Nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the genetic material of cells, their names derived from type of sugar, ribose, contained within the molecules.
Nucleic Acid Structure
Nucleotides are the building blocks of these nucleic acids. Each nucleotide is a monomer of nucleic acid and consists of 3 portions:
- a pentose sugar
- one or more phosphate groups
- one of five cyclic nitrogenous bases
Phosphate-Sugar Backbone
Nucleotides are linked together by covalent bonds between phosphate of one nucleotide and sugar of next. These linked monomers become the phosphate-sugar backbone of nucleic acids. Nitrogenous bases extending from this phosphate-sugar backbone like teeth of a comb.
The Nucleic Acid “Ladder”
Hydrogen bonds form between specific bases of two nucleic acid chains, forming a stable, double-stranded DNA molecule, which looks like a ladder
Three H bonds form between bases cytosine (C) and guanine (G), which always pair up together between two nucleic acid chains. Two H bonds form between adenine (A) and thymine (T) in DNA or adenine and uracil (U) in RNA molecules.
The structure is analogous to a ladder, with the two deoxyribose-phosphate chains as side rails and the base pairs, linked by hydrogen bonds, forming the rungs.
How Do Nucleotides Put Themselves Together Into Nucleic Acids?
The construction of a nucleic acid is an anabolic polymerization process. Anabolic reactions build bigger molecules. Polymerization is the process of taking nucleotide monomers and putting them together into polymers (large molecules composed of many monomers).
These three-phosphate nucleotide building blocks of DNA bring their own energy for polymerization within their phosphate bonds. When the triphosphate bond of the nucleotide is broken, it contributes the energy required add another nucleotide to the growing nucleic acid.
Replication, Transcription and Translation
The three main activities that nucleic acids are involved in are:
- Replication: The duplication of genetic material.
- Transcription: Transcribing the genetic code of DNA into RNA molecules.
- Translation: RNA following DNA’s instructions for building proteins from amino acids.
Replication
Before a cell divides, it must make a copy of its DNA so that both parent and daughter cell have a complete copy of genetic information. This process of copying the double-stranded DNA molecule is called replication. This process takes place in the nucleus of eukaryotic cells and the nucleoid region in prokaryotes.
Semiconservative Replication
Each DNA strand holds the same genetic information, so both strands can serve as templates for the reproduction of the opposite strand. The template strand is preserved and the new strand is assembled from nucleotides (semiconservative replication). The resulting double-stranded DNA molecules are identical.
Transcription
This is the process by which a DNA sequence is copied to produce a complementary strand of RNA. In other words, it is the transfer of genetic information from DNA into RNA. The process is similar to replication, but in this case, RNA is being built, rather than DNA. Transcription is the beginning of the process that ultimately leads to the translation of the genetic code into a peptide or protein.
Three types of RNA are transcribed from DNA:
- Messenger RNA (mRNA)
- Ribosomal RNA (rRNA)
- Transfer RNA (tRNA)
Messenger RNA is a copy of the genetic information that was transcribed from the DNA.
This copy is brought to the ribosome and “decoded” by tRNA and rRNA. Ribosomal RNA (rRNA) is part of the structure of ribosomes, the cellular protein factories where peptides are built. Transfer RNA (tRNA) brings to the ribosome the amino acids that mRNA coded for.
Translation
Ribosomes (which contain rRNA) make proteins from the messages encoded in mRNA.
Each three nucleotide group, called a codon, encodes one amino acid. This is the genetic code. In other words, the triplet code of genetic instructions for a polypeptide chain is ‘written’ in the DNA as a series of 3-nucleotide ‘words.’
These genetic instructions are brought to the ribosome by mRNA, decoded by rRNA, and tRNA brings the amino acid monomers that were coded for in the base triplet of mRNA. Amino acids are monomers that, when linked together with peptide bonds, ultimately become a protein molecule, the end product of translation.
Additional Cell Biology Information
To learn more about cell biology and organic chemistry, see the websites Science Prof Online, the Virtual Cell Biology Classroom or Cells Alive.
Sources
Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.
