DNA and RNA Structure, Function, and the Genetic Code Explained

DNA: Genetic Material

DNA is the genetic material made up of chains of deoxyribonucleotides (A, G, C, T). DNA, except in viruses, comprises two chains of nucleotides. It is found in viruses, in the nucleus of eukaryotic cells (in the form of linear molecules), and in the cytoplasm of prokaryotic cells, some viruses, and some organelles such as mitochondria and chloroplasts (circular shape).

Structure

Primary: consists of a linear sequence of nucleotides.

Secondary: double helix model (Watson & Crick, 1953):

1. Double chains of deoxyribonucleotides.
2. Complementary chains regarding the sequence of bases (A = T, G = C, hydrogen bridge bonds). Each nucleotide in one chain is linked to a nucleotide in the other.
3. In the double helix structure, the nitrogenous bases are arranged in the interior (in parallel).
4. To separate the chains, they have to unroll.

Tertiary: three-dimensional arrangement of secondary structure, forming chromatin or chromosome.

Functions

DNA contains and transmits the hereditary information, in the form of a sequence of bases, which will be used to synthesize proteins. It governs cellular functions.

RNA: Ribonucleic Acid

RNA is made up of chains of ribonucleotides (A, G, C, U). Generally, RNA consists of a single chain of nucleotides. It is found in the nucleus of eukaryotic cells, the cytoplasm of prokaryotic and eukaryotic cells, ribosomes, and some viruses.

Functions

RNA is responsible for synthesizing proteins, using DNA information (DNA → RNA → protein synthesis).

Types of RNA

• mRNA (messenger): copies DNA information. It takes this information to the ribosomes, which collaborate in the synthesis of proteins (transcription, protein synthesis).
• rRNA (ribosomal): forms part of the structure of ribosomes (translation, P.S.).
• tRNA (transfer): transports amino acids to ribosomes, where protein chains are formed (translation).

The Genetic Code

The genetic code is the correspondence between mRNA codons and the amino acids that form proteins (the genetic language). There are 64 correspondences (words).

1. Triplet code: each codon, or triplet, codes for specific amino acids.
2. Degenerate: amino acids can be coded by more than one codon or triplet (some triplets are synonyms; there are 64 codons for 20 amino acids).
3. Unambiguous: each codon encodes only one type of amino acid.
4. Universal: apart from a few small exceptions, it is identical in all living things.
5. Start and stop signals: AUG (Met) marks the beginning of translation. The end of translation is marked by UAA, UAG, or UGA, which do not code for any amino acid.

Stages of DNA Replication

1. The DNA unwinds and the two chains separate.
2. Each chain is duplicated independently, serving as a template for a new complementary DNA chain (A = T, G = C), by the enzyme DNA polymerase or replicase.
3. Two identical double helices are obtained, each made up of a new chain and an old one. This is known as semiconservative replication.