In the nucleus of living things, there is a substance that is easily stained by alkaline dyes, called chromatin. Chromosomes are just another form of chromatin. Their composition is the same, but there are still some differences due to different configurations. Chromosomes are formed by chromatin spirals during cell mitosis. The chromatin of prokaryotic cells used for chemical analysis contains naked DNA, that is, it is not connected with other molecules. However, the chromosomes of eukaryotes are much more complicated, and they are composed of four types of molecules: DNA, RNA, histone (low molecular weight alkaline protein rich in lysine and arginine, with at least five different types) and non-histone (acidic). The ratio of DNA to histone is close to 1: 1.
The number of somatic chromosomes in normal people is 23 pairs, which has a certain shape and structure. The abnormality of chromosome in morphological structure or quantity is called chromosome abnormality, and the diseases caused by chromosome abnormality are chromosome diseases. At present, there are 100 chromosome diseases, which can often cause abortion, congenital stupidity, congenital multiple malformations, cancer and so on. The incidence of chromosomal abnormalities is not uncommon, which can reach 0.5%~0.7% in the general newborn population. For example, there may be 15~20 cases of chromosomal abnormalities based on the average number of 3000 newborns born in our hospital every year. In early spontaneous abortion, about 50%~60% is caused by chromosome abnormality. The common causes of chromosomal abnormalities are ionizing radiation, chemical contact, microbial infection and heredity. The purpose of clinical chromosome examination is to find chromosomal abnormalities and diagnose diseases caused by chromosomal abnormalities.
Chromosome examination is that peripheral blood is cultured at 37℃ for 72 hours under the action of cell growth stimulating factor-phytoagglutinin (PHA) to obtain a large number of splinter cell, and then colchicine is added to stop the splinter cell at the metaphase for chromosome observation. Then, the cells were enlarged by hypotonic treatment to reduce the entanglement and overlap between chromosomes. Finally, the cells were fixed on the glass slide with methanol and glacial acetic acid, and the structure and number of chromosomes were observed under the microscope. The chromosome karyotype of normal men is 44 autosomes plus 2 sex chromosomes X and Y, which are often expressed as 46, XY in examination reports. Normal women have the same autosome as men, and the sex chromosome is 2 XX, which is usually represented by 46, XX. 46 represents the total number of chromosomes, and any number greater than or less than 46 is abnormal. The missing sex chromosome is usually represented by O.
Each cell in human body has 23 pairs of chromosomes, including 22 pairs of autosomes and 1 sex chromosomes, including X chromosome and Y chromosome. A fertilized egg with a pair of X chromosomes develops into a female, while a fertilized egg with an X chromosome and a Y chromosome develops into a male. In this way, for women, the normal sex chromosome composition is XX, and for men it is XY. This means that the gametophyte produced by meiosis of female cells contains an X chromosome; Half of the sperm produced by men contain X chromosome and the other half contains Y chromosome. The chromosomes of sperm and eggs carry genetic genes and record the genetic information passed on from parents to their children. Similarly, when sex chromosomes are abnormal, genetic diseases can also be formed. Chromosome abnormalities account for about 2% ~ 2 1% of male infertility, especially oligozoospermia and azoospermia.
The sex chromosome pair of mammalian male individual cells is xy; The woman is XX.
The sex chromosomes of birds are different from those of mammals: ZZ for males and ZW for females.
The platypus has 5 pairs of sex chromosomes and 25 sexes. Function: Chromosome research is the basis of clinical genetics research. The sequencing results showed that X chromosome contained as many as 1 100 genes. Surprisingly, there are more than 100 related diseases, such as fragile X chromosome, hemophilia, autism, obese muscular dystrophy, leukemia and so on. It seems that this chromosome will never be underestimated!
The other half of the X chromosome is the Y chromosome. The sequencing of human Y chromosome was also completed, and it was found that it was not as fragile as people thought before. There is a "testis" determining gene on Y chromosome, which is very important for sex determination. At present, there are more than a dozen diseases related to Y chromosome. If we compare the human genome to a heavy book, this book consists of 23 chapters, and each chapter has its own story. Up to now, autosomes that have completed gene sequencing also include chromosomes 5, 6, 7, 9, 10, 13, 14, 16, 19, 20, 2 1 and 20. Chromosome diseases are characterized by large defects or duplication of genes, which make patients' intelligence and appearance development and even many organs of the body obviously abnormal, such as Down syndrome and microdeletion. New Progress in Genome Sequencing This genome research, represented by the International Human Genome Project, is a "hot spot" in biotechnology research today. The completion of the draft human genome indicates a new era-post-genome era. At present, other animals that have completed genome sequencing include Caenorhabditis elegans (1998), fruit flies (2000), dogs (2004) and chickens (2004). China researchers independently completed the whole genome sequencing of rice, silkworm, chicken, blood sucking worm and other species. Clinical indications of chromosome examination Edit this paragraph 1. Persons with reproductive dysfunction
At least 7%~ 10% of women with reproductive dysfunction such as infertility, repeated abortions and teratoma are carriers of chromosomal abnormalities. Common chromosomal structural abnormalities, such as balanced translocation and inversion, are abnormal in number, such as 45, XO caused by one less X chromosome or 47 XXXY caused by one more Y chromosome in women. Balanced translocation and inversion, because there is no gene loss, carriers themselves often do not get sick, but they can cause infertility, abortion, teratogenesis and other reproductive dysfunction due to chromosome abnormality of their germ cells. Abnormal number of sex chromosomes will not only lead to infertility, but also often lead to abnormal secondary sexual characteristics.
2. People with abnormal secondary sexual characteristics
Common in women, such as primary amenorrhea, sexual dysplasia, short stature, cubitus valgus, shield chest and mild mental retardation, little or no pubic hair and armpit hair, low hairline, infertility and so on. It is necessary to consider whether there is an X chromosome abnormality. The common X chromosome abnormalities are Turner syndrome and circular X chromosome. Turner syndrome patients have one less X chromosome than normal women, and the karyotype is 45, XO. For some reason, patients with circular X chromosome break both ends of X chromosome at the same time and rejoin at the break. The smaller the ring chromosome, the more serious the clinical symptoms. Early detection of these abnormalities and appropriate treatment can improve the secondary sexual characteristics to some extent, and may also gain fertility.
Third, hermaphroditism of external genitalia
For patients with ambiguous differentiation of external genitalia, such as penile hypospadias, clitoral hypertrophy is penis-like, and it is often difficult to correctly determine gender according to the appearance of genitals. Sex chromosome examination helps to make a definite diagnosis. According to the results of chromosome examination and other clinical examinations, hermaphroditism can be divided into true hermaphroditism, false hermaphroditism and sex reversal syndrome.
1. True hermaphroditism: the internal genitalia is bisexual, that is, there are testicles, vas deferens, ovaries and fallopian tubes in the body. Chromosome examination shows two types: 1, 46, XX/46, XY, that is, there are two cell lines in an organism, and the proportion of each cell determines sexual orientation. The reasons are: X sperm and Y sperm are fertilized with two eggs at the same time, or X sperm and Y sperm are fertilized with eggs and polar bodies that have just formed and have not been excreted. 2. The karyotype is 46, XX, but some genes or fragments of Y chromosome are easy to locate on X chromosome, or autosomal genes are mutated to have the function of Y chromosome.
2. Pseudohermaphroditism: It can be divided into female pseudohermaphroditism and male pseudohermaphroditism. Female pseudohermaphroditism is characterized by female internal genitalia, including uterus, ovary and fallopian tube, and the chromosome examination is 46, XX. The internal genitalia of male pseudohermaphroditism is male, that is, the gonad is testis, and the chromosome karyotype is 46, XY.
3. Sex reversal syndrome: that is, the chromosome karyotype is opposite to the phenotype, such as the female karyotype 46, XX, but the phenotype is like male; Or the karyotype is male karyotype 46, XY, but the phenotype is like female. 46. The main clinical manifestations of XX men are testicular dysplasia, cryptorchidism, hypospadias, little or no sperm, and possibly laryngeal whiskers. Axillary hair is sparse, and the incidence rate of the population is1/20,000. 46. The main clinical manifestations of XY women are tall, cord-shaped ovaries, no uterus, vaginal blindness, primary amenorrhea and underdeveloped breasts.
4. Chromosome diseases of children with congenital multiple deformities and mental retardation and their parents are characterized by multiple deformities and mental retardation. The common clinical manifestations are small head, sparse hair, wide eye distance, low ear position, short neck, short nose, hypoplasia of external genitalia, cleft palate, hypotonia or hyperthyroidism, epilepsy, palm piercing, anal atresia, short stature, developmental retardation, small eye fissure and low hairline. Chromosome examination can find abnormalities such as 2 1- trisomy syndrome.
Fifth, the temperament is abnormal.
Tall, fierce and aggressive men may have sex chromosome abnormalities. For example, in XYY syndrome, chromosome examination shows that there is one more Y chromosome than normal men, and the karyotype is 47, XYY. Most of the patients have normal phenotype, that is, they are healthy and often have fertility, but the male post-algebra is also 47, and the probability of XYY is greater than normal. The incidence of this disease accounts for 1/750 of the general male population. Men with slender figure, slender limbs, small penis, underdeveloped testicles and azoospermia may sometimes be accompanied by mental disorders. Chromosome examination is needed to determine whether they have Coriolis syndrome. Patients with this disease have one more X chromosome than normal men, and the karyotype is original 47, XXY. Its incidence rate is 65,438+0/800 in general men, 65,438+0% in mentally retarded men and 65,438+0/65,438+00 in infertile men.
6. Those who have been exposed to harmful substances
Radiation, chemicals, viruses, etc. Will lead to chromosome breakage. If the original fragment is not reconnected in the original position after chromosome breakage, various abnormal chromosomes will be formed, such as deletion, translocation, inversion, duplication, ring chromosome and so on. If these distortions occur in somatic cells, they will cause some corresponding diseases, such as tumors. If the distortion occurs in germ cells, genetic effects will occur, affecting future generations, resulting in abortion, stillbirth and deformed children.
Seven, premarital examination
Premarital examination can find abnormal chromosome carriers with normal phenotype, such as balanced translocation and inversion. Balanced translocation and inversion of chromosomes are normal phenotypes, because genes are not lost, but they can easily lead to abortion, teratogenesis and stillbirth. Blind abortion will lead to an increase in the birth rate of deformed children. Premarital examination can also find that the phenotype is basically normal, but the sex chromosome is abnormal. These patients can show sexual dysfunction and infertility. Therefore, premarital examination is of great significance to prenatal and postnatal care.
Eight, leukemia and other tumor patients
Chromosome abnormalities in leukemia and other tumors will cause the expression of oncogenes in blood cells and make blood cells grow uncontrollably. Different leukemias often have their own characteristic chromosomal abnormalities, so chromosome examination is helpful for the diagnosis and prognosis of leukemia.
1. Chronic myeloid leukemia: The Ph chromosome is its marker chromosome, which is formed by mutual translocation of some fragments of chromosome 9 and chromosome 22. The appearance of Ph chromosome is a definite diagnostic index of chronic myeloid leukemia, and the appearance or disappearance of Ph chromosome during treatment can also be used as a reference index for curative effect and prognosis.
2. Acute non-lymphocytic leukemia: Chromosome changes are mainly the mutual translocation of chromosome 8 and chromosome 2 1, and the mutual translocation of chromosome 15 and chromosome 17, forming four abnormal chromosomes and adding one chromosome 12.
3. Acute lymphoblastic leukemia: Chromosome examination showed that chromosome 8 was translocated with 14, chromosome 4 was translocated with 1 1, and chromosome 9 and chromosome 22 were translocated with each other to form six abnormal chromosomes and add an extra chromosome 2 1.
Chromosome and genetics
Because of the particularity of Y chromosome, it is also used as a means to find genealogy in many new branches of anthropology such as molecular anthropology, such as Y-SNP and Y-STR detection. At present, there are authoritative doctors in this field in China, such as Dr. AARON Li from Fudan University, Mr. Wenbo and Mr. Li Jing. Chromosome packaging actually refers to the further structural changes of nuclear DNA on the basis of double helix. The packaging of huge DNA chains into chromosomes can only be achieved through multi-level structural changes. These structural changes are generally the formation of higher-order supercoils. The nucleosome discussed above can be regarded as the primary packaging of chromosomal DNA. That is, the "beaded" structure of nucleosome with a diameter of 1 1nm is formed from DNA double helix winding protein with a diameter of 2nm. If the rising distance of each base pair along the central axis of the helix is 0.34nm, the stretching length of 200bpDNA (a DNA fragment of nucleosome) is 68nm, and the nucleosome is only 1 1nm (nucleosome diameter) after formation. If the ionic strength is increased and H 1 is kept, it can be observed by electron microscope that 10nm fiber will be transformed into thicker 30nm fiber, which is the secondary packaging of chromosome DNA. At present, the recognized model of secondary packaging structure is solenoidalfiber, which is a hollow solenoid wound with nucleosome fibers, with an outer diameter of 30nm and six nucleosomes in each turn. Therefore, due to the formation of solenoid, the primary packaging of DNA is reduced by 6 times. According to the fully extended DNA double helix theory, each solenoid contains a DNA chain with a length of 408nm(6×68nm), and the length of each solenoid is almost equal to the diameter of nucleosome, that is, 1 1nm, so the secondary packaging of chromosomes is equivalent to compressing the DNA length by nearly 40 times. H 1。
A more advanced package based on solenoid fiber is to form a ring solenoid. The results of electron microscope observation show that the structure is 30 nanometers, and the fibers are wound on the central skeleton composed of some non-histone proteins. That is, some parts of the spaced solenoid fibers are "pulled" and fixed on the protein axis. As a result, many rings protruding from the skeleton are produced. Every ring in animal cells contains 5- 10× 104bpDNA, which obviously compresses solenoid fibers to a great extent. It is believed that the formation of loop is an ideal structure for gene expression. These circular regions are active units of gene expression. It seems reasonable to understand this from the fact that annulus fibrosus DNA has a more extended structure than other regions. The process of DNA double-stranded helix passing through the three-stage packaging circular solenoid is shown in figure 1-2 1. After the above three-stage packaging is completed, the compression degree of DNA chain is far from enough to form chromosomes that can be accommodated by the nucleus. Solenoid fibers with annular regions need to be further curled and folded in some way, and finally complete chromosome packaging at different stages of cell growth and reproduction. At present, there is no clear conclusion about how this higher-level complex packaging is carried out. But it is undoubtedly an important content of chromosome DNA packaging. From solenoid fiber ring to packaging to chromosome formation, DNA compression should be the highest stage, estimated to be 200-240 times. After packaging at all levels, chromosome DNA is compressed by thousands of times, so that DNA molecules several centimeters long in each chromosome (for example, the average length of DNA molecules in human chromosomes is 4cm) can be accommodated in nuclei with a diameter of several microns (for example, the diameter of human nuclei is 6-7μm).