Replication, Transcription, and Translation

1. Replication, Transcription, and Translation

2. Review Questions • Who discovered DNA’s Structure? • How did they determine the structure of DNA? • What is the name of the photograph in which they looked at? • What is the structure/shape of DNA? • What are the 4 bases that make up DNA? • What bases pair with each other? • Draw a Nucleotide • What carbon does the phosphate group come off of the sugar molecule? • What carbon does the 5' end of one nucleotide bond with to form a sugar-phosphate backbone? • What carbon does the nitrogenous bases attach to on the sugar molecule?

3. Review Questions • Adenine bonds with guanine – True or False • Pyrimidines bonds only with Pyrimidines – True or False • If a DNA strand has the nucleotide sequence of CCGAGATTG, what is the nucleotide sequence of the complementary strand? • What is the 5-carbon sugar in DNA nucleotides called? • What holds the nucleotides together, A with T and C with G. • Which nitrogenous bases is in RNA that is not in DNA? • How many strands is RNA? • Why are the two strands that make up DNA considered to be antiparallel? • Molecules of DNA are composed of long chains of? • Structure of a DNA Molecule – Understanding 5’ to 3’ and 3’ to 5’

4. How are the structures of DNA and RNA related to their functions?Coding for Traits • Genetic information is encoded in the order of nucleotide bases in DNA. The order of bases in DNA is analogous to the order of letters in written text. Just as the English language uses 26 letters arranged in patterns to form words that convey meaning, DNA uses a 4-letter alphabet to carry genetic information. • DNA is a very long molecule that contains intermittent “coding” segments called genes. In general, each gene codes for a different protein. An organism’s DNA contains the genes to code for all of the proteins that the organism needs to produce structures and carry out life processes.

5. Genes are made up of sequences with three-base segments called codons. • Codon - Definition the portion of a DNA or mRNA molecule that encodes for a specific amino acid or marks the starting or stopping of protein production. • Key Context A codon consists of a series of three nucleotides in a segment of DNA or mRNA. The codons determine which amino acids will be assembled to form proteins. The codons also denote where protein building should begin and end. A sequence of three nitrogen bases that codes for an amino acid. Such a sequence is called a codon. Codons determine the sequence of amino acids in a protein. The term codon refers to a coding triplet on DNA or mRNA.

6. Different codons code for unique amino acids. However some amino acids may be coded for by more than one codon. For example the amino acid leucine is coded for by six different codons. Other codons signal the stop or start of the gene. Amino acids are the monomers that make up proteins. • There are 20 different amino acids. Proteins can contain hundreds of amino acids. • The order of amino acids in a protein determines the protein’s structure and function. • To make a protein the gene containing the DNA code for that protein must first be converted into an RNA code.

7. The genes you inherit from your parents determine not only your traits, but also how every structure in your body from your cells to your organ systems works. But DNA is not the end of the story—DNA just contains a code that needs to be rewritten and decoded to make all the structures in your body function. • Although DNA contains all your genetic information, this information needs to be processed and decoded to make proteins, which are what make you tick. These proteins are made in the cytoplasm on ribosomes, but DNA is in the nucleus of the cell.

8. How do cells make copies of DNA?Cell Division • Cells pass along their genetic information each time they divide. This ensures that every non-reproductive cell in an organism contains a complete copy of the organism’s genetic code. In asexual reproduction, an organism passes its complete genetic code to offspring. • In sexual reproduction, about half of an organism’s genes are passed to its offspring. In order to pass genetic information, the cell copies its DNA in the process of replication. • DNA replication is a biological process that occurs in every living organism.

9. Before Replication - DNA Helicase Happens First • Imagine for a minute that you are shopping and interested in a new pair of jeans. You want to try the jeans on, so you need to figure out how to get into them. The first thing you notice is the button. Opening the button gives a little room, but not enough for you to get into the jeans. You then pull down the zipper, which opens the jeans up even more, giving you the space you need to get into them. • This is very similar to the role that DNA helicase plays in DNA replication: DNA helicase is the zipper. It unzips our DNA strands to allow space for attachment and to expose the nucleotides that are used as a template during DNA replication and transcription. • Before replication can take place, an enzyme called helicase unwinds the DNA molecule from its tightly woven form. This opens up or "unzips" the double-stranded DNA to give two single strands of DNA that can be used as templates for replication.

10. DNA Replication • DNA replication begins when the enzyme helicase unwinds a segment of DNA and separates the two strands of the double helix, forming a structure called a replication fork. Each strand of the double helix serves as a template for duplicating the sequence of bases found in the original molecule of DNA. • Replication fork Y-shaped structure that forms during the process of DNA replication; the unseparated double stranded DNA represents the base of the Y; the separated single strands are the arms of the Y • Key Context During DNA replication, helicases “unwind the DNA”, separating the strands and forming a replication fork. Replication fork is a point where the two strands of DNA separate during the process of DNA replication. The leading strand replicates its complementary strand, followed by the lagging strand.

11. DNA Replication • During cell division, DNA replicates by breaking the hydrogen bonds that hold two nucleotide strands together. The enzyme DNA polymerase then adds complementary nucleotides to the exposed bases at the same time. • https://app.discoveryeducation.com/player/view/assetGuid/c25b5055-9093-4c50-8fb3-7bb62993d418 • Both strands of the DNA double helix act as templates for the new DNA strands. Incoming DNA is unraveled by the enzyme helicase, resulting in the 3' strand and the 5' strand. The 3' strands and the 5' strands are replicated by a DNA polymerase enzyme but in different ways. • https://app.discoveryeducation.com/player/view/assetGuid/4ba65452-a39d-46da-9711-8d7c46707a6a

12. DNA Replication - Polymerase • A different enzyme, DNA polymerase, catalyzes the synthesis of complementary strands of DNA by attaching the correct deoxyribonucleotide having the correct complementary base (A if the template reads T, G if the template reads C, etc.) to the new strand of DNA being made. • DNA polymerases are enzymes that synthesize DNA molecules from deoxyribonucleotides, the building blocks of DNA. These enzymes are essential to DNA replication and usually work in pairs to create two identical DNA strands from a single original DNA molecule. During this process, DNA polymerase "reads" the existing DNA strands to create two new strands that match the existing ones. • Every time a cell divides, DNA polymerases are required to help duplicate the cell's DNA, so that a copy of the original DNA molecule can be passed to each daughter cell. In this way, genetic information is passed down from generation to generation.

13. DNA Replication • The synthesis of new DNA proceeds differently on each complementary strand. DNA polymerase can add nucleotides only to a free 3' end of a growing segment, never to a 5' end, so DNA synthesis can occur in the 5' to 3' direction only. As a result, only one strand—the leading strand—can be continuously assembled. Synthesis of the other strand—the lagging strand—occurs in segments. The enzyme DNA ligase joins these segments, called Okazaki fragments, by their sugar-phosphate backbones to form a continuous strand of DNA. • DNA Ligase - DNA ligase is used in both DNA repair and DNA replication. It is a specific type of enzyme that facilitates the joining of DNA strands together. • Okazaki fragments - are short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication

14. Applying DNA To you • Hand out

15. How DNA Relates To You - GMO’S • What Is a Genetically Modified Food? • Genetically modified foods have been demonized in recent years by health advocates and environmentalists alike. If we look at the history of food cultivation, however, it is apparent we've been eating them all along. • https://www.scientificamerican.com/video/what-is-a-genetically-modified-food2013-07-24/# • Advantages of Genetically Modified Foods Genetically modified foods have been around for a long time and have shown to be beneficial. Why would people oppose eating them? • https://app.discoveryeducation.com/player/view/assetGuid/276ba1d9-9163-496b-a58e-657c6acfb96b

16. The Importance of Solving Crimes with DNA • In this episode of SciShow, we’re going to investigate a murder. But first, we’re going to have to learn all about forensics, the use of science in criminal law -- and the real-life version is a little different from what you might see on TV. • https://youtu.be/h3-Pj-zbEq8 • Genetics and Criminal Investigations: The Story of Sam Shepard DNA can be used to prove the guilt or innocence of a suspect. If DNA technology had been available at the time, it could have been used to unarguably exonerate murder suspect Sam Shepard. Comparing DNA fragments from a crime scene with those of suspects can be used to determine guilt or innocence. • https://app.discoveryeducation.com/player/view/assetGuid/4e2bf820-ad0e-4c42-8237-70614e0db99b

17. Using DNA in Modern Medicine • Recombinant DNARecombinant DNA is one of the core techniques of genetic engineering. It is the process of removing DNA from one organism and inserting it into the DNA of another organism, giving it new traits. Recombinant DNA can be used to make crops resistant to pests or disease, it can be used to make livestock leaner or larger. In medicine, the technique can be used to develop drugs, vaccines, and to reproduce important human hormones and proteins. By engineering human DNA into a host organism, that organism can be turned into a factory for important medical products. Insulin production is an excellent example of the recombinant DNA process. Host organisms can range from bacteria like E. coli, to plants, to animals. • Restriction Enzymes - Unravels the catalyst that clips DNA strands for use in genetic modification. The production of insulin is used as an example for the use of restriction enzymes. • https://app.discoveryeducation.com/player/view/assetGuid/1f1b2afa-ad17-4ea8-bcbe-a09854644fbd

18. Genetically Engineered Pharmaceuticals • insulin for diabetics • factor VIII for males suffering from hemophilia A • factor IX for hemophilia B • human growth hormone (GH) • erythropoietin (EPO) for treating anemia • three types of interferons - fight viral infections • several interleukins • granulocyte-macrophage colony-stimulating factor (GM-CSF) for stimulating the bone marrow after a bonemarrow transplant • tissue plasminogen activator (TPA) for dissolving blood clots • adenosine deaminase (ADA) for treating some forms of • severe combined immunodeficiency (SCID) • angiostatin and endostatin for trials as anti-cancer drugs • parathyroid hormone

19. Review • What are the components of DNA and RNA? • Nucleic acids are biological molecules that contain specialized components. • Both DNA and RNA are polymers of nucleotide monomers that are each made of a sugar (Deoxyribose for DNA; ribose for RNA) attached to a phosphate and one of four bases (A, G, C, or T in DNA; A, G, C, or U in RNA). • Two strands of nucleotides join to form a DNA molecule. The molecule has a double helix shape. • RNA contains a single strand that can form folded structures that can bond with other molecules. • In chromosomes DNA is coiled around proteins.

20. Review • How are the structures of DNA and RNA related to their functions? • The structures of nucleic acids relate to their functions of carrying and transmitting genetic information. • The order of the nitrogenous bases in a nucleotide determines the genetic traits that are transferred by a strand of DNA. • DNA is a long molecule composed of sections. These sections are called genes. Genes carry coding information for proteins. These proteins determine specific traits in an organism. This information is copied to form an mRNA strand. • These single strands of mRNA carry the information to the ribosomes. • Transfer RNA (tRNA) carries amino acids to the ribosomes and attaches them in the order coded on the mRNA molecule. These chains of amino acids form a protein.

21. Review • How do cells make copies of DNA? • The process of replication duplicates DNA before cell division occurs. • Enzymes separate the two strands of the DNA double helix. • Other enzymes synthesize complementary nucleotides on the old strand. • Two new double-stranded DNA molecules are formed, each identical to the original. • DNA – Practice Worksheet

22. How does the information in DNA get its information to the ribosomes to make proteins? • The molecule RNA • One type of RNA carries the information supplied by the DNA to the ribosomes. At the ribosomes, another type of RNA uses this information to assemble proteins from amino acids. These proteins then function as enzymes that enable your body processes. • The process of copying the DNA code is called transcription. • The process of translating this code into proteins is called translation. • Without these processes, no life would exist.

23. Have you ever shared a secret? If so, you might have sent messages to a friend using a secret code. To create the code, you may have changed around the letters of the alphabet to form new words or eliminated letters altogether and just used special symbols instead. DNA has a sequence of nucleotide bases akin to that secret code. • How much DNA is needed to code for an entire human being? In fact, each of your cells has enough genetic material in it that if the DNA molecule was laid end to end, it would be taller than you are! If you typed out all those bases on printer paper, how high would that stack of paper be? • The DNA in One Cell How much DNA is there in just one cell in your body? Of that, how much of it actually works to code for proteins? https://app.discoveryeducation.com/player/view/assetGuid/8143d1a4-dc3c-4d4b-9b61-7c446dc006ea

24. Do you like to look at old photographs? Perhaps you have a faded dog-eared photo of long-gone relatives, such as your great-great-great grandparents. It’s fun to think about the people who came before us, but if you go back far enough in time, you soon lose track of your ancestors. Most people cannot trace their ancestry further back than a few hundred years, and the further back you go, the more and more difficult it becomes to unravel your origins. However, this challenge has not deterred scientists from reaching back into deep time to the earliest life forms to ask the question: How did life begin? To address this question, researchers are investigating the molecular structure of DNA and RNA and how these molecules evolved into all forms of life that we observe around us today. • RNA’s Role in Creating Life RNA is one of the two primary nucleic acids found within the cell. What role does it currently play, and what is it believed to have done in primitive life on Earth? https://app.discoveryeducation.com/player/view/assetGuid/52bb26e4-1267-4614-aa1e-69a7df0928c9

25. DNA and RNA are both classified as nucleic acids, but they have some specific differences between them. Classify the following as either a characteristic of DNA or RNA • RNA • DNA • DNA • RNA • DNA • DNA • RNA • RNA • ribose • thymine • double helix • Uracil • blueprint for protein synthesis • Deoxyribose • Single strand • Molecule that carries the coded message for protein synthesis

26. What is transcription? • Transcription Definition the process of synthesizing RNA from a DNA template • Key Context The transcription of RNA from a DNA template occurs in the nucleus of eukaryotic cells. Transcription is the process in which DNA is unwound and one of its strands is used as a template to synthesize RNA. • The discovery of DNA’s structure and function was the first step toward understanding the information held within the molecule. The discovery of RNA, or ribonucleic acid, revealed the way in which DNA code is unraveled. • DNA and RNA are similar and different. Enzymes synthesize RNA from DNA. Think of RNA as a disposable copy of the original DNA molecule. RNA reads the DNA code and controls the production of proteins. Different types of RNA are responsible for various aspects of protein production.

27. Transcription - Protein Synthesis • Protein synthesis is a biological process that allows individual cells to build specific proteins. Both DNA (deoxyribonucleic acid)and RNA (ribonucleic acids) are involved in the process, which is initiated in the cell's nucleus. The actual process of protein synthesis takes place in the cell cytoplasm, and it occurs on multiple ribosomes simultaneously. RNA travels from the nucleus to the cytoplasm, carrying the instructions for forming proteins. The original DNA molecule stays inside the nucleus. • During protein synthesis, amino acids arrange in a linear fashion through an intricate interaction between ribosomal RNA, transfer RNA, messenger RNA and a variety of enzymes. The amino acids connect to each other in a specific order. This order is determined by nucleotide sequence in the DNA. • If protein synthesis fails to occur, cells have difficulty dividing, repairing themselves or contributing to the organism as a whole. It is crucial to repairing damage to organelles and adding new ones after the cells divide. An example is how the protein hemoglobin, which is essential for distributing oxygen throughout the body, only exists because it is made by the blood stem cells in red bone marrow. Without this interaction, the entire organism suffers. A living cell has the ability to synthesize hundreds of separate proteins per second.

28. Transcription – RNA Polymerase • Before RNA can be used in protein synthesis, it must be made from DNA in a process called transcription. During transcription, segments of the DNA molecule serve as a template. The template is used to produce a complementary RNA strand. In eukaryotes, transcription takes place in the nucleus. The completed RNA molecule (called a transcript) then moves to the cytoplasm. Here protein synthesis takes place. In prokaryotes, RNA synthesis and protein synthesis occur in the cytoplasm. • Transcription begins when the enzyme RNA polymerase binds to DNA. The enzyme separates the DNA strands. Using the DNA strand as a template, RNA polymerase adds complementary base pairs to form a strand of RNA. Recall that one base pairing differs between RNA and DNA. In an RNA molecule, adenine pairs with uracil instead of the adenine-thymine base pair in DNA. • RNA polymerase • Definition enzyme that transcribes RNA from a DNA template • Key Context In all organisms, RNA synthesis is carried out by enzymes called RNA polymerases. RNA polymerases bind at the promoter of a gene, unwind the DNA strand and synthesize the RNA using base pairing rules RNA polymerase is an enzyme that attaches to the promoter region of the DNA template and catalyzes the synthesis of an RNA strand.

29. Transcription – RNA Polymerase • How does RNA polymerase determine where it should begin to make the RNA molecule? Regions along the DNA template with specific sequences act as signals to RNA polymerase to begin transcription. Similar terminal sequences also signal RNA polymerase where to stop building the new RNA molecule. • Protein Synthesis • https://youtu.be/2zAGAmTkZNY • Read - The Five Discoveries of RNA Polymerase Hand out.

30. Types and Functions of RNA • RNA has a single strand, has ribose as its sugar, and uracil instead of thymine. Transcription and translation are two processes that are involved in making protein. In transcription, a chemical signal switches on a gene, leading messenger RNA to carry protein making instructions to ribosomes in the cytoplasm, where the proteins are made. In translation, which starts the process of making protein, transfer RNA picks up amino acids from the cytoplasm and transfers them to the messenger RNA on the ribosome. There, the protein chain is assembled in the correct order. • https://app.discoveryeducation.com/player/view/assetGuid/ad1bf56e-4990-410f-a877-ff0e68dbc3e9 • Since RNA carries out so many jobs for protein synthesis, there are several different types of RNA, each with a specific role. • Messenger RNA • Transfer RNA • Ribosomal RNA

31. Messenger RNA (mRNA) • Definition messenger RNA; RNA transcribed from a protein coding gene that travels to the ribosome and is translated into a protein. • Key Context A gene that codes for a protein is transcribed into mRNA. This message is than translated into a protein on the ribosome. • It contains a sequence of nucleotides that direct the assembly of amino acids into proteins. Messenger RNA carries this information from the cell nucleus to ribosomes in the cytoplasm. • The mRNA Template - Discover how the cell creates triplet codons to ensure enough combinations to make all the necessary amino acids. https://app.discoveryeducation.com/player/view/assetGuid/706b2c46-825c-4d3d-840e-e7af399f72d7

32. Transfer RNA (tRNA) • Definition transfer ribonucleic acid; tRNA transfers amino acids to a growing protein chain during protein synthesis; different tRNA molecules have different anticodons and carry amino acids specific to their anticodon • Key Context A tRNA molecule with the anticodon CCG will carry the amino acid glycine and bind to the mRNA codon GGC. • It transfers amino acids to the ribosome as the protein is built. Transfer RNA connects each three-letter genetic code carried in mRNA to the corresponding 20-letter code of amino acids.

33. Ribosomal RNA (rRNA) • Definition ribosomal RNA; the RNA component of ribosomes, the site of protein synthesis. • Key Context Ribosomes, composed of rRNA and protein, are essential to protein synthesis. Their enzymatic function binds new amino acids to the growing polypeptide chain. • It forms subunits on a ribosome that allow for decoding of mRNA. Ribosomal RNA also interacts with tRNA during protein synthesis to help form peptide bonds between amino acids.

34. Translation • What is translation? • Once mRNA has been transcribed in the nucleus, the job of deciphering the genetic message can begin. Decoding the message carried by mRNA into the proteins needed to carry out life processes is called translation. Translation plays a major role in gene expression. • Translation • Definition the process of building a protein based on a RNA template • Key Context Gene expression requires transcription to build mRNA from the DNA template and translation to build the protein from the mRNA template. Translation is the process in which mRNA is translated by the ribosome to produce a specific amino acid chain, which folds to form a protein. • Discover translation and gain the ability to separate it from transcription. • https://app.discoveryeducation.com/player/view/assetGuid/ff50c900-056e-4d57-9977-90b26bafb943

35. Translation – Gene Expression • Gene Expression • Definition the result of coding information determined by DNA • Key Context Gene expression includes phenotype, the detectable traits related to an organism’s genetic makeup. It also includes all tissue development such as protein building that is a controlled by DNA coding. Individuals within a species exhibit different traits. These differences in the expression of genes is a result of the information encoded in a gene.

36. Translation • Recall that the code carried by RNA is made up of just four nucleotides represented by the letters: A, U, G, and C. The code is read in three-letter increments, called codons, one at a time. Each codon corresponds to an amino acid. There are more codons than amino acids (64 codons, only 20 amino acids) so most amino acids have more than one codon specific to them. For example, four different codons—ACG, ACA, ACC, ACU—specify the amino acid threonine. Some codons specify specific signals, such as “start” or “stop,” for building the amino acid chain, or polypeptide. A table of the genetic code shows which codons specify each amino acid or the start or stop signals. Sequences along DNA that signal the RNA polymerase to begin transcription are called promoter sequences.

37. Translation • Degenerate Code System – Discover the role tRNA plays in translation. https://app.discoveryeducation.com/player/view/assetGuid/cbe0afef-f9f2-4c4e-87e1-7ceae2caf9b0 • The Genetic Code - The genetic code is the sequence of bases in DNA, which form three-letter triplets that specify particular amino acids in a particular order to make a protein. https://app.discoveryeducation.com/player/view/assetGuid/c5972a60-2986-4550-889c-38976d72e72b

38. mRNA Codon Table. DNA codons are converted into mRNA codons during transcription. These mRNA codons  code directly for amino acids in translation. Like DNA, RNA has codons that correspond to more than more than one amino acid. Which amino acid does the codon UUU code for?

39. Translation • Following transcription, the mRNA transcripts are transported from the nucleus to the endoplasmic reticulum (ER), specifically the rough ER. The rough ER is studded with ribosomes where translation takes place. • Translation occurs in sequence and only in one direction, known as the 5’ to 3’ direction. The codons would be different if read in the other direction, which would synthesize a different protein. • The following steps outline the process of translation: • A ribosome attaches to the 5’ end of mRNA at a specific, modified codon that is located upstream from a “start” codon. • Once the start codon is read, construction of the polypeptide begins. The ribosome moves along the mRNA strand “reading” the codons. tRNAs with corresponding anticodons bring the correct amino acids to the ribosome. Anticodons are a complementary set of three nucleotides in the anticodon region of tRNA.

40. Translation - Anticodon • Definition a sequence of 3 nucleotides on a tRNA molecule that is complementary to a codon on mRNA • Key Context During translation the tRNA with the complementary anticodon binds to the codon on the mRNA. Each tRNA with a particular anticodon is bound to a specific amino acid. This is how the sequence of the mRNA nucleotides determines the sequence of the amino acids in a protein. An anticodon is a sequence of three nitrogen bases on a tRNA molecule which binds to codons on the mRNA strand and codes for an amino acid. Anticodons are complementary to the mRNA codons.

41. Translation 3. One at a time, the ribosome attaches amino acids from each tRNA together with a peptide bond. Simultaneously, the ribosome breaks the bond between tRNA and its amino acid. The tRNA is released and the ribosome is available to bind the next tRNA dictated by the mRNA code. 4. The ribosome continues to move from the 5’ end of mRNA toward the 3’ end, reading each codon as it moves along. Construction of the protein continues until the ribosome reaches a “stop” codon. At that point, both the protein and mRNA strand are released from the ribosome. Exploration Student Worksheet: Translation

42. Gene Expression • But how do proteins result in blue eye color or yellow plant flowers? • Recall that many proteins are enzymes. Enzymes catalyze a wide range of biochemical reactions in an organism’s cells. The cellular instructions to make the enzymes that produce a flower’s yellow pigment are encoded, or stored, in the information of the organism’s DNA. The flower cells then follow these instructions to make the enzymes responsible for yellow pigmentation, in a multi-step process called gene expression.

43. Gene Expression • Some genes code for RNA only. Most genes code for proteins, including some proteins that make up cell structures and other proteins that are enzymes. In humans, there are an estimated 20,000 to 25,000 pairs of genes that code for proteins. Gene expression is the process of using the information stored in each gene to instruct the synthesis of each gene product.  However, protein is not expressed directly from DNA, there are multiple steps involved in getting from the starting information (DNA) to the final end product (protein). • gene expression • Definition • the result of coding information determined by DNA • Key Context • Gene expression includes phenotype, the detectable traits related to an organism’s genetic makeup. It also includes all tissue development such as protein building that is a controlled by DNA coding. • Individuals within a species exhibit different traits. These differences in the expression of genes is a result of the information encoded in a gene.

44. Gene Expression • In biology, there is a widely accepted idea about how the DNA gets expressed as traits in an organism. This is called the Central Dogma. • The Central Dogma states that DNA provides the template of instructions for the synthesis of RNA (by a process called transcription), and messenger RNA (mRNA) provides the template of instructions for the synthesis of proteins (by a process called translation). • All cells, whether they are bacterial cells or human cells, share these two basic steps of gene expression, transcription and translation. The end products produced, for example proteins, determine the traits of an organism. • A visible or measurable trait of an organism, for example eye color, is called a phenotype.

45. Gene Expression • The total DNA of a cell or of an organism is called its genome, arranged in structures called chromosomes made of double-stranded DNA spooled around special proteins. The smallest unit of information stored in DNA is called a gene. • The size of a genome, the number of chromosomes, and the total number of genes is specific to the organism.  Most normal human cells (except eggs and sperm) contain two sets of 23 chromosomes (46 total), one set from one’s mother and one set from one’s father. Therefore, each cell has two copies of each gene, one from each parent. This totals around 6 billion base pairs of DNA per cell, packed to all fit within the cell nucleus.

46. Gene Expression • Genes in an organism only become active when protein synthesis is needed. Some genes lie dormant for most of the organism’s life, while others are active from the time the organism starts its existence. Tadpoles develop into frogs, which look and behave very differently. Yet the tadpole and the frog have the exact same genetic code. The difference is that in the tadpole, the genes are activated that cause a tail and gills to grow. When the tadpole becomes a frog, these genes are turned off, and genes that cause legs and lungs to grow are turned on. The proteins responsible for the developmental process that produces these traits directly result from the sequence of DNA in the animal’s body cells. • The sequence of bases in the DNA molecule governs the process of protein synthesis. Studying the structure of DNA and its sequence of nitrogen bases enables scientists to better understand the processes of transcription and translation and therefore how proteins give rise to the traits of living things. • From Cell to DNA, through transcription and translation, DNA produces the traits expressed by an organism. Sometimes certain genes can turn on or off, resulting in different traits that enable response to environmental changes.

47. Environmental Factors Affecting Gene Expression • Gene expression can be influenced by the environment. This environment includes the world in which the organism is living in, as well as its internal world. This internal world may include factors such metabolism or gender. External influences could include factors such as food, substances in the environment, light, and temperature. Some of these will influence which genes are switched on or off, and may alter how an organism functions and develops. • There are many examples of the environment influencing gene expression. Genes that are influenced by internal and external environmental factors may provide certain adaptive advantages and will be selected for through the process of natural selection. • Different genders express autosomal traits (those not carried on the sex chromosomes) in different ways. One example of this is baldness in men.  Usually this type of baldness only occurs when levels of testosterone and dihydrotestosterone—male sex hormones—are high. This is because the allele for baldness is only expressed at these high concentrations. Women carry this gene but it is not usually expressed because they have very low levels of these hormones.

48. Environmental Factors Affecting Gene Expression • Food availability can influence gene expression. Certain gut bacteria in mammals produce specific enzymes only when certain types of food are present. The bacteria can alter the types of enzymes according to the nutrients present in the food. Imagine that you had cereal and milk for breakfast. The bacteria produce enzymes specifically to digest carbohydrates and fats from this meal. Imagine that you then have a hamburger and salad for lunch. The bacteria now produce enzymes to digest fats and fiber. These bacteria produce different enzymes by turning on and off certain genes due to changes in the food substances in their environment. This helps the bacterium and its host.

49. Environmental Factors Affecting Gene Expression • Chemicalsor drugs in the environment can also change gene expression. One such example involved a drug called thalidomide. Thalidomide was a sedative drug that was used in the 1950 and 1960s.   In adults it was shown to have no measurable effect on gene expression and was widely prescribed. However it was later discovered that it altered gene expression in fetuses. Many pregnant women who had been prescribed the drug gave birth to infants with missing or malformed arms and legs. About 10,000 infants were impacted in this way.

50. Environmental Factors Affecting Gene Expression • Temperature can alter gene expression. The best known example of the influence of temperature is the Himalayan rabbit. Himalayan rabbits carry a gene which is needed for pigment production. The expression of this gene depends upon the temperature. Those parts of the rabbit that are cold generally express the gene, whereas those that are not have no pigment and are white. This gives rise to the characteristic black ears and noses of this breed of rabbit. • Lightmay also alter gene expression. For example exposure of some caterpillar species to different colors of light will change the color of the wings they develop when they become butterflies. Those exposed to green light had dark wings, whereas those raised under red light had very brightly colored wings.