Chapter 03: Genes, DNA Structure, and the Genetic Code

                

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Overview of Genetic Control in Cells

The nucleus of every cell houses DNA (deoxyribonucleic acid), which serves as the master blueprint controlling virtually all cellular structures and functional activities. Genes, arranged in long chains within DNA, dictate cell function by regulating the synthesis of structural proteins and functional enzymes. This genetic control operates primarily through a two-step pathway: transcription, where the DNA code is copied into messenger RNA (mRNA) inside the nucleus, and translation, where ribosomes in the cytoplasm read the mRNA sequence to assemble specific amino acids into functional proteins.

Beyond everyday protein synthesis, genetic control governs the cell lifecycle through tightly regulated gene expression and DNA replication. Specific regulatory proteins, such as transcription factors, bind to gene promoter regions to turn synthesis on or off in response to internal signals or external hormones. When a cell prepares to divide, the entire DNA molecule undergoes semi-conservative replication, duplicating every chromosome so both daughter cells receive a complete genetic set. Precise feedback mechanisms ensure that cellular growth, enzyme activity, and cell division proceed in a balanced, highly controlled manner.

• Genes located in cell nuclei control parent-to-child heredity.
• Genes control day-to-day functions of all body cells.
• Genes determine synthesized intracellular structures, enzymes, and chemicals.
• Deoxyribonucleic acid is abbreviated as cellular DNA.
• DNA automatically controls ribonucleic acid formation.
• Ribonucleic acid is abbreviated as cellular RNA.
• RNA controls formation of specific cellular protein molecules.
• The process from transcription to translation is gene expression.
• Each cell contains approximately 30,000 different genes.
• Structural proteins form frameworks for various intracellular organelles.
• Most cellular proteins act as catalytic enzyme molecules.
• Enzymes catalyze oxidative reactions supplying energy to cells.
• Enzymes promote synthesis of lipids, glycogen, and ATP.

DNA Double-Helical Structure in the Nucleus:

• Genes attach end-to-end in long double-stranded DNA.
• DNA molecules have molecular weights measured in billions.
• DNA contains simple chemical compounds bound regularly together.
• DNA contains phosphoric acid backbone components.
• DNA contains a sugar called deoxyribose.
• DNA contains four nitrogenous purine and pyrimidine bases.
• Purine bases include adenine and guanine molecules.
• Pyrimidine bases include thymine and cytosine molecules.
• Phosphoric acid and deoxyribose form helical strand backbones.
• Nitrogenous bases lie between and connect helical strands.

Basic Nucleotide Units of DNA:

• Combining acid, sugar, and base forms acidic nucleotides.
• Four separate nucleotides exist for four nitrogenous bases.
• Deoxyadenylic acid forms the adenine nucleotide unit.
• Deoxythymidylic in acid forms the thymine nucleotide unit.
• Deoxyguanylic acid forms the guanine nucleotide unit.
• Deoxycytidylic acid forms the cytosine nucleotide unit.

Organization of DNA Strands:

• Multiple nucleotides bind forming two long DNA strands.
• Alternating phosphoric acid and deoxyribose form backbones.
• Bases attach directly to sides of deoxyribose sugars.
• Two DNA strands loosely bond via hydrogen linkages.
• Adenine on one strand always bonds with thymine.
• Guanine on one strand always bonds with cytosine.

The Genetic Code and Triplet Words:

• DNA controls protein formation via the genetic code.
• Splitting DNA strands exposes projecting base code sequences.
• Genetic code consists of successive three-base triplets.
• Base triplets act as specific genetic code words.
• Triplets control amino acid sequences in synthesized proteins.
• Successive triplets determine placement of specific amino acids.
• Triplets code for proline, serine, and glutamic acid.

Transcription and RNA Synthesis:

Overview of Transcription:

• DNA resides inside the cell nucleus.
• Cellular functions occur mostly in the cytoplasm
• Nuclear DNA controls cytoplasmic functions using RNA.
• DNA code transfers to RNA via transcription.
• RNA diffuses through nuclear pores into cytoplasm.
• Cytoplasmic RNA controls cellular protein synthesis.

Basic Building Blocks and Nucleotides of RNA:

• DNA strands separate temporarily during RNA synthesis.
• One DNA strand serves as the template.
• DNA code triplets form complementary RNA codons.
• RNA uses ribose sugar instead of deoxyribose.
• Ribose contains an extra hydroxyl group ring.
• RNA uses uracil base instead of thymine.
• RNA forms four distinct types of nucleotides.
• RNA nucleotides contain adenine, guanine, cytosine, uracil.

Activation of RNA Nucleotides:

• RNA polymerase enzymes activate new RNA nucleotides.
• Activation adds two extra phosphate radicals.
• High-energy phosphate bonds derive from cellular ATP.
• Activation produces high-energy triphosphate nucleotides.
• Released ATP energy drives RNA chain linkage.

Assembly of the RNA Chain:

• RNA polymerase enzyme regulates RNA chain assembly.
• Polymerase recognizes promoter nucleotide sequences on DNA.
• Promoter binding initiates RNA chain formation process.
• Polymerase unwinds two turns of DNA helix.
• Polymerase separates two unwound DNA strands.
• Polymerase moves sequentially along the DNA strand.
• Hydrogen bonds link DNA bases with RNA bases.
• Polymerase breaks two high-energy phosphate bonds.
• Released energy covalently links ribose and phosphate.
• Polymerase reaches DNA chain-terminating nucleotide sequence.
• Polymerase and completed RNA release from DNA.
• Weak DNA-RNA hydrogen bonds break spontaneously away.
• Releasing RNA enters nucleoplasm for export.
• Complementary bases pair predictably during transcription.
•  Guanine pairs with cytosine.
•  Cytosine pairs with guanine.
•  Adenine pairs with uracil.
•  Thymine pairs with adenine.