Q. 26

(a) Why are colou

Answer :

(a) Color blindness and thalassemia are categorised as Mendelian disorders because in these disorders single gene altered or mutated. Their mode of inheritance follows the principles of Mendelian genetics.

Mendelian disorders are transferred to offspring’s on the same liners too. They may be dominant or recessive.

Mendelian disorders can be autosomal dominant, autosomal recessive like thalassemia or sex linked like color blindness.

The symptoms of thalassemia are as follows:

- Facial bone deformity

- growth problems – not putting on weight or growing in height.

- dark urine

- anaemia – red blood cell deficiency, leading to tiredness, weakness and shortness of breath.

- jaundice – yellowing of the skin and whites of the eyes i.e., pale coloration.

- swollen abdomen (tummy) – this is caused by an enlarged liver of spleen.

- Fatigue

The symptoms of color blindness are as follows:

- Difficulty distinguishing between colors due to missing of single color receptive cones.

- Inability to see shades or tones of the same color

- Rapid eye movement (in rare cases)

(b) Colour blindness is a genetic disorder i.e., X-linked recessive disorder and is caused by a recessive gene located on X-chromosome.

Females have two X chromosomes, so for a female to get color blindness, both the X chromosomes should be have mutant gene. This is not always possible.

While males have just one X chromosome and they are affected by color blindness when only one X chromosome have mutant gene. So they have higher chances of getting affected in comparison to females.

This is because atleast one of the two X chromosomes can have normal gene gene for each type of cone. And as the male has just one X chromosome, so missing gene can lead to color Blindness.

X chromosome comes from mother and never from father, so color blindness is passed from mother to son. And mother is color blindness carrier, hence 8% of human male population suffers from colour blindness whereas only about 0 �4% of human female population suffers from this disease.


Transcription is a process of transferring genetic information from one strand of DNA to RNA. It is done by formation of RNA over the template of DNA. In transcription complementary takes place, except that adenosine now form base pair with uracil instead of thymine.

The segment of DNA that takes part in transcription is known as Transcription unit. In transcription process the transcription unit consists of three regions of DNA namely – a promoter, a structural gene and a terminator. The main enzyme in transcription is RNA polymerase. RNA polymerase has a sigma factor; which recognizes the start signal of the promoter region.

Process of transcription:

In prokaryotes, the DNA-dependent RNA polymerase catalyses transcription of all types of RNA

RNAs are of three types – mRNA (messenger RNA), tRNA (transfer RNA) and rRNA (ribosomal RNA).

Initiation: The RNA polymerase binds to the promoter. The Sigma factor recognises the promoter site and initiates transcription. The RNA polymerase uses nucleoside triphosphates as substrate for initiation and polymerises in a template depended fashion. This happens based on particular rule of complimentary. It facilitates the opening of the helix and promotes elongation.

Elongation: The activated ribonucleotides come to lie opposite to the template strand, complementarily & start pairing, and result in releasing of phosphate radical & energy. With the help of energy and Mg2* ions, the core enzyme bonds the adjacent nucleotides to form RNA chain. As the enzyme moves along the DNA, the RNA chain elongates till terminator region is reached.

When the polymerase reaches terminator region, nascent RNA falls off and termination process starts.

Termination: By means of Rho (ρ) factor & NUS G the termination starts. The rho (ρ) factor has ATPase activity. After RNA separation, the sense & antisense strands of DNA re-establish Fl-bonds.

Difference in Eukaryotic Transcription:

- In eukaryotes in addition to RNA polymerase there are at least three RNA polymerases in the nucleus.

- RNA polymerase I help to transcribe rRNA.

- RNA polymerase II helps to transcribe precursor of mRNA, i.e., hnRNA.

- RNA polymerase III helps to transcribe tRNA.

- In Eukaryotes the primary transcripts contains non-functional intron and exons. So it is subjected to splicing where introns are removed and exons are joined. This is followed by capping and tailing. In capping unusual nucleotide is attached to 5’-end of hnRNA. And in tailing, adenylate residues (are joined at 3'-end in a template independent manner. Now the hnRNA is fully developed and is known as mRNA. This mRNA is used for translation.

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