Complementarity of nitrogenous bases
The structure of the double helix is based on strict correspondence between nucleotides of opposite strands. The function of various chemical substituents in purine and pyrimidine ring molecules is to ensure the most accurate and specific pairing of nitrogenous bases. This interaction is carried out exclusively through hydrogen bonds formed between strictly defined pairs.
The fundamental principle of complementarity states that pair formation occurs according to an invariable rule:
- Adenine (A) always binds to Thymine (T). To hold this pair together, the spatial configuration of the molecules allows for the formation of exactly two hydrogen bonds.
- Guanine (G) always pairs with Cytosine (C). In this case, the chemical structure of the rings dictates the formation of three hydrogen bonds, making this bond somewhat stronger.
When a new polynucleotide chain is assembled, each successive nucleotide can be incorporated into it only if it is able to form the correct number of hydrogen bonds with the corresponding nucleotide located on the template (parent) strand.
DNA replication mechanism
In order for genetic information to be passed from the mother cell to daughter cells during cell division, prior doubling of the genetic material is necessary. This vital process is called replication.
The doubling process proceeds in several consecutive stages:
- Initiation of the process. At this stage, local unwinding of the DNA double helix occurs. Hydrogen bonds between complementary nitrogenous bases break, exposing the nucleotide sequence.
- Template synthesis. After the strands have separated, each of the two resulting single polynucleotide chains begins to function as a template. Based on these templates, strictly following the principle of complementarity described above, a new polynucleotide chain is synthesized.
- Final result. Upon completion of synthesis, instead of one original parental molecule, two identical DNA double helices are formed. Each of them contains one "old" (template) and one newly synthesized strand.
Gene expression and protein biosynthesis
In addition to ensuring cell division, DNA is essential for the cell's ongoing vital activity, which is manifested in the process of gene expression. On one of the DNA strands, which in this case serves as a template, the synthesis of messenger RNA (mRNA) is carried out. This process of rewriting information from the language of DNA to the language of RNA is called transcription.
The further pathway of the mRNA molecule is transport from the nucleus to the site of direct protein assembly in specialized organelles—ribosomes. It is there that the realization of the information embedded in the genetic code takes place. A crucial feature of the genetic code is that the amino acid sequence in the future protein is strictly encoded by triplets—groups of three nucleotides.
Molecules of transfer RNA (tRNA) play a critical role in translating information from the language of nucleotides into the language of amino acids:
- Each existing amino acid has its own specific tRNA molecule.
- The chemical attachment of an amino acid to its corresponding tRNA occurs through the formation of a strong ester bond.
- This delicate process does not occur spontaneously; it is catalyzed by a special, highly specific enzyme that is individual for each separate amino acid.
Historical background
All the processes described above—from the storage of information to its realization in the form of protein structures—constitute the subject matter of the fundamental science of molecular biology.
As an independent scientific discipline, molecular biology was formed relatively recently—in the mid-20th century. Its emergence and rapid development were made possible by a key event in the history of natural science: the successful deciphering of the spatial structure of DNA. This epoch-making work was performed by researchers James Watson and Francis Crick in 1953, which forever changed our understanding of the nature of living things.
Briefly: answers to topical questions
Which nitrogenous bases make up DNA nucleotides?
DNA nucleotides contain four nitrogenous bases, which are divided into purines and pyrimidines.
- Adenine is a purine base (6-aminopurine), contains no hydroxyl group, and forms two hydrogen bonds with thymine.
- Guanine is a purine base (2-amino-6-hydroxypurine), characterized by lactim-lactam tautomerism, and forms three hydrogen bonds with cytosine.
- Cytosine is a pyrimidine base, complementary to guanine.
- Thymine is a pyrimidine base, complementary to adenine (unlike RNA, where uracil is present instead).
What are the main differences in chemical structure between DNA and RNA?
The main differences in the chemical structure between DNA and RNA lie in the composition of the carbohydrate component, the set of nitrogenous bases, and the chain structure.
| Sign | DNA | RNA |
|---|---|---|
| Carbohydrate (pentose) | D-deoxyribose (lacks -OH at the 2nd carbon atom) | D-ribose (contains -OH at all carbon atoms) |
| Nitrogenous bases | Adenine, guanine, cytosine, thymine | Adenine, guanine, cytosine, uracil |
| Structure | Double helix | Single-stranded linear heteropolymer |
Which enzymes participate in DNA replication and what is their role?
More than 20 enzymes participate in the DNA replication process, ensuring the unwinding of the helix and the synthesis of new strands.
- Helicase unwinds the DNA double helix (an energy-requiring process).
- Topoisomerase (gyrase) prevents the formation of secondary coils (supercoiling) during strand unwinding.
- DNA polymerase carries out the synthesis of new polynucleotide chains. In eukaryotes, 7 types of DNA polymerases are known ($\alpha, \beta, \gamma, \delta, \epsilon, \zeta, \eta$), which operate in the nucleus, mitochondria, and plastids.
What types of RNA exist in the cell and what are their functions?
Several types of RNA exist in prokaryotic and eukaryotic cells.
- Information RNA (iRNA) turns into messenger RNA (mRNA) upon maturation; mRNA moves to ribosomes for protein synthesis.
- Transfer RNA (tRNA)—every amino acid has its own tRNA; the amino acid attaches to tRNA via an ester bond, a process catalyzed by a specific enzyme for each amino acid.
- Ribosomal RNA (rRNA) is a type of RNA in prokaryotic and eukaryotic cells; the basis of ribosome subunits, around which proteins cluster.
- Small nuclear RNAs (snRNAs) are present only in eukaryotic cells.
General functions of RNA include the transfer of genetic information with the formation of proteins, specialized endonuclease functions, and regulation of gene expression stages.
What is the triplet genetic code and what are its properties?
The genetic code is a method of recording information about the amino acid sequence of proteins using a sequence of nucleotides. The main properties of the genetic code:
- Triplet nature—one amino acid is encoded by three nucleotides (a codon).
- Universality—the code is identical for all living organisms.
- Degeneracy (redundancy)—one amino acid can correspond to multiple codons (exceptions: methionine and tryptophan).
- Specificity—one triplet encodes strictly one amino acid.
- Non-overlapping: reading occurs sequentially, and a single nucleotide cannot be part of two adjacent codons.
- Continuity: reading proceeds triplet by triplet without gaps.
- Presence of punctuation marks: there is a start codon (AUG) and stop codons (UAA, UGA, UAG) that do not encode amino acids.
To memorize base pairs: "Apple — Tree" (Adenine-Thymine, 2 bonds), "Car — Garage" (Cytosine-Guanine, 3 bonds).
Class and exam questions
How many hydrogen bonds are formed between guanine and cytosine?
Exactly three hydrogen bonds are formed between guanine (G) and cytosine (C) in the DNA double helix.
What is the result of replication?
As a result of replication, the original double helix unwinds, and a new strand is built upon each template. This produces two identical DNA molecules.
What chemical bond holds the amino acid to the tRNA?
An amino acid is attached to its transfer RNA (tRNA) via an ester bond.
Who deciphered the structure of DNA and when?
The spatial structure of the DNA molecule was deciphered by J. Watson and F. Crick in 1953.
What else to study on this topic
- Mechanisms of double helix unwinding initiation during replication
- Functional features of enzymes catalyzing the attachment of amino acids to tRNA
- Structure and properties of the ester bond in the transfer RNA molecule
- Nature of the triplet genetic code and its degeneracy
- Structure of purine and pyrimidine rings in nitrogenous bases
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