Biology - B2 5.4 From Mendel to DNA


From Mendel to DNA

Until about 150 years ago people had no idea how information was passed from one generation to the next. Today we can identify people by the genetic information in their cells.

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Mendel's discoveries

Gregor Mendel was born in 1822 in Austrian Silesia. He was clever but poor, so he became a monk to get an education. 
He worked in the monastery gardens and became fascinated by the peas growing there. He carried out some breeding experiments using peas. He used smooth peas, wrinkled peas, green peas and yellow peas for his work. Mendel cross-bred the peas and counted the different offspring carefully. He found that characteristics were inherited in clear and predictable patterns. Mendel explained his results by suggesting there were separate units of inherited material. He realised that some characteristics were dominant over others and that they never mixed together. This was an amazing idea for time time.
Mendel kept records of everything he did, and analysed his results. This was almost unheard of in those days. Eventually in 1866 Mendel published his findings.
He had never seen chromosomes nor heard of genes. Yet he explained some of the basic laws of genetics using mathematical models in ways that we still use today.
Mendel was ahead of his time. As no one knew about genes or chromosomes people simply didn't understand his theories. He died 20 years later with his ideas still ignored - but convinced that he was right.
Sixteen years after Mendel's death, his work was finally recognised. By 1900, people had seen chromosomes through a microscope. Other scientists discovered Mendel's papers. 

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DNA - The molecule of inheritance

The work of Gregor Mendel was just the start of our understanding of inheritance. Today, we know that our features are inherited on genes carried on the chromosomes found in the nuclei of our cells. 
These chromosomes are made up of long molecules of a chemical known as DNA (deoxyribonucleic acid). This has a double helix structure. Your genes are small sections of DNA. The DNA carries the instructions to make the proteins that form most of your cell structures. These proteins also include the enzymes that control your cell chemistry. This is how the relationship between the genes and the whole organism builds up. The genes make up the chromosome in the nucleus of the cell. They control the proteins, which make up the different specialised cells that form tissues. These tissues then form organs and organ systems that make up the whole body. 

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The genetic code

The long strands of your DNA are made up of combinations of four different chemical bases. These are grouped into threes and each group of three codes for an amino acid.
Each gene is made up of hundreds or thousands of these bases. The order of the bases controls the order in which the amino acids are put together so that they make a particular protein for use in your body cells. Each gene codes for a particular combination of amino acids, which make a specific protein.
A change or mutation in a single group of bases can be enough to change or disrupt the whole protein structure and the way it works.

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DNA fingerprinting

Unless you have an identical twin, your DNA is unique to you. Other members of your family will have strong similarities in their DNA. However, each individual has their own unique pattern. Only identical twins have the same DNA. That's because they have both developed from the same original cell. 
The unique patterns in your DNA can be used to identify you. A technique known as 'DNA fingerprinting' can be applied to make the patterns known as DNA fingerprints
These patterns are more similar between people who are related than between total strangers. They can be produced from very tiny samples of DNA from bodily fluids such as blood, saliva and semen.
The likelihood of two identical samples coming from different people (apart from identical twins) is millions to one. As a result, DNA fingerprinting is very useful in solving crimes. It can also be used to find the biological father of a child when there is doubt.  

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