Question

SHORT ANSWER 1. Explain how the technique of DNA fingerprinting works. 2. What is the purpose of using a restriction enzymes? 3. What is the technique of gel electrophoresis used for? 4. What is the overall charge on DNA and what gives DNA this charge? 5. If both the parents are carriers of sickle cell disease, what is the probability of having a child that has sickle cell disease? Draw a punnett square to determine the probabilty.

Answer 1

ANSWER 1

In DNA fingerprinting, the DNA sample is first acquired from blood, semen, hair or saliva. The DNA is digested by restriction enzymes which cuts the DNA at specific sites, thus producing fragments of different lengths. These fragments are separated by gel electrophoresis. The pattern of DNA bands is called DNA fingerprint.

DNA fingerprint is unique to an individual. The probability of two people having the same DNA fingerprint is very small unless they are identical twins. This is how DNA fingerprint works.

ANSWER 2

The purpose of restriction enzymes is to cut the DNA. Restriction enzymes cut the DNA at specific sites called restriction sites. Generally, restriction sites are 4-8 bp long sequences. Cutting by restriction enzymes either produce blunt ends or sticky ends with single stranded overhangs.

ANSWER 3

Gel electrophoresis is a technique used for separating DNA, RNA and proteins based on their molecular size.

Agarose gel electrophoresis is used to separate DNA fragments based on their molecular size/weight. An electric field is applied across the gel and the DNA is allowed to move through the pores of the gel. DNA is negatively charged, therefore it migrates from negatively charged cathode to positively charged anode. The smaller fragments move faster than the larger fragments. Thus, the fragments get separated.

ANSWER 4

The overall charge on the DNA is negative. This negative charge comes from the phosphate backbone of the DNA. Since DNA is negatively charged, it moves towards anode in gel electrophoresis.

ANSWER 5

Sickle cell disease is caused by a mutation in the β globin gene which is a part of the hemoglobin gene. It is an autosomal recessive disorder.

Let us call the normal gene as Hb^A and the mutated gene as Hb^S.

Parents HbAHbs Carrier HbAHbs Carrier Gametes НЬ НЬs x HbA Hbs HbA Hbs HbA HbAHbA Normal HbAHbs Carrier Children HbAHS HbsHbs Hbs Carrier Sickle celled

From the punnett square, it is clear that if both parents are carriers of sickle cell disease, the probability of a child having sickle cell disease is 1/4 or 0.25.