Discovering DNA The principles of genetics were discovered and described long before scientists knew anything about the molecules that make up genes. With the discovery of DNA, and continued investigation of its structure and function, we are learning more and more about genetics. Explore the timeline below to see some of the important discoveries that have led up to what we currently know about DNA. DNA Replication

Important characteristic of DNA’s structure is that base pairing allows it to be copied, or replicated. Each base on one strand of the molecule can form a hydrogen bond with only one type of base on the opposite strand. This means that each strand of the DNA double helix contains the coding needed to reconstruct the other half through base pairing. The two strands are described as complementary. Complementary: Making a pair or whole because each can be used to build the other strand. Before a cell divides, it copies its DNA in a process called replication Replication: The process of copying of DNA during interphase, before cell division occurs. During replication, the DNA molecule separates into two strands and then follows the rules of base pairing to build two new complementary strands. The nitrogenous bases on the original strands code for the arrangement of nucleotides in the new strands. For example, if the original strand contains guanine, then cytosine is added opposite it in the newly forming strand. DNA replication involves several different enzymes. Enzymes break the hydrogen bonds between the nitrogenous bases to separate the two strands of the DNA molecule. Then, an enzyme called DNA polymerase joins individual nucleotides together to form the new complementary strands of DNA. Lastly, the DNA polymerase "proofreads" each new strand to make sure that each DNA molecule is perfect, or a nearly perfect copy of the original molecule DNA Replication RNA and DNA DNA serves as the master copy of an organism's genes, but another nucleic acid, ribonucleic acid (or RNA), copies sections of the DNA molecule, and then carries the copies outside the nucleus. The genetic instructions provided by these copies direct the construction of protein molecules. Both DNA and RNA are nucleic acids made up of nucleotides, but there are three important differences between DNA and RNA: 1

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RNA is single-stranded, and DNA is double-stranded.

2. The sugar in RNA’s nucleotides is ribose instead of deoxyribose. 3. RNA contains the nitrogenous base uracil instead of the thymine in DNA. When constructing a new building, the valuable master plan remains in a safe location. Blueprints are less expensive copies of some or all of that master plan that can be taken to the construction site as a reference. In the same way, a cell’s DNA is the “master plan” that remains safely in the nucleus. RNA “blueprints” are copied from the DNA and taken to the ribosome “construction sites” where proteins are constructed. DNA Replication Flow of Genetic Information

Genes are instructions for making specific proteins, but they do not build the proteins directly. The flow of genetic information within a cell goes from DNA to RNA to protein. The two major steps in this passing of information are transcription and translation. In transcription Transcription: The synthesis of an RNA molecule from a DNA template, the base sequence within a gene is used to build an mRNA molecule. During translation Translation: The process in which the base sequences in mRNA are used to direct the arrangement of amino acids in protein synthesis. , the genetic code within the mRNA molecule is used to direct the assembly of amino acids into a protein.

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Transcription

Transcription is when a segment of DNA is used to produce a complementary mRNA molecule. This process uses an enzyme called RNA polymerase, similar in form and function to DNA polymerase. Because transcription makes the mRNA copies from DNA, it occurs inside the nucleus in a eukaryotic cell. 1. First, the RNA polymerase binds to a DNA molecule and separates its two strands. 2. One strand of the DNA serves as a template to assemble nucleotides together into a complementary mRNA strand. 3. Specific base sequences on the DNA molecule, called promoters Promoters: Specific base sequences on the DNA molecule that signal where the RNA polymerase should begin transcribing a section of mRNA, signal where the RNA polymerase should begin transcribing a section of mRNA. Other base sequences on the DNA then signal that the new mRNA molecule is complete, and transcription stops. Like in DNA base pairing, each base on the DNA strand has only one base it will pair with on the new mRNA strand. RNA contains the nitrogenous base uracil (U) instead of thymine. This means that wherever the DNA strand has adenine (A), the complementary mRNA strand will have uracil (U). Notice that RNA is a single strand; it does not exist as a double helix like DNA. The Genetic Code We keep saying that DNA and RNA contain information for building a protein molecule, but what is this genetic code and how is it interpreted? Transcription used base pairing to build a strand of complementary mRNA from a section of DNA. Translation will interpret the sequence of nitrogenous bases on the mRNA as instructions for assembling amino acids together to build a protein molecule. There are many different amino acids, 20 of which are commonly used to build proteins. The specific amino acids, and the order in which they are bonded together, determines the shape and properties of the protein. If there are only four different nitrogenous bases in a nucleic acid (DNA or RNA) molecule, how can those bases code for 20 different amino acids? The genetic code is read three bases at a time, where each set 3

of three bases corresponds to a specific amino acid. Each set of three nitrogenous bases in an mRNA molecule that codes for a specific amino acid is called a codonCodon: A group of three nitrogenous bases in mRNA that codes for a specific amino acid to be added to the protein molecule. With four different bases in RNA, there are 64 possible codons (4 × 4 × 4 = 64) in the genetic code. There are only 20 amino acids used in protein molecules, but some amino acids can be specified by more than one codon. There are also codons that signal where to start and stop the assembly of the protein molecule. For example, the methionine codon AUG serves as a start codon for protein synthesis. After a start codon, the base sequence is read three bases at a time until reaching a stop codon. At that point, the protein synthesis stops because the new protein molecule is complete. This chart lists all of the codons found on an mRNA molecule. Notice that some amino acids are specified by more than one codon. Also, notice that some are start or stop codons.

Translation After transcription, the new mRNA strand leaves the nucleus and enters the cytoplasm. The mRNA is then used in the process of translation, which occurs at the cell’s ribosomes. The process of translation uses the genetic code on the mRNA strand to direct the construction of a protein molecule. 1. A ribosome attaches to an mRNA molecule in the cell’s cytoplasm. 2. As each codon passes through the ribosome, tRNA molecules bring the corresponding amino acid into the ribosome. Each tRNA molecule has a set of three unpaired nitrogenous bases, called the anticodon. An anticodon sequence is complementary to a specific codon on the mRNA. The other end of the tRNA

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molecule can only carry one type of amino acid. In this way, tRNA translates the code in the mRNA molecule into a pattern of amino acids. 3.The ribosome and the rRNA molecules it contains attach the amino acids together as they are brought by the tRNA molecules. The protein chain continues to grow until the ribosome reaches a stop codon on the mRNA molecule. At that point translation is complete. The ribosome releases both the mRNA molecule and the newly formed protein molecule. DNA Replication

Summary DNA molecules contain coded instructions for the synthesis of protein molecules, but they are not directly involved in this process. This is why the various forms of RNA are so important. Transcription builds mRNA strands that are complementary to the base sequence of a section of the DNA molecule. Those mRNA strands are then able to leave the nucleus and provide instructions for protein synthesis at the cell’s ribosomes. The mRNA’s codons provide instructions for the arrangement of amino acids in the new protein molecule in a process called translation, with tRNA playing an important role in the process.

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