Question

Answers the following questions in your own words.

a. What is meant by ‘chromosome’. Explain its types.

b. Describe the structure of the DNA molecule.

c. Express your opinion about the use of DNA fingerprinting.

d. Explain the structure, function and types of RNA.

e. Why is it necessary for people to have their blood examined before marriage?

Answer 1

a. Chromosomes are thread-like structures found in the nucleus of all living cells. Chromosomes are divided into four types based on the position of the centromere.

(1) Metacentric chromosomes: In these chromosomes, the centromere is present in the middle, which gives rise to two equal arms.

(2) Sub-metacentric chromosomes: In sub-metacentric chromosomes, the centromere lies slightly away from the middle region. As a result, it has one arm slightly longer than the other.

(3) Acrocentric chromosomes: In acrocentric chromosomes, the centromere is located close to the end of the chromosome. As a result, it has one arm, which is extremely long and the other, which is extremely short.

(4) Telocentric chromosomes: In telocentric chromosomes, the centromere is present at the terminal end. As a result, the chromosome has only one arm.

b. DNA is a very large single molecule also called as macromolecule. It has a double helix structure, similar to a ladder, which is twisted at both ends. The DNA molecule is made up of basic materials called nucleotides and each nucleotide is made up of three components :

• Sugar
• Phosphate groups

Sugar and phosphate are arranged lengthwise.

• Nitrogen bases are attached sugar from inwards that extends to join hydrogen bond and the complimentary nitrogenous base from other strand.

The nitrogen bases are :

• Adenine(A)
• Guanine(G)
• Cytosine(C)
• Thymine(T)

Adinine pairs with Thymine with two hydrogen bonds. Guanine pairs with Cytosine with three hydrogen bonds.

During the interphase of cell cycle, DNA molecules duplicates for their equal distribution in the daughter cells during mitosis. The double helix opens at one end, makes the two strands free and from there new strands are formed.

c. DNA fingerprinting is a method for comparing the DNA sequences of any two individuals. 99.9% of the base sequences in all human beings are identical. It is the remaining 0.1% that makes every individual unique.

The applications of DNA fingerprinting are as follows:

• DNA fingerprinting is widely used in forensics since DNA of every tissue from an individual has the same degree of polymorphism.

• DNA fingerprinting forms the basis of paternity testing since a child inherits polymorphism from both its parents.

• It can be used for studying evolution and genetic diversity in a population.

d. A RNA nucleotide has three main components − a nitrogenous base, a ribose sugar and a phosphate group.

• The ribose sugar and the phosphates form the backbone of a polynucleotide chain with nitrogenous bases linked to sugar moiety and projecting from the backbone.

• Two types of nitrogenous bases are present i.e. Purines (Adenine and Guanine) and Pyrimidines (Cytosine and Uracil).

• A nitrogenous base is linked to the ribose sugar through N-glycosidic linkages to form a nucleoside (like adenosine, guanosine or cytidine and uridine).

• A phosphate group is linked to 5'-OH of a nucleoside through phosphoester linkage to form a corresponding nucleotide.

• Every nucleotide residue has an additional −OH group present at 2' -position in the ribose.

• Many nucleotides are linked through 3'-5' phosphodiester linkages to each other to form the polynucleotide chain.

• The end of the chain which has a free phosphate moiety at 5'-end of ribose sugar is referred to as 5’-end and the other end of the chain having a free 3'-OH group at the ribose sugar is referred to as 3' -end of the polynucleotide chain.

There are 3 types of RNA:

1. mRNA (messenger RNA) − It serves as a template for protein synthesis. DNA is transcribed to form an mRNA, which in turn is translated to form protein.

2. tRNA (transfer RNA) − It brings amino acids during translation and reads the genetic code.
3. rRNA (ribosomal RNA) − These are the work benches of translation. They play a structural and catalytic role during translation.


e. It is necessary for people to have their blood examined before marriage because the genetic disorders are transmitted only by reproduction. If a carrier/sufferer of a genetic disorder marries a person who is also a carrier/ sufferer of the disorder, then there are chances that disorder will be passed on to the offsprings. In order to prevent this transmission, people should get their blood examined before marriage to know if they are a carrier of any genetic disorder.

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