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Name ___________________________ Date ________ Block__________ DNA and Genetics DNA - The Double Helix Recall that the nucleus is a small spherical, dense body in a cell. It is often called the "control center" because it controls all the activities of the cell including cell reproduction, and heredity. Chromosomes are microscopic, threadlike strands composed of the chemical DNA (short for deoxyribonucleic acid). DNA controls the production of proteins within the cell. Proteins are the building blocks of an organism. The proteins that will be made for your body are determined by the sequence of DNA in the nucleus. Chromosomes are composed of genes, which is a segment of DNA that codes for a particular protein, which in turn codes for a trait. Hence you hear it commonly referred to as the gene for blue eyes. Meanwhile, DNA is the chemical that genes and chromosomes are made of. DNA is a nucleic acid found not only in the nucleus but also in the mitochondria and chloroplasts. It is the DNA in the nucleus that actually controls the cell's workings. After the components of DNA were figured out, a race among scientists began to determine the structure of DNA. Three scientists are given credit to the structure. Rosalind Franklin, a female scientist was actually the first to determine the structure however James Watson and Francis Crick are credited to establishing it. X-ray photographs were used to describe the shape and dimensions of the molecule. DNA is made up of alternating nucleotides. Each nucleotide consist of a nitrogenous base, a phosphate molecule and sugar. The shape of DNA is a double helix (double stranded), which looks like a twisted ladder. The sides of the helix are made of alternating sugar and phosphate molecules. The sugar is named deoxyribose. The rungs of the ladder consist of 4 different are pairs of 4 types of nitrogen bases - adenine, thymine, cytosine, and guanine. The bases are known 1 by their coded letters - A, T, C, and G. The strands of the molecule are connected by complementary nucleotide pairs (A & T and C & G) like rungs on a ladder. These bases always bond in a certain way. Adenine will only bond to thymine. Guanine will only bond with cytosine. This is known as the "BasePair Rule." The bases can occur in any order along a strand of DNA. The order of these bases is considered the genetic code. A strand of DNA contains millions of bases. Color all the deoxyribose sugars blue (light) ("D"). Color all the phosphates pink (P). Color Color Color Color the thymines orange. the adenines green. the guanines purple. the cytosines yellow. Note that that the bases attach to the sides of the ladder at the sugars and not the phosphate. The two sides of the DNA ladder are held together by their bases, by hydrogen bonds. Hydrogen bonds connect the complementary bases. The hydrogen bonds are represented by small circles. Color the hydrogen bonds grey or black. The DNA can actually "unzip" at the hydrogen bonds when it needs to replicate - or make a copy of itself. DNA needs to copy itself when a cell divides, so that the new cells each contain a copy of the DNA DNA Replication Cells pass on their genetic code by replicating their DNA. When DNA replicates, certain steps must be followed correctly in order to prevent a mutation. Three enzymes are used for the process. Helicase opens the double helix up into two separate strands. DNA polymerase reads the original DNA strand and lays down 2 Name ___________________________ Date ________ Block__________ complementary bases for each strand. Ligase glues the newly formed DNA together, thus creating two separate strands of identical DNA. Protein Synthesis Transciption It is the sequence of bases that determine which protein is to be made. The sequence determines which proteins are made and the proteins determine which activities will be performed. Because DNA is too big to go through the nuclear pores a chemical called messenger RNA (mRNA) is brought into the nucleus to interpret the DNA code. Like replication, the DNA molecule unwinds and splits apart. The mRNA reads it, and like replication, matches up complementary bases to the ones on the DNA. There is one difference. RNA does not use the base thymine. Instead, a nitrogenous base called uracil matches up with adenine. Another difference in RNA is that it is single stranded, not double. In addition to that difference, mRNA has the sugar ribose instead of deoxyribose. RNA stands for Ribonucleic Acid. Color the mRNA as you did the DNA, except Color the ribose a DARKER BLUE, and the uracil brown. Translation: Once the mRNA has the coded strand from the DNA it leaves the nucleus and finds a ribosome. The bases on the strand are broken up into groups of three. This is known as a codon. There a molecule of transfer RNA (tRNA) that matches with complementary base pairs. Each tRNA is grouped by 3 nucleotide pairs. At the top of each tRNA molecule is an amino acid. As a new tRNA arrives and binds to the 3 next codon on the mRNA strand, a peptide bond forms between the first amino acid and so on, until it reaches a stop codon, which will indicate the end of the amino acid chain. This chain of amino acids will create the template for a protein. Amino acid chart The genetic code consists of 64 triplets of nucleotides. These triplets are called codons. They come from the mRNA. With three exceptions, each codon encodes for one of the 20 amino acids used in the synthesis of proteins. Amino acids are the "building blocks" of the body derived from protein. Besides building cells and repairing tissue, they form antibodies to combat invading bacteria and viruses; they are part of the enzyme and hormonal system; they carry oxygen throughout the body and participate in muscle activity. Eight are essential (cannot be manufactured by the body) the rest are non-essential (can be manufactured by the body). The codons are arranged in a chart, each which code for a specific amino acid. Note that for each table, the left-hand column gives the first nucleotide of the codon, the 4 middle columns give the second nucleotide, and the last column gives the third nucleotide. 4 Name ___________________________ Date ________ Block__________ Questions: 1. Write out the full name for DNA ________________________________________ 2. What is a gene? _____________________________________________________ 3. Where in the cell are chromosomes located? ________________________________ 4. DNA can be found in what other organelles in the cell? ________________________ 5. What two scientists are credited with establishing the structure of DNA? ___________________________________________________________________ 6. What is the shape of DNA? ____________________________________________ 7. The sides of the DNA ladder are composed of what? __________________________ 8. The "rungs" of the DNA ladder are made of what? ____________________________ 9. What sugar is found in DNA? In RNA? ____________________________________ 10. How do the bases bond together?________________________________________ A bonds with _____ G bonds with _______ 11. DNA is made of repeating units called ____________________________________ 12. If the sequence of nitrogenous bases in one strand of DNA is GAGTC, what is the sequence of bases in the complementary strand of DNA? ____________________ 13. Why is RNA necessary to act as a messenger? Why can't the code be taken directly from the DNA?________________________________________________________ 14. Which of the nitrogenous bases pairs with uracil in an RNA molecule? _____________ 15. Explain how codons determine the amino acid sequence of a protein. ___________________________________________________________________ 16. An essential amino acid is an amino acid that _____________________________ 17. Use the chart to determine which amino acids, the codons code for? a) UCA ______________________ b) CAU_______________________ C) AAG ______________________ d) GCA _______________________ 5 DNA Molecule 6 Name ___________________________ Date 7 ________ Block__________