mineral ions?
- What is active transport?
- Why do mineral ions not diffuse into root hair cells?
Ebola, HIV, rabies and the Corona virus. They all are caused by germs that can spread between animals and humans, also called zoonotic diseases.
HIV most likely originated from chimpanzees that were hunted and eaten for meat. Similarly, Ebola is linked to the consumption of bush meat, especially bats.
The infamous Corona virus is thought to have jumped species first at a ”wet market” in Wuhan, China. Wet markets are traditional places that sell dead and live animals out in the open. These markets pose a good opportunity for a virus to jump species because hygiene standards are low and they are densely packed with people. However, the exact animal source of the Corona virus is still unknown. But bats are suspected to be involved here as well by infecting chicken, which was then consumed by humans. These winged mammals
Normally, it is not that easy for a virus to jump from one species to another. When an organism gets infected a virus hijacks its cells to make copies of itself. To enter a cell, the virus has a key-like structure on its surface that will only let it into the cells of one single species. However, during the copying process mistakes are made and mutations occur in the key-like structures. With some luck for the virus one of these mutations will enable it to enter the cells of another species, for example humans. The virus has jumped species. This process is easier if hygiene standards are low and places are densely packed.
Research by scientists from the universities of Bonn and Ulm (Germany) also suggests that the destruction of ecosystems like rain forests may enable infections to jump species more easily. The researchers looked at ecosystems in Panama comparing undisturbed rain forest, smaller rain forest islands in the Panama Canal and small islands of rain forest within in an agricultural landscape.
Biodiversity is reduced in these small rain forest islands compared to intact rainforests. For example, there are fewer species of bats and rats. Therefore, individuals of the same species live closer together and are less dispersed. The results of the German research team showed that this also made it easier for different kinds of virus to spread within the populations of the remaining rats and bats giving these germs larger reservoirs. This could in turn make it easier for a virus to jump species and infect humans.
It is quite astonishing that the destruction of ecosystems could indeed influence our health by increasing the risk of zoonotic diseases like Ebola, HIV and the Corona virus.
Image: D. Keeno, 17 November 2017.
How do muscles work?
Muscles work by getting shorter. We say that they contract.
Muscles are attached to bones by strong tendons. When a muscle contracts, it pulls on the bone, and the bone can move if it is part of a joint.
Muscles can only pull, but not push. This would be a problem if a joint was controlled by just one muscle. As soon as the muscle had contracted and pulled on a bone, that would be it, with no way to move the bone back again. The problem is solved by having muscles in pairs, called antagonistic pairs.
To move the bone back the second muscle of the antagonistic pair contracts. Examples of antagonistic pairs are biceps and triceps.
How does your forearm move?
The elbow joint lets our forearm move up or down. It is controlled by two muscles, the biceps on the front of the upper arm, and the triceps on the back of the upper arm.
The biceps and the triceps are an antagonistic pair.
- When the biceps muscle contracts, the forearm moves up.
- When the triceps muscle contracts, the forearm moves down.This solves the problem. To lift the forearm, the biceps contracts and the triceps relaxes. To lower the forearm again, the triceps contracts and the biceps relaxes.
1. Box how muscles work.
2. Circle what attaches muscles to bones.
3. Underline what happens when muscles contract.
4. Underline why muscles need to work in antagonistic pairs.
5. Describe in your own words how antagonistic pairs work.
6. Explain in your own words how your forearm moves.
The following text is written for pupils with low reading ages to help them study the topic of antibiotics and bacteria resistance.
Think back to the last time you cut yourself. Can you imagine that cut becoming infected with bacteria – so seriously infected that you would die?
Before the discovery of antibiotics, there was nothing anybody could do. Bacteria could kill 80 percent of people with infected wounds.
Who would have thought a mouldy plate would lead to this?
In 1928, Alexander Fleming, a doctor at London’s St. Mary’s Hospital, found that a mould on a discarded plate had antibacterial properties. This mould was ‘penicillin’. Penicillin is an antibiotic.
Antibiotics kill bacteria and slow down their growth. A bacterium consists of one single cell and antibiotics disturb their cell functions. Antibiotics do not work against viruses because a virus consists of a DNA fragment instead of a cell.
Human life expectancy increased rapidly by eight years when antibiotics were first introduced in the 1930s.
Within four years of penicillin being introduced onto the market, bacteria resistance was being reported. Bacteria resistance means that an antibiotic no longer kills the bacteria.
Today bacteria resistance against commonly used antibiotics is increasing rapidly around the world and a growing problem.
In this lab you will isolate starch from potatoes and investigate if different food samples contain starch. This is done with the help of Lugol’s solution (iodine/ potassium iodide solution).
Starch is an organic compounds that belongs to the carbohydrates. Carbohydrates are an energy storage for both plants and animals. Starch molecules are very long and the building blocks repeat themselves. They form long chains and belong to the so-called poly saccharides. The two building blocks of starch are amylose which forms spiral chains and amylopectin which forms branched chains. Both are built up from glucose rests which is why the chemical formula can generally be written as (C6H10O5)n. Starch can be found, e.g. in root crops and grains.
The presence of starch can be detected with with the help of Lugol’s solution which is a mixture of iodine and potassium iodide dissolve din water. Potassium iodide is added to increase iodine’s solubility in water. Iodine molecules (I2) are stored in the spiral chains of amylose when Lugol’s solution (brown solution due to the iodine) is added to the starch. This storage compound (”iodine starch”) causes a blue-black colour. A schematic of the compound is shown to the right in the figure above.
3 Materials and Chemicals
- Two Potatoes
- Different other foods: Flour, bacon, cheese, apples, pasta and rice are recommended
- Lugol’s solution (= iodine/potassium iodide solution)
– Preparation: Dissolve 10 g potassium iodide in 100 ml of distilled water. Then slowly add 5 g iodine crystals, while shaking. Finally, lter and store in a tightly stoppered brown glass bottle.
- Knife and Spoon
- Several Test Tubes with Gummy Plugs
- One 250 ml-Beaker and one 800 ml- or 1000 ml-Beaker
- Heating plate
- Linen cloth
- Two Bowls
4.1 The isolation of starch
First, the potatoes need to be cleaned and peeled. Thereafter, they are grated and put into a bowl. 500 ml of water are added and the mixture is stirred thoroughly with a spoon for at least five minutes. A linen cloth is placed in a funnel and the mixture is pressed through into a large beaker (800 ml or 1000 ml). A part of the grated potatoes, for example the cellulose, will stay behind in the linen cloth. The liquid in the large beaker needs to stand and rest for approximately ve minutes. Then more water is added, approximately 100 – 200 ml. The finely dispersed, solid starch particles will slowly settle at the bottom of the beaker. Afterwards, the
water is decanted (= poured off). Then 100-200 ml of water are added again to
the large beaker and decanted when the solid starch particles have settled on the
bottom a second time. This cleaning step is repeated until the starch particles have
a completely white colour. Afterwards, the starch is dried in a at bowl in air and
at room temperature.
4.2 The detection of starch
The foods are crushed in a mortar and and small amounts of each are put into their respective test tube. The test tubes are filled up to a third with water and shaken vigorously. In case not all the food particles are suspended, the test tubes are heated in a water bath (water bath = a 250 ml-beaker filled with water and the test tube inside is heated carefully on a heating plate, the test tube is afterwards cooled under owing, cold water). Then one drop of Lugol’s solution is added to each test tube and the test tubes are shaken with a gummy plug on top.
5 Questions for Discussion
1. What is observed macroscopically when iodine is built into starch molecules? What happens when no starch is present?
2. Which function does starch have?
3. In which foods do you expect to detect starch? In which foods should there be no starch?
4. Do the results in the table match your expectations? If not, why could they be different?
Image retrieved from: Petra Mischnick, Skolan för kemivetenskap , Kungliga Tekniska Högskolan, Stockholm, 11 January 2013 (https://www.kth.se/che/archive/arkiv/molnov-1.272910, 30 August 2017).
Plants are truly amazing. To produce energy they basically ingest sunlight, water and carbon dioxide. As a result energy is chemically stored in sugars like glucose. (Scientists call them carbohydrates.) We all have heard about this process in school, photosynthesis. The carbohydrates can be further modified by the plants into fats or proteins. Animals like humans rely on all three as food, but also on the oxygen produced as a byproduct. In fact, there was no oxygen in the Earth atmosphere before the first cyanobacteria invented photosynthesis. These cyanobacteria later evolved into the chloroplasts inside plant cells where photosynthesis takes place.
Photosynthesis is very effective in transforming the energy of sunlight into chemical energy in sugars without creating any toxic or polluting waste. For this reason scientists today are trying to artificially create photosynthesis. The goal of these systems is to produce hydrogen or other fuels for engines and electricity. Another advantage would be that carbon dioxide released by the use of fossil fuels could be ”mopped up” from the atmosphere by artificial photosynthesis.
The main difference between artificial and natural photosynthesis is that plants produce carbohydrates, fats and proteins while humans are looking for suitable fuels that can power airplanes or cars. These fuels should ideally resemble fossil fuels and thus enable the use of already existing combustion motors. For this reason, chemists are trying to create different end-products than plants while using the same energy source (sun) and building blocks (carbon dioxide and water).
Plants use their chlorophyll to capture the sunlight while a collection of enzymes and proteins uses this energy to split water molecules into hydrogen, electrons and oxygen. Hydrogen and electrons then form carbohydrates (sugars) with the carbon dioxide and oxygen is released.
For artificial photosynthesis, scientists are mainly interested in the first two steps above. Capturing sunlight is the easy part, as there are plenty of solar-power systems available. Splitting water, however, is trickier and the main challenge. Water is a very stable compound and catalysts are required to initiate the splitting reaction. Catalysts are materials that can accelerate chemical reactions, without being depleted in the process. A great amount of research is being carried out in order to find suitable catalysts for artificial photosynthesis. Among recently published very succesful catalysts were cobalt-based materials. Nevertheless, these systems still require more work and research in order to be optimized for commercialization.
Theoretically, the produced hydrogen could be directly used as a fuel. However, right now it is still more practical to transform hydrogen and carbon dioxide to fuels that closely resemble fossil fuels. This last step can be carried out either with the help of bacteria or other inorganic catalysts like copper. The conversion makes it possible to use the products of artificial photosynthesis in already existing car and airplane engines.
Maybe it will not take too long before we are able to drive cars with fuels directly produced with sunlight and carbon dioxide.