![]() ![]() The chains of amino acids establish the primary structure of a protein, but interactions (both attractive forces and repulsive forces) among the components of the chain reshape the protein into its filnal three-dimensional structure. Proteins are sometimes referred to as polypeptides because they consist of chains of amino acids linked together with peptide bonds. Now imagine that dozens or even hundreds of amino acides are linked together in chains of varying length to create the primary structure of a protein. The primary structure of proteins results from linking together various combinations of these 20 amino acids with peptide bonds, which link the carboxyl group of one amino acid to the amino group of another amino acid. What distinguishes one amino acid from another is the side chain of atoms that is also bonded to the alpha carbon (designated "R-group on the right). ![]() As shown below, each of these 20 amino acids has a central carbon (the alpha carbon) bonded to an amino group (-NH 2 i.e., nitrogen bonded to two hydrogens) at one end and a carboxyl group ( -COOH) at the other end. All proteins are made from combinations of the 20 amino acids show below. Proteins are another class of enormously diverse organic molecules that are made from multiple units of simpler molecules arranged in chains. Enzymes also attach to the ribosomal complex and aid in the process of translation, in which the coded sequence of bases on the mRNA is translated and directs the synthesis of a chain of amino acids, which are the building blocks of proteins. Once the mRNA emerges from the nucleus, it attaches to a two part structure called a ribosome, which consists of ribosomal RNA (rRNA). The mRNA then leaves the nucleus through special pores in the membrane of the nucleus. Transcription takes place inside the cell nucleus where chromosomal DNA is located. Transcription is the process by which a gene, segment of DNA that encodes for a specific protein, serves as a template for the synthesis of a messenger RNA (mRNA) for that specific protein. This short animation from the Discovery Channel provides a nice overview of the transcription and translation. An Overview of Transcription and Translation For the time being the video below provides an overview of this process that will be helpful. This process will be clearer when we explore it in more detail in another online module. This newly synthesized messenger RNA will then leave the cell nucleus and move to the cytoplasm of the cell where the RNA will in turn be used as a template to synthesize a specific protein. In a highly regulated process, cellular enzymes can unwind a particular segment (gene), and other enzymes move along a gene using one strand of DNA as a template to synthesize a complementary strand of messenger RNA. ![]() A single chromosome contains thousands of genes, segments of DNA that encode for specific proteins. In essence, each chromosome is a gigantic molecule of double stranded DNA wound tightly into a double helix. The cells of living organisms have chromosomes which contain an inherited code for synthesizing all of the proteins that the organism produces. The bases found in DNA are limited to adenine, cytosine, guanine, and thymine RNA has adenine, cytosine, and guanine, but hase another base called uracil instead of thymine.The double strands are wrapped to form a double helix. DNA consists of two nucleotide chains that are bonded to together by weak hydrogen bonds between complementary base pairs.DNA contains the sugar deoxyribose, while RNA contains the sugar ribose.There are several differences between DNA and RNA. Alternating sugar molecules and phosphate groups are bonded together to form the backbone of the nucleic acid, and a purine or pyrimidine base is bonded to each of the sugars, as illustrated below. These molecules are also polymers of smaller units called nucleotides each nucleotide consist of a sugar (ribose or deoxyribose), a phosphate group, and one of several "bases" that are either purines or pyrimidines. There are two types of nucleic acids that are important to living things. ![]()
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