DNA STRUCTURE AND FUNCTION PROTEIN SYNTHESIS

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DNA STRUCTURE ANDFUNCTIONPROTEIN SYNTHESISSELF STUDY GUIDEGRADE 12

1Exploring inside the cellBefore teaching learners about DNA & RNA, revise the structure of a cell (Grade 10content) and in particular the structure of the nucleus and the position of ribosomesin the cytoplasm. Learners start with a familiar larger structure and then look atprogressively smaller structures i.e.nucleuschromosomesDNAgenesThe NucleusThe nucleus is the most conspicuous organelle in all eukaryotic cells. The nucleusstores all the genetic information in the genes of the chromosomes. It is the CEO ofthe cell directing all the functions for life and, in addition prepares the cell for growthand replication.The us.jpg)How is the nucleus constructed?1. Location and shape in animal cells: rounded and in the centre of the cell

22. Location and shape in plant cells: lens shaped and pushed to the side of thecell by the vacuole.3. Nuclear membrane or envelope – surrounds the nuclear contents and is adouble membrane.4. Nuclear pores – many and control the passage of molecules and structuresinto and out of the nucleus.5. Nucleoplasm – the ‘cytoplasm’ of the nucleus.6. Nucleolus – this is an extra dense area of DNA and protein where theribosomes (rRNA is synthesized) are produced.7. Chromatin – is made up of DNA (a nucleic acid) and proteins called histones.When the cell is about to divide the chromatin condenses into separatechromosomes.Points to ponder:* Suggest how you could model the eukaryotic cell nucleus.* Consider – Could you draw up a table of the structures related to their function in terms of a factory?The cell cycle, chromatin, chromosomes and DNACells pass through a cell cycle consisting of mitosis (cell division) and interphase(phase between divisions).In higher organisms, most actively dividing cells take 18 to 24 hours to complete thecell cycle. During this cell cycle, mitosis is completed in ½ to 2 hours. Most of thetime is spent in interphase.http://www.biology.arizona.edu/cell BIO/tutorials/cell cycle/cells2.html

3Interphase consists of G1, S and G2 stages.(It's not necessary for learners to remember these terms but they should understand whatis happening to the DNA)G1 phase: before DNA replicationAfter mitosis, the cells grow, may differentiate and there is intense metabolicactivity. The DNA is active, mRNA is produced and protein synthesis takes place.xActively dividing cells e.g. the cells in a developing embryo and meristematiccells in plants, spend hours in this phase before moving to the next phase ofDNA replication.xSome cells mature, specialise and continue to be metabolically active but donot continue with DNA replication, the G2 phase and cell division. As theymature, they lose their ability to divide e.g. red blood cells, muscle cells andnerve cells.xSome cells, once they mature and specialise, divide only occasionally e.g.cortex cells in plant stems. They may spend years in this phase and onlyreenter the cell cycle when stimulated.In all human cells (except the sex cells & rbc's), the chromatin consists of 46chromosomes. Each chromosome consists of a long ribbon-like structure, the DNA(double helix), wrapped around histone molecules.(Nucleosome – a group of histone molecules with DNA wrapped around it).(C. Still; Wits Univ.)

4http://www.biology.arizona.edu/cell BIO/tutorials/cell cycle/cells1.htmlS1 phase: DNA replicationEach of the 46 DNA strands makes a copy of itself, so that there are now twostrands of DNA (2 double helices), each wrapped around histones. The two strandsare held together at the centromere. The double structure is a chromosome andeach strand is called a chromatid.G2 phase: after DNA replication:The cells continue to grow, synthesise proteins and undergo other metabolicactivity. The cell begins to prepare for mitosis.At the start of mitosis, the chromatids makingup each chromosome prepare for mitosis bycondensing and becoming very folded and coiledso that at the start of mitosis the chromosomeslook short and thick and one can see the twochromatids held together at the centromere.If you place the 46 chromosomes end-to-end,the length of the DNA in those chromosomesin one cell is almost 2metres.Imagine trying to pack this DNA into onemicroscopic nucleus!

5http://www.biology.arizona.edu/cell BIO/tutorials/cell cycle/cells1.htmlLearners often find it difficult to understand how DNA is packaged inside thechromosome. The following series of diagrams illustrates that packaging.(nm nanometres).Fig X: Levels of DNA dia/chromosome packing.gif)Activity 1 A simple model of DNA packagingTake the material (string and presstick) out of the packet labeled Activity 1.1. Take the two pieces of string and twist them around one another to represent aDNA double helix (or use two stranded string)2. Roll the presstick into ten balls of equal size. These are the 'nucleosomes' madeup of 'histones'.

63. Now wind your 'DNA' twice around each of the ten 'nucleosomes' .4. Bend your strand backwards and forwards (2nd diagram from bottom) to create asimplified version of a thick chromatid.5. Join your chromatid together with another group's chromatid using presstick asthe centromere. You now have a 'chromosome'!(Alternatively, untwist (unzip) and separate your string, add on complementarystrings, and join them by a centromere.)Deciphering the Three Dimensional Structure ofDNA – a brief historyLet us begin in 1856:Gregor Mendel was an Austrian monk. He worked in the small monastery gardenwith pea plants and did a series of experiments hybridizing pea plants. The resultsof Mendel’s crosses allowed him to conclude from the consistent ratios he obtainedthat plants transmitted ‘elementen’ or discrete units.Mendel did not know that his‘elementen’ were found on chromosomes and were in fact renamed genes in 1909.1930’s – 1940’sIt was found by various researchers between these years that DNA, a nucleic acid,is the biochemical responsible for transmitting traits. In 1928 Frederick Griffithcontributed to the initial understanding that DNA was the genetic material. He foundthat genetic information can be transferred from heat-killed bacteria to live bacteria.This process, known as transformation, was the first clue that genetic information isa heat-stable compound. Then in 1944 the Avery and Hershey-Chase Experimentsclearly showed that the active principle for transforming a bacterium calledStreptococcus is DNA.Their evidence confirmed that DNA is the hereditarymaterial. This led to questions regarding the molecular nature of DNA.Then in 1949 and early 1950’sErwin Chargaff, a biochemist, showed that DNA contains equal amounts of thebases adenine (A) and thymine(T) and equal amounts of the bases cytosine (C) and

7guanine(G). He also showed that the DNA composition varies from one species toanother, that is, it is species specific.Morris Wilkins and Rosalind Franklin, a physicist and chemist respectively, showedby using X-ray diffraction a pattern of regularly repeating nucleotides. This was thefirst clue to the three-dimensional structure of DNA.Finally in 1953James Watson, an American biochemist and Francis Crick an English physicistbegan their collaborative work to try to solve the puzzle of the molecular structure ofDNA. Using data provided by Maurice Wilkins and Rosalind Franklin, they made anaccurate model of the molecular structure of DNA. This discovery they called ‘thesecret of life’. In 1962 Crick, Watson and Wilkins received the Nobel Prize fordetermining the molecular structure of DNA – a double-stranded, helical,complementary, anti-parallel model for deoxyribonucleic acid.

8“The Secret of ges/WatsonCrick.jpg)CSStill (2009)DNA structureActivity 21. Did Watson and Crick use 'the scientific method' to decipher the structure of DNAand construct their model? Justify your answer.xDiscuss in your group.xAsk one person to give feedback.2. What did Watson & Crick discover? What do we now know?a. In pairs, brainstorm the topic 'DNA structure' and in any order write down asmany important terms or phrases that you believe relate to the detailed structure ofDNA.You might include some of the following terms in your list:StructureLadder-likeDouble helixAnti-parallel ·end (containing a phosphate group) ·end (containing a hydroxyl (-OH) group)Make-up of DNA helix2 outer strandsPhosphate sugar linkBackboneRungs of ladderPairs of basesWeak hydrogen bondsComplementary base pairingA only pairs with TC only pairs with GMonomers of DNANucleotidesSugar deoxyribosePhosphate moleculeNitrogen basesPurine – adenine & guaninePyrimidines – cytosine &thymineTypes of bondsCovalent bonds, phosphor-diester(sugar-phospate) bondsHydrogen bonds2 hydrogen bonds between A & T3 hydrogen bonds between C & G

9Fig X: The Structure of logy-edited/chap8/b0808202.asp)b. Which terms are essential for matric learners to remember?

10DNA, a nucleic acid and nucleotidesWhat do learners need to know?DNA is a nucleic acid made up of two strands, wound around one another to form adouble helix. Each DNA strand is made up of nucleotides.1.Each DNA nucleotide consists of:x deoxyribose sugarx phosphatex nitrogenous base (adenine, thymine, guanine or cytosine)2.Nucleotides join to each other by sugar-phosphate bonds between thephosphate of one nucleotide and the deoxyribose sugar of the nextnucleotide. Many nucleotides join to form a single DNA strand.3.The two strands are connected by weak hydrogen bonds betweencomplementary nitrogenous bases.adenine always bonds with thymineguanine always bonds with ecturesf04am/nucleotides.jpg)

11Learners don't need to know the structures of these molecules but it might be usefulto show them diagrams on a chart/ OHT etc so they understand why differentshapes are used to represent these nitrogenous bases.They also don't need to remember how many hydrogen bonds link A to T, & C to G.Some concepts and statistics:xthe human genome is all the DNA in an organism including its genesxthe human genome is made up of just over 3 billion pairs of basesxeach chromosome has 50 million – 250 million base pairsxeach gene is a section of DNA with a specific sequence of bases that actsas the 'instructions' or code for the production of a specific protein.xthe human genome has 20 000-25 000 genesxthe average gene has about 3000 basesxthe genes make up only 2%* of the human genome; the rest of the DNA ismade up of non-coding regions, some of which regulate chromosomalstructure and where, when and in what quantity proteins are made. Thefunction of 50% of the DNA, made up of repeated sequences and knownas 'junk DNA', is not known.xchromosome 1 has the most genes i.e. 2968, whilst the Y chromosomeshas the fewest i.e. 231.http://www.ornl.gov/sci/techresources/Human Genome/home.shtml(*5% according to Dr Carolyn Hancock)Activity 3: DNA modelsModel 1: Cardboard modelsUse any simple models of nucleotides such as the ones on the next page toconstruct a 'DNA molecule'.You can print the diagrams onto thin cardboard or onto paper which you stick ontocardboard, or you can trace the outline onto different colour cardboard.Put presstick on the back of each nucleotide and let learners construct the moleculeon a wall or the board (or in groups).Alternatively use cellotape as 'hydrogen' and 'sugar-phosphate' bonds, connectingthe nucleotides. Twist your helix and suspend it from the ceiling of your classroom.

12PGDPCDPADPTDCardboard cutout models: C. Still

13Model 2: Cardboard models (more detail, free-standing 3D model)(C. Still)Make a model of a DNA molecule using:xA stand or basexA dowel rod/stickx2 outer phosphate strandsxBases that form the rungs of the ladder, A, T, C and G.xSpacersMake copies of the next 3 pages. (2 copies each of cytosine- guanine and adenine– thymine; 1 copy of S-P)1. Remove the pages of base pairs and paste them on a piece of stiff cardboard.2. Cut each base pair along the dark border. You should have a total of 20 basepairs.3. Use a crayon or marker pen and colour each of the bases using a suitable colourscheme eg cytosine – red; thymine - blue; adenine – green; guanine – orange.4. Use a punch or cork borer (or any suitable instrument) to remove the circle in thecentre of the base pairs. Stack the base pairs so that all the base names are facingin the same direction.5. Cut out the nine strips of paper containing the diagrams of the phosphatemolecules.6. Using a razor blade, cut along each dotted line to make a slit in the paper band.7. Tape each strip to the end of another strip with just enough overlap to keep theslits evenly spaced.and then.(PTO)(Models & activity modified from Montgomery, R.J. and Elliott, W.D. (1994). Investigations in Biology,pp151-171)

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18DNA Replication or DNA synthesisWhen a cell is ready to divide, each DNA molecule duplicates or replicates itself, ina process we call DNA replication. In this way each new cell or daughter cellreceives an identical DNA copy.Both strands of DNA in the parent cell acts as a template for the formation of twonew complementary strands.Thus the daughter cell receives a DNA double helix, where one strand is from theoriginal DNA and the other strand is newly formed. We term this way of replicatingDNA as semi-conservative.Replication requires: Parental double-stranded DNA known as the template Complex enzymes and proteins to open up the helix An enzyme knows as DNA polymerase Free nucleotides with adenine, thymine, cytosine or guanine bases.The Replication process1. An enzyme or protein complex opens up the two DNA strands, that is, theDNA helix unwinds.2. Weak hydrogen bonds between A and T and C and G break, allowing thetwo strands to part or unzip.3. This exposes the bases on the

xeach gene is a section of DNA with a specific sequence of bases that acts as the 'instructions' or code for the production of a specific protein. xthe human genome has 20 000-25 000 genes xthe average gene has about 3000 bases xthe genes make up only 2%* of the human genome; the rest of the DNA is

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