Àcids Nucleics: ADN i ARN

Nucleic Acids

• Nucleic acids contain information relevant to the cell.
• Nucleic acids are complex molecules that carry genetic information and are made of carbon, oxygen, hydrogen, nitrogen, and phosphorus.
• Nucleic acids were discovered in 1869 by Friedrich Miesner.
• He isolated a substance inside the nucleus of white blood cells and named it nuclein and it was later named nucleic acid due to its acidic nature.

Structure of Nucleic Acids

• Nucleic acids are macromolecules formed by the union of simpler molecules called nucleotides.
• Nucleotides consist of three types of molecules: a phosphoric acid residue (H3PO4), a five-carbon monosaccharide (pentose), and a nitrogenous base.

Types of Nucleic Acids

• There are two types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
• DNA contains the genetic information for living beings- viruses contain either DNA or RNA.
• Some viruses, such as SARS-CoV-2 and other coronaviruses, do not have DNA; their genetic information is in the form of RNA.
• Composition of DNA: deoxyribonucleotides consisting of phosphoric acid residue, deoxyribose, and nitrogenous bases (adenine, guanine, cytosine, and thymine).
• Composition of RNA: ribonucleotides consisting of phosphoric acid residue, ribose, and nitrogenous bases (adenine, guanine, cytosine, and uracil).
• DNA has two polynucleotide molecules joined together by the nitrogenous bases forming a double helix structure.
• RNA has one polynucleotide molecule forming a single-stranded structure.
• DNA is found in the nucleus, mitochondria, and chloroplasts.
• RNA is located in the cytoplasm either freely or bound to ribosomes.
• DNA carries the genetic information of the organism.
• RNA is involved in protein synthesis.

The Gene

• Genes are the bearers of biological information located inside cells.
• A gene is a segment of DNA that is a unit of hereditary transmission, responsible for the information that provides characteristics to a cell and an organism.
• The concept of a gene has evolved greatly over the last two centuries.

DNA Structure

• The DNA molecule is double-stranded, formed by two chains of polynucleotides joined by hydrogen bonds between their nitrogenous bases.
• In DNA, adenine (A) always binds with thymine (T), and cytosine (C) with guanine (G) - the chains are complementary, maintaining the same distance between the two strands.
• The structure resembles a spiral staircase, with the bonds as the steps: it curls up onto itself like two intertwined springs.

DNA Replication

• Replication or self-duplication is a unique ability of DNA, where it makes copies of itself - allowing cells to divide and pass on the genetic information.
• The mechanism involves obtaining two daughter molecules from the original molecule and it occurs in the cell nucleus.
• The new molecules are formed by one of the nucleotide chains of the original DNA and a newly created one, which is complimentary.
• Replication is semiconservative where original strands remain intact and serve as templates for new strands.

Gene Expression

• The expression of a gene is the process by which organisms transform the information contained in nucleic acids into proteins.
• The gene expression happens through mechanisms that are similar for different living beings.
• The Central Dogma of Molecular Biology: DNA -> RNA -> Protein

Transcription

• Transcription is the process by which a part of the genetic message is copied from its original form, DNA, into messenger RNA (mRNA).
• It occurs in the cell nucleus - the resulting messenger RNA (mRNA) exits to the cytoplasm through the pores of the nuclear envelope.
• This mRNA is complementary, in its sequence of bases, to the DNA fragment that has been transcribed, corresponding to the sequence encoding for a specific protein. that the cell needs.

Translation

• Translation is the process of synthesizing proteins from the information contained in the mRNA formed during transcription.
• The mechanisms that originate the new proteins take place inside the ribosomes.
• The following elements are needed for the translation to take place: a molecule of mRNA, ribosomes, transfer RNA (tRNA), and amino acids.

The Genetic Code

• The genetic code is the relationship between the sequence of nitrogenous bases of mRNA and that of the amino acids that constitute a protein.
• Every three nucleotides (triplet) are needed to encode for an amino acid- triplets of DNA are called gene codons, and those of mRNA, codons.
• There are 64 different codons: 61 code for amino acids, one indicates the start of translation, and two indicate the end of protein synthesis.
• The genetic code was deciphered between 1955 and 1966 where Severo Ochoa and his team identified an enzyme capable of synthesizing RNA (RNA polymerase).

Characteristics of the Genetic Code

• Organized in triplets or codons, each three bases determine an amino acid.
• Degenerate: some amino acids can be coded for by more than one triplet of bases, because there are more possible triplets (64) than amino acids (20).
• No overlapping: each nucleotide of the nucleic acid belongs to a single triplet.
• Without spaces: there are no spaces in the reading of the code.
• Universal: in all species, the same triplets code for the same amino acids.

Genetic Engineering Techniques

• Recombinant DNA: an artificial DNA molecule obtained from the gene of an organism and a vector.
• Vectors: agents that transfer genetic information between two organisms,
• PCR: Polymerase Chain Reaction

Cloning

• Cloning is the process of obtaining identical copies of a gene (gene cloning) or of an organism (organism cloning).
• Gene cloning has multiple applications, such as the large-scale production of proteins, like insulin which is synthesized in modified bacteria.
• Animal Cloning Techniques: involves an egg, from which the nucleus is extracted, and another cell from the individual to be cloned.
• Therapeutic Cloning: cloning tissues or organs to transplant them into sick patients.
• Reproductive Cloning: cloning genetically identical individuals.