Abstract
Two types of nucleic acids, DNA and RNA, carry genetic information of organisms across generations. Many researchers are credited with the early work that laid the foundation of the discovery of the structure of DNA. During cell division, the cell replicates its DNA and organelles during the synthesis (S) phase of the cell cycle. Four main steps are involved in the processes of replication. DNA replication errors and cells have evolved a complex system of fixing most (but not all) of those replication errors proofreading and mismatch repair. With repeated cell division, the DNA molecule shortens with the loss of critical genes, leading to cell death. In gonads, a special enzyme called telomerase lengthens telomeres from its own RNA sequence which serves as a template to synthesize new telomeres. Although most DNA is packaged within the nucleus, mitochondria have a small amount of their own DNA called mitochondrial DNA. This chapter explores this aspect of genes.
TopChapter Outline
7.1 Discovery of Nucleic Acids
7.2 The Elucidation of DNA Structure
7.3 How DNA Makes Copies (Replication)
7.4 When DNA Makes Mistakes Copying
7.5 End of the DNA Molecule
7.6 Mitochondrial DNA
7.7 DNA Organization in the Nucleus
Chapter Summary
TopLearning Outcomes
- •
Outline the nature and types of nucleic acids.
- •
Explain how DNA was discovered.
- •
Summarize the process of DNA replication.
- •
With examples demonstrate how DNA corrects mistakes during replication.
- •
Explain why DNA shortens after each replication.
- •
Explain inheritance of mitochondrial DNA.
- •
Describe how DNA is organized in the nucleus of a cell
Top7.2 The Elucidation Of Dna Structure
Many researchers are credited with the early work that laid the foundation of the discovery of the structure of DNA. In 1948, Linus Pauling, an American theoretical physical chemist, using X-ray diffraction techniques found out that proteins assume an alpha-helix shape spiraled like a spring coil. In the mid-1950s, Alfred Hershey and Martha Chase, doing experiments on viruses, discovered that when viruses infected bacteria, they injected DNA. Around the same time, Columbia University biochemist, Erwin Chargaff discovered two features of DNA: 1) that the amount of guanine bases was equal to those of cytosine bases; while those of adenine were equal to those of thymine; 2) the four bases A, T, C and G while constant for all living organisms, varied in amounts in each species. Note that two pairs of bases have similar structures—A and G both have two carbon-nitrogen rings (purines); while C and T have a single carbon-nitrogen ring (pyrimidines) (Figure 1).
Key Terms in this Chapter
Autoradiography: A technique that utilizes X-rays to visualize radioactively labeled molecules.
DNA Primase: Enzymes that create RNA primers complementary to DNA strands.
Endosymbiosis: A form of relationship in which one cell lives inside another to form and acts as a single organism.
Primer: Short RNA sequences created from DNA primase.
Primosome: A protein complex involved with DNA primases to synthesize RNA primers.
Initiator Proteins: Proteins that bind to the replicator and signal to begin replication.
Protein-DNA Complex: A structure of protein and DNA that have bound together to regulate cell replication.
Replication Origins: The starting point for DNA replication to begin its process.
X-Ray Diffraction: A method that determines the molecular structure of a substance using a beam of X-rays which diffract or spread out in specific patterns.
Mitochondrial DNA: Genetic material (DNA) in the mitochondria that carries the code to convert chemical energy into ATP.
Binding Proteins: Proteins that act as a link to bring two molecules together.
Apoptosis: Programmed cell death.
Leading Strand: A new DNA strand that is synthesized continuously in the 5’ to 3’ direction.
DNA Polymerase I: Enzyme that has proofreading ability, where it removes nucleotides from the 5’ end and replace with the correct DNA.
Mutation: An alteration in the DNA sequence due to an insertion, deletion or rearrangement of a single base pair or fragments.
Mismatch Repair: A repair system that corrects any errors in base pairing.
Telomerase: An enzyme that extends the telomeres in chromosomes to reverse shortened telomeres.
Chromatin: A DNA and protein complex that forms a chromosome.
Semi-Conservative Replication: A type of DNA replication in which the two new strands of DNA produced each contain an original strand and a new strand.
Nucleic Acids: Polymers that are made of nucleotides and form either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA).
Nuclein: A term which was previously used to describe a nucleic acid.
3’ OH End: Refers to the 3’ carbon of DNA and RNA sugar backbone that has a hydroxyl group attached to it.
Okazaki Fragments: Term used to label the synthesized DNA fragments on the lagging strand.
Telomeres: Protective chemical structures located at the end of a chromosome to prevent the loss of genes.
Nucleosome: DNA wrapped around histones, forming a repeating unit of chromatin.
Lagging Strand: A new DNA strand that is synthesized discontinuously in the 5’ to 3’ direction.
5’ Carbon End: Refers to the 5’ carbon of DNA and RNA backbone that has a phosphate group attached to it.
DNA Helicase: An enzyme that initiates the unwinding (or unfurling) of DNA during DNA replication.
Proofreading: Editing and correcting the errors that enzymes may have made while they were creating daughter strands.
DNA Polymerase III: A holoenzyme that catalyzes the synthesis of the complementary strand of DNA in the 5’ to 3’ end direction.
DNA Ligase: Enzyme that joins the fragmented nucleotides together via a phosphodiester bond.