(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 ? Whatwould 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 concepts of the theory.

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

• DNA replication is the process by which DNA molecule replicate itself.

• Each strand in a parental duplex DNA acts as a template for synthesis of a daughter strand and remains base paired to the new strand, forming a daughter duplex by semi-conservation mechanism.

• Replication begins at a sequence called an origin and new strands are formed in 5’ to 3’ direction.

• DNA replication require many enzymes and proteins are as follows:

a) DNA polymerase and DNA primase to catalyze nucleoside triphosphate polymerization.

b) DNA helicase and single strand DNA binding(SSB) proteins to help in opening up the helix so that it can be copied.

c) DNA ligase for joining of DNA fragments.

d) DNA topoisomerase for unwinding of DNA helix.

e) Helicases to separate the parental DNA strands.

• DNA replication generally occurs by a biodirectional mechanism in which two replication forks form at an origin and move in opposite directions, with both template strands being copied at each fork.

• At a replication fork one daughter strand that is leading strand is elongated continuously.

• The other daughter strand that is lagging strand is formed as a series of discontinuous okazaki fragments from primers synthesized every few hundred nucleotides.

• Finally, adjacent okazaki fragments are joined by DNA ligase.

(c) DNA replication occurs in S-phase of cell cycle in eukaryotes. Replicated chromosomes would not be distributed to daughter nuclei if the cell division is not followed after DNA replication.

A repeated replication of DNA without any cell division, results in the accumulation of DNA inside the cell and results in cell expansion.

OR

(a)

• Charles Darwin was an English naturalist who proposed and provided evidence for the scientific theory that all species have evolved over time from one or a few common ancestors.

• This theory became widely accepted by the scientific community in the modern evolutionary theory.

• Darwin stated that variation exists among individuals of a species.

• He stated that scaricity of resources in a burgeoning population would lead to competition between individuals of the same species because all use the same limited resources.

• Such competition would lead to the death of some individuals, while others would survive.

• From this reasoning Darwin concluded that individuals having advantageous variations are more likely to survive and reproduce that those without the advantageous variations.

• Darwin coined the term natural selection to describe the process by which organism with favourable variations survive and reproduce at a higher rate.

For eg. Giraffe’s neck

During times of food scarcity the longer-necked individuals are able to exploit food resources that are unavailable to the shorter- necked individuals. In this way longer- necked individuals penetrate their kind in much greater numbers that the shorter-necked individuals.

Two key concepts of the theory are-

1. Evolution- This theory states that the world is not constant or recently created nor perpetually cycling, but rather is steadily changing, and that organisms are transformed with time.

2. Natural selection- the relatively few individuals who survive, owing to a particularly well-adapted combination of inheritable characters, give rise to the next generation.

b. Characterstics of Neatherland man are-

• They lived from 400,000 to 40,000 years ago.

• Height about 1.50-1.75m

• Weight about 64-82kg

• Brain size at least 1,200cm³ to 1,750cm³.