5 = DNA. < I Thymine. ~ Adenine. = Guanine ... = sugar (deoxyribose) = phosphate :>.~>~;:<.. DNA. o, I', . :..:. Key to the 4 Nucleotides...

Transcription

1 ":: I.. ".0._.. ':.. ".. :.. o -:, I::.. :.:. :..:. : :.. " (Of DNA : '.:;:".. :.U, :....1:... :: ".. i'.-;. "0 : :: :...: :>.~>~;:<.... : : f :'.-..:..... : ~.; :..~..;... ".. "...: ~.. : : :: 0.:.':.0; 0 : : :; :...:!~'::.:.:..;. '.. '..:.:.:: ;:. -:.:.. ::: :. ::.....:: "..,,. DNA... Key to the 4 Nucleotides....JI."'."'... G = Guanine o,. I', 5 = I < I Thymine T = ~ Adenine,. 0 = sugar (deoxyribose),. = phosphate Ch 36 The Chemical Structure of the Gene: DNA 67

2 36-3 Lab Investigation DNA Replication 1. Name the four nucleotide bases that make up DNA. Objectlve When you have finished this brief lab activity, you should be able to describe how DNA makes exact copies of itself. (replicates) DNA Replication To replicate means to produce a copy of itself. DNA is the only known molecule that can do this. When chromosomes double and pull apart in both mitosis and meiosis, the DNA that makes up the chromosome must make an exact copy of itself (replicate). DNA is able to make an exact replica of itself because of the base pairing characteristics stressed earlier (A with T and C with G). When DNA makes a duplicate molecule of itself, the two strands unwind. See how this is shown in the drawing on page 67. L.-.J After the 2 strands have pulled apart, new bases (A, T, C & G) as well as new sugar and phosphate units come into place according to the base pairing rules. A comes in opposite the T and C opposite the G. When this has occurred, 2 identical DNA molecules will be created. This has been an introduction to DNA replication. You will demonstrate this with paper clips in the following lab investigation. This lab investigation will help you to better understand the process. DNA (Deoxyribonucleic acid) is a vital molecule in living organisms. The GENE is made of DNA. DNA is one of the few molecules that can REPLICATE ITSELF. (To replicate means to make exact copies of itself.) Some of the most interesting research taking place today is being done with DNA. Before the student can understand genetic engineering research, he or she must first understand the basic structures of DNA and understand how this molecule replicates itself. It is assumed that you now already know the basic structure of DNA. Recall that DNA is the GENE and is a double-stranded nucleic acid molecule composed of two strands, twisted into a double helix (double spiral). Each DNA strand is composed of four molecular sub-units called NUCLEOTIDE BASES. In this activity, you will construct a DNA molecule out of colored paper clips. You will then learn and review how the molecule replicates itself to form an exact copy of itself. Each different colored paper clip will represent a different nucleotide base. Use the following colored clips to represent the four nucleotide bases as follows: Ch 36 The Chemical Structure of the Gene: DNA 68

3 I EACH PAIR OF STUDENTS WILL NEED THE FOLLOWING: 14 Black clips = Adenine (A) The time needed for 14 White clips= Thymine (T) this activity is about 9 Red clips = Cytosine (C) minutes. 9 Green clips = Guanine (G) Remember that each DNA molecule has a unique structure that makes it different from other DNA molecules (or genes) in that the sequence of A, T, C, and G vary from one molecule (gene) to another. You will be making a short sequence of a human gene that controls the body's production of growth hormone which causes growth during childhood and adolescence. Genetic engineers call this gene the hgh (human Growth Hormone) gene. This gene is actually made of 573 nucleotide base pairs. You will only construct the first ten bases in the gene. WORK IN TEAMS OF TWO STUDENTS STEP ONE: Use the colored paper clips according to the key above and construct the top strand of the hgh gene according to the diagram of the gene below. Link the 10 appropriate colored clips for the top chain shown below. L..J A -A -G -C - T - T - A - T - G - G T -T -C -G - A - A - T - A - C - C Notice that the bottom strand of the DNA molecule follows the "rule of complementarity", which means that A bonds with T, and C bonds with G. STEP TWO: Now construct the bottom strand of the hgh gene by linking 10 more clips into a chain according to the pattern above. The entire sequence of this gene is known. Your DNA model should resemble the following: L-J ( ( 0 ( 0 ( { ( ij ( () ( (J ( ( 0 ( 0 ( ( ( () ( ~ ( (J STEP THREE: Set the two chains side-by-side as shown in the drawing above so that A bonds with T and C with G. You now have a model of the hgh gene. (first 10 bases only). Compare the 2 chains within each other side-by-side to verify that C bonds with G and A with T. When this gene replicates in the nucleus of a cell, the double-strand begins to separate at one end and as it separates, new nucleotide bases are moved into place by enzymes so as to form the beginning of 2 new identical molecules. These A, T, C, and G bases are present in the nucleus of each cell and come from food molecules. When these new bases are brought into place, the A bonds with T and the C bonds with G. Ch 36 The Chemical Structure of the Gene: DNA 69

4 STEP FOUR: Open your hgh DNA molecule as shown below: «O~( ~ ( ( ( 0 ( Q ( 0 ( 0 ( ( ( () ( Q ( 0 ( 0 «)...(--~ STEP FIVE: Now use the other available clips to create the beginning of 2 new strands. Remember A with T and C with G. Connect the clips as follows: :( ~ ( ( ( ( () ( &( Q ( 0 ( ( [ ( () ( &( () ( 0 ~ ( i ~ ( ( ( STEP SIX: Continue separating the strands and bring in appropriate new bases (clips) to create 2 complete new double-stranded hgh gene molecules. Remember that A bonds opposite to T and C is opposite to G. You should have 6 clips left. Save them for ~ later..' Answer the questions that follow. 2. Examine the 2 double-stranded DNA molecules. Are they identical or different in any way? 3. If you were asked to replicate each of the 2 DNA molecules on your table to create 4 identical DNA molecules, how would you go about doing this? \ 4. You now have 2 copies of a segment of the hgh gene on your table. During periods of growth and cell division, the chromosomes, which are made-up of genes, must divide. What features about DNA replication causes each new DNA molecules to be exactly like the original? Cells can divide, along with their DNA, in this manner without any errors for thousands of replications. On occasion an error can occur. When this type of error occurs in a cell that is destined to become an egg or a sperm cell, it is called a MUTATION WHAT IS A GENE MUTATION? To demonstrate a gene mutation, place one of your paper clip hgh DNA strands in front of you. L-J Identify the second nucleotide base called Adenine (A) which is black. To_ cause a mutation. remove this 2nd black clip and replace it with a red cytosine (C) clip. ~' Ch 36 The Chemical Structure of the Gene: DNA 70

5 LJ 5. different chemical 6. Since different the Please ~~ fit PI' You have just demonstrated how a mutation occurs. This replacement usually occurs when the DNA is replicating. When this mutated DNA molecule replicates, will the resulting new DNA be similar or from the hgh gene? Explain. This type of mutation can be caused by a variety of circumstances including radiation, exposure or it can occur spontaneously without known cause. Draw structures of the new double-stranded mutated gene you created by using the letters A, T, C, and G in a manner similar to that shown earlier. the sequence of bases is different in this molecule, the genetic code is This gene CANNOT direct the cell to produce normal human growth hormone. In your own words, explain how mutations can occur in cells and how this might affect new organism with the mutation. separate all the paper clips and organize them by color in appropriate containers. LJ Count them. You need to have 14 black and 14 white, 9 red and 9 green. L..J Call your teacher to check your table. LJ Ch 36 The Chemical Structure of the Gene: DNA 71

6 1. What contributions did Watson, Crick and Griffith make to 'the field of genetics? 36-2 The Double Helix 1. What is the significance of DNA to the cells and bodies of organisms? Objective You will be expected to describe the basic structure of DNA, its components and its relationship to the chromosome. This activity is a prerequisite to all other labs and reading topics on DNA and genetic engineering. DNA is the molecule that makes up the chromosomes. The different segments along a, the DNA molecule comprise the genes 'that govern body functions and physical WI"'. characteristics. The diagram on the next page shows the relationship of DNA to the chromosomes in the nucleus. Study the diagram on the next page and be sure to read all captions and then continue reading below. L-J Notice that the X-shaped chromosomes in the nucleus are composed of long tightly coiled strands of DNA. The diagram is artificial in that the DNA is shown pulled out of a chromosome and nucleus and progressively magnified. This is done to show details of DNA and how it makes up the chromosome. The DNA molecule itself is a long double stranded molecule. The two long strands are twisted around each other to form what geneticists call a double helix (double spiral). The two strands are held together by cross-piece subunits called nucleotide bases. Examine these cross-pieces on the next page again. L-J You will notice at the bottom of the page, where the enlargement of DNA is its greatest, that the nucleotide bases are molecule subunits symbolized by A, T, C and G. These subunits are arranged in a different order in each specific gene. The arranged order of these 4 subunits spells out the specifications for gene expression. This is the genetic code found in all genes. Ch 36 The Chemical Structure of the Gene: DNA 64

7 ~/~ The 46 chromosomes in cell nuclei are composed of DNA. Enlarged section of the tightly ~ coiled DNA that makes up the chromosome. ( dso)(yribonucisic acid ) As the DNA is enlarged further and uncoiled, the double helix structure is ::::~ble along with the nuci80tij. If the DNA in all 46 human chromosomes from 1 cell were uncoiled and stretched out end to-end, it would be more than 6 feet long. In the cell's nucleus, this DNA is so small it is not vis':ible to the unaided eye. Thousands of genes are contained on each DNA molecule. These genes regulate every characteristic and aspect of body function. As the strands of DNA separate in replication, new bases from the cell are incorporated into new molecules of DNA SUbu\ The genes on DNA are made of 4 ~~~ide bases (or I\~ tj ~tj~ The Chemical Structure of the Gene: DNA 65 The genetic code. and thus a genetic trait. is determined by the order of the bases on DNA.

8 To understand how genes express themselves and how scientists splice genes from one organism into another, it's essential to understand specific details about the structure of IG:\. DNA. For this purpose, the DNA molecule has been redrawn on the next page in an.r ' untwisted form to cause the molecule to appear as a ladder (lower right). Study this drawing carefully and return to the next paragraph. [-l Notice that the vertical rails of the ladder are composed of alternating sugar-phosphate subunits. [-l The horizontal rungs or steps of the ladder are composed of the nucleotide bases called Adenine (symbolized by A), Thymine (T), Cytosine (C), and Guanine (G). Examine the DNA ladder. Certain nucleotide bases always pair with others. What pairs with what? [-l You should have observed that A always pairs with T and C with G. This is important to remember. The following chart shows some important comparisons between the structures of DNA and RNA, another important cellular molecule: Component ~t~~ded Strand number double helix SingleR:r:nded ~... ' Nucleotide bases A Adenine A Adenine T = Thymine U = Uracil (instead of Thymine) C = Cytosine C = Cytosine AiiilIi G = Guanine G = Guanine..., Sugar Deoxyribose Ribose tilt Name DeoxyriboNucleic Acid RiboNucleic Acid... RNA, as you will see, is an important helper nucleic acid molecule needed in order for the ~ ~ gene (DNA) to express itself. Study the above chart and mentally note the differences between RNA and DNA. [-l... You should remember that RNA has a different nucleotide base not found in DNA. In RNA, U (Uracil) substitutes for the T (Thymine) found in DNA. This means that when base pairing occurs in RNA, C pairs with G, but that U pairs with A. Some other ~ important differences are that RNA is single stranded, whereas DNA is double stranded ~ and wound in a helix (double spiral). The sugar in RNA is ribose rather than ~ deoxyribose which is reflected in the differences in names; DeoxyriboNucleic Acid as opposed to Ribonucleic Acid. :1 3. If the DNA found in all 46 chromosomes were unwound and lined up end-to-end, how long would the DNA 2. Chromosomes are made from what molecule? be? ~ 4. What are the full chemical names of the 4 nucleotide bases found in DNA? ~ 5. What are the full chemical names of the 4 nucleotide bases found in RNA? 6. What is the genetic code? Explain. 7. How is one gene different from another gene? 8. What is the chemical name of the sugar found in DNA? In RNA? 9. What is the complete chemical name of DNA? Of RNA? 10. Which nucleotide base is found in RNA but not DNA? 11. In DNA, which bases pair with which other bases? 12. In DNA, what bases makeup the vertical strands of the ladder? 13. Describe the 3-dimensional structure of the DNA molecule. How do all the parts fit together? tm:i i fill Ch 36 The Chemical Structure of the Gene: DNA 66