Chromosome_Cell Biology_Undergraduate.pptx

Chromosome_Cell Biology_Undergraduate

1. What is chromosome? Chromosome is a package that contain DNA molecule into thread-like structures. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

2. Morphological structure of chromosome Size The size of chromosome vary from species to species and relatively remain constant for a particular species. The length of a chromosome may vary from 0.2-50 µm. The diameter of chromosome may be from 0.2-20 µm. For example, human chromosome are upto 6 µm in length.

3. Shape Each chromosome contains a clear zone commonly known as centromere or kinetochore. This centromere divides the chromosome into two parts, each part is known as chromosome arm. The position of centromere vary from chromosome to chromosome. It provides different shape to chromosome.

4. Type of chromosome depending on location of centromere

5. The main function of chromosomes is to carry the genetic material from one generation to another Chromosomes play an important role and act as a guiding force in the growth, reproduction, repair and regeneration process, that is important for their survival Chromosomes protect the DNA from getting tangled and damaged Histone and non-histone proteins help in the regulation of gene expression Spindle fibres attached to centromere help in the movement of the chromosome during cell division Each chromosome contains thousands of genes that precisely code for multiple proteins present in the body Functions of Chromosomes

6. Eukaryotic Chromosome One of the main differences between prokaryotic and eukaryotic cells is the nucleus. Prokaryotic cells lack an organized nucleus but eukaryotic cells contain membrane-bound nuclei (and organelles ). The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. Chromatin describes the material that makes up chromosomes, which are structures within the nucleus that are made up of DNA, the hereditary material. DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nuclei of its body’s cells.

7. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. In order to organize the large amount of DNA within the nucleus, proteins called histones are attached to chromosomes; the DNA is wrapped around these histones to form a structure resembling beads on a string. These protein-chromosome complexes are called chromatin.

8. Eukaryotic Chromosome

9. Chemical organization of eukaryotic chromosome Short stretches of the DNA double helix wrap around a core of eight histone proteins at regular intervals along the entire length of the chromosome. The DNA-histone complex is called chromatin. The beadlike, histone DNA complex is called a nucleosome , and DNA connecting the nucleosomes is called linker DNA. A DNA molecule in this form is about seven times shorter than the double helix without the histones, and the beads are about 10 nm in diameter, in contrast with the 2-nm diameter of a DNA double helix. The next level of compaction occurs as the nucleosomes and the linker DNA between them are coiled into a 30-nm chromatin fiber. This coiling further shortens the chromosome so that it is now about 50 times shorter than the extended form. In the third level of packing, a variety of fibrous proteins is used to pack the chromatin. These fibrous proteins also ensure that each chromosome in a non-dividing cell occupies a particular area of the nucleus that does not overlap with that of any other chromosome.

10. Prokaryotic Chromosome Vs. Eukaryotic Chromosome Prokaryote cell (right) showing the nucleoid in comparison to a eukaryotic cell (left) showing the nucleus

11. Prokaryotic Chromosome The genome of prokaryotes is often significantly larger than the cell itself. Prokaryotes compress their DNA using fiber rolled into small rolls – supercoiling . The fibers are twisted so tightly that the final consequences loops overlap to form one big ball. Distinguishes two types of collapse – Positive (DNA turns are in the same direction as the helix) or Negative (DNA is coiled in the opposite direction than the helix) Most bacteria during normal growth is negatively coiled .

12. Allocycle : Allocycle is a pattern of condensation and expansion through the cell cycle that is out of synchrony with the bulk of the chromosomal material. Allocycle and Heteropycnosis Heteropycnosis : It is a phenomenon shown by chromosomes in which certain regions of chromosomes stain more intensely than others. There are two different types of heteropycnosis . Positive heteropycnosis : In this, heterochromatic regions may be thicker and more dark-staining than the euchromatic ones. Here heterochromatin has synthesized more nucleic acid and possibly more protein as well. A positively heteropycnotic chromosome in a prophase nucleus looks like a metaphase chromosome. Negative heteropycnosis : In this, heterochromatic regions may appear thinner and stain more weakly than euchromatic regions in the same nucleus. Negatively heteropycnotic chromosomes usually have a fuzzy outline and at metaphase they still look like prophase chromosomes.

13. Special type of Chromosomes In Eukaryotic organism, certain chromosomes are found only in certain special tissues and are not seen in other tissues. Among them some chromosomes are larger in size and are called giant chromosomes. In certain plants, they are found in the suspensors of the embryo. There are two types of giant chromosomes – Polytene chromosome Lamp brush chromosome

14. Observed by C.G. Balbiani in 1881 in the salivary glands of Drosophila. These are larger in size comparison to any other chromosome. Along the length of the chromosome there is a series of dark bands alternate with clear zones called inter bands. The light bands or interbands are fibrillar , feulgen negative, heterochromatic regions containing small amount of DNA, large amount of RNA and acidic proteins. The polytene chromosome has extremely large puff called Balbiani ring also known as chromosomal puff. As this chromosome occurs in the salivary gland it is known as salivary gland chromosomes. Polytene chromosomes

15. Lamp brush chromosome Lamp brush chromosomes were first observed by Flemming in 1882. The name is given due to its resemblance with a brush that is used for cleaning lamp, glass chimneys, etc. They occur at the diplotene stage of oocytes in vertebrates and invertebrates They are present as a bivalent with 4 chromatids Chromosomal axis is formed from highly condensed chromatin and lateral loops extend from the row of chromomeres Lateral loops of DNA are always symmetrical and formed due to intense RNA synthesis The centromere doesn’t bear any loops

16. Sex chromosome, either of a pair of chromosomes that determine whether an individual is male or female. The sex chromosomes of human beings and other mammals are designated by scientists as X and Y. In humans the sex chromosomes consist of one pair of the total of 23 pairs of chromosomes. The other 22 pairs of chromosomes are called autosomes. Sex chromosome

17. Sex chromosome Individuals having two X chromosomes (XX) are female. Individuals having one X chromosome and one Y chromosome (XY) are male. The X chromosome resembles a large autosomal chromosome with a long and a short arm. The Y chromosome has one long arm and a very short second arm. During meiosis the male XY sex-chromosome pair separates and passes on an X or a Y to separate gametes; the result is that one-half of the gametes (sperm) that are formed contains the X chromosome and the other half contains the Y chromosome. The female has two X chromosomes, and all female egg cells normally carry a single X. The eggs fertilized by X-bearing sperm become females (XX), whereas those fertilized by Y-bearing sperm become males (XY). Typically in mammals, the gender of an organism is determined by the sex chromosomes

18. Sex chromosome

19. Isochromosomes Isochromosome formation is a relatively frequent chromosomal aberration, mainly in X chromosomes. This is generally created by the incorrect division of the centromere. Normally centromere divides vertically. In this case it divides horizontally. The result is usually the lost of one arm. It means that newly created chromosome has just two long arms or two short arms which are connected normally at the centromere. We can find it on X chromosome very often. It is a huge problem during fertilization. Because fetus then get a trisomy for one arm and a monosomy for the second arm. A= Normal X Chromosome B= Identical daughter chromosome A=Normal X chromosome B 1 = Isochromosome X B 2 = Isochromosome X

20. Diplochromosomes Diplochromosomes are consisted with four chromatids lying side-by-side, instead of the normal two, are produced when cells go through two rounds of DNA replication without separation of chromatids. They are an indication of the failure of the normal chromosome separation mechanism.

21. Changes in chromosomal morphology Chromosomes are structure with definite organization. They carry genes in a linear order. But due some natural or artificial conditions, some structural or numerical changes may occur. These changes are collectively called chromosomal aberrations. Most of the chromosomal aberrations are due to accidental, natural or induced breakage of chromosomes. The induced chromosome breakage may be caused by radiation, chemicals such as Lysergic acid diethylamide (LSD). The chromosomal aberrations are classified into following two groups: Intra-chromosomal aberrations Inter-chromosomal aberrations

22. Intra-chromosomal aberrations Deletions: A portion of the chromosome is missing or deleted. 2. Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material.

23. Intra-chromosomal aberrations 3. Inversions: A portion of the chromosome has broken off, turned upside down, and reattached. As a result, the genetic material is inverted.

24. Inter-chromosomal aberrations Translocations: A chromosomal rearrangement in which part of one chromosome becomes attached to a non-homologous chromosome (non-reciprocal), or in which parts of two non-homologous chromosomes trade places (reciprocal).

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