Gene is DNA fragments which possess specific hereditary effect. It determines the synthesis of RNA & proteins (enzyme) in the cells, thus determining the hereditary characteristics of organisms.

Gene is a complete fragment of DNA that is needed to synthesize a functional polypeptide strand or RNA molecule. Gene can be classified as protein-gene & RNA gene based on the types of their products; And basing on the products' function, gene can be classified as structural gene (The proteins or enzymes synthesized have no influence on the expression of other genes) and regulatory gene (regulateing the expression of structural genes through encoding repressor protein or transcription-activation factor).

Gene modifications such as DNA methylation or histone acetylation may also lead to gene expression activity changes, resulting in changes in phenotypes determined by gene. and this modification can be passed on to the offspring for a few generations. The phenomenon that non-DNA sequence information was transmitted through mitosis or meiosis was known as epigenetic inheritance.

For most of organisms, the chemical nature of their gene is DNA, which is the genetic material. But for some viruses only possessing RNA, RNA is their genetic material. The molecular structure of DNA shows the following biological significance:

(1) The nucleotide sequence of DNA molecule encodes and determines the genetic information. If there are n bases, then there will be 4n kinds of combinations. On a long-chain of DNA molecule, three adjacent bases constitute a triplet, which forms a unit of genetic code, 4 kinds of bases can form 64 kinds of genetic code. Among them, 61 kinds are encoding the 20 amino acids, and the other 3 kinds, which are called stop code, do not encode any amino acid. .

(2) The double-stranded DNA complementary base is the foundation of DNA replication and repairment, each strand in the DNA double strand contains the same information as the complementary strand, which can be used as a template for the synthesis of a new strand. When DNA molecule is damaged, with the aid of DNA repair enzymes and the complementary strand as a template, the damaged bases can be repaired and replaced according to the principle of complementary bases.

(3) Complementary bases of DNA double-strand is the basis of “hybridization”, just because of the complementary DNA bases, the single-stranded DNA can find its complementary DNA sequence in a complex DNA mixture.

(4) The major groove and minor groove caused by the DNA double helix are important for the proteins (enzymes) which possess special function to identify the genetic information of the DNA. As the surface of the double helix structure are phosphate group and deoxyribose skeleton (backbone), which can not be recognized by proteins or enzymes. It is only in the grooves that they can identify the different base sequences. The interaction between DNA and various transcription factors is through the motif of the transcription factors combined with the major groove of DNA.

The human genome is the sum of all the genetic information, including the nuclear genome and the mitochondrial genome. Nuclear genome refers to all the genetic information in 24 DNA molecule of haploid cells, which is the sum of 3.2×109 bp genetic information in the 24 DNA molecules, about 20~25 thousand genes. Mitochondrial genome (mtDNA) is a double-stranded circular DNA molecule, which has a total length of 16,569 basepairs (16569bp), encoding 37 genes. Of which, 2 are rRNA, and 22 are tRNA. Besides that, there are 13genes encoding respiratory chain proteins as well as proteins inoxidative phosphorylation system.

Solitary gene also called unique sequence, which has only 1 copy in the genome of haploid.

In human genome, some genes with similar functions arranged in clusters are called gene family, and these genes can work at the same time, and can also be expressed in different developmental stages. There are two types of gene family:

① Gene Cluster:

Gene family members are arranged in clusters on the same chromosome, and the proteins expressed either at the same time or at different time are functionally relative. For example: α -globin gene cluster is located on chromosome 16 (16p13), including a ζ gene, two α genes and two pseudogenes (ψζ and ψα); β -globin gene cluster is located on chromosome 11 (11p15), including five functional genes (ε, Gγ, Aγ, δ, β) and a pseudogene (ψβ), constituting the α and β-globin gene cluster, which are expressed at embryo, fetal or adult stages, respectively.

② Gene superfamily:

Different members of gene family clusters distributed on different pairs of chromosomes, but the proteins encoded by them are functionally related. For example, human globin gene superfamily members distributed on both chromosomes 16 and 11, constituting globin gene superfamily.

Pseudogene is the gene without functions, the sequence of Pseudogene is similar to the encoding genes, but it can't express a product. Such as, ψζ and ψα gene in α -globin gene family as well as ψβ genes in β -globin gene family.

45SrRNA, 5SrRNA and various tRNA genes are arranged in tandem repeats, encoding the same or almost the same kind of RNA or protein. Each copy of them is almost or completely identical, but their linker DNA are very different.

There is only a single copy or a few copies exist in the genome, with a length is of 800 ~ 1000bp, constituting about 45% of the human genome. There also exit single copy for the non-gene sequences.

Repetitive multiple copy sequence is highly or moderately repetitive DNA sequences in genome, constituting 55% of human genome, which can separate different genes.

According to the frequency of repetition, it can be divided as follows:

(1) Highly repetitive sequence DNA

① Satellite DNA: 5bp, 10bp, 20bp or 200bp as a repeating unit. Satellite DNA can be serially repeated many times, with a length of a few hundred kb. Satellite DNA are located on the heterochromatin region of centromeres, telomeres or chromosome arm. For example, telomere sequences -TTAGGG- can be repeated 250 to 1500 times.

② Minisateliite DNA: repeating units are about 15 ~ 100bp (rich in GC). it can be repeated 20 to 50 times, with a total length of 0.1~20kb.

③ Microsatellite DNA: located in the non-coding region of intervening sequences or introns of genome, with repetitive sequence of 1 ~ 6bp, and a total length of less than 100bp, such as (CA)n and (CCG)n and so on. The number and frequency of microsatellite DNA regions appears are different in human genome, namely polymorphism. Instability of (CAG)n and (CTG)n in microsatellite DNA, such as trinucleotide repeat expansion mutation, has been found in spinocerebellar ataxia and fragile X syndrome (FraX), etc. Marking microsatellite DNA can be used in parentage test.

(2) Intermediate repetitive DNA

It is the interspersed repetitive sequences in genome. DNA distributed in an interspersed manner. According to the length of the repetitive sequences, it is classified into short interspersed nuclear elements and long interspersed nuclear elements.

① Short interspersed nuclear elements (SINEs)

SINE has a length of 100 ~ 500bp, with repetitive copy number of up to 106 times. Alu family is the most abundant intermediate repetitive sequences in human genome, with DNA length of 282bp, about 300,000 to 500,000 copies. Alu sequence has a recognition sequences of restriction enzyme Alu Ⅰ, that is AG ↓ CT, and this site is cut into fragments of 170bp and 130bp. On average, by the mean interval of 5kb DNA there is an Alu sequence.

② Long interspersed nuclear elements (LINEs)

LINE has a length of 6000bp ~ 7000bp, with repetitive copy number of 102~104. KpnⅠ family is the second largest family in the intermediate repetitive sequence family, with DNA length of 6.5kb (6500bp) and copy number of 3000 to 4800. Kpn Ⅰ sequence can be digested to 1.2kb, 1.5kb, and 1.9kb fragments by restriction enzyme KpnⅠ.

Structural genes of eukaryote are split genes. They are made up of coding sequences and non-coding sequences, which are in alternate arrangement and can be exchanged with each other according to the different shearing or modification ways of mRNA.

Coding sequence of a gene is exon, while non-coding sequence of a gene is intron.

The linker region between exons and introns is highly conserved consensus sequence, that is, the initial two bases in 5' end of each intron are GT, and the last two bases in 3' end are AG. GT-AG is a splice recognition signal for cutting introns and splicing exons in gene expression.

At both upstream of the first exon and downstream of the last exon of each split genes, there is a non-coding region called flanking sequence, which include promoters, enhancers, terminator that cannot be transcripted but play regulatory roles in gene expression.

(1) Promoter: located at 100 ~ 200bp upstream of the transcription initial point, which can bind with transcription factors and start gene transcription.

(2) Enhancer: It is located at the upstream or downstream of the transcription initial point, which can specifically bind with transcription factors and enhance the transcriptional activity.

(3) Terminator: It is made up of AATAAA and a segment of palindromic sequence. AATAAA is an additional signal of poly A. Palindromic sequence can be transcribed to a hairpin-like structure, preventing the movement of RNA polymerase and terminating transcription.

Gene expression is the process by which genetic information stored in DNA molecule is transcribed and then translated to a protein or enzyme, or only transcribed to functional RNA.

Transcription is a process by which single-stranded RNA is synthesized using DNA 3' → 5' single (template) strand according to complementary base pairing (A= U, C ≡ G pairing ).

The product of transcription is only mRNA precursors, namely heterogeneous nuclear RNA (hnRNA), which includes exons, introns and part of flanking sequence. Because of immaturity, only hnRNA is processed and modified, it can become a functional mRNA.

(1) Splicing: the process by which non-coding intron sequences are removed, and then the coding sequences of exon are spliced together by ligase with the help of splicing enzymes. Splicing occurred at GT and AG junction site between exon and intron.

(2) Capping: spliced hnRNA 5' end is covered with a 7-methyl guanosine (m7GpppN) cap to cover the 5' end of RNA, which helps to be identified by ribosomes in cytoplasm.

(3) Tailing: Poly A tail was formed by adding 200 adenylic acid (AMP) at 3' end to keep 3' end of RNA stable. The transcription of tRNA and rRNA are functional only after the process of processing and modification.

The process by which proteins are synthesized using mRNA as a template. The spatial structure of a protein is determined by its post-translational modification.

Gene mutation refers to the changes in the types or order of nucleotides in DNA sequence, resulting in the change of expression product of protein, thus hereditary character changes. The types of gene mutation includes base substitution, frame shift mutation and dynamic mutation, etc.

Germinal mutation: mutated genes can be passed on to the offspring by sexual reproduction, present in every cell of the offspring, thereby hereditary character of the offspring accordingly changes.

Somatic mutation: mutated genes cannot be passed on to the offspring, but can be passed on to the cells which derived from cell division of the mutant cells, forming mutated cell clones locally as the basis of tumor lesions.

Genes on the same locus can have multiple different mutations independently to form a new allele. For example, A mutated to allele a1, a2, a3, a4…an, forming multiple alleles eventually.

Mutated genes can mutate again and form a new allele. A can mutate to allele a; then a can mutate to a new allele a1, a1 can also mutate to allele a2, and a2 may also mutate to a3. The repetition of gene mutation is also the reasons for multiple alleles formation.

Gene mutation occurs in a random manner. Mutation rate is the probability of an allelic form of a gene in one generation mutates to another allelic forms.

Spontaneous mutation rate is very low. Mutation rate of human genes is approximately: 10-6~10-4/ germ cell / site / generation.

Forward mutation is that wild-type gene mutate to an allelic mutant gene; Reverse mutation is that the mutant gene mutates to its corresponding wild-type gene. Forward mutation rate is always much higher than the reverse mutation rate.

The formation of genetic traits is the result of both long-term evolution and natural selection. Gene mutation is harmful. Gene mutation of germ cell or fertilized egg can cause genetic diseases; Somatic cell mutation can cause tumors.

Physical, chemical and biological factors that interfered and damaged DNA, leading to DNA structure changes, which is called induced mutation

① Ultraviolet

UV radiation can induce adjacent thymines in DNA sequence to form thymine dimers (TT), which changes the local structure of DNA. When DNA replication or RNA transcription reaches this region, base pairing goes wrong, causing the change of bases in the newly synthesized strand.

② Ionizing radiation

X or γ-ray directly hit DNA strand, and DNA molecule absorbs energy then causes DNA strand to break. Intensity and dose of ionizing radiation can be accumulated.

① Hydroxylamine

Fried foods contain more hydroxylamine (HA), which can cause changes in DNA base pairs.

② Nitrite

Pickled food contains more nitrite, which can also cause changes in DNA base pairs.

③ Base analogues

Base analogue is strong mutagenic substance, similar to nucleotide bases in structure. For example, 5-bromo- uracil (5-BU), which is similar to dTMP in structure and is wildly used in laboratory, can be incorporated into DNA molecule and substitute for T, then pair with nucleotide G. After only one replication of DNA, A-T pair becomes G-C pair.

Toxins and metabolites of viruses, fungi (aflatoxin ) and bacteria.

It is also known as point mutation, which means a single base or a base pair changes in DNA sequence.

(1) Base substitution: It refers that a base is replaced by another.

① Types:

I, Transition: It refers to substitution of purine for purine, and pyrimidine for pyrimidine. II, Transversion: It refers to substitution of purine for purine, and pyrimidine for purine. ② Effect:

Base substitution can occur in any part of the genomic DNA sequence. When it occurs in the regulatory region of a gene sequence, it will alter the gene expression, thereby affecting the functions of these proteins; when it occurs at the coding sequence of a gene, the mutation effects that may occur are: I, Synonymous mutation: Due to the degeneracy of the genetic code, there is no change in the encoded amino acid even though the substitution of nucleotide bases. Synonymous mutation usually occurs in the third base of the genetic codon. For example, genetic codon CCA encodes proline, but even when A mutates to G, CCG can also encode proline.

II, Nonsense mutation: After base substitution, the encoded genetic codon turns to a stop codon (UAA, UAG or UGA), resulting in early termination of the polypeptide chain synthesis and producing a shorter polypeptide chain.

III, Missense mutation: After base substitution, encoded amino acid changes to another, resulting in the changes of amino acid sequence. Missense mutation usually occurs at the first or the second base in the genetic codon. For example: Sickle cell anemia is caused by the transversion of hemoglobin β -globin gene in the sixth codon, resulting in the sixth amino acid of β -globin chain glutamic acid (GAG) changes to valine (GTG) (GAG→GTG, A→T), and β -globin of hemoglobin changes from HbA ( normal ) to HbS ( abnormal ). Sickle shaped cell can not easily pass through the capillaries, thus resulting in thrombosis.

IV, Termination (Stop) codon mutation: Termination codon in DNA molecule can turn into an encoded codon for amino acid. Synthesis of polypeptide chain will continue till the next termination codon occur, forming a prolonged abnormal peptide chain.

(2) Frameshift mutation: It refers that loss or insertion of one or several bases in the DNA coding sequence could cause the change of DNA encoding frame behind the insertion or deletion point. Effect: ① Insertion or deletion of one or two bases in DNA molecule could cause the change of all of the codon combination behind the insertion or deletion point. ② Insertion or deletion of three bases could lead to increase or decrease of a triplet code at the insertion or deletion point, causing only one genetic code before or behind the insertion or deletion point. ③ One or two bases are inserted or deleted, and meanwhile at another site the same number of bases are deleted or inserted correspondingly, thus only resulting in the change of the genetic code between the two points.

Dynamic mutation is due to repeated amplification of trinucleotide in gene encoding sequence or flanking sequence, and the repetition number of trinucleotide has an accumulative effect which increases along with the generation. This disease is known as trinucleotide repetitive expansion diseases (TREDs).

For example: Fragile X syndrome (FRAX). At the Xq27.3 point of X chromosome of patients, there is a fragile site, existing in the form of filament. The genetic basis of FRAX is DNA amplification. At the 5' end untranslated region of FMR -1 (fragile X mental retardation-1) gene, there are tandem repeat sequences (CGG) n, which is polymorphic.

The copy number of (CGG)n of a normal human is only 6~50, and the copy number of a carrier is 50 to 200, but a patient has more than 200.

If the copy number of CGG is large, transcription of FMR-1 gene stops (gene was closed), and abnormal methylation occurs at the 5' end of FMR-1 gene which leads to deactivation of gene transcription and causes disease. Male patients exhibit mental retardation, large testis, long face, big head, big ears, protruding forehead, mandibular prognathism, large mouth, thick lips and eccentric temperament, but have normal function of testicular to procreate. The longer the (CGG) n is, the more severe the symptom is.

A variety of factors including ultraviolet, ionizing radiation can cause the damage of DNA, but in eukaryotic cells there are a variety of DNA damage repair system in the eukaryotic cells to help damaged DNA to be repaired. UV-induced DNA damage repair

UV radiation can cause two adjacent thymine nucleotides on the same DNA strand to form thymine dimer (TT) and thus influence the replication of DNA and transcription of RNA.

Repair can be achieved in the following ways:

(1) Photoreactivation repair

There exists a type of photoreactivation enzyme in the cell. The photoreactivation enzyme can be activated under visible light radiation, which can specifically recognize and bind to the pyrimidine dimer, forming enzyme-DNA complex. Then the enzyme utilizes the visible light to provide energy, to separate the pyrimidine dimers, to complete the repair, the photoreactivation enzyme is dissociated and released from the DNA.

(2) Excision repair

It is also known as dark repair, which occurs before DNA replication. Firstly a specific site of pyrimidine dimers 3' end is cut by the endonuclease, and with DNA polymerase, a complementary base fragment is synthesized using a normal complementary strand as a template, then pyrimidine dimers' damaged fragment is excised by exonuclease at the 5' end of the pyrimidine dimers, finally the DNA ligase connects the incisions together, and the excision repair is completed.

(3) Recombination repair

Repair occurs after DNA replication. Damaged DNA still can be replicated, but when replication reached the damaged site, a gap forms in the daughter DNA strand which is corresponding to the damaged site. After replication, the daughter chain with a gap exchanges fragment with the complete mother chain (the exchanged mother chain and the daughter chain have the same polarity), in that way the gap is transferred to the mother chain. DNA polymerase synthesizes complementary fragment at the gap of the mother chain, then the gap was connected by a DNA ligase, so that the DNA produced by replication is normal in structure. Recombination repair cannot eliminate the DNA damage, but the proportion of damaged DNA decreases after multiple replications, thus it plays a role in diluting the mutation.

Repair system itself is composed of a series of gene-encoding enzymes. The damage of DNA repair system prevents the damaged DNA from being repaired as soon as possible. Mutation exists in various forms and will be inherited, eventually leading to disease.

Such as xeroderma pigmentosum disease: The sunshine of UV (ultraviolet) causes pyrimidine dimerize.Due to the gene mutation of endonuclease enzyme, endonuclease dysfunction causes DNA repair defect, and DNA damaged by the sunlight cannot be repaired normally. It manifest as light allergy, skin epithelial cell or basal cell cancer and nervous system disorder.