1 The Blueprint of Life DNA & Protein Synthesis Why Do You Look Like That? Hair on your head grows up to 25 inches, but hair on our eyebrows grows only an inch or less. Why? Humans have five fingers on each hand instead of six. Why? Dogwood leaves always have smooth edges. Why? People rarely grow over 6 1/2 feet tall. Why? How do our bodies know just what to do? We are not surprised that all people have two arms and tow legs, but how did the developing fetus know to grow two arms and two legs? The Answers Are in the Nucleus Our bodies, and the bodies of all living things, are made of cells. A cell is a tiny living structure that contains a set of directions for what to do, when to do it, and how much of it to do. All of these directions are tightly packed in the cell s nucleus, a large, dark structure in the cell. Inside the nucleus are long, thin strands of chromatin, a molecule made of deoxyribonucleic acid, or DNA. The chromatin is a nucleic acid made up of monomers called nucleotides, which are composed of a nitrogen base, a sugar, and a phosphate group (see Figure A). Figure A.
2 Let s take a look at the DNA inside of a cell s nucleus. If we had a magic microscope, we could see that a strand of chromatin is made of two strands of DNA that are arranged in a spiral shape, or DOUBLE HELIX (see Figure B). Figure B.
3 A Molecule That Has a Familiar Shape If we could take a strand of chromatin and straighten it out so that the helix shape is gone, it would look like a ladder (see Figure C). the outside parts of the ladder are made of alternating sugar (S) and phosphate (P) molecules. The sugar in a DNA molecule is deoxyribose. Figure C. The rungs of the DNA ladder are made of pairs of nitrogen bases. There are only four nitrogen bases in any DNA molecule: thymine, Adenine, guanine, and cytosine. Thymine and adenine fit together like two puzzle pieces, and guanine and cytosine fit together. Because of this special fit, thymine never pairs with anything but adenine, and guanine never pairs with anything but cytosine. The DNA Code How does DNA direct the cell and tell it exactly what to do? The sequence of the nigrogen bases in a DNA strand forms a code. This code determines which protein will be made. Proteins, important molecules that make up most of a living thing, are composed of strings of amino acids. A triplet, the sequence of three nucleotides on the DNA strands, codes for a particular amino acid.
4 Protein Synthesizers The parts of the cell that make products, like ribosomes, are out in the cytoplasm. Ribosomes assemble proteins. DNA is in the nucleus and is too large to leave the nucleus through pores in the nuclear membrane. Therefore, DNA has to send a message to a ribosome to tell it what kind of protein to assemble. The messenger that DNA makes and sends is called messenger ribonucleic acid, or mrna. DNA also makes other types of RNA called transfer RNA (trna) and ribosomal RNA (rrna). RNA molecules are very similar to DNA but differs from DNA in four ways: RNA is single stranded and can leave the nucleus by passing through the nuclear pores, while DNA is double stranded and stuck in the nucleus The sugar ribose is found in RNA, while deoxyribose is found in DNA RNA does not have any of the nitrogen base, thymine (found in DNA), but instead has uracil; uracil can pair with adenine RNA exists in three forms (trna, rrna, mrna), while DNA only has one form DNA s Message To make RNA, the enzyme RNA polymerase spreads the DNA molecule in the cell s nucleus in a V formation. RNA polymerase then causes RNA nucleotides floating around in the nucleus to line up along one side of the open DNA the side which contains the code for the desired protein. They arrange themselves so that adenine can pair with uracil and guanine can pair with cytosine (see Figure D). This process of creating a complementary copy of RNA from one side of a DNA strand is called transcription. Figure D.
5 When the mrna strand is complete, it leaves the nucleus and enters the cell s cytoplasm, where it attaches to a ribosome. Within the cytoplasm are all the parts necessary for the construction of a new protein. Amino acids, the monomers of proteins, are floating around in the cytoplasm (see Figure E). The role of mrna is to arrange amino acids in the correct order to make a particular protein. This process is called protein synthesis, but is also known as translation. There are 22 different amino acids, and their arrangement in a protein is of vital importance. Just one mistake in amino acid sequencing can produce a defective protein. Figure E. Codons and Anticodons On the mrna, three nucleotides that correspond to a particular amino acid are called a codon. The mrna is carrying the correct sequence of codons needed to make a protein. The problem is that something needs to deliver the correct amino acids to their spot on the mrna. This is the role of transfer RNA, or trna. There are 22 amino acids and more than 22 trnas. On the trna, the spot that matches or base pairs with the mrna s codon is called the anticodon.