John E. McMurry et al. - Fundamentals of General, Organic and Biological Chemistry in SI Units Chapter 26

26.1 DNA, Chromosomes, and Genes

Learning Objective: Explain the role of chromosomes, genes, and DNA in the human body.

• Chromatin is the form of DNA found in non-dividing cells, consisting of DNA wrapped around proteins called histones that help package the DNA into a compact, organized structure.

• During cell division, chromatin undergoes a condensation process to form visible chromosomes, with each chromosome containing a distinct DNA molecule that carries genetic information.

• Humans typically have 46 chromosomes, arranged in 23 pairs, where one chromosome of each pair comes from each parent, contributing to genetic diversity.

• Each chromosome is composed of many genes, which are segments of DNA that code for proteins essential for various functions in the body. Not all genes translate into proteins; some genes code for functional RNA molecules, which have roles in regulating gene expression and other cellular processes.

26.2 Composition of Nucleic Acids

Learning Objective: Describe and identify components of nucleosides and nucleotides.

• Nucleic acids, including DNA and RNA, are polymers made up of repeating units called nucleotides.

• Nucleotide Structure: Each nucleotide is comprised of three components: a five-carbon sugar (deoxyribose in DNA and ribose in RNA), a nitrogenous base (either a purine or pyrimidine), and a phosphate group.

• Nucleoside: A nucleoside is a molecule consisting of a sugar and a nitrogenous base without the phosphate group.

• The nitrogenous bases are classified into two groups: purines (adenine [A] and guanine [G]) and pyrimidines (cytosine [C] and thymine [T] in DNA; cytosine [C] and uracil [U] in RNA).

26.3 The Structure of Nucleic Acid Chains

Learning Objective: Identify nucleic acid chains in DNA and RNA.

• Nucleotides bond together through phosphate diester linkages, which involve the 3′ -OH group of one nucleotide connecting to the 5′ phosphate group of another nucleotide.

• Each nucleic acid strand has directional ends—the 5′ end has a phosphate group, while the 3′ end has a hydroxyl (-OH) group, which is crucial for the directionality of the replication and transcription processes.

26.4 Base Pairing in DNA: The Watson–Crick Model

Learning Objective: Interpret the structure of DNA and write complementary sequences.

• DNA consists of two strands that coil around each other to form a double helix structure.

• According to the Watson-Crick base pairing model, adenine (A) pairs with thymine (T) through two hydrogen bonds, while cytosine (C) pairs with guanine (G) through three hydrogen bonds, providing stability to the DNA molecule.

• The complementary nature of the strands ensures that they run in opposite directions (antiparallel), with one strand oriented 5′ to 3′ and the other 3′ to 5′, allowing for accurate replication and transcription processes.

26.5 Nucleic Acids and Heredity

Learning Objective: Describe genetic information and its processes.

• Heredity is determined by the DNA inherited from parents, which contains the genetic blueprint for an organism's traits and characteristics.

• This DNA dictates traits through three vital processes: replication (copying the DNA), transcription (synthesizing RNA from DNA), and translation (synthesizing proteins from RNA).

• Genetic variation among individuals contributes to evolutionary processes and diversity within populations.

26.6 Replication of DNA

Learning Objective: Explain the process of DNA replication.

• Replication begins with the unwinding of the double helix at specific origins of replication, creating Y-shaped structures called replication forks.

• DNA polymerases play a critical role in synthesizing new strands by adding nucleotides that are complementary to the template strands.

• In DNA replication, the leading strand is continuously synthesized in the direction of the replication fork, while the lagging strand is synthesized in short segments called Okazaki fragments, which are later joined together by the enzyme DNA ligase to create a continuous strand.

26.7 Structure and Function of RNA

Learning Objective: List types of RNA and their functions.

• RNA, or ribonucleic acid, is typically single-stranded and shorter than DNA, featuring ribose as the sugar and uracil in place of thymine.

• Major types of RNA include:

  • mRNA (messenger RNA): Carries genetic information transcribed from DNA to ribosomes, where protein synthesis occurs.

  • tRNA (transfer RNA): Transfers specific amino acids to the ribosome during protein synthesis, where it matches the amino acid to the appropriate codon on the mRNA.

  • rRNA (ribosomal RNA): A structural component of ribosomes, rRNA plays a critical role in the assembly of amino acids into proteins.

26.8 Transcription: RNA Synthesis

Learning Objective: Explain transcription processes.

• Transcription is the process by which mRNA is synthesized. RNA polymerase binds to a specific region of the DNA (the promoter) and unwinds a portion of the double helix to access the template strand.

• As RNA polymerase moves along the template strand, it synthesizes complementary RNA nucleotides, resulting in the formation of a single-stranded mRNA.

• Before the mRNA can leave the nucleus, it undergoes several modifications, such as 5′ capping, polyadenylation, and splicing (the removal of introns), to form a mature mRNA molecule ready for translation.

26.9 The Genetic Code

Learning Objective: Interpret mRNA codons and the protein sequence.

• The genetic code is composed of codons, which are three-nucleotide sequences found in mRNA that correspond to specific amino acids in proteins.

• Of the 64 potential codons, 61 codons encode amino acids, while three codons serve as stop signals, signaling the end of translation and the release of the newly synthesized polypeptide.

• The universality of the genetic code among most organisms highlights its evolutionary importance.

26.10 Translation: tRNA and Protein Synthesis

Learning Objective: Identify the steps of translation.

• Translation occurs at ribosomes, where the mRNA is read in sets of three nucleotides (codons) by tRNAs that are attached to specific amino acids.

• The process consists of three main steps:

  • Initiation: mRNA and the first tRNA carrying methionine bind to the ribosome, marking the start of translation.

  • Elongation: tRNAs sequentially add amino acids to the growing polypeptide chain as the ribosome moves along the mRNA, ensuring correct codon-anticodon pairing.

  • Termination: Translation concludes when a stop codon is encountered. The completed polypeptide chain is released from the ribosome, folding into its functional form with the help of chaperone proteins.

• This sequential addition of amino acids determines the final structure and function of the protein, which plays vital roles in the body.