Chromosome Structure PDF

STRUCTURE OF CHROMOSOME Learning outcomes At the end of this topic, students should be able to Explain the structure of viral, bacterial, and eukaryotic chromosomes Distinguish eukaryotic chromosomes based on the number and position of the centromere Explain the organization of chromoso...

STRUCTURE OF CHROMOSOME Learning outcomes At the end of this topic, students should be able to Explain the structure of viral, bacterial, and eukaryotic chromosomes Distinguish eukaryotic chromosomes based on the number and position of the centromere Explain the organization of chromosome ORGANIZATION OF DNA IN CHROMOSOMES Chromosomes are considered as the physical basis of heredity, as their main chemical constituent is DNA, the hereditary material. It occurs in all living organisms in specific number & organization. The nature of chromosomes varies in viruses, bacteria & eukaryotes. VIRAL CHROMOSOMES It is single & may be made up of double stranded DNA, single stranded DNA, double stranded RNA or single stranded RNA. A virus is called a DNA virus or RNA virus according to the type of nucleic acid that make up its genome. The genome may be a linear or circular molecule.  Viral chromosomes are tightly packed in the capsids of mature virus particles or virons, or occurs freely inside the host cell, or are incorporated in the host DNA. Small positively charged ions (Na, Mg, K, etc.), and nucleic acid binding proteins help in packaging the genome inside the virus. Viral genomes vary in size from a few thousand to a hundred thousand nucleotides. Animal Virus PROKARYOTIC CHROMOSOMES They contain a single circular chromosomes in the nucleoid. The chromosome is extensively supercoiled resulting in a highly compact molecule. Several DNA binding protein helps in packaging the bacterial DNA in the cell. There are two types of super coiling, negative super coiling in which the DNA is twisted about its axis in the opposite direction from the clock wise turn of the right handed double helix & positive super coiling where DNA is super coiled in the same direction as the clockwise turn of the helix. EUKARYOTIC CHROMOSOMES Eukaryotes are characterized by a great amount of genetic material, which are organized into many different units called chromosomes enclosed in the nucleus. In contrast to prokaryotes, most eukaryotes are diploid, with each somatic cell containing one set of chromosomes from the maternal parent & a comparable set of chromosomes from the paternal parent. The number of chromosomes in a dual set of a diploid somatic cell is called the diploid number (2n). In contrast the sex cells of a diploid eukaryotic cell contain only half the no: of chromosome & is known as haploid (n) cells. Fertilization restores the diploid number. Different species of eukaryotes have different but always constant & characteristic numbers of chromosomes. During sexual reproduction, the chromosome number (2n) reduces to half (n) in the gametes and again the original number (2n) is restored in the offspring after fertilization.  Organism Chromosome number Human 46 Ant (Myrmecia pilosula) 2 Chimpanzees 48 Potato 48 Fern (Ophioglossus 1260 reticulatum) Genome of an organism is the complete set of DNA. Human genome is made up of more than 3 billion nucleotide base pairs. Karyotype is the number and appearance (size, shape) of chromosomes in the nucleus of a eukaryotic cell. Karyogram is a photograph of the chromosomes of a cell, arranged in homologous pairs and in a numbered sequence.  Human Karyotype Medical specialists use this display of karyotyped chromosomes to diagnose abnormalities in patients. HUMAN METAPHASE CHROMOSOMES 1 2 3 4 5 6 7 8 9 10 11 12 16 17 18 13 14 15 23 19 20 21 22 x y Morphology of eukaryotic chromosome The chromosome shape changes during the process of cell growth & cell division. During interphase chromosomes appear as long coiled, elastic, contractile thread like structure called chromatin thread. During metaphase of mitosis or prophase of meiosis chromatin thread becomes highly condensed & is seen as distinct chromosomes. These chromosomes contain two strands called chromatids and a constricted region at one point along its length called the centromere. The region contain kinetochore to which spindle fibers are attached during cell division.  MEIOSIS MITOSIS METAPHASE STAGE PROPHASE The number and position of centromere vary in different chromosomes but are characteristic of a specific chromosome. Based on the no:of centromere, chromosomes are classified into Monocentric- 1centromere, Dicentric-2 & polycentric- more than 2 centromere. Based on the position of the centromere chromosomes are classified Into: Telocentric- Centromere truly terminal. Chromosomes are rod shaped. Acrocentric- Centromere is located closer to one end of chromatid therefore the chromatids on opposite side are very long. Submetacentric- Centromere is subterminal, chromosome is j-shaped. Metacentric- Centromere is in the center. Chromosome is V-shaped.  Some chromosomes can have a secondary constriction besides the centromere to which a short segment of a chromosome is attached. Such chromosomes are called SAT chromosomes. Ex. Chromosome 13, 14, 15, 21, 22 in humans. SAT chromosomes are associated with the formation of the nucleolus. Molecular structure of chromosomes The chromosome is made up of linear, double-stranded DNA molecules associated with histone and non-histone proteins. The mixture of DNA and protein is called chromatin. 2 kinds of chromatin are seen: Euchromatin is condensed during division and extended or uncoiled during interphase.  Heterochromatin remains condensed throughout the cell cycle. It is genetically inactive and contain no genes or genes are repressed. Most of the heterochromatic regions however tend to be repetitive DNA, or other functional regions of the chromosome such as centromeres and telomeres. Heterochromatin is of two types: Facultative heterochromatin- Here some chromosomes become condensed in one sex, or cell types while remaining euchromatic in the other. Constitutive heterochromatin- It remains heterochromatin at all times.  BARR BODY EXAMPLE OF FACULTATIVE HETEROCHROMATIZATION In mammals, males are heterogametic (XY) and females homogametic (XX). Dosage compensation between male and female sex with respect to X chromosomes is achieved by inactivation at random of one of the two X chromosomes in females. The heterochromatized X chromosome appears as a darkly-staining body attached to the nuclear membrane called Barr body. The discovery of X inactivation is generally attributed to British geneticist Mary Lyon, and it is therefore often called “lyonization.” Telomeres Telomeres are the ends of the chromosomes. They consist of many tandem repeats of short (generally hexamer) sequences. In humans the telomeric sequence is TTAGGG. This sequence is repeated over 50 times at the end of each chromosome. These sequences are added to the ends of the chromosomes by an enzyme called telomerase. The reason telomeres exist is to provide stability to the chromosomal ends.  When aging cells stop dividing, they become “senescent.” Scientists believe one factor that causes senescence is the length of a cell's telomeres, the protective caps at the end of the chromosomes. Every time chromosomes reproduce, telomeres get shorter. As telomeres dwindle, cell division stops altogether. Chromatin structure Histone and Non-Histone proteins The Proteins present in chromatin are Histones and Non histones. Histones are small basic proteins that are positively charged and are bound to the negatively charged DNA. 5 basic histones are recognized in eukaryotes: H1, H2A, H2B, H3 & H4. Chromatin contains an equal amount of histone and DNA.  Non-histone proteins – They are acidic found in smaller proportions and are of different kinds. They vary from tissue to tissue and between species. They are responsible for specific gene expression. Some non-histone proteins that have structural and metabolic functions are common to all chromatin. Ex. DNA & RNA polymerase enzyme. The DNA double helix is found coiled within the nucleus. There are several levels of packaging seen in a chromosome. The simplest level involves winding of DNA around a core of histones in a structure called nucleosomes. It appears as beads on a string. The beads are made of two each of H2A, H2B, H3 & H4 ( histone octamer). This particle is called nucleosome core particle. Loosely bound to it is H1 histone.  146 base pair of DNA is wound around the core particle. In living cells chromatin is not kept in a beads–on–a-string structure but is a more highly compacted structure. The next level of packaging above the nucleosome is 30 nm chromatin fiber. The nucleosome is folded in a zig zag manner. Next level is 300nm fiber followed by 700nm fiber & 1400nm fiber of metaphase chromosome.