Chromatin Structure & Human Chromosome PDF

Chromatin structure Human chromosome Chromatin structure Dr Safa Hmid  DNA &Chromatin structure Human chromosome  Learning Objectives To Identify what are histones, chromatin, chromosomes and karyotyping Recognize structure , types and functions of chromosomes Ide...

Chromatin structure Human chromosome Chromatin structure Dr Safa Hmid  DNA &Chromatin structure Human chromosome  Learning Objectives To Identify what are histones, chromatin, chromosomes and karyotyping Recognize structure , types and functions of chromosomes Identify and understanding the principles of karyotyping, its steps and its use in detecting chromosomal anomalies and diagnosis of genetic diseases  ORGANIZATION OF EUKARYOTIC DNA A typical human cell contains 46 chromosomes (23 p pair), whose total DNA is approximately 2m long. Each chromatid contain a DNA molecule Each chromosome is composed of 2 chromatids joint at the centromer.. The end of chromatids are called telomeres. Human genome is the total DNA content in the chromosomes of the cell.  Steps of backing of DNA into chromosome 1-DNA is wrapped tightly around proteins known as histones to form structures called nucleosomes Definition of nucleosome It is the basic structural unit of the chromatin (DNA wrapped twice around histone core forming negatively charged super coils Nucleosome is formed of : 8 Histone proteins (octamer) + DNA double helix  2-This nucleosome is linked to the next one by a short strand of DNA (linker DNA) that is free from histones. This is also known as the “beads on a string” structure; the nucleosomes are the “beads” and the short lengths of DNA between them are the “string  Linker DNA: Core of DNA connects adjacent nucleosomes& produces the beads on a string form of chromatin 3- The nucleosomes, with their DNA coiled around them, each other to form chromatin fiber by the help.of many protine 4-Chromatin fibers are further coiled into a thicker structure and more compact 5-Then they are coiled into chromosomes  Chromatin Chromatin is a complex of DNA, proteins (histones and non histones) and small quantity of RNA found inside the nuclei of eukaryotic cells chromatin (condensed DNA) is consist of 1. long double strand DNA 2. Histones which are basic proteins, positively charged with high content of lysine & arginine 3. other basic proteins 4. small quantity of RNA strengthening the DNA for mitosis and meiosis to take place. prevents damaging the DNA controls the gene expression and replication of the DNA  Histone proteins Histones are basic proteins interact with DNA helping its backing inside the cell. Histones have high contents of lysine and arginine. They are positively charged at the physiologic pH. There are five classes of histones, designated H1, H2A, H2B, H3, and H4. Because of their positive charge, they form ionic bonds with negatively charged DNA (DNA is negatively charged due to the presence of phosphate groups  DNA is wrapped twice around 8 histone proteins (octamer) two copies of each H2A, H2B, H3, and H4 to form nucleosome. Histone octamer is defined as the eight histone protein complex found at the center of a nucleosome core particle. Histone H1, is not found in the nucleosome core  Site of histones :1 On the top of the nucleosome. H 1protein binds to the "linker DNA" (approximately -20 80nucleotides in length) region between nucleosome. Functions of histones 1 are Keeps the DNA wrapped around the nucleosome. -2Helps in packing of DNA  The N-terminal ends of histones can be acetylated, methylated, or phosphorylated. This modefication can be collectively called histone code These reversible covalent modifications influence the binding of histones to the DNA, thereby affecting the expression of specific gene. Steps of backing of DNA into chromosome  H1 Histone is the least Attached and the least tightly bound to the chromatin. Easley removed by salt solution ❑ The chromatin becomes soluble (The linker histone) 2-Dec-23 19 Chromatin : They are two varieties of chromatin Euchromatin Light-staining, decondensed and transcriptionally accessible regions of the genome. Heterochromatin Dark-staining, condensed and gene-poor regions of the genome. EUCHROMATIN Euchromatin is the lightly packed form of chromatin that is rich in gene concentration.92% of the human genome is euchromatic. The structure of euchromatin is reminiscent of an unfolded set of beads represent nucleosomes Euchromatin participates in the active transcription of DNA to mRNA products. Types of heterochromatin: 1. Constitutive heterochromatin is always condensed and thus essentially inactive it is fixed and irreversible in form and function. it does not reverse to the Euchromatic stage. Chromosomes 1 ,9 ,16 and Y chromosome contain regions of constitutive heterochromatin It is found in the regions near the chromosomal centromere and at chromosomal ends (telomeres). 2. Facultative heterochromatin is at times condensed, but at other times it is actively transcribed and, thus, uncondensed and appears as euchromatin. The inactive X chromosome is made up of facultative heterochromatin is almost completely inactive transcriptionally. CHROMOSOMES  CHROMOSOMES Chromosomes are the carriers of the genes (units of heredity) that present in the nucleus. They consist of thread-like structures , known as chromatin. - Chromosomes are not visible in resting (non dividing ) cells , but they are clearly seen during cell division (where it become so condensed and tightly coiled )  Chemical composition * Composed of thin chromatin threads called Chromatin fibers *Interphase chromatin consists of about 30-40 % DNA, 50-60%protein and 1-10% RNA *Metaphase chromosomes contain 15-20%DNA ,10- 15%RNA and 65-75% protein.  Single stranded chromosome The usual form of chromosome present in all non- dividing cell , is Single stranded chromosome “S chromosomes, which is composed of a long thread of double stranded DNA that is coiled on itself forming DNAhelix - In non dividing somatic cell, the normal chromosome number is 46 S chromosomes differentiated as [44 Autosomes + pair of sex chromosome], either ( 44 autosomes + XY) in males or (44 autosomes +XX ) in females..-  In the germ cell of human, the normal chromosome number is 23 S chromosomes (Haploid number), differentiated as [22 Autosomes + one sex chromosome], either (22 autosomes + X) or (22 autosomes + Y)  Number of Chromosomes *Normally all the individuals of a species have the same number of chromosomes. *Closely related species usually have similar chromosome number. * The full complement (normal set) of chromosomes are said to be Euoploid *It includes haploids, diploids, triploids, tetraploids etc. *The condition in which the chromosomes sets are present in multiples of “n” is Polyploidy  Double stranded chromosome "d" chromosomes In the preparation for cell division either mitosis or meiosis, the chromosomes become duplicated at the S-phase of the cell cycle to enter cell division. At S- stage, each d-chromosome actually consists of 2 chromatids joined together at a centromere “primary constriction” that divides classic d- chromosome into 2 short arms usually directed upwards (P) and 2 long arms (q) directed downwards. Upper and lower ends are terminated by telomeres, that have a unique DNAsequence which protects the end of the chromosome from damage deterioration or from fusion with neighboring chromosomes is required for the stability of the chromosome. -help the cell recognize the difference between the end of a chromosome and a broken chromosome that needs repair  Centromere The region where two sister chromatids of a chromosome appear to be joined during cell division is called Centromere 1-Also termed as Primary constriction 2-Darkly-stained region 3-In human ,the centromere contains 1-10 million base pairs of DNA. 4- First part of chromosomes to be seen moving towards the opposite poles during anaphase It performs two role -holding the two chromatids and binding the spindle fibers. -It may be single at one stage, double or four stranded at other stage like Meiosis, Prophase I. Normal human chromosome The areas of the p and q regions close to the telomeres are the subtelomers, or subtelomeric regions. The areas closer to the centromere are the pericentronomic regions.  Types of Chromosomes There are two types of eukaryotic chromosomes: autosomes and sex chromosomes 1-Autosomes The two chromosomes in a homologous pair are: very similar to one another have the same size and shape. centromere location They carry the same type of genetic information  -2Allosomes/ Hetrosome- sex chromosomes *These chromosomes are directly associated with reproduction and differ from autosomes in size, form and behavior *usually there is a single pair of allosomes in mammals termed as ‘X’ and ‘Y’ chromosomes  Identification of Chromosomes -1According to the position of the centromere at each chromosomes II. Denver classification III. Karyotyping IV. Banding Identification V. Fluorescent In Situ Hybridization (FISH)  Identification of Chromosomes According to the position of the centromere at each chromosome, chromosomes may be: 1-Metacentric : When the centromere is centrally placed (half way between the two ends) (Parm =q arm). 2-Submetacentric :When the centromere is closer to one end than the other (P arm is shorter than q arm).  TYPES OF CHROMOSOMES SHORT ARM 2. SUB-METACENTRIC:- Here the centromere is not at the middle position of the CENTROMERE chromosomes. So the arms are unequal and it is ‘L-Shaped’ in appearance. LONG ARM TWO EQUAL ARMS 1. METECENTRIC:- The centromere is at the middle position. So the arms are equal and it is ‘V-Shaped’ in appearance. CENTROMERE  Identification of Chromosomes 3-Acrocentric: When the centromere is placed very close to one end. (P arm is very short). Small mass of chromatin “Satellite” is attached at the upper part of the short arm of chromosome 4-Telocentric: When the centromere is terminal, with no short arm (there is only long arms). This type not present in human.  TYPES OF CHROMOSOMES TYPES OF CHROMOSOMES 1. 4-TELOCENTRIC:- The CENTROMERE centromere is present at the end of the chromosomes.i LONG ARM SHORT ARM CENTROMERE 3. ACROCENTRIC:-The centromere is almost terminal. It has LONG ARM one large and another very small arm.J  Banding Technique Using special stains as Geimsa and produces a characteristic pattern of dark bands alternating with light bands within regions of a given chromosome through which small changes in chromosomal structure can be identified. general aspects of (molecular) cytogenetics Chromosome Size *The size of chromosomes shows a remarkable variation depending upon the stages of the cell division *Longest and thinnest in Interphase. *Progressive decrease in their length with an increase in thickness in prophase. *Most easily observed during metaphase when they are very thick, quite short and well spread in the cell. *Chromosomes are smallest in anaphase. Therefore, chromosomes measurements are generally taken during mitotic metaphase Mitotic Metaphase  Karyotypes The chromosomes from a dividing cell can be photographed and organised into the pairs – one from the mother and one from the father Each pair has a characteristic size, shape and banding pattern (except for the sex chromosomes where the X and Y chromosomes are different) Karyotype from a Autosomes female 22 pairs Make a karyotype Try it Heterosomes yourself 1 pair  Information obtained from a karyotyping Number of chromosomes Sex chromosome content Chromosomal anomalies; either numerical or Structural abnormalities.  Chromosome Reading The regions immediately adjacent to the centromere are designated as “1” (p1 and q1) Region numbers increase distally to the centromere Regions are divided into bands and bands into sub-bands Xp22.3 = X chromosome, short arm region 2, band 2, sub-band 3 Band is read as X q two-two point three, not X q twenty-two point three Chromosome Banding Patterns and Nomenclature X Chromosome Band Pattern Centromere Locations Normal male karyotype  Diagnostic Karyotype New Clinical Genetics 2e Andrew Read and Dian Donnai Diagnostic karyotype