MindMap Gallery Medical Biology Chapter 8 Nucleus (2)
Medical cell biology mind map, including the components of chromatin, Euchromatin and heterochromatin, Chromatin is assembled into chromosomes and other contents.
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Chapter 8 Nucleus
Section 2
Overview
Chromatin: is the form of genetic material in interphase cells. It is a filament-like composite structure composed of DNA, histones, non-histone proteins and a small amount of RNA. It has an irregular shape and is diffusely distributed in the nucleus.
Chromosome: It is a strip or rod-shaped structure formed by repeated winding and condensation of chromatin during mitosis or meiosis.
Chromatin and chromosomes are different forms of genetic material in different stages of the cell cycle.
1. Components of chromatin
Chromatin components: DNA and histones account for 98%, a small amount of RNA and non-histone proteins
(1) DNA is the carrier of genetic information
DNA in chromatin is the carrier of genetic information
Each unreplicated chromosome of a eukaryotic cell is composed of a linear DNA molecule
The entire genetic information contained in a eukaryotic cell's haploid chromosome set is called the genome.
Single and repeating sequences
①Unique sequence: Generally there is only a single copy or a few copies in the genome
②Middle repetitive sequence: The number of repetitions is between 101 and 105, and the sequence length ranges from several hundred to several thousand base pairs (bp).
③Highly repetitive sequence: short in length, generally a few to dozens of bp, but with a repeat copy number exceeding 105, distributed in the telomeres and centromeric regions of chromosomes
Three types of functional sequences of chromatin DNA
①Replication origin sequence
It is the starting point for DNA replication in cells.
Maintain the continuity of chromosomes in generational transmission
② Centromere sequence
It is the connection site between the two sister chromatids that have been replicated.
During metaphase, it is connected to the spindle fibers so that the replicated chromosomes are evenly distributed into the two daughter cells.
③Telomere sequence
A simple G-rich repeat sequence present at the end of eukaryotic chromosomes
Maintain the integrity of replication at both ends of the DNA molecule
Maintain chromosome stability
(2) Histones are the basic structural proteins in eukaryotic cell chromatin
1. Characteristics of histones
Basic structural proteins of eukaryotic chromosomes
Rich in positively charged basic amino acids such as Arg and Lys, it is an alkaline protein
Can bind tightly to acidic DNA (non-specific binding)
2. Classification of histones
⑴ Nucleosomal histone
Nucleosomal histones include H2A, H2B, H3 and H4
No species or tissue specificity, highly conserved in evolution
Helps DNA curl into a stable structure of nucleosomes
⑵ H1 histone
There are species-specific and tissue-specific
Related to further packaging of nucleosomes
(3) Non-histone proteins can affect the structure and function of chromatin in many ways
Concept: General term for chromatin-binding proteins other than histones
Features
Less quantity, more variety
Can bind to specific DNA sequences and affect the structure and function of chromatin in many ways
Assists in DNA folding, participates in initiating DNA replication, and regulates gene transcription and expression.
2. Euchromatin and heterochromatin
(1) Euchromatin is a functionally active and stretched chromatin fiber.
Definition: It is a chromatin filament that is lightly stained with basic dyes in interphase nuclei, has a low degree of spiralization, is in an extended state, and contains active sites for gene transcription.
Intracellular distribution: evenly distributed in the nucleus, mostly located in the center of the nucleus
(2) Heterochromatin is a functionally inert and condensed chromatin fiber.
Definition: The chromatin component in the interphase nucleus that is in a condensed state, has a dense structure, has no transcriptional activity, and is darker when stained with a basic dye.
Intracellular distribution: mostly located around the nucleus and near the nuclear membrane
Classification
Constitutive heterochromatin (constitutive heterochromatin): is the main type of heterochromatin and is in a condensed state in all cell types and all stages of development.
Facultative heterochromatin: In certain cell types or at certain developmental stages, the original euchromatin is condensed and transformed into heterochromatin and loses its transcriptional activity. It can be converted into euchromatin.
3. Chromatin is assembled into chromosomes
(1) Nucleosome—the primary structure of chromatin (basic structural unit)
Nucleosome: The basic structural unit of chromosomes. It is a disc-shaped particle composed of a DNA molecule of about 200 bp and a histone octamer.
Assemble
Two molecules each of nucleosomal histones H2A, H2B, H3 and H4 form an octamer
The 146 bp DNA molecule coils around the histone octamer 1.75 times to form a nucleosome
Two adjacent nucleosomes are connected by connecting DNA, with a typical length of 60bp
Histone H1 binds to the connecting DNA and is located at the entry and exit ends of the DNA double strands in the core of the nucleosome, stabilizing the nucleosome.
The formation of nucleosome beads compresses the DNA molecule about 7 times
(2) Solenoid - the secondary structure of chromatin
Concept: A solenoid is a hollow structure composed of nucleosome bead structures spiraled with the help of histone H1
Assembly: The solenoid has 6 nucleosomes per turn, pitch 11nm, outer diameter 30nm, inner diameter 10nm
Histone H1: consists of a spherical center and two amino acid arms. The spherical center can bind to specific sites on nucleosomes, and the two arms bind to corresponding sites on the histone core of adjacent nucleosomes. To assist nucleosomes in packaging into regular repeating arrangements
Solenoid formation compresses the nucleosome bead structure approximately 6-fold
(3) Further packaging of solenoid
Regarding how solenoids are further packaged into chromosomes, there are currently two models: the multi-level helicalization model and the chromosome skeleton-radiating ring model.
1. Multiple coiling model
Primary structure - nucleosome
Secondary structure - solenoid
Tertiary structure - supersolenoid: a cylindrical structure formed by further spiralization of the solenoid
Quaternary structure - chromatid: supersolenoid is further spirally folded to form
2. Chromosome skeleton-radial loop structure model (scaffold radial loop structure model)
Primary structure - nucleosome
Secondary structure - solenoid (30nm chromatin fiber)
high level structure
loop structure
chromatids
Higher-order structures beyond the solenoid are composed of loops formed by folding of 30 nm chromatin fibers.
The loops extend from the center to the periphery along the longitudinal axis of the chromosome, forming radial rings.
Each 18 loops are arranged radially to form a miniband, which is the component unit of the advanced structure of chromatin.
About 106 microbands are arranged along the longitudinal axis to form chromatids
4. Morphological structure of chromosomes
(1) Centromere
(2) The centromere-kinetochore complex mediates the combination of spindle fibers and chromosomes
(3) Secondary constriction does not exist on all chromosomes
(4) Satellites are globular structures located at the ends of chromosomes
(5) Telomeres are specialized parts at the ends of chromosomes
5. Karyotype and banding type
Karyotype: refers to the phenotype of all chromosomes in a certain individual cell during mitotic metaphase, including the number, size and morphological characteristics of chromosomes
Band: refers to the chromosomes that have been processed and dyed with specific dyes so that the chromosomes display bands of varying depths and widths along their long axis.
The most important application of chromosome banding technology is to clearly identify any chromosome or even a certain translocation fragment