Long Answer Question: Explain the process of DNA replication. - Biology

Answer in Brief

Long Answer Question:

Explain the process of DNA replication.



The process by which DNA duplicates to form identical copies is known as replication.

Semi-conservative method of replication:

1. After replication, each daughter DNA molecule has one old and other new strands.

2. As parental DNA is partly conserved in each daughter's DNA, the process of replication is called semi-conservative.

3. The model of semi-conservative replication was proposed by Watson and Crick.

4. The semi-conservative model of DNA replication using the heavy isotope of nitrogen N15 and E. coli was experimentally proved by Meselson and Stahl (1958).

Mechanism of replication is as follows:

a. Activation of Nucleotides:

i. The four types of nucleotides of DNA i.e. dAMP, dGMP, dCMP and dTMP are present in the nucleoplasm.

ii. They are activated by ATP in presence of an enzyme phosphorylase.

iii. This results in the formation of deoxyribonucleotide triphosphates i.e. dATP, dGTP, dCTP and dTTP. This process is known as Phosphorylation.

b. Point of Origin or Initiation point:

i. Replication begins at a specific point ‘O’ origin and terminates at point ‘T’.

ii. Origin is flanked by ‘T’ sites. The unit of DNA in which replication occurs is called replicon.

iii. In prokaryotes, there is only one replicon however in eukaryotes, there are several replicons in tandem.

iv. At the point ‘O’, enzyme endonuclease nicks one of the strands of DNA, temporarily.

v. The nick occurs in the sugar-phosphate backbone or the phosphodiester bond.

c. Unwinding of DNA molecule:

i. Enzyme DNA helicase breaks weak hydrogen bonds in the vicinity of ‘O’.

ii. The strands of DNA separate and unwind. This unwinding is bidirectional and continues as ‘Y’ shaped replication fork.

iii. Each separated strand acts as a template.

iv. The two separated strands are prevented from recoiling (rejoining) by SSBP (Single-strand binding proteins).

v. SSB proteins remain attached to both the separated strands for facilitating the synthesis of new polynucleotide strands.

d. Replicating fork:

i. The point formed due to the unwinding and separation of two strands appears like a Y-shaped fork, called replicating/ replication fork.

ii. The unwinding of strands imposes strain which is relieved by the super-helix relaxing enzyme.

e. Synthesis of new strands:

i. Each separated strand acts as a mould or template for the synthesis of a new complementary strand.

ii. It requires a small RNA molecule, called RNA primer.

iii. RNA primer attaches to the 3’ end of the template strand and attracts complementary nucleotides from the surrounding nucleoplasm.

iv. These nucleotides bind to the complementary nucleotides on the template strand by forming hydrogen bonds (i.e. A=T or T=A; G = C or C = G).

v. The newly bound consecutive nucleotides get interconnected by phosphodiester bonds, forming a polynucleotide strand.

vi. The synthesis of a new complementary strand is catalyzed by enzyme DNA polymerase. 7. The new complementary strand is always formed in 5’→ 3’ direction.

f. Leading and Lagging strand:

i. The template strand with free 3’ end is called a leading template and with free 5’ end is called a lagging template.

ii. The process of replication always starts at the C-3 end of the template strand and proceeds towards C-5 end.

iii. As both the strands of the parental DNA are antiparallel, new strands are always formed in 5’ → 3’ direction.

iv. One of the newly synthesized strands which develop continuously towards the replicating fork is called the leading strand.

v. Another new strand develops discontinuously away from the replicating fork and is called the lagging strand.

vi. Maturation of Okazaki fragments: DNA synthesis on the lagging template takes place in the form of small fragments called as Okazaki fragments (named after scientist Okazaki).

vii. Okazaki fragments are joined by the enzyme DNA ligase.

viii. RNA primers are removed by DNA polymerase and replaced by DNA sequence with the help of DNA polymerase-I in prokaryotes and DNA polymerase-α in eukaryotes.

ix. Finally, DNA gyrase (topoisomerase) enzyme forms a double helix to form daughter DNA molecules.

g. Formation of daughter DNA molecules:

i. At the end of the replication, two daughter DNA molecules are formed.

ii. In each daughter's DNA, one strand is parental and the other one is totally newly synthesized.

iii. Thus, 50% is contributed by mother DNA. Hence, it is described as semiconservative replication.

Concept: DNA Replication
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Chapter 4: Molecular Basis of Inheritance - Exercises [Page 93]


Balbharati Biology 12th Standard HSC for Maharashtra State Board
Chapter 4 Molecular Basis of Inheritance
Exercises | Q 5.1 | Page 93


Explain replication of bacteriophage with the help of a suitable diagram.

What is Bacteriophage?

As the base sequence present on one strand of DNA decides the base sequence of other strands; this strand is considered as ______.

Very Short Answer Question:

When does DNA replication takes place?

Semiconservative mechanism of DNA was detected using:

Enlist the names of enzymes used in semiconservative replication of DNA?

Describe the process of semiconservative replication of DNA with the help of a neat and labelled diagram.

Okasaki fragments are joined together by ______.

What are Okazaki fragments?

E. coli cell grown on 15N medium are transferred to 14N medium and allowed to grow for two generations. DNA extracted from these cells is ultracentrifuged in a cesium chloride density gradient. What density distribution of DNA would you expect in this experiment?

What is the basis for the difference in the synthesis of the leading and lagging strand of DNA molecules?

Which of the following statements about DNA replication is not correct?

Which of the following statements is not true about DNA replication in eukaryotes?

Meselson and Stahl’s experiment proved ____________.

Differentiate - Leading stand and lagging strand.

From their examination of the structure of DNA, What did Watson and Crick infer about the probable mechanism of DNA replication, coding capability, and mutation?

a) Identify the figure given below

b) Redraw the structure as a replicating fork and label the parts

c) Write the source of energy for this replication and name the enzyme involved in this process.

d) Mention the differences in the synthesis of protein, based on the polarity of the two template strands.

Identify the direction of DNA replication in eukaryotes.

Match Column I with Column II and select the correct option among the following.

  Column I   Column II
i. DNA replication a. RNA polymerase
ii. Translation b. DNA polymerase
iii. Transcription c. Reverse transcriptase
iv. Reverse transcription d. t-RNA- amino acid complex

For which of the following reason RNA primer is must for initiation of DNA replication?

Assertion (A): DNA replication is said to be semi-conservative.

Reason (R): After DNA replication, each daughter molecule has one old and one new strand.

Which of the following is present at the sticky ends of a fragmented DNA molecule?

The addition of nucleotides on the lagging strand occurs ____________ during DNA replication.

In a segment of DNA with 10 base pairs, the numbers of sugar molecules are _________.

Select the mis-matched pair.

When the DNA molecule appears like inverted Y, it is ______

Nucleolus is a major center for:

"DNA is considered as genetic material" - Why?

Extension of the plasma membrane in a prokaryotic cell is ______.

Which of the following statement is incorrect?

Wobble position means:

Match the following column.

Column - I Column - II
(A) DNA structure 1. Muller and stadder
(B) Semiconservative replication of DNA 2. Beadle and partum
(C) One gene-one enzyme theory 3. Watson and crick
(D) Induction of mutation 4. Massillon and stahl

Okazaki is known for his contribution to the under tanding of:

During replication of DNA Okazaki fragments are formed in the direction:

Identify the biomolecule that is capable of self-replication.

Okazaki segments are formed during ______.

Which of the following reaction is required for proofreading during DNA replication by DNA polymerase III?

What background information did Watson and Crick had available with them for developing a model of DNA? What was their own contribution?

During the replication of DNA, the separated strands are prevented from recoiling by using ______.

Draw a suitable diagram of replication of eukaryotic DNA and label any three parts.

In prokaryotes, the primers of lagging strands are removed by ______.

Enzyme involved in synthesis of RNA primer.


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