DNA.pdf

DNA DNA (or deoxyribonucleic acid) is the molecule that carries the genetic information in all living things. It belongs to a class of molecules called the nucleic acids, which are polymers of nucleotide units. Each nucleotide consists of three components:  a phosphate molecule  a five-carbon...

DNA DNA (or deoxyribonucleic acid) is the molecule that carries the genetic information in all living things. It belongs to a class of molecules called the nucleic acids, which are polymers of nucleotide units. Each nucleotide consists of three components:  a phosphate molecule  a five-carbon sugar molecule (deoxyribose in the case of DNA)  a nitrogenous base, one of: cytosine (C), guanine (G), adenine (A) or thymine (T) DNA is the molecule that holds all the information about every living creature. It acts like the plans of a house – it’s a code that decides the shape and functions of every living thing as it grows. Whereas a blueprint is a set of instructions for building a house, DNA is a set of instructions for building an organism. It provides all the details about what should be included, the order in which these parts should be assembled and how they should work together. DNA is often called the "blueprint of life". Like the blueprint for building a complex skyscraper, the DNA of an organism contains all the details about what should be included, the order in which these parts should be assembled and how they should work together. However, DNA is more than just a set of instructions that is used once then thrown away. The DNA blueprint remains in every living cell and continues to provide information about how the cell must function. It does this by ensuring that only the proteins that are required for the particular cell type are produced. Each type of cell in an adult organism contains a unique collection of proteins that causes the physical characteristics we can observe. The blueprints for making these proteins are found in the nucleus of each cell in the form of DNA. Muscles tissue cells look and function completely differently from bone tissue cells – and we say that these cells are specialised – this is due to the specific set of proteins produced in each cell type. So even though all cells in an organism contain the same genetic information, these genes used (expressed) by the cell may be different. While we can work out what a lot of the DNA code is for – for example, whether it holds the design instructions for an eye or a leg – there are vast amounts of it inside plants, insects, animals, and us, that we just can’t decipher. Scientists are arguing about what the extra DNA is for. Some say that it is “junk”, with no real purpose. Others say that it is another code – our genome’s equivalent of a computer's high-level operating system. Recently, it seemed we had an answer when a worldwide project looked at every one of the three billion letters of DNA that make up the human genome. The results showed that 80% of our DNA code is “functional”. Sometime, somewhere, one cell or another in the body is reading almost every bit of the genome. But even then, some scientists noted that just because the DNA code is being read, that doesn’t mean it's useful. The problem for scientists is that they can’t check for sure what this “junk” DNA does by taking it out of people. But recently scientists discovered a new pond plant that may give us a better idea of which side of the argument is right. 1 The plant, the bladderwort, has almost no “junk” DNA – every piece has a purpose that we can work out. And the plant does just fine. So our “junk” DNA has no purpose, too? Not necessarily. Perhaps the extra DNA in more complex plants and animals has helped them evolve their advanced traits. DNA and the production of proteins DNA carries the genetic information in the cells of all living organisms. It contains codes for the assembly of amino acids into all the proteins required in the body Structure and function of deoxyribonucleic acid (DNA) Deoxyribonucleic acid (DNA) is the information-carrying molecule found in all living organisms. In most animal, plant and fungal cells DNA is stored in the nucleus coiled up in thread-like structures called chromosomes. The nucleus of one of your skin cells contains about two metres of DNA, so a chromosome is a very large molecule compacted into a very small space. Ref:Bing images The information DNA contains is the instructions that the cell uses to make proteins. Proteins play a big part in determining the characteristics of specialised cells and whole organisms. Your eye colour, muscle mass, height and even your ability to learn new skills all result from the activity of specific proteins. The building blocks of DNA are called nucleotides. There are four different types of nucleotide. The part of a nucleotide that can make it different from others is called the base. The four DNA bases are adenine, thymine, cytosine and guanine. These base names are abbreviated to A, T, C and G. 2 Nucleotides are linked together to form strands. DNA consists of two strands of nucleotides twisted around each other to form a shape called a double helix. The two strands are held together by weak bonds between pairs of bases. Only certain pairs of bases have complementary shapes that let them form bonds (The two DNA strands are held together by interchain hydrogen bonds. H-bonds pair the bases in one chain to the complementary bases in the other chain. (A/T and G/C) with each other to make the double helix. Base A bonds with base T and base G bonds with base C. These are complementary. DNA is long and shaped in a twisted ladder kind of way. Ref: Bing images Each DNA molecule consists of two complementary strands that twist around each other to form a double-stranded helix. The nitrogenous bases link across the two strands very specifically such that cytosine (C) on one strand only base pairs with guanine (G) on the other and adenine (A) on one strand only base pairs with thymine (T) on the other. One single strand in the DNA molecule is called a chromosome and that chromosome generally has many genes in it. These genes are sections of the DNA that code for certain proteins. 3 Genes are the functional units of genetic information in cells.A gene consists of a specific sequence of nucleotides which codes for a specific protein. A human being has 20,000 to 25,000 protein-coding genes located on 46 chromosomes (23 pairs). These genes along with all the in-between bits of DNAare known, collectively, as the human genome. Every sequence of three bases on a DNA molecule is called a codon, and those codons contain codes for specific amino acids. Those amino acids then code for different proteins with in turn make up a gene. The sequence of bases in each gene provides instructions for building proteins. Proteins are molecules. They are often very large, made up of many thousands of atoms, and they are critical to all living organisms. They make up most of the tissue in the organisms' bodies and help control all of the chemical reactions that occur in the organisms. When the machinery in a cell reads the bases that make up a gene it interprets them in groups of three. Each group of three is called a codon. Codons "code" for amino acids – the building blocks of proteins. So, to summarise:  genes are made up of sequences of bases  bases are grouped into codons  codons code for amino acids  amino acids make up proteins. Genes and Chromosomes DNA in chromosomes found in the nucleus carries a permanent copy of all the genetic information available to a cell. Specific sequences of DNA nucleotides, called genes, are located along the chromosomes. These genes carry the instructions for making the proteins that make an organisms function. We say that genes code for proteins. NOTE: Bacterial cells (prokaryotes) do not have a nucleus- their DNA is in the cytoplasm. Eukaryotic cell Genetic information and the genetic code The sequence of bases in a DNA molecule can determine the order of amino acids in a protein molecule. Groups of three bases called triplets represent different amino acids. 4 This is the basis of the genetic code. A sequence of bases (genetic information) on DNA that codes for a protein is called a gene. Messengery RNA (mRNA) In animal, plant and fungal cells, the instructions for making proteins and the structures where proteins are made are found in two different locations. DNA is stored in the nucleus, while proteins are assembled from free amino acids in the cytoplasm in structures called ribosomes. A chemical called messenger RNA (mRNA) is made in the nucleus and carries a copy of the DNA base sequence of a specific gene to the cytoplasm. Ribosomes attach to the mRNA and the instructions it carries are used to assemble amino acids in the correct order to make a specific protein. mRNA is the temporary information messenger molecule that carries genetic information from DNA in the nucleus to the part of the cell in the cytoplasm that makes the proteins. Like DNA, RNA(or ribonucleic acid) is a type of nucleic acid and is a polymer of nucleotides. 5  Its nucleotides, however, are different from those of DNA. RNA contains the sugar ribose (instead of deoxyribose) and uracil (instead of thymine) as one of its nitrogenous bases. It is also shorter and single-stranded. One type of RNA, called messenger ribonucleic acid, or mRNA for short, is a short-lived molecule that exists to carry a small portion of genetic information from the chromosomal DNA to the part of the cells which manufactures proteins. In contrast, DNA has a long life span, is much larger and carries the instructions for making all possible proteins needed by the cell. Inside the cell in the CYTOPLASM, Ribosome produces Proteins Organisms are made up of proteins that are, in turn, made up of amino acids. The amino acids needed for protein synthesis by each organism is encoded in their DNA. Using the processes of transcription and translation, you can, theoretically start with a strand of DNA and calculate the amino acid chains for which an organism is coded. 6 Summary CELL In animal, plant and fungal cells, the instructions for making proteins and the structures where proteins are made are found in two different locations. DNA is stored in the nucleus, while proteins are assembled from free amino acids in the cytoplasm in structures called ribosomes. DNA TO RNA 7 A chemical called messenger RNA (mRNA) is made in the nucleus and carries a copy of the DNA base sequence of a specific gene to the cytoplasm. Ribosomes attach to the mRNA and the instructions it carries are used to assemble amino acids in the correct order to make a specific protein. PRODUCTION OF PROTEIN: RNA combines with Amino Acids in the cytoplasm of the cell via the Ribosome to produce proteins organised in just the right shape in accordance with the DNA genetic code. The sequence of bases in a DNA molecule can determine the order of amino acids in a protein molecule. 8 These proteins (derived from RNA transcription and translation using amino acids) form living cells which form tissues and which, in turn, form organs which result in the living organism e.g. human beings CHECKPOINT  The above text mentions that humans have 46 chromosomes. How many chromosomes come from the mother? From the father? (23 chromosomes come from each parent.)  Every time a cell divides, what do you think happens to the chromosomes? (They replicate and each new cell gets a copy of the same 46 chromosomes.)  The DNA of chromosomes is described as a double helix. What does that mean? (There are two strands that wrap around each other in a helical fashion, held together by bases.)  What is each organ made of? (Each organ is made of cells.)  Are the cells identical to one another in their DNA? (Yes.)  Are the cells identical to one another in their function? (No.) 9   You may have learned that half of the genetic information of a human being comes from the mother and half from the father. What is this genetic information? (It is DNA.)  How does half the DNA come from the mother? (It comes from the egg.)  How does the other half come from the father? (It comes from the sperm.) When the egg and sperm unite, they form one embryonic cell that contains a complete set of DNA.  How does that first one embryonic cell become two or three or four and so on? (It begins to divide.)  Does the DNA also divide or does it get duplicated in all the new cells? (The DNA duplicates before the cell divides. The daughter cell receives the copy from the parent cell.) * 10 MUTATIONS Your DNA directly affects how your body is built and functions, and small changes in the information it contains–mutations–can have a big impact. Often these mutations occur because the processes used to copy DNA are imperfect. Very occasionally the misspellings originate in your own body, but most often they are inherited from parents, who in turn likely inherited them from their parents. The mutations can alter the construction of proteins vital to our bodies, with significant harmful results. On the other hand, many mutations have no noticeable effects at all. BRCA2 is a very well-studied gene because it has a mutation that increases a woman's chance of breast cancer five to 10 times. In fact, the gene's name stands for Breast Cancer 2. But recently scientists found a new mutation on the same gene. It also increases the risk of breast cancer but is dangerous in a different way as well – it significantly increases the risk of lung cancer in smokers. Which goes to show how two small tweaks in the same gene can carry two very different – and dangerous – ways. 11 POINT MUTATION: There are many different types of mutations, but some involve a change to just a single base. These are called point mutations.  Substitution: substitution is when one base is replaced with another, different one.  Insertion: insertion is when a new base is added to the sequence of bases already there.  Deletion: deletion is when one base is removed from the sequence. For your tests 1. What is the largest structure inside the cell? The nucleus is the largest structure. 2. Why is the nucleus an important cell structure? It stores the information and instructions for a living organism to grow and live. 3. How many chromosomes are found within the human cell nucleus? There are 46 chromosomes in the human cell nucleus. 4. What is the relationship between chromosomes and DNA? DNA is the molecule that makes up the larger chromosome structures. 5. What do the initials DNA stand for? They stand for Deoxyribonucleic acid. 6. What type of bond exists between the bases? Hydrogen bonds exist between the bases. 7. What is the backbone of the double helix? Sugar phosphate is the backbone. 8. What are the four letters of bases and how are they paired? The four letters are A, T, C, and G. A is paired with T and C is paired with G. 9. What do the base letters stand for? A is for adenine, T is for thymine, C is for cytosine, and G is for guanine. 10. What are genes? They are a “sentence” of letters in a DNA strand that tell the cell to make proteins. 11. What do proteins do? They enable the cell to perform specific functions. 12