The Structure and Function of Genetic Information (DNA)
Nucleic acids play a crucial role in transmitting genetic information.
There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are composed of nitrogenous bases bound to a five-carbon sugar and phosphate groups.
Table of Contents
Roles of Nucleic Acids and Genetic Information (DNA)
【Nucleic Acids】
Nucleic acids play a crucial role in transmitting genetic information. There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are composed of a five-carbon sugar, nitrogenous bases, and phosphate groups.
The nitrogenous bases that constitute nucleic acids are composed of carbon and nitrogen atoms, and there are two types: purines and pyrimidines. Both DNA and RNA are made up of four combinations of these bases.
- Purines include adenine (A) and guanine (G), which are present in both DNA and RNA.
- Pyrimidines in DNA are cytosine (C) and thymine (T), while in RNA, they are cytosine (C) and uracil (U).
【Structure and Function of Genes】
Genes are composed of DNA, and the sequence of bases in DNA is precisely read by RNA. This sequence serves as a template for the synthesis of proteins. Additionally, DNA, which carries genetic information, accurately replicates its base sequence to pass on to offspring.
DNA-Protein Complex Chromatin
【Structure of Chromatin】
Long, thread-like DNA molecules exist folded and compacted within the nucleus of cells, bound to proteins. This DNA-protein complex is known as chromatin.
Chromatin plays a crucial role in folding DNA into a regular and compact structure, which is essential for gene expression. During cell division, the nuclear membrane disappears, and chromosomes condense.
Within chromosomes, DNA is bound to histones. Histones assemble into octamers (histone octamers) consisting of two copies each of four types of histone proteins. Each histone octamer wraps approximately 1.65 turns of left-handed DNA, encompassing about 146 base pairs. This structure is called a nucleosome and represents the basic unit of chromatin structure.
A nucleosome consists of four types of histones: H2A, H2B, H3, and H4, with each type present as two copies, totaling eight histone proteins. The length of DNA contained in a single human cell is approximately 1.5 meters, folded and stacked in a highly organized manner within the nucleus.
Genetic Composition
【DNA Replication】
DNA exists in a double helix structure. During DNA replication, one strand of the DNA double helix serves as a template to create one strand of new DNA. This process is known as semi-conservative replication.
The two strands of DNA are held together by hydrogen bonds. When replication occurs, the hydrogen bonds between the parent DNA strands are first broken, resulting in two separated single strands. From each single strand, new DNA synthesis begins, producing two new DNA double helices. The sequence of bases on the single strand acts as a template for the synthesis of a daughter strand with a complementary sequence.
【Structure of Genes】
A gene is a segment of DNA that includes regions coding for proteins called exons, interspersed with non-coding sequences called introns. Introns do not contribute to the protein product. Exons are the gene segments that determine the amino acid sequence of proteins. These introns and exons are alternately arranged.
In the human genome, most genes contain at least one (usually several) introns. The total length of all introns combined in many genes is longer than the total length of all exons combined.