When DNA is replicated, the two strands are first separated through ; each strand then becomes a against which two new strands are made.

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Explanation:

Deoxyribonucleic acid, DNA, replication is the process whereby DNA makes a copy of itself and it occurs during cell division.

When DNA is replicated, the two strands are first separated through unzipping at the origin of the replication fork. This is catalysed by the enzyme helicase. Helicase breaks down the hydrogen bonds which hold the complementary base pairs of DNA together.

Each strand of DNA to then becomes a template against which two new strands are made. One of the strands is called the leading strand and the other is called the lagging strand. Both are oriented in opposite directions.

explanation:

b,bacteria convert nitrates

into nitrogen gas.

d,plants and animals take in

and use nitrogen compounds.

e,bacteria break down waste    

products and return nitrogen  

to the soil.

a,bacteria convert atmospheric

nitrogen to ammonia.

c,bacteria convert ammonia

to nitrites and nitrates.