CHROMOSOMES AND GENE LINKAG

EUKARYOTIC CHROMOSOMES

The chromosome is composed of DNA and histone protein. DNA is the genetic material. It exists with protein in the fix form. The chromosome shows following phases:

  1. Chromatin

The highly dispersed state of chromosomes is called chromatin. The chromosome remains in chromatin state during most of the life cycle of cell. Therefore, the genes actively participate in the formation of protein.

  1. Chromosome

The highly condensed form of chromatin during division of cell is called chromosomes. This condensed state distributes chromosomes equally between newly formed cells.

Organization of chromosome

Chromatin consists of DNA and histone proteins. This association of DNA and protein helps in the complex packing of DNA into chromosomes. So it regulates the DNA activity. There are five different histone proteins. These are:

  • H1
  • H2A
  • H2B
  • H3

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  1. Nucleosomes

Four types of Histone protein form a core particle called nucleosome. These proteins are H2A, H2B, H3, H4. The•nucleosome is composed of eight molecules of histones. DNA coil around the nucleosome. Histories have a high proportion of positively charged amino acids. So they bind tightly to the negatively charged DNA and form chromatin. The unfolded chromatin shows beads on a string under electron microscope. Chromatin organization controls the transcription of DNA. Therefore, nucleosome influences the gene expression.

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Linker protein H1

The  fifth histone HI is called the linker protein. It is not needed to form the nucleosome. Rather it fixes the DNA with nucleosomes.

3         Solenoid

e  beaded string undergoes higher order packing. The chromatin coils and Ids further to produce the thickened, compact chromosomes during mitosis.

e  beaded string can coil tightly with the help of histone HI. It makes a cylinder 31 nm in diameter. It is called solenoid.

4          Looped domains

The solenoid fiber forms loops called loop domains.

5          Metaphase chromosomes

The metaphase chromosome themselves coil and fold in a mitotic chromosome. It further compact all the • chromatin. Thus it produces the characteristic metaphase chromosomes. Not all chromatin is equally active. Some human g nes are active only after adolescence. In other cases, entire chromosomes may not function in particular cells. So there are two areas of chromosomes:

  • Heterochromatic region: These are inactive portions of chromosomes. They show a banding pattern with certain stains. So they are called heterochromatin. The DNA of heterochromatin is not transcribed.
  • Euchromatic region: The active portions of chromosomes are called euchromatic regions.

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SEX CHROMOSOMES AND AUTOSOMES

Chromosomes that are represented differently in females and males and function in sex determination are called sex chromosomes. Chromosomes that are alike and not involved in determining sex are called autosomes. Work was started on the sex determination in the early 1900s. This work was done on insect Protenor. It gave evidences about the chromosomal basis for sex determination. The Protenor has one darkly stained chromosome. It is called the X chromosome. It is represented differently in males and females.

  • Al! somatic (body) cells of males have one X chromosome (XO). Therefore, half of all sperm contain a single X, and half contain no X chromosomes
  • All somatic cells of females have two X chromosomes (XX). All female ‘gametes contain a single X.

This pattern suggests that if X-bearing sperm fertilize an egg it produces female offspring. If a sperm with no X chromosome fertilize an egg. It produces a male off pring. This sex determination system gives 50:50 ratios of females to males in his insect species.

Th re are following systems of sex determination:

1 XO system: The system of sex determination in Protenor is called the X0 system. It involves only one kind of chromosome. So it is the simplest system of sex determination.


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  1. XY system: In the XY system, males and females have an equal number of chromosomes. But the male is XY and female is XX. This system is present in many animals like man and drosophila.
  2. ZW system: The XY system in birds and butterfly is called ZW system. In this case the male is XX and female is XY.LINKAGE RELATIONSHIPST e genes present on the same chromosomes are called linked gene and the phenomenon is called linkage. Number of genes in a cell is greater than the number of chromosomes. Therefore, each chromosome contains hundredso  thousands of genes. These genes are part of a single chromosome. They are passed as a unit. Genes located on the same chromosome tend to inherit together in genetic crosses. Such genes are linked genes. The results of linked gene deviate from expected results of Mendelian principle of .independent a sortment.

    Morgan performed experiments on Drosophila. He studied the affect of linkage

    o  the inheritance of two different characters. He followed two characters, body color and wing size of drosophila:

    • Wild type: Wild-type flies have gray bodies and normal wings.
    • Mutants: Mutant phenotypes for these characters are black body and vestigial wings. The vestigial wings are much smaller than normal wings. e alleles for these traits are represented by the following symbols: V = gray, b= black; vg + = normal wings, vg vestigiel wings. They are not sex linked. Their loci are on autosomes.est crossorgan crossed female di-hybrids (V b vg+ vg) with males. These males were utant with black bodies and vestigial wings (b b vg vg). It is a Mendeliant stcross. Arc ilinc to Mendel’s law of independent assortment, Morgan’s rosopiiiict testcross should produce four phenotypic classes of offspring. These lasses should be equal in number:
      • 1 gray-normal (wings) . 1 black-vestigial

      .    1 gray-vestigial

      .    1 black-normal.

      he actual results were very different. There were disproportionate numbers of wild-type (gray-normal) and double mutant (black-vestigial) flies among the offspring. These two phenotypes corresponded to the phenotypes of the two parents. Morgan concluded that body color and wing shape are inherited together in a specific combination. The genes for these two characters are located on the same chromosome. The other two phenotypes (gray-vestigial and black-normal) had much less number. These phenotypes were present among the offspring of Morgans cross. These new combinations of the two characters were formed by crossing over.

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The Recombination of Linked Genes: Crossing Over

Linked genes are located on the same chromosomes. So they do not assort in ependently. They move together during meiosis and fertilization. Therefore, Ii ked genes should not recombine by the assortments of alleles. But, in fact,

r combination between linked genes occurs.        ”

( ) The offspring of tire testcross did not give 1:1:1:1 phenotypic ratio. This ratio comes if the genes for these two characters were present on different chromosomes and assorted independently.

) But if the two genes were completely linked then we should observe a 1:1 ratio. The parental phenotypes should be present among the offspring.

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The actual results are different from both these expectations. Most of the offspring had parental phenotypes. It suggests linkage between the two genes. But about 17% of the flies were recombinants. Thus the linkage appeared incomplete. Morgan proposed the mechanism of crossing over for these different results. Crossing over breaks the linkage between the two genes. Crossing over occurs when the homologous chromosomes are paired in synapsis during prophase of meiosis 1. The non- sister chromatids break at some points and exchange fragments. A crossover between chromatids of homologous chromosomes breaks linkages in the parental chromosomes. It forms recombinants. The recombinants bring together alleles in new combinations The later stages of meiosis distribute the recombinant chromosomes to gametes.

 

 

 

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