Chapter 7: Nucleic Acids and Proteins Fifteen Summary Facts

Worksheet 7.1 Chapter 7: Nucleic acids and proteins – fifteen summary facts 1 DNA has a rather simple structure with great capacity for variation. It is a double helix, made up of nucleotides. Its nucleotides are attached at the 5' and the 3' carbons of the pentose deoxyribose. 2 DNA includes covalent and hydrogen bonds. The hydrogen bonds allow linkage of the two chains of DNA between a purine and a pyrimidine nitrogenous base. Covalent bonds occur everywhere else within the DNA molecule. The covalent bonds are much stronger than the hydrogen bonds. Because of the weak hydrogen bonds between the bases of the two DNA chains, the DNA can be opened down the middle thus exposing the bases on both chains. 3 In eukaryotic cells, the DNA is wrapped around molecules of protein called histone. This allows effective packaging of DNA so that its relatively large amount may fit in the nucleus of a cell. DNA and histone together form nucleosomes. 4 The centromere of chromosomes is largely made up of highly repetitive sequences of DNA. 5 DNA replication occurs in a 5' to 3' direction and involves semiconservative replication. The two strands of DNA are said to be antiparallel. Therefore, replication of each strand occurs in opposite directions. Normally, DNA replication results in two exact copies of the parent DNA being produced. Changes in DNA are usually harmful and are called mutations. 6 The enzymes necessary for DNA replication are helicase, primase, DNA polymerase III, DNA polymerase I, and DNA ligase. 7 There are three types of RNA involved in protein synthesis within the cell. They are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). The central dogma of biology is summarized as: DNA  RNA  protein The first arrow represents transcription and the second arrow represents translation. 8 The antisense strand of DNA is the strand that is copied during transcription. The other strand is referred to as the sense strand; it has the same base sequence as the newly transcribed RNA except that thymine is present in the place of uracil. 9 The promoter region for a particular gene determines which DNA strand is the antisense strand. The promoter region is a short sequence which is not transcribed. The terminator region causes the RNA polymerase to detach from the DNA, thus stopping transcription. Nucleoside triphosphates provide the energy for polymerization of the mRNA strand to occur. 10 Translation occurs at the ribosomes and involves mRNA, rRNA, and tRNA. The end result of translation is a protein produced to the specifications of the DNA in the nucleus. Each ribosome is made up of a large and a small subunit.

© Pearson Education Ltd 2010. For more information about the Pearson Baccalaureate series please visit 1 www.pearsonbacc.com 11 There are three binding sites for tRNA in the ribosome between the two subunits. These are the A, P, and E sites. The translation process involves initiation, elongation, translocation, and termination. 12 Proteins have four levels of organization:  primary organization is the unique sequence of amino acids  secondary structure occurs as a result of hydrogen bonds between components of the carboxyl and the amino groups  tertiary structure involves the polypeptide chain bending and folding because of interactions among R-groups and the peptide backbone  quaternary structure occurs when multiple polypeptide chains combine to form a single structure. 13 Proteins may be fibrous or globular, polar or non-polar. 14 Enzymes are quite specific and work in a way explained by the induced-fit model. This model states the need for a close fit between an enzyme’s active site and the substrate. Once the active site and substrate are together, a conformational change occurs allowing the formation of an enzyme–substrate complex. This lowers the activation energy of a reaction thus allowing the reaction to proceed more rapidly. 15 Inhibition of an enzyme may occur in several ways: competitive inhibition, non-competitive inhibition, and end-product inhibition. Active sites and allosteric sites of an enzyme are involved in different types of inhibition.