Chromosomes

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Chromosomes

  1. 1. Chromosomes
  2. 2. • Chromosomes are present in nucleus. • They are long threads made up of chromatin material. • The chromatin is deoxyribonucleic acid (DNA) associated with protein. • The chromatin material is in two forms.
  3. 3. Euchromatin • It is dispersed and poorly stained. • It lies in the central region of nucleus. • Nuclei with euchromatic appearance are known as open-face nuclei and are larger in size. • This state or form is associated with high degrees of synthetic activity. • Euchromatin is also called active chromatin.
  4. 4. Heterochromatin • It is clumped and densely stained. • It is peripheral and lies close to the nuclear envelope, leaving gaps at the nuclear pores. • Nuclei with heterochromatic appearance are known as closed-face nuclei and are relatively smaller in size. • This state is associated with low degrees of synthetic activity. • Heterochromatin is also called inactive chromatin.
  5. 5. • This is the diagram of a nucleus showing heterochromatin and euchromatin. Look at the nuclear envelope and nuclear pores. The heterochromatin is peripheral and lies close to nuclear envelope. Heterochromatin is absent in the region of nuclear pores.
  6. 6. • The number of chromosomes in each cell is fixed for a given species. There are 46 chromosomes in each somatic cell of insan (human being). • These chromosomes are in pairs. So there are 23 pairs of chromosomes in each somatic cell of insan. In other words, there are two sets of chromosomes. In each set there are 23 chromosomes.
  7. 7. • 46 chromosomes are the diploid number or double number of chromosomes and 23 is haploid number or half number of chromosomes. • There are two haploid sets of chromosomes. The number of sets is called ploidy. • If more than two sets are present the cell is said to be polyploid.
  8. 8. • Each chromosome in a haploid set is unique in size and shape. • There are 23 different chromosomes. There size and shapes are different from one another.
  9. 9. • In the diploid set there are two identical chromosomes of each type of chromosome. They are called homologous pairs. • However the sex chromosomes are not identical in man. In woman the sex chromosomes are also identical.
  10. 10. • So in man there are 22 pairs of chromosomes that are identical. • are homologous pairs of chromosomes. These are called autosomes. • And there is one set of sex chromosomes. They are not identical. • They are heterologous chromosomes.
  11. 11. • So man is heterogametic sex. • There is one X and an unequal Y chromosome giving a diploid complement of 44 autosomes + X and Y sex chromosomes.
  12. 12. • The woman (female) is the homogametic sex. Here the sex chromosomes are identical or homologous. Both sex chromosomes are X chromosomes. • The diploid complement of a woman is 44 autosomes + 2 X sex chromosomes.
  13. 13. • During interphase the chromosomes are usually highly extended filaments and form a diffuse network collectively termed chromatin. • During cell division the chromosomes shorten and thicken.
  14. 14. • They become characteristically visible during metaphase, of course under microscope. • Each pair of homologous chromosomes has a characteristic common basic structure.
  15. 15. • Each chromosome consists of two parallel and identical filaments called chromatids. • They are joined together at a narrowed region called centromere or kinetochore. Here is the primary constriction of the chromosome.
  16. 16. • There is a pale-staining region in centromere. • The centromere is attached to the spindle fibers during cell division. • The free ends of the chromatids are known as telomeres.
  17. 17. • The diagram of a chromosome during metaphase showing two chromatids. • Major parts of the chromosome are highlighted. • A pale staining area is visible in the centromere. • Each chromatid is divided by the centromere into two arms. • One is p arm and the other is q arm.
  18. 18. • The centromere may be near the middle of the chromosome. • Here the two arms of chromatids are nearly equal. • This type of chromosome is called metacentric chromosome.
  19. 19. • The centromere may not be near the middle of the chromosome. • Here the two arms of chromatids are not equal. • One arm is short and the other is long. • This type of chromosome is called submetacentric chromosome.
  20. 20. • The centromere may be near one end of the chromosome. • Here the short arm is very short and the long arm is very long. • This type of chromosome is called acrocentric chromosome.
  21. 21. • In some mammals the centromere may be placed at the extreme end of the chromosome. • Here the short arm is negligible. This type of chromosome is called telocentric chromosome.
  22. 22. • In certain chromosomes, in addition to primary constriction, there is another narrowing (the secondary constriction) near one end of each chromatid, separating a portion of chromatid from main arm. • This portion is given the name satellite body.
  23. 23. • These constrictions and terminal satellite bodies are believed to be often associated with the organization of nucleoli, which may be attached to the chromosome at this point in interphase.
  24. 24. • Each complete diploid set of chromosomes contains the cell’s hereditary instructions, or genome, and each somatic cell has an identical genetic complement. • The human genome contains an estimated 50,000 to 100,000 structural genes per haploid set or three billion base pairs.
  25. 25. • Each chromosome has a linear sequence of genes. • Every gene is situated at a characteristic position called gene locus. • This sequence is highly stable.
  26. 26. Mutation • Mutations are permanent and inheritable changes in genome. • They are brought about by external forces such as ionizing radiation, or exposure to certain chemicals.
  27. 27. • When the affected region is restricted these are called gene mutations. • When whole arms or relatively large segments are involved, they are termed chromosomal mutations.
  28. 28. Alleles • Such mutations are some of the principal factors responsible for the inheritable variations which all populations show. • Within members of such a population, homologous chromosomes may determine a number of alternative characters since they may carry some mutant genes; such groups of alternative genes are termed alleles.
  29. 29. • In any genome homologous chromosomes may possess, at a particular locus, identical alleles (the homozygous condition) or non-identical ones (the heterozygous condition). • Since, in the formation of gametes, the homologous chromosomes are segregated into haploid sets, the alleles which they bear are also segregated. • The various types of recombination occur at fertilization.
  30. 30. • Normal resolution chromosome banding reveals 350 bands per haploid set, • whereas high-resolution chromosome banding reveals up to 1300 bands per haploid set.

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