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Class 12th Biology 2014 Set1 Delhi Board Paper Solution

Question 29

(a) Explain the process of DNA replication with the help of a schematic diagram.

(b) In which phase of the cell cycle does replication occur in Eukaryotes? What would happen if cell-division is not followed after DNA replication.

OR

(a) Explain Darwinian theory of evolution with the help of one suitable example. State the two key concept of the theory.

(b) Mention any three characteristics of Neanderthal man that lived in near east and central Asia.

Answer

(a) DNA Replication DNA replication is the phenomenon in which a duplicate copy of DNA is synthesised. The steps involved in the process of DNA replication are as follows:

DNA replication occurs in S-phase of the cell cycle. Enzyme involved: DNA polymerase (DNA dependent DNA polymerase) Replication requires energy. Source of energy -Deoxyribonucleoside triphosphates (DNTPs) DNTPs have dual purposes: act as substrates as well as provide energy Replication initiates at specific regions in DNA called the origin of replication. DNA polymerase polymerises a large number of nucleotides in a very short time. During the course of replication, two parent strands do not completely open, but a small opening forms in which replication occurs. This small opening forms a replication fork. DNA polymerase can polymerise only in one direction. Therefore, replication occurs smoothly in to end of DNA (continuous replication, but occurs discontinuously in to end). The discontinuous fragments so formed are joined by DNA ligase.

 

(b) DNA replication occurs in S phase of cell cycle in eukaryotes. If cell division is not followed after DNA replication then the replicated chromosomes (DNA) would not be distributed to daughter nuclei. A repeated replication of DNA without any cell division results in the accumulation of DNA inside the cell. This would increase the volume of the cell nucleus, thereby causing cell expansion.



OR

(a) Darwinian theory of Evolution

According to Darwin, evolution took place by natural selection. The number of life forms depends upon their life span and their ability to multiply. Another aspect of natural selection is the survival of the fittest where nature selects the individuals, which are most fit to adapt to their environment. An example of such a selection is the antibiotic resistance in bacteria. When a bacterial population was grown on an agar plate containing antibiotic penicillin, the colonies that were sensitive to penicillin died, whereas one or few bacterial colonies that were resistant to penicillin survived. This is because these bacteria had undergone chance mutation, which resulted in the evolution of a gene that made them resistant to penicillin drug. Hence, the resistant bacteria multiplied quickly as compared to the non-resistant (sensitive) bacteria, thereby increasing their number. Hence, the advantage of an individual over the other helps in the struggle for existence. Darwin also observed that variations are inheritable and the species fit to survive the most leaves more offsprings. Hence, the population characteristics change, giving rise to the evolution of new life forms.

         The two key concepts of the theory are:

         (1) Branching descent : According to this concept, various species have come into existence from a common ancestor.

         (2) Natural selection : According to this concept, nature selects the individuals, which are most fit to adapt to their environment.

(b) Characteristics of Neanderthal man

(i) They possess a brain capacity of 1400 cc.

(ii) They were short but very strong with outward curved thigh bones.

(iii) They used hides to protect their body and to bury the dead.

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