Nucleic Acids, DNA Replication, and the Human Genome
1.1. Types of Nucleic Acids
- DNA: Contains deoxyribose as the pentose sugar; nitrogenous bases are adenine, guanine, cytosine, and thymine.
- RNA: Contains ribose as the pentose sugar; nitrogenous bases are adenine, guanine, cytosine, and uracil.
1.3. Deoxyribonucleic Acid (DNA)
DNA contains all the genetic information necessary for the operation and development of a living being. In most eukaryotic organisms, DNA is found in the nucleus as chromatin or chromosomes, depending on the cell cycle stage; it is also present in mitochondria and chloroplasts. Its structure was established by Crick and Watson.
Features:
- Each molecule consists of two polynucleotide chains coiled into a double helix. The pentose and phosphate groups form the external skeleton, while the nitrogenous bases are arranged inwards.
- The two chains are antiparallel, arranged in opposite directions.
- The chains are held together by hydrogen bonds. They are complementary: adenine always pairs with thymine, and guanine with cytosine.
1.4. Ribonucleic Acid (RNA)
RNA participates in the expression of information contained in DNA by synthesizing proteins, which regulate most life processes. In eukaryotic cells, RNA is localized in the nucleus and cytoplasm and consists of a single polynucleotide chain.
Types of RNA:
- Messenger RNA (mRNA): Copies genetic information from nuclear DNA and transports it to ribosomes.
- Ribosomal RNA (rRNA): Associates with proteins to form ribosomes, where proteins are synthesized.
- Transfer RNA (tRNA): Binds and transports amino acids to the ribosome to form proteins.
2. DNA Replication
DNA replication ensures that daughter cells resulting from mitosis receive the same genetic information as the parent cell. It takes place at the end of interphase and requires nucleotide molecules and a series of enzymes that control the process.
Process:
- Hydrogen bonds linking the nitrogenous bases of both strands break, and the two strands begin to unwind.
- Once separated, specific enzymes read the information and join the complementary nucleotides.
- As the new chains form, hydrogen bonds between the bases are established. The result is two DNA molecules identical to each other and to the parent molecule.
The process is semiconservative, as each new helix retains one original strand and one new chain. In cells, repair enzymes exist that detect incorrectly paired nucleotides, withdrawing and replacing them with the correct ones.
13. The Human Genome Project
The main objective was to determine the order of nucleotides in the DNA sequences of human chromosomes. Other objectives include:
- Identifying all human genes and locating them on the chromosomes.
- Sequencing each gene to determine the order of nucleotides.
- Determining the function performed by each gene.