Genes

Genes are the basic units of hereditary information, or segments of DNA or RNA sequences that are capable of being inherited and have functional roles, composed of nucleotides.

The structure and function of life forms, as well as their most fundamental behavioral traits, are constructed based on the gene template and passed on to the next generation during reproduction, ensuring biological continuity.

Additionally, when genes are transmitted to the next generation, they are not 100% accurate; random mutations occur. These random mutations can lead to changes in basic functions of life, and those that adapt to environmental changes may continue to be passed on (failure to adapt may lead to extinction). This mechanism also ensures that organisms can explore different evolutionary directions, trying various structural and functional adaptations.

It's important to note that evolution does not always move "forward"; it can also regress, depending on whether it adapts to the current environment, rather than on whether new functions are more advanced. It's entirely possible for evolution to reach a local optimum, akin to a dead end. If the environment undergoes drastic changes, species may not adapt in time and could face extinction, as was the case with the dinosaurs.

DNA

DNA stands for Deoxyribonucleic Acid, a polymer consisting of two intertwining strands forming a double helix.

DNA is composed of nucleotides, whose core components are nitrogenous bases, of which there are four:

• Adenine (A)
• Thymine (T)
• Guanine (G)
• Cytosine (C)

According to base pairing, A pairs with T, and G pairs with C. Thus, the double helix structure has complementary bases, meaning the two strands contain identical information.

Genes are encoded in DNA through the ATGC sequence. The human genome contains approximately 3 billion base pairs, forming about 25,000 genes.

Chromosomes

Chromosomes are composed of long chains of DNA wrapped around histones, forming nucleosomes, which further coil and fold into highly ordered structures. Chromosomes are the actual structures that store and transmit DNA, protecting and ensuring its stability and accuracy. To use a computer analogy, DNA is like a disk that stores genetic information, while chromosomes are the hard drives that encapsulate the disks.

Chromosomes typically appear in pairs, with the entire human genome distributed across 23 pairs, totaling 46 chromosomes. Genes related to sexual differentiation are stored on the 23rd pair, the X and Y chromosomes.

Expression

The function of genes is primarily realized through expression, which refers to reading the encoded information of genes in DNA and using that information to produce specified proteins.

The two most crucial steps in expression are transcription and translation. In simple terms, in the first step, DNA is transcribed into RNA, including mRNA (messenger RNA) and ncRNA (non-coding RNA); in the second step, mRNA is translated into proteins.

RNA

RNA, or ribonucleic acid, is a long-chain molecule made of ribonucleotides linked by phosphodiester bonds. A ribonucleotide consists of a phosphate group, ribose, and a base. The bases of RNA are:

• Adenine (A)
• Uracil (U)
• Guanine (G)
• Cytosine (C)

Note that uracil (U) replaces thymine (T) found in DNA.

Codons

A codon is a sequence of three nucleotide residues on mRNA (or DNA). Each codon consists of three adjacent nucleotides and specifies a particular amino acid during translation, forming proteins.

For example, the codon marking the start of protein synthesis is: ATG; the codons marking the end of protein synthesis are: TAA, TGA, TAG.

Replication

The growth, development, and reproduction of organisms rely on cell division—the process where a single cell divides into two typically identical daughter cells. This requires the replication of DNA so that both daughter cells inherit the same DNA copy. DNA replication occurs during the synthesis phase (S phase) of the cell cycle.

DNA replication is carried out by a special enzyme called DNA polymerase, which "reads" one strand of the DNA double helix (the template strand) and synthesizes a new complementary strand. Because the double helix is held together by base pairing, the sequence of one strand fully specifies the sequence of its complementary strand; thus, the enzyme needs only to read one strand to produce an identical copy. Each daughter cell inherits a DNA copy containing one original and one newly synthesized strand.

After DNA replication, the cell divides, and chromosome separation occurs during the mitotic phase (M phase).

Mutations

DNA replication is highly accurate in most cases, but mutations can occur. In eukaryotic cells, the error rate for each nucleotide during replication is 0.000001%; for some RNA viruses, it can be as high as 0.1%. This means that for humans, about 30 new mutations accumulate in each generation and each human genome.

Small mutations can arise from DNA replication errors and DNA damage, including:

• Point mutations changing a single base,
• Frameshift mutations resulting from the insertion or deletion of a single base.

Any of these mutations can alter a gene through missense (changing a codon to encode a different amino acid) or nonsense (premature stop codon) mutations.

Larger mutations may result from recombination errors, leading to chromosomal abnormalities, including large segments of chromosomes being duplicated, deleted, rearranged, or inverted.

Additionally, DNA repair mechanisms may introduce mutations when fixing physical damage to molecules. Although repair mechanisms can introduce mutations, they are crucial for survival compared to not repairing at all.

Most mutations are neutral and do not affect the organism's phenotype (silent mutations). Some mutations do not change the amino acid sequence because multiple codons can encode the same amino acid (synonymous mutations). Others, while leading to changes in the amino acid sequence, result in proteins that still have similar functions (conservative mutations).

Some mutations are harmful or even lethal, leading individuals and their descendants to disappear from the population, but harmful mutations do not always propagate within the population, protecting it from such harm. For example, genetic diseases are the result of harmful mutations, which can be due to spontaneous mutations or hereditary factors.

Finally, a small fraction of mutations are beneficial, enhancing an organism's adaptability, which is crucial for evolution. Protecting biodiversity is essential for safeguarding various evolutionary possibilities, thereby increasing biological adaptability.

Heredity

Genes are passed on to the next generation through reproduction, known as heredity. For asexual organisms, heredity requires only DNA replication, while sexual reproduction involves a more complex process.

In sexual reproduction, haploid cells (gametes) are produced through a specialized form of cell division called meiosis, containing only half of the DNA.

Gametes are reproductive cells, with female gametes called eggs and male gametes called sperm. When an egg and sperm unite, a fertilized egg can form a complete DNA sequence, allowing the next generation to inherit traits from both parents. This reproductive method can further enhance biodiversity and lead to evolutionary directions better suited for environmental adaptation.

Translation Note

The original text is in Chinese, and the English translation was automatically generated by ChatGPT. There may be inaccuracies or errors in the expression; please refer to the original text for accuracy.