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CHAPTER 4- GENETICS AND CELLULAR FUNCTION The Nucleus and Nucleic Acids Nuclear Structure. The nucleus is surrounded by a nuclear envelope with passages in it called nuclear pores. The material within the nucleus is nucleoplasm. Nuceloplasm contains one or more masses of RNA, called nucleoli DNA and associated proteins make up the chromatin in the nucleus. Organization of the Chromatin Chromatin looks like a granular thread with each granule (Nucleosome) consisting of a cluster of proteins called histones and DNA wound around each cluster. Histones serve as spools to organize the DNA. Other proteins (nonhistones) provide structural support for the chromatin and regulate gene activity. DNA Structure and Function Nucleic acids are made of monomers called nucleotides. Each nucleotide is made up of a monosaccharide, a phosphate group and a nitrogenous base. Cystosine, thymine and uracil are pyrimidine bases because of their single carbon- nitrogen ring. Adenine and guanine are the purines and have double rings. The ladder-like structure of DNA is arranged with a monosaccharide- phosphate backbone as the “Uprights” and base pairs as the “rungs”. The “ladder” is twisted, making the DNA molecule a double helix. In the base pairs, adenine always pairs with thymine, and cytosine with guanine. This arrangement is called the law of complementary base pairing. DNA serves as a code for the structure of the proteins manufactured within the cell. A gene is a sequence of DNA nucleotides that codes for a specific protein.
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The human genome contains 30,000 genes accounting for only 3% of the DNA, the rest is “junk”. RNA Structure and Function RNA is a single strand of nucleotides with the base uracil instead of thymine. It also contains ribose instead of the deoxyribose found in DNA. The primary functions of RNA are to carry the instructions from DNA to the cytoplasm and to direct the synthesis of proteins. Protein Synthesis and Secretion Preview: 1. DNA contains the genetic code that specifies which proteins a cell can make as well as the synthesis of all enzymes. 2. Although each cell (except sex cells) has the same DNA, different genes are activated in different cells. 3. Messenger RNA (mRNA) copies the DNA template and carries it to the cytoplasm to a ribosome, where the genetic code is “read”. 4. Transfer RNA molecules deliver the correct amino acids to the
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