Lesson 1: Genes, the Genetic Code, DNA Replication

I. Genes

• Definition: A gene is a segment of DNA that carries the coded information for a specific polypeptide or an RNA molecule.

• Function: Genes play a crucial role in:

• Storing genetic information: Genes contain the genetic code that determines an organism’s traits.

• Transmitting genetic information: Genes are copied and passed down from one generation to the next.

• Controlling protein biosynthesis: Genes serve as templates for the production of RNA, which in turn synthesizes proteins.

II. DNA (GENE) Structure

• Structural principle:

• Polymer: DNA is made up of monomers called nucleotides.

• Complementary: The nitrogenous bases on the two single strands of DNA bond together according to the principle of complementarity:

• Adenine (A) binds to Thymine (T) with two hydrogen bonds.

• Guanine (G) binds to Cytosine (C) with three hydrogen bonds.

• Spatial structure: DNA has a double helix structure, consisting of two polynucleotide chains spiraling around a common axis in a counter-clockwise direction.

• The diameter of a DNA molecule is 20 A^o.

• The distance between two consecutive base pairs is 3.4 A^o.

• One helical turn comprises 10 base pairs.

III. Nucleotides

• Structure: A nucleotide consists of three components:

• Phosphoric acid (H₃PO₄): Binds to the pentose sugar, forming the backbone of the DNA molecule.

• 5C sugar (pentose): This is deoxyribose (C₅H₁₀O₄) in DNA and ribose (C₅H₁₀O₅) in RNA.

• Nitrogenous base: There are four types of nitrogenous bases:

• Adenine (A)

• Thymine (T)

• Guanine (G)

• Cytosine (C)

• Size and weight:

• The length of a nucleotide is 3.4 A^o.

• The average weight of a nucleotide is 300 daltons.

IV. The Genetic Code

• Definition: The genetic code is the system for encoding genetic information on DNA into the sequence of amino acids in a protein.

• Characteristics:

• Triplet: Each triplet (codon) of three nucleotides codes for a single amino acid.

• Specific: Each codon codes for only one specific amino acid.

• Degenerate: Multiple codons can code for the same amino acid.

• Universal: The genetic code is nearly identical across all living organisms.

• Non-overlapping: Codons do not overlap with each other.

• No punctuation: There are no codons that mark the start or end of translation.

V. DNA Replication

• Definition: DNA replication is the process of creating two identical DNA molecules from a single parent DNA molecule.

• Principles:

• Semi-conservative: Each daughter DNA molecule consists of one old (parent) strand and one newly synthesized strand.

• Complementary: New nucleotides are attached to the new strand according to the principle of complementarity with the nucleotides on the template strand.

• Stages:

• Stage 1: Unwinding and separation of the double helix: The enzyme helicase unwinds and separates the two single strands of the parent DNA.

• Stage 2: Synthesis of the new strand: The enzyme polymerase attaches free nucleotides to the template strand according to the principle of complementarity.

• Stage 3: Joining of Okazaki fragments: The enzyme ligase joins the Okazaki fragments on the discontinuous strand into a complete single strand.

Note:

• DNA replication occurs in the nucleus of the cell.

• The process is highly accurate, ensuring the precise replication of genetic information from one generation to the next.

• DNA replication is the basis for cell division and the transmission of genetic information.

VI. The Role of Genes in Biology

• Genes play a critical role in:

• Biodiversity: Genes are the basis for the diversity in morphology, structure, and function of living organisms.

• Evolution: Changes in nucleotide sequences within genes are the driving force behind evolution.

• Individual development: Genes control the process of development from zygote to mature organism.

• Disease manifestation: Gene mutations can lead to genetic diseases.

VII. Applications of Knowledge about Genes

• Medicine:

• Diagnosis and treatment of genetic diseases.

• Development of new drugs.

• Agriculture:

• Breeding of high-yielding, disease-resistant crops.

• Development of high-productivity livestock breeds.

• Industry:

• Production of biological products such as insulin, interferon, etc.

• Applications in biotechnology and genetic engineering.

VIII. Conclusion

Genes are the fundamental unit of life, carrying genetic information and controlling protein biosynthesis. Understanding genes is fundamental to research in genetics, molecular biology, and applications in various aspects of life.