In writing nucleotide sequences for nucleic acids, the convention is to write the nucleotides (usually using the one-letter abbreviations for the bases, shown in Figure 10.5 "Structure of a Segment of DNA") starting with the nucleotide having a free phosphate group, which is known as the 5′ end, and indicate the nucleotides in order. For amino acid sequences in proteins, the convention is to write the amino acids in order starting with the N-terminal amino acid. Unlike proteins, which have 20 different kinds of amino acids, there are only 4 different kinds of nucleotides in nucleic acids. Like proteins, nucleic acids have a primary structure that is defined as the sequence of their nucleotides. We begin by looking at the small molecules needed to form DNA and RNA (ribonucleic acid)-the nucleotides.įigure 10.5 Structure of a Segment of DNAĪ similar segment of RNA would have OH groups on each C2′, and uracil would replace thymine. If genes are segments of DNA, we need to learn more about the structure and physiological function of DNA. But what really are genes and how is the information they contain expressed? One definition of a gene is that it is a segment of DNA that constitutes the code for a specific polypeptide. A new individual receives half its hereditary material from each parent.Ĭalling the unit of heredity a “gene” merely gives it a name. Thus, in sexual reproduction, the entire complement of chromosomes is achieved only when an egg and sperm combine. Sperm and egg cells contain only a single copy of each chromosome that is, they contain only one member of each chromosome pair. Human body cells have 23 pairs of chromosomes having 20,000–40,000 different genes. The number of chromosomes (and genes) varies with each species. These complex structures, consisting of DNA and proteins, contain the basic units of heredity, called genes. What accounts for this specificity at all levels of reproduction? How does a fertilized egg “know” that it should develop into a kangaroo and not a koala? What makes stomach cells produce gastric acid, whereas pancreatic cells produce insulin? The blueprint for the reproduction and the maintenance of each organism is found in the nuclei of its cells, concentrated in elongated, threadlike structures called chromosomes. Furthermore, within each multicellular organism, every tissue is composed of cells specific to that tissue. From viruses to humans, each species reproduces after its own kind. Foxes have kits that grow up to be foxes. “Mutations and Health” by U.S.Dogs have puppies that grow up to be dogs. This type of mutation can cause the resulting protein to function in a completely different way than it would have originally. A repeat expansion is a mutation that increases the number of times that the short DNA sequence is repeated. For example, a trinucleotide repeat is made up of 3-base- pair sequences, and a tetranucleotide repeat is made up of 4-base-pair sequences.
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